introductio1

INTRODUCTION

Printed circuit board (PCB) is now an important item for any mechanical, electrical and electronics device. It is used mainly for the maintenance of device as well as to minimize the maintenance problem. The PCB are classified in three category.

Single Sided Double Sided Multilayer Flexible

In ESTC as PCB manufacturing lab with advance facilities have been setup. The lab have PCB production (Single sided & Double side only) capacity of 3 meter square per shift max. 40cm x 40cm PCB size.

The raw materials for PCB productions are basically copper clad sheet, various chemicals & films of different kinds. The PCB of moderate & lower quality.

Glass epoxy sheet have single & both sided copper and used for manufacturing the single, double & multi PCB of high quality. For processing single side PCB general chemical are required but for double sided PCB specialized chemical are essential. In India generally Growl & Well brand name chemicals are used for PCB production line.

SINGLE SIDED BOARD

The term single sided means the insulating substrate contains circuit pattern on only one side which is called as solder side. The other side is called component side. This kind of PWPB is used in professional and non-professional grades and are manufactured by Print and Etch method.

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DOUBLE SIDED BOARD

The term Double Sided means the circuit pattern is present on the either wide of the insulating material. Circuit pattern is present both on components side and solder side. In this board, the components density and the conductor lines are higher than the single sided boards. There are two types of double sided boards commonly available. They are:-

Double sided board with plated through hole connection (PTH) Double sided board without plated through hole connection (Non

PTH)Double Sided PTH Board has plated through hole connection between the top and bottom layer. This is a new concept of technology. In this board all the component mounting holes and inter-layer connecting holes are drilled and metallised with through hole electro plating. The top side is called component side and bottom side is called solder side. The component side has less dense circuits and solder side has more dense circuits in horizontal direction. Double sided NON PTH board is only an extension of single sided board and there is not much change in manufacturing method.MULTILAYER BOARDSIt consists of a number of layers of separate conductors bonded together to produce a homogenous unit. It can be constructed as a number of double sided boards stocked together. In the board, the circuit density is high compared to double sided board.The interconnections between the inter layers are obtained by means of plated through hole. The through hole connection are of two types:

Blind via ( One side visible) PTH Buried via ( Invisible ) PTH.

Totally the size of the multiplayer board is drastically reduced. The board is manufactured like double sided board with plated through hole techniques after layers are built up.

FLEXIBLE CARDS

Flexible boards are similar to rigid boards excepts the boards are made from a flexible substrate materials. The base materials are very thin and the common thickness is 0.1mm. Plastic fillers such as Polyester, Polyimide, Fluorinated Ethylene Propylene resin (FEP), Poly Vinyl Chloride (PVC) and thin Epoxy glass with excellent electrical and mechanical properties are used

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for flexible boards. Laminates are available with or without filler reinforced material and with copper either on one or both the sides in rolls. BASE MATRIALSThe Base A thin insulating material which is the mixture of filler and resin on which all conductors and the components are mounted, is called base material .This can be either rigid or flexible material.FillerThese are continuous webs of base, such as organic, Non organic, metallic or non- metallic formAramide fibers ( Organic)Graphite( Non- Metallic)Copper clad invar ( Metallic)Molybdenum (Metallic) ResinsThese are based on synthetic types of organic base materials, either thermo plastic or thermosetting plastic formed by poly mersation process.Resins are polymeric material which are obtained in their natural form the trees. On exposure to the atmosphere, they dry into hard semi- transparent masses.There are two types of resins

Natural resins Synthetic resins

Natural Resins: - They are obtained from natural sources like plants and animals. On exposure to atmosphere, they dry into hard semi- transparent masses.Examples:- Amber wood resins and Shellac.Synthetic Resins: - There are obtained from the chemical reactions of a simple organic compound through polymerization reactions. Synthetic resins are of two types

Thermo Plastics Thermosetting Plastics

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Thermo PlasticThere plastic are softened on heating Plastics are available in their

additive or semi- additive form and are used for general purpose surface mount applications. The example for thermo plastics are.Cellulose Derivatives oly hydro carbon ( Polyethylene, polytetra flouro ethylene)Polystyrene , Polypropylene, etc.Vinyl polymers (Polyvinyl) acetate, chloride alcohol, polyamide, etc. Polycarbonate resinsThermosetting Plastics

These resins do not soften or melt after its manufacturing process. When heated they only change irregularly.Examples of thermosetting plastics :- Formaldehyde resins ( Phenol formaldehyde, urea melamine resins) Polyester resins. Epoxy resins, Silicone resins, Polyamide resins.COPPER FOIL

A layer of very thin copper foil of thickness of 17.5 microns, 35 microns or 70 microns is bonded to one or both sides of the base material. 17.5 micron copper foil is also denoted as half ounce since half ounce of copper is used to get a 1 sq. ft of copper clad sheet with 17.5 micron thickness of copper.

LAMINATES

Phenolic Laminates Phenolic laminates are reinforce with paper fillers and the copper foil

is pressed to the base material. Hence they are also called as paper phenolic copper clad laminates. Commercially available grades are 1.6 0.8, 1.6 18/18 Fr-2, 1.6 35/0.Epoxy Laminates he base material is reinforced with glass fiber or paper fiber as filler and epoxy resin and the copper foil is pressed over it to get copper clad laminates. Hence they are called glass epoxy copper clad laminates or paper epoxy copper clad laminate depending on the type of filler used. Type of epoxy laminates :-

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Thickness :-

1.6, 35/0 - S/S0.8, 18/18 - D/S1.6, 18/18 - D/S2.4, 18/18 - D/S3.2, 18/18 - D/S

Teflon Laminates

Teflon is reinforce with glass fiber to get a laminate of low dielectric constant and it is used to Radio frequency (RF) applications with small leadless components. Due to the high Co- efficient of Thermal Expansion, the use is limited.

