Chapter 19Chapter 19

Electronic Electrochemical Electronic Electrochemical Chemical and ThermalChemical and Thermal Machining Processes Machining Processes

(Review) (Review)

EIN 3390 Manufacturing ProcessesEIN 3390 Manufacturing ProcessesFall, 2010 Fall, 2010

19.1 Introduction19.1 IntroductionNon-traditional machining (NTM) processes

have several advantages◦Complex geometries are possible◦Extreme surface finish◦Tight tolerances◦Delicate components◦Little or no burring or residual stresses◦Brittle materials with high hardness can be

machined◦Microelectronic or integrated circuits (IC) are

possible to mass produce

NTM ProcessesNTM Processes Four basic groups of material removal using NTM processes

◦Chemical: Chemical reaction between a liquid reagent and

workpiece results in etching◦Electrochemical

An electrolytic reaction at workpiece surface for removal of material

◦Thermal High temperature in very localized regions evaporate

materials, for example, EDM◦Mechanical

High-velocity abrasives or liquids remove materials

Limitations of Conventional Limitations of Conventional Machining ProcessesMachining Processes

Machining processes that involve chip formation have a number of limitations◦Large amounts of energy◦Unwanted distortion◦Residual stresses◦Burrs ◦Delicate or complex geometries may be difficult or impossible

Conventional End Milling vs. NTMConventional End Milling vs. NTMTypical machining parameters

◦Feed rate (5 – 200 in./min.)◦Surface finish (60 – 150 in) AA – Arithmetic

Average◦Dimensional accuracy (0.001 – 0.002 in.)◦Workpiece/feature size (25 x 24 in.); 1 in. deep

NTM processes typically have lower feed rates and require more power consumption

The feed rate in NTM is independent of the material being processed

19.2 Chemical Machining 19.2 Chemical Machining ProcessesProcessesTypically involves metals, but ceramics

and glasses may be etchedMaterial is removed from a workpiece by

selectively exposing it to a chemical reagent or etchant◦Gel milling- gel is applied to the workpiece in

gel form.◦Maskant- selected areas are covered and the

remaining surfaces are exposed to the etchant. This is the most common method of CHM.

Table 19-1 Summary of NTM ProcessesTable 19-1 Summary of NTM Processes

MaskingMasking

Several different methods◦Cut-and-peel◦Scribe-and-peel◦Screen printing

Etch rates are slow in comparison to other NTM processes

Figure 19-1 Steps required to produce a stepped contour by chemical machining.

Defects in EtchingDefects in Etching

If baths are not agitated properly, defects result

Figure 19-2 Typical chemical milling defects: (a) overhang: deep cuts with improper agitation; (b) islands: isolated high spots from dirt, residual maskant, or work material inhomogeneity; (c) dishing: thinning in center due to improper agitation or stacking of parts in tank.

Advantages and Disadvantages Advantages and Disadvantages of Chemical Machiningof Chemical MachiningAdvantages

◦Process is relatively simple

◦Does not require highly skilled labor

◦ Induces no stress or cold working in the metal

◦Can be applied to almost any metal

◦Large areas◦Virtually unlimited

shape◦Thin sections

Disadvantages◦Requires the handling

of dangerous chemicals

◦Disposal of potentially harmful byproducts

◦Metal removal rate is slow

Photochemical MachiningPhotochemical MachiningFigure 19-4 Basic steps in photochemical machining (PCM).

Design Factors in Chemical Design Factors in Chemical MachiningMachiningIf artwork is used, dimensional variations can

occur through size changes in the artwork of phototool film due to temperature and humidity changes

Etch factor (E)- describes the undercutting of the maskant◦Areas that are exposed longer will have more metal

removed from them◦E=U/d, where d- depth, U- undercutting

Anisotropy (A)- directionality of the cut, A=d/U, and Wf = Wm + (E d), or

Wm = Wf - (E d)where Wf is final desired width of cut

d/3

Chemical-Mechanical Polishing Chemical-Mechanical Polishing (CMP)(CMP)

Uses the synergy of chemistry and mechanical grinding to obtain flatness on the order of 50 nm.

CMP is used to fabricate integrated circuits (ICs) Figure 19-6 Schematic of chemical-mechanical

polishing (CMP).

Photochemical Machining for Photochemical Machining for ElectronicsElectronicsMost common method for creating maskantsInvolves the use of UV (Ultra-Violet) light-sensitive

emulsions, called photoresistsPhotoresists are applied to the surface of the

workpiece and selectively exposed to an intense ray of UV light

ICs use semiconductor materials that can be made to be either electrically conducting or insulating ◦Doping modifies these electrical properties by

introducing impurity atoms into semiconductors.Silicon is the most widely used semiconductor

material

How How ICsICs are Made are MadeAbility to selectively modify the electrical properties of semiconductors is the backbone of microelectronic manufacturing

On Fig 19-8, the sequence of processes or steps required to manufacture a simple metal-oxide-semiconductor (MOS) is show up.

Photolithography shown on Fig 19 - 9 is used to produce a polymeric mask over the oxide layer, which allows only selected areas of the oxide layer to be etched.