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PROCESS FLOW CHART OF S/S PCB

Film Master Preparation Brushing Screen Fabrication

Screen Printing Etching Manual Drilling

Ink Stripping Masking Roller Tinning or HAL

Legend Printing Final Cutting Quality Control.

PROCESS FLOW CHART OF D/S PCB

Film Master Preparation Sheet Cutting CNC Drilling

Brushing Electro less Photo Printing

Pattern Plating or Electroplating Dry film Stripping

Etching Tin Stripping Masking

HAL Legend Printing Final Cutting Quality Control.

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Mechanical Operation

Under mechanical operation. We understand PCB production steps such as drilling, laminate cutting to processing size or final size. Slot cutting etc. In PCB manufacturing basically two type of material used. Base material with glass enforced show typically 10 times liner abrasives effect on the cutting tool used than those with paper. Therefore tools made of tungsten carbide materials are preferred for glass reinforced.

SHEARING

Shearing is suitable cutting method for almost any king of base material of less than 2 mm thickness. If boards thicker than 2 mm are cut the finish of the edges become coars and unclean and there for unacceptable.

The shearing of glass reinforced laminate is less difficult in obtaining a perfect edge finish. Paper reinforced laminates required after heating up to a temp. in the rang up 60- 1000 C to obtain a clean edge.

SHEARING MACHINE

Laminate shearing is preferably done on manual or motor drive plate shear plate shears used for laminate cutting should not be used for sheet metal cutting because perfect blades are a newest for clean laminate shearing when shearing start the board to be cut is first firmly pressed down with & spring loaded hold down device before shearing is executed.

This hold down is an absolute must be to prevent the otherwise unavoidable shifting of the board. While shearing it through. The hold down device should be at a very close distance to the cutting blades.

SHEARING BLADES

Both blades can be of rectangular shape but the lower one is often provided with a free angle of up to 70 for shearing of different kind of laminates a length wise angle between. The blades of 1-15 0 will give best results for glass epoxy materials up to 4 0 can be tolerated for a clean edge finish the gap between the cutting edges of the two blades of the two blades has to be les than 0.25 mm for the cutting to other final size the board to be cut is aligned is such away that corner marks are flush with the fixed shearing blade.

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SAWING

The sawing of laminates by means of a circular saw is the preferred method of many laminate manufacturers. PCB manufacturers, too. Often use these sawing machine. Which are specially designed for this purpose. The edge finish is smooth and acceptable for clean cutting. The dimension tolerances are of the same order as in shearing unless a jig with registration pins is used, which provides a 0.2 mm tolerance field.

CIRCULAR SAWING MACHINE

Sawing machine of the moving- table type give the best results. The design should be such that no noticeable drop in cutting speed occurs during operation. This is achieved by heavy pulleys with more than one V- belt. The optimum saw blade speed can very in the range of 2,000-6,000 rpm which calls for suitable provisions for speed control.

CIRCULAR SAW BLADES

For paper phenolic materials, HSS blades with a diameter of approximately 300 mm are used at a speed of 2,000- 3,000 rpm. Too thing should be around 1.2-1.5 tooth per cm circumference. The time between re sharpening is multiplied by a factor of 3-5 if tungsten carbide tipped saw blades are used at a speed of 2,500-6,000 rpm.

Glass- epoxy materials are cut with tungsten- carbide tipped blades or for still better performance, with diamond wheels. The higher initial costs of such blades are offset by the much longer life and improved edge finish.

OPERATION

For safety purposes, the blade should always be covered by a suitable guard device. The circular saw is adjusted in such a manner that the free height of the blade above the boards to be cut is in the range of 10-14 mm. A bad edge finish can be caused by a blunt or badly sharpened blade, by too much play in the bearings, by a too coarse too thin or by an incorrect feed rate. Thick materials need a lower feed rate while thin materials can be cut faster.

PUNCHING

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Punching of holes into PCB’s ( also called pressing) is a widely used method in high- volume production of consumer- type boards made of paper phenolic and paper epoxy laminates. The punching of glass epoxy boards is not common because of cracks formed inside the hole.

A cross – section of a punched hole will show a conic shape and partially a rough surface. Punching of holes is therefore not compatible with professional PCB plated through hole processes. For plated through hole, only drilling provides an exactly cylindrical shape and a high surface smoothness.

Where paper phenolic material is punched, the cold punching varieties are usually preferred. This is not only because heating up of the boards is adverse to a rationalized production but also because of dimensional accuracy which can be better controlled in cold punching. In warm punching, an expansion of the boards or 20-40 ppm/c has to be considered during the tool design stage.

The actual punching is performed on ex center presses with a capacity of 10-30 t and 100-200 strokes/min. The punching is always done from the copper foil side in case of single-sided PCBs. The punches should have a diameter which is 0.1-0.12mm bigger than the desired whole diameter in boards of 1.6mm thickness. The conicity of the hole depends on the difference between punch and die hole diameter. The smaller the difference, the lesser the conicity, but at the same time tool manufacturing costs will rise enormously with the higher precision required. A very typical diameter tolerance of punched holes is 0.1mm

BLANKING

When the clean cutting operation is done with a punching toll (either than with a shear or a saw), it is called blanking.

In some cases, hole punching and blanking are done with the same tool in the same operation. Therefore most of what has been said under punching applies equally to blanking.

Blanking is sometimes also used for glass epoxy materials but the edge finish does not look very clean although it is functionally acceptable. (with blanking tools, PCB dimensions can be kept within a tolerance field of (0.1-0.2)mm.

Where standard PCB formats with a pin registration system are prevalent, blanking can be very efficient and economic solution for clean- cutting of PCBs. This also includes PCBs with plated through holes.

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MILLING

Clean- cutting of PCBs by milling is preferred where a good finish and exact overall dimensions are a required feature. Milling of PCB controlCan be carried out on wood or metal working milling machines, either of horizontal or vertical type. The usual cutting speeds are in the range of 1,000-3,000 rpm with straight or spiral tooth HSS milling cutters. Here too use of tungsten carbide tools will result in a much longer life of the cutter, especially for glass epoxy materials. For milling a strong support to the PCB must be given, if necessary with backing plates. This is to avoid possible delimitation .

ROUTINGIn routing, the clean cutting is done with the help of a pin router

machine . The boards are moved past a vertical side mill with the aid or a routing jig. The routing jig is guided in relation to the mill by holding it against a bushing which is concentric with the mill. Internal registration holes are used to position the PCBs on the routing jig.

Routing provides an excellent edge finish similar to milling. The dimensional tolerances are within. (0.1-0.2)mm. Compared with blanking, it is much slower, its speed depends on the quantity of material to be removed. But a routing jig is much cheaper than a blanking too. Sometimes blanking with slight oversize is applied followed by routing to get a smooth surface finish.

DRILLING

Drilling of component mounting holes into PCBs is by far the most important mechanical machining operation in PCB production processes. Holes are made by drilling wherever a superior hole finish for plated through hole processes is required and where the tooling costs for a punching tool cannot be justified. Therefore drilling is applied by all the professional grade PCB manufactures and generally in all smaller PCB production plants and laboratories.

DRILLING MACHINE

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Drilling machine PCB application are available in a wide range of designs and principles. But one common feature of all PCB drilling machine is the high speed range which is required for an economic and efficient drilling of the various base material. The speeds mostly used are around 20,000-50,000 rpm. The minimum spindle speed available should not be less than 15,000 rpm for glass reinforced laminates ( tungsten carbide drill bits ) or 8,000 rpm for paper reinforced laminates and HSS drill bits. Very sophisticated drilling machines have spindles operating up to 1,00,000 rpm. While very simple machines use belt drives of fast running DC- motor spindles, there are electronically controlled high frequency spindles employed in almost all the more sophisticated drilling machines. The spindle speed can thus be controlled over a wide range and is kept constant. In some cases, the spindles are water cooled with the help of a water circulation and cooling system built into the machine.

JIG DRILLIN

With jig drilling also called semi automatic drilling, the productivity goes up to 30-50 strokes per minute. The resulting accuracy is comparable to drilling by optical sight( 0.10mm) but more results are obtained.

A jig can be made our of 3mm acrylic glass or brass. The holes in the jig are drilled by optical sight using a film master of the respective pattern on the top of the plate , followed by drilling. The drilling of the side of the holes into the jig is done with almost care because of their reference function. Counter sinking is carefully done to suit the type of position sensor (spring tracer) used in the drilling machine.

The machines and methods discussed so far make use of only one spindle which permits simultaneous drilling of maximum five boards in a stack. To increase productivity , specially designed jig drilling machines are available , operating with up to 8 spindles , all governed by the same jig and sensor . In certain design , the movement of the tables is controlled via the jig sensor while other designs utilize the control of the spindle position via the sensor.

The ensure that all the holes are drill in jig drilling operations , the routes corresponding to the various drill bit diameters can be indicated on the jig by different colors and controlled by a reset able stroke counter.

DRILL BITS

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The drilling of holes into paper phenolic laminate with drill bits made of high speed steel (HSS) does not cause much difficulties . However if the same drill bits are used for glass epoxy materials, the cutting edges will become blunt after a short time and re sharpening has to be done. Therefore drill bits made of tungsten carbide are widely used in PCB drilling operations, even for paper phenolic laminates.

Drill bits made of tungsten carbide are also preferred because of a very high operation time between re sharpening and also excellent quality of holes. Based on this fact, the present section will deal exclusively with drill bits made of tungsten carbide materials.

The tungsten carbide drill bits employed are usually of the solid types straight shank drill bits are used in drilling with bushes which give perfect guidance very close to the point where the bit enters the boards and thus enable very accurate hole positions. Standard shank drill bits are used without bushed. They are costlier because of more tungsten carbide material involved. On the other side, the standardization on one particular shank diameter will be cheaper on the drilling equipment side (only one collect type required)

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CNC DRILLING PROGRAMME

Firstly take the solder side positive film. Paste the miler sheet on the film make the path making i.e. identify the different tool for different colors.

After path making the film is put on drill bed put glass on the film, Camera is fixed on drills spindle. Firstly you have checked the film is paste on drill bed is state line or not. If the film is state line than you have start the programming.

Press the programming switch program LED is glow. Set tool switch (1) You have firstly feed the speed and feed rate for auto tool changing feed and speed is feed by using Joy Stick. Top display of the DRO to the required spindle RPM i.e. X- axis & feed State in Y- axis.

The top display reads directly in RPM x 1000 i.e. reading of +000.50 equal to 50,000 RPM. The Max. speed which can be set is 60,000 and any display above +000.60 will set to Max spindle speed of 60,000 RPM.

Now use the joys till to set the bottom display of the DRO to the required Down feed. The down feed is entered as a number from +000.00 to +000.99. Any display above +000.99 will be taken as 99 and stream the max. down feed of 150 hits/min. Do not set any negative value. Press the SP/FD button above the tool switch the machine will response with a beep sound. You have feed every tool. After the speed and feed is feeded than you have set the tool switch (01). Move the camera with the help of path making on the film and centering the pad and press the hole switch. When tool is completed press tool (02) start same as above after completion of programme press END switch one time. For I.C. or connector holes you have set the first hole of the IC press hole switch than move he camera of the last hole of the IC press hole switch than move the camera of the last hole of the IC. Set the number of holes by using thumb wheel of SIL that press SIL switch. The machine automatic take Co- ordinate of each holes.

If you want to see previous position of hole Co-ordinate used BST switch. If you are press the BST switch the camera goes to the previous hole position. SST used for forward position.