IC Manufacturing and EconomicsIC Manufacturing and Economics

Small circuits are inexpensive, but the cost of packaging, testing, and assembling the completed circuits into an electronic system must be taken into account

Ways to improve the economics◦Increase wafer size

Increases the usable area◦Increase the number of chips per wafer by decreasing chip dimensions

◦Improve die yield

IC PackagingIC PackagingServes to distribute electronic signals

and power Provides mechanical interfacing to test

equipment and printed circuit boards (PCBs)

Protect the delicate circuitry from mechanical stresses and electrostatic discharge during handling and in corrosive environments

Dissipate heat generated in the circuits

Steps in IC PackagingSteps in IC PackagingTwo main methods in which components are

connected to the circuit on the PCBDIP is an example of through-hole (TH)

technology, or pin-in-hole (PIH)◦ IC packages and discrete components are inserted

into metal-plated holes in the PCB and soldered from the underside

Surface mount (SM) technology places electronic components onto solder paste pads that have been dispensed onto the surface of the PCB

IC PackagingIC PackagingSM technology

◦Packages are more cost-effective than TH◦Designed for automated production◦TH components have only one lead geometry

and SM have many different designesLead geometries

◦Butt lead or J-lead◦Gull wing leads◦Solder balls

PCB Fabrication ProcessPCB Fabrication Process

Figure 19-13 Typical base materials used may be epoxy-impregnated fiberglass, polyimide, or ceramic. Epoxy-impregnated fiberglass is the cheapest substitute for interconnecting leaded packages. Fiberglass is used to increase the mechanical stiffness of the device for handling, while epoxy resin imparts better ductility. The fiberglass is impregnated on a continuous line where resin infiltrates the fiberglass mat in a dip basin, and the soaked fabric passes through a set of rollers to control thickness and an oven where the resin is partially cured. The resulting glass resin sheet is called prepreg. Multiple prepregs are then pressed together between electroformed copper foil under precise heat and pressure conditions to form a copper-clad laminate or PCB.

19.3 Electrochemical Machining 19.3 Electrochemical Machining ProcessProcess

Electrochemical machining (ECM) removes material by anodic dissolution with a rapidly flowing electrolyte

The tool is the cathode and the workpiece is the electrolyte

Figure 19-17 Schematic diagram of electrochemical machining process (ECM).

Table 19-3 Material Removal Rates for ECM Alloys Table 19-3 Material Removal Rates for ECM Alloys Assuming 100% Current EfficiencyAssuming 100% Current Efficiency

Electrochemical ProcessingElectrochemical ProcessingPulsed-current ECM (PECM)

◦Pulsed on and off for durations of approximately 1ms

Pulsed currents are also used in electrochemical machining (EMM)

Electrochemical polishing is a modification of the ECM process◦Much slower penetration rate

Table 19-4 Metal Removal Rates for ECG for Various Table 19-4 Metal Removal Rates for ECG for Various Metals (Electrochemical Grinding – ECG)Metals (Electrochemical Grinding – ECG)

Advantages and Disadvantages Advantages and Disadvantages of Electrochemical Machiningof Electrochemical Machining

Advantages◦ECM is well suited for the

machining of complex two-dimensional shapes

◦Delicate parts may be made

◦Difficult-to machine geometries

◦Poorly machinable materials may be processed

◦Little or no tool wear

Disadvantages◦ Initial tooling can

be timely and costly

◦Environmentally harmful by-products

19.4 Electrical Discharge 19.4 Electrical Discharge MachiningMachiningElectrical discharge machining (EDM)

removes metal by discharging electric current from a pulsating DC power supply across a thin interelectrode gap

The gap is filled by a dielectric fluid, which becomes locally ionized

Two different types of EDM exist based on the shape of the tool electrode◦Ram EDM/ sinker EDM◦Wire EDM

Figure 19-21 EDM or spark erosion machining of metal, using high-frequency spark discharges in a dielectric, between the shaped tool (cathode) and the work (anode). The table can make X-Y movements.

EDM ProcessesEDM Processes

Slow compared to conventional machining

Produce a matte surface

Complex geometries are possible

Often used in tool and die making

Figure 19-22 Schematic diagram of equipment for wire EDM using a moving wire electrode.

Effect of Current on-time and Effect of Current on-time and Discharge Current on Crater SizeDischarge Current on Crater SizeMRR = (C I)/(Tm

1.23),Where MRR – material removal rate in in.3/min.; C – constant of proportionality equal to 5.08 in US customary units; I – discharge current in amps; Tm – melting temperature of workpiece material, 0F.

Example:A certain alloy whose melting point = 2,000 0F is to be

machined in EDM. If a discharge current = 25A, what is the expected metal removal rate?

MRR = (C I)/(Tm1.23) = (5.08 x 25)/(2,0001.23)

= 0.011 in.3/min.

Figure 19-25 The principles of

metal removal for EDM.

Effect of Current on-time and Effect of Current on-time and Discharge Current on Crater SizeDischarge Current on Crater Size

From Fig 19 – 25: we have the conclusions:◦Generally higher duty cycles with higher

currents and lower frequencies are used to maximize MRR.

◦Higher frequencies and lower discharge currents are used to improve surface finish while reducing MRR.

◦Higher frequencies generally cause increased tool wear.

Considerations for EDMConsiderations for EDMGraphite is the most widely used tool

electrodeThe