After completion of programme save the programme on magnetic Tap(Audio Cassettes ) for, further used. Press play & Record button of tap records and that press save switch blinking the LED of save switch indicate that programme is save on magnetic tape after completion of saving green LED will be glow and beep sound will be create.

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DRILLING OPERATION

After completion of program put the laminate on the bed paste the film on laminate to sheet the full program will be drill or not.

If the laminate is ok than move the camera corner of the laminate and press X, Y button of DRO. At that time the display of DRO X & Y both direction is 00. Move the camera on the first hole of the programme press M/Exp button. At that time the tool setting will be zero position.

Firstly you have checked the program on monitor it is fully drill in laminate or not. With the help of RUN & check switch. If program is ok. Than press reset & home button the machine goes in home position. Set the drill bit on drill pot in sequence remove camera release the program switch press the RUN button.

After completion of drilling the machine goes in home position and beep sound will be create and END switch will be glow up. That’s indicate drilling is completed. If you want to drill other panel once again press reset home remove drill laminate & place new laminate and press RUN switch again.

PHOTO FILM PROCESSING

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Dark room section is an important area of PCB and screen printing with the help of this area for PCB and Screen Printing with the help of this area, the Film master (neagative and positive) are prepared. Negative or positive which is used directly on PCB coated by photo resist film or light sensitive screen is called film master.

Requirement for the preparation of film master: Following are required for the preparation of film master.

Lith Film ( Ortho graphic film) Developer Hardener Retouching brush Retouching ink PCV tray Hanging clip.

Orthographic filmIt is used for the preparation of film master. An photographic material consists basically of two layer, the emulsion and the base is myler.

The emulsion under unexposed conditions, contains the light sensitive material Silver Bromide is coated in emulsion side. The base material can be accelerate, polyester on glass. In PCB Technology, the use of polyester based film has become standard. The film used in PCB manufacturing must have high contrast capability.DEVELOPER

It is used for developing the exposed film. It is available in market as Lith film developer. Its container/box consists two packets on which part (A) and part(B) are printed. The equal ratio of part A & part B is used for developing the exposed film. Hardener

It is used for hardening and fixing the picture on film. It is basically sodium thio-sulphate and dissolved in water for after developing the film. Retouching Brush

It is used to cover the pin holes developed on film master generally “0” number brush is used.Retouching Ink

It is basically ink of gray colors which is used to cover pin holes with the help of brush so that light can not be passed on.

Hanging Clip

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These are steel clips used for hanging the film master while drying.

FILM PROCESSING

Following steps are involved in film processing:-

Exposing Developing Hardening Drying Retouching

At the time of exposing every bulb and tube light is OFF position only RED bulb Zero watt should be On position. For negative artwork and emulsion side of lith film and artwork will be paste and put between two glass ( non- emulsion side of lith film is up side) and on the lamp with 2 to 3 sec. After exposing the lith film is taken out developed in developing solution by manual shaking when the negative are visible the film is taken out from developing solution and kept in hardening solution for some time so that the visible negative and positive become hard.Then the film is taken out from hardening solution and kept for drying in air. In case any pin holes observed it is retouched with proper ink and brush.

In case of positive preparation the lith film and negative are kept emulsion to emulsion. After exposing the film is taken out and developed in developing solution. After this process the film is kept in hardening solution for some time so that the visible negative and positive become hard.

Then the film is taken out from hardening solution and kept for drying in air. In case any pin holes observed it is retouched with proper in brush.

SCREEN PRINTING

Process:-

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In Screen Printing techniques, there are three types of stencil making methods namely

1. Direct stencil with film2. Direct stencil with emulsion and film3. Direct stencil with emulsion.

Step 1 Screen Mesh SelectionMesh size of 90 T to 110T is used for indirect methods and mesh size

of 120T is used for direct method.Mesh size of 90 T to 110T is used for indirect method and mesh size

of 120T is used for direct method.

Step 2 Stencil Making

1. The Direct method2. Indirect method3. Direct- Indirect

Step 3 Exposing :-

1. Carry out step exposure test .2. Maintain the distance between halogen lamp and vacuum

Step 4 Developing :-

1. The exposed screen must be moisturized both the sides with cold water and then thoroughly washed from the squeegee side.

2. Check the screen opening.

Step 5 Drying

1. The developed screen is dry at 30 0c2. Inspect the screen and touch up the pin holes with proper super

coat solution.3.

Step 6 Printing :-

1. Clean the substrate

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2. Set the printing machine

3. Check the ink viscosity

4. Check the squeegee blade.

5.(A) The Squeegee blade should not project more than

approximately 35mm. From the squeegee handle.

(B) The squeegee hardness should be 70-75 shore

(C) Squeegee angle should be 750c

(D) Squeegee length should be exceeding the image by 2-3 cm on each side.

6. Check the temperature and humidity in the working area.

(i) Humidity should be 50-60 0c

(ii) Temperature should be 18-21 0c

Direct and indirect Film Using Charomoline Film

Process :-

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Step 1:-(i) Fixing the charomoline film(ii) Cut the charomoline film more than the actual circuit size(iii) Keep the film emulsion side in contact with the printing side

of the screen.(iv) Apply coating solution (P.V.A.) on the squeegee side.(v) Remove excess solution with squeegee.

Step 2:- Drying(i) Dry the coated screen(ii) Remove the packing shut carefully

Step 3:-(i) Keep the film master emulsion side in contact the printing

side.(ii) Expose the screen for 3-4 min. under a halogen lamp.

Step 4:- Developing :

(i) Develop with cold water on both sides.(ii) Removes excess water.(iii) Dry the screen under a fan(iv) Inspect the opening area and retouch up the pin holes.

Direct Stencil Making Process(Using Ply vinly Acetate (PVA)

Procedure :-

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Step 1 :- Mixingi. Weigh the emulsion and catalyst ii. Mix the above according to the supplier

recommendediii. Keep if for 30 min.

Step 2 :- Coatingi. Two coats on the print and four to five coats on the

squeegee side all wet on wet. ( It means the next coat is given when the previous coat is still wet.)

ii. The screen should be dried on a horizontal position print side down, for half hour at 30oc

iii. Make the emulsion thickness between 14 to 18 microns

Step 3 :- Exposingi. Keep the film master emulsion in contact with the

screen printing side.ii. Use metal halogen lampiii. Provide 1.5 m distance between lamp and vacuum

frameiv. Exposing time approx. 5 minutes

Step 4 :- Developingi. Spray cold water both sides and then thoroughly

was from the squeegee side.ii. Check the open area.iii. Dry the screen

Step 5 : Printing

Indirect Method Direct Stencil with emulsion and filler using Five Star Film ProcessStep 1:-

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i. Take a five star film and cut to a required size.Step 2 :- Exposing

i. Use U.V. lampii. Keep the film master on the five star film in to

reverse directioniii. Deep the non emulsion side of the five star film in

contact with the emulsion side of the film master.iv. Expose 2-3 min.

Step 3 :- Developingi. Mix water with hydrogen peroxide in the ration 3:1

i.e. 3 part water and 1 part H2O2

ii. Keep the exposed film in the solution for 20 sec.iii. Wash the film with warm water (400c) on the

emulsion side.Step 4 :- Fixing

i. The developed film should be fixed on the screen.ii. Keep the emulsion side in contact with printed

side.iii. Remove the excess wateriv. Dry the screen using warm (4000oc) air or with

fanv. Inspect the pin holes and air gapsvi. Retouch the unwanted openings.

ETCHING

Etching techniques used today are

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(a) Immersion etching(b) Bubble etching(c) Splash etching(d) Spray etching

The spray etching is the most commonly used technique due to high productivity and fine line definitions.

Immersion Etching

Immersion etching technique involves a deep tank containing etching solution into which the boards are immersed. They are kept immersed until the etching is complete. This requires a long process time. The solution can be heated to speed up the etching process. This method is suitable for small boards or prototype. In this method, the etch rate is low. Normally ammonium per sulphate or hydrogen peroxide with sculpture acid etching medium is used for immersion etching.Bubble etching

In this technique air is bubbled through the etching solution. This technique is a modified form of immersion etching and this gives high etch rate. Air passing through the solution has two functions.

(a) It ensues fresh etch ant contact on the surface.(b) It enhances the oxidation power and regenerates the etch ant.Rate of etching to a certain extent depends on the air pressure to get good quality of etching. Chromic sulphuric acid.

Splash EtchingSplash etching or paddle etching is better than the bubble

etching with regards to even etching and minimum under cut. But at a time, only a limited number of boards can be etched. Ferric chloride and chromic sulphuric acid solutions are commonly used in this type of technique. The machine principle involves a paddle or cup attached to a motor driven shaft. When the motor rotates, the etchant is thrown by centrifugal force towards the boards being etched. The contact of the solution with the boards depends on the shaft rotation and paddle design.

SPRAY ETCHING

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In spray etching technique, there is no sludge formation and the etching rate is constant. There are two types of spray etching. They are:-

(a) Horizontal Spraying(b) Vertical Spraying

The etch ant is pumped under pressure from the reser through a pipe network to the nozzles and splashed on to the boards. The rate of etching depends on etch ant chemistry, the pump pressure and nozzle configuration and placement. The boards are etched continuously in a closed loop system.

The etch rate is high in this system with minimum undercut and fine line definition. Ammonimu chloride etch ant is commonly used in this technique for double sided PTH boards.

The spray etching machines are available in automatic semi automatic modes for vertical or Horizontal type of spray. The automation contains pressure control, heating element, specific gravity indicator and automation solution regeneration.

The equipment fabricated should be made of acid and alkali resist material like PVA. However, equipment for sulphuric acid hydrogen peroxide etch ant system require stainless steel, poly carbonate or poly propylene material.

Etching Solution

Ferric Chloride as etching solution. Ferric chloride etching solutions are used in PWB industry. It still uses composition or ferric chloride in water and hydrochloride. It has a high etch rate and high copper dissolving capacity. It is quite inexpensive.Ferric chloride is used with screen inks, photo resist and gold plated boards. But this is not suitable for tin or tin lead plated boards.At the initial stage of etching process, the concentration of copper dissolved increases and the etch rate also increases linearly, after which there is a fall in the concentration. The better dissolution of copper occurs when the etch ant is sprayed perpendicular to the copper surface, when the boards is moved. The rate of dissolution copper depends on the ferric chloride concentration, temperature and agitation.

Chemistry

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The Cupric chloride further reacts with copper to form cuprous chloride. The overall reaction is

CU+ + Fe 3+ Cu2++ +Fe 2+Hydrochloric acid is added to increase the etching rate and depress the formation of ferric hydroxide.Composition of Ferric Chloride Etch ant

FeCl3 (Ferric Chloride) - 350-500 gm/ ltr.Specific Gravity - 1.4Copper Dissolving Capacity - 120 gm/ ltr.HCL( Hydrochloric Acid) - 10m/ltTemp. - 20-45 Deg.Antifoaming Agent -` 3ml/lt.

Hydrogen Peroxide Sulphuric Acid Etchant :-

This system is extensively used for copper surface preparation which is also called micro etching. It is compatible with organic and metallic resists and provides a steady etch rate with optimum undercut.Chromic Sulphuric Acid Etch Ant:-

Chromic acid mixed with sulphuric acid is used because of its strong oxidizing power and suitability for all kinds of metal resist ( tin, tin-lead, gold vinyl lacquer and dry or liquid photo resist) It has fast etching rate with little under cutting. But the use is now limited since it is difficult to regenerate and it is highly toxic polluting and hazardous to health. Hence it is not recommended for use.Ammonical Etchant/ Ammonium Chloride Etch Ant:-

Alkaline ammonical etching system is used in both batch and conveyor spray etching system and is compatible with metallic and organic resists. The advantages of this etch ant are minimum undercut, high copper dissolving capacity and fast etch rates. It is easily replenishable.

Chemistry

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Duty of high copper dissolving capacity, the system is increasingly used in PWB industry. Function of different ingredients are:-

Cupric chloride as copper ion Ammonium hydroxide complexion agent and also to increase

the pH Ammonium chloride improves the etch rate Ammonium bicarbonate as a betterTo preserve the solder metal surfaceAmmonium Nitrate increase the etching speed.The etch rate is increased with increase in temp. The cupric ion is oxidized to cuprous complexion by oxygen in air.CU+CU(NH3)42+ - 2CU(NH3)2+

4CU(NH3)2+5NH3+O2+2H20 - 4CU(NH4)42+4OH

CompositionAmmonium Bicarbonate - 75g/lAmmonium Nitrate - 80g/lCupric Chloride - 200g/lAmmonium Chloride - 100 to 110g/lpH - 8.5 to 9Temp. - 45 to 55ocSpecific Gravity - 1.2 to 20ocCopper Content - 150 to 160g/l

Under Cut

The reduction of line width more than the designed width during etching is called under cut. This happens due to more dissolution of copper.

Over hang

Excess metal growth on the side of the line is called over hang. Excess overhang may induce electrical shorting like the silvers.

BRUSHIGN

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Laminate Surface Preparations

For the image transfer to take place, the laminate should be free from oil, grease, dust finger prints and foreign particles. Any contamination on the surface of the laminate may impair the adhesion of the photopolymer or decrease the bonding of the electro deposited copper. Hence very good cleaning methods are required. The methods commonly followed are :-

Chemical Cleaning Mechanical Cleaning

Chemical CleaningThe chemical cleaning uses concentrated alkali chemicals to remove

the oil, grease and soil particles on the surface of the laminate. The concentration of the alkali chemical which is between 80- 100% at a temp. between 60o c to 70oc is used for 20 to 30 minutes for cleaning the laminates. After this process of alkaline soda, the laminates is effectively rinsed with filtered tap water which is oil and soil fee. Water immersion followed by strong water spray ensures complete removal of cleaners. Neutral or acidic cleaners are sometimes preferred because of the attack of hot alkaline solutions on exposed epoxy or polyamide substrates.

Mechanical Cleaning :-

In the mechanical cleaning process, the laminates are cleaned by using abrasives such as emery corundum aluminum oxide and silicone carbide. The mechanical cleaning operations are of different types. They are :

Polishing Brushing Buffing or Sanding Debarring or Scrubbing

PolishingThis process involves, smoothening of the metal surface to improve

appearance. To do it polishing wheels are generally used.

Brushing

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This process is a metal smoothening operation which involves non metallic wire brushes such as nylon. The wire brushes are generally run wet. For very dull finishes pumice slurry of grade 3F or 4F with water is used.Buffing

Buffing involves the metal surface buffed with the muslin wheel, to obtain low gloss finish to high gloss finish. They can be done in two ways.

(a) Satin finishing(b) Cut down buffing

DeburringAfter the board is drilled or punched, the burr formation is noticed.

Burr is the extra projection of metal when a hole is drilled through PWB. These sharp edges should be eliminated. The machine used for debarring consists of a debarring wheel (fused silicone carbide). This is used to remove the burr on the drilled laminate. This process is carried on only in the presence of wet conditions. The sanding process wet debarring involves a water resistant abrasive of 280 to 600 grid.

In all the above methods, after laminate is cleaned, the boards are rinsed with water at high pressure and dried with hot blow air. Normally all mechanical processes involve automatic or semiautomatic machines. During these processes, the base material may be removed which is turn degrades the electrical properties of the boards. Hence strict process monitoring is required.

PHOTOPRINTING

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Pre-laminating Cleaning (Surface Preparation) :-

The laminate surface is cleaned, using any one of the method mentioned under surface preparation techniques discussed at the beginning of this chapterPre-lamination Drying

The laminate surface whether it is drilled or un drilled will absorb water or moisture during surface cleaning process. The absorbed moisture is removed by moisture during surface cleaning process. The absorbed moisture is removed by 15-20 minutes bake in an oven set at 110deg._+5 deg. CWater must be physically removed or blown off from the surface and via holes before oven drying. A high volume air turbine dryer should be used at this stage.Lamination :-

The cleaned and dried laminates are coated with dry film photo resists using hot roll laminator. The hot roller is preheated at 120deg.c. and the lamination is done at high pressure. The dry film is coated on the laminate surface. The temp. pressure and fee space varies from one laminator to the other based on the recommendation of the dry film photo resist manufacturers. The distance between top chamber and the surface heating which is called as the free space must be at the required level. Less free space may lead to air entrapment between resist film and the laminate surface. The entrapment may bead to metal chip age during plating or under cut during etching.

Hold Time After Lamination

The hold time between lamination and exposure should be controlled carefully for optimum results. Dry film photo resist after lamination is immediately cooled for 10 to 15 minutes. For better dimensional stability. This period is called the normalizing period.

During the a lamination the resist is coated at a temp. of 120 deg. c. If no time is allowed for cooling its dimensional properties will vary . If the polymer is exposed, the resolution of the circuit image is varied. So hold time after lamination is very important.Exposing :-

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Exposure is carried out by registration of the emulsion side of the photographic film like diazo film or sliver halide film on the polyester sheet. The two are then placed in a vacuum frame for intimate contact between surfaces. The proper exposing time depends on light intensity, temperature, thickness of photopolymer, and type of equipment being used. To determine the required exposure time, hold time and the suitable development conditions a trial production work may be carried out. The required exposure level varies with the type of film and photopolymer manufacturer normally supplies suitable data.

Developing

Developing the exposed laminate after stabilization starts with the peeling off the protective polyester cover sheet. The developing process washes away the unexposed or un polymerized photo polymer without affecting the exposed polymerized portions of the film.Developing time is dependant on the chemistry and temperature of the developer solution and the thickness of the photo polymer. Recommended chemistries and temperatures must be maintained before the starting the developing process. If not controlled, the developer affects the polymerized resist causing degradation of the surface and the side wall of the film.Thorough removal (development of unexposed resist from surface is necessary to ensure good etching and plating.The total time between lamination and development should not be more than 8 hours to minimize difficulties in developing process.

PLATING OPERATIONS

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The purpose of metal deposition in electronics is to protect the surface from corrosion, to increase the solder ability and to interconnect the components.

Different processes are followed to deposit metal in PCBs. They are :-

(A) Electro less Plating.

(B) Electro Plating.Electro less Plating

Electro less plating mechanism is similar to the electroplating but the electrons are obtained from chemical reduction process. In electro plating the electrons are obtained from a DC power source to reduce metal ions to the metallic state. But in case of electro less plating, external source of energy is not used. The reaction takes place only on a catalytic surface.A catalytic surface is a surface on which a noble metal like palladium is seeded to trigger the chemical reduction process. Plating would occur only on the catalyzed substrate.

The Process for Electro less Plating :-

The following operations are carried out in sequence to complete the electro less plating on a drilled hole in a PCB to create PTH.

(a) Laminate drilling(b) Laminate debarring(c) Laminate cleaning either by solvent degreasing or alkaline soak

cleaning(d) Water rinse(e) Micro etch or etch back multiplayer boards(f) Water rinse(g) Dipping in sulphuric acid of 20% concentration(h) Water rinse(i) Sensitizer dipping(j) Activation mixed catalyst(k) Water rinse(l) Accelerator dip (m) Water rinse(n) Electro less plating ( copper electro less plating)(o) It the laminate is already, there is no need for step (i) and step (j)

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DebarringThe drilled board surfaces are cleaned using debarring machine which

uses a 320 grade silicon carbide brush.

Alkaline Soak Cleaning for Laminate Cleaning

In PTH process, instead of vapor degreasing, hot alkaline soak cleaning is most used. In this process oil, grease layers and epoxy smears are removed. The bath contains mixed, alkali chemicals. The temperature of the bath is 60-70deg.C and the dipping time is 20 minutes.

Micro Etching for Surface CleaningAmmonium per sulphate and sulphuric acid solution is used for micro

etching or cleaning of copper surfaces already existing on the laminate. In this process some amount of copper is removed from copper surfaces and dissolved in the solution used to achieve uniform surface. The important of micro etching is to get uniform metal surface and to increase the bonding between the copper layers. The etch back method is used multilayer PTH processing to cleaning the hole walls.Acid Dip

20% sulphuric acid is used for the dipping process. This helps to get active copper surface on the board.Pre- Activator

Pre Activator is a mixture of stannous chloride and hydrochloric acid. The function of this bath is to initiate the reaction of catalyst.

HCL AR Grade - 30%Remaining - D.I. WaterTime - 1 Min.Tem. - R.T.W.R. - No.Activator

Activator or catalyst is a solution of palladium chloride, hydro chloric acid and some non ionic wetting agent. The function is to activate the entire surface including metal and base.37% Hole AR grade - 2000ml/l

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Ginplate Additivae - 5ml/lt.Ginplate Activator - 76ml/lt.Time - 5-6min.Temp. - 25-32deg.C

Post ActivatorPre activator and activator are missed in a bath to get mixed catalyst

solution. The mixed catalyst produces the catalyst film only on the surface of insulator but the copper foil of the laminate remains un activated. The thin palladium film will improve adhesion between the base copper and plated copper.Ginplate PA-491 - 17%Remaining - D.I. waterTime - 3-5 Min.W.R. - 0.5 Min.The pre activating step involves immersion of the board in an acidic solution containing stannous ions which are absorbed by the surface of the resin. Kthe stannous ions reduces the palladium ion and the stannous ions are oxidized to stannic ions according to a reduction reaction.

Electro less Copper Plating

Ginplate CU 406A - 100ml/lt.Ginplate CU 406B - 100ml/lt.Ginplate CU 406C - 10ml/ltr.Water - D.I. RemainingTime - 10-12 Min.

Principal of Electro Plating

It is the process by which the coating metal is deposited on the base metal by passing a direct current through an electrolytic solution containing the solvable salt of the coating metal.

The metal to be plated is used as the cathode in the electrolytic cell.The anode in the cell can be same as the metal to be coated on cathode or any other inert metal having good conductivity. The reaction which take place at the junction between the electrolyte and the electrodes is called electrolysis. It is accompanied by transfer of electrons.

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Electro Chemical Terms.

Anode :- It is a positively charged electrode. During electrolysis, positive ions are formed at the electrode. In other words, oxidation reaction takes place.

Anion:- Negatively charged radical which on electrolysis is attracted towards the anode.

Cathode :- It is a negatively charged electrode. During electrolysis, negative ions are formed at the electrode. In other words reduction reaction takes place.Anodes :

In electroplating, the positive electrode or anodes in the from of bars, plates, rods or pallets are used. It may be the same metal of electrolyte or different, but it should be free from any impurities. The contaminated anode will affect the electroplating solution and cathode deposition. So the anode used in electroplating should be 99.9% pure. The size of the anode is also very important and it depends on the current carrying capacity.Anode Bags :-

Anode bags are used to cover the anodes and filter the dissolved particles. The anode bags are made up of poly propylene materials and are available in various grades (1 micron to 10 micron).Electro PlatingAcid CleanerD.I. - 25%Temp. - RemainingTime - 5MinW.R. - 0.5MinMicro EtchGinplate AD 481 - 40/LH2So4 AR Grade - 5CCTime - 1Min.Temp - 2Min.W.R. - 0.5Min.Acid DipH2So4 - 10%Remaining - D.I. WaterTime - R.T.W.R. - 1Min.

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Electrolytic Copper PlatingH2SO4 - 10%CuSO4 - 80g/lt.Cupper Acid - 7CC/lt.Brightener - 2CC/ltDipping Time - 15min.Current Density - 2-3Amp/Sq.dm.Voltage - 3-9VFiltration - Const.Agitation - Low or moderate air pressureAnode - Phosphoresced Cu Min. 0.04-0.08Anode Bag - Napped polypropyleneAnode to Cathode Ratio - 2:1 MoreW.R. - 0.5-1min

Electrolytic Tin Plating

Stannous Sulphate - 30g/lH2SO4 - 1000ml/lStannous Carrer - 30ml/lStannolume Brightner - 2ml/lTime - 1minW.R. - R.T.Anode Current - 1-2 Amp/dm2Filtration - Recommended

GENERAL PLATING DEFECTS

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1. Voids :- If the holes are not drilled and cleaned properly, during electro less plating gases or organic chemicals are entrapped or accumulated in the holes walls. When the boards are wave soldered or pr heated, the entrapped chemicals or gases escape with high pressure and give discontinuity or cracks on the hole walls. These are called voids2. Blow Holes :-

During plating and copper clad manufacturing, some air entrapped between the copper layers or between copper and base material. This air comes out when the boards is kept in high temperature. This will give cracks in the plating pattern. That result is blow holes , small voids also give blow holes.3. Out gassing :-

Pinholes are formed during tin-lead surface finished. This is due to co-deposition of organic additives during electroplating of tin lead.

SOLDERMASK OVER BARE COPPER

This is one of the major solder resist application technology. SMOBC stands for Solder Mask Over Bare Copper. One of the major disadvantages of conventional tin lead electroplating undermeath the solder mask is blistering of solder mask, during wave soldering or infrared reflow or vapor phase soldering operations.

The flow of the molten tin lead under meath the solder mask leads to the development of a strong hydraulic force which forces its way through the solder mask, thereby reputing the solder mask which can seriously bring down the reliability of the PCB, by acting as a direct conduit for the flow of liquids and excessive soldering flux on to the base material.

The SMOBC process overcomes these problems by totally eliminating the tin-lead deposition on the conductor undermeath the solder mask. An all copper plated PCB can often the prepared by the tin and etch process. In this case the PCB is initially drilled and the holes are moralized by the conventional electro less copper plating process. Then the copper is built up to the required thickness by panel plating. After this copper build up the dry film is laminated and exposed using a negative photo tool which polymerizes the photo resist in the exposed areas there by forming a tent over the hole. Then the board is directly taken for etching where the unwanted copper around the hole and the conductor areas are removed

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leaving only the holes and the conductor pattern. Later on the photo resist is stripped and solder masked where only on the pad areas copper will be exposed and the copper on the tracks will be covered by the photo resist. The copper exposed are later on prevented from getting corroded by applying a thin coating of tin lead by a process of hot air leveling (HAL).

The other alternative process is to process the PCB by the conventional pattern plating process. However, the metal etch resist is stripped and the PCB is solder masked. In this case also, the pads are prevented from getting corroded by the process of HAL, there by increasing the solder ability of the Printed Circuit Board.

LEGEND PRINTING

This is the printing done generally on the component side of a PWB, which gives the details of the components and their assembly position on the board. Legend printing is done by the process of screen printing. Epoxy based ink is used for this purpose. These inks when once printed and cured, bond permanently with the epoxy of the base material. This marking which gives the component outline with their identification numbers helps in the identification of the component during the rework and troubleshooting of the PCBs.

Legend printing is generally the last process done in the fabrication of a printed circuit board. This is done by the screen printing process. Generally used colors for legend printing are white, black and yellow.

Roller Tinning

This is a process for putting the tin layer on copper pad to improve solder ability and protect from atmospheric attack. This process is suitable for single sided.

Hot Air Leveling

HAL i.e. Hot Air Leveling is an advance machining process, applied on double sided & multilayer PCB for putting tin layer of required thickness on copper pads. This process increases the production & reduces the rejection percentage as earlier there was heavy rejection in tin plating.

Quality Control of PCB

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This is last process for PCB production to minimize the rejection percentage in process quality control is very essential. Following are required for quality control of PCB.

Hole dia gauge. (To measure the size of a hole) Beta Scope. ( To measure the thickness of plating) Double adhesive tap.( For peel off test) Bare board tester.( For finding out the short and open circuit of the

board)

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The process of HAL, thereby increasing the solder ability of the Printed Wiring Board.

LEGEND PRINTING

This is the printing done generally on the component side of a PWB, which gives the details of the components and their assembly position on the board. Legend printing is done by the process of screen printing. Epoxy based ink is used for this purpose. These inks when once printed and cured, bond permanently with the epoxy of the base material. This marking which gives the component outline with their identification numbers helps in the identification of the component during the rework and troubleshooting of the PCBs.

Legend printing is generally the last process done in the fabrication of a printed circuit board. This is done by the screen printing process. Generally used colors for legend printing are white, black and yellow.

Roller TinningThis is a process for putting the tin layer on copper pad to improve

solder ability and protect from atmospheric attack. This process is suitable for single sided.Hot Air Leveling

HAL i.e. Hot Air Leveling is an advance machining process, applied on double sided & multilayer PCB for putting tin layer of required thickness on copper pads. This process increases the production & reduces the rejection percentage as earlier there was heavy rejection in tin plating.Quality Control of PCB

This is last process for PCB production to minimize the rejection percentage in process quality control is very essential. Following are required for quality control of PCB.

Hole dia gauge ( To measure the size of a hole) Beta Scope ( To measure the thickness of plating) Double adhesive tap ( For peel off test)

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Bare board tester ( For finding out the short and open circuit of the board)

2.

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introductio1

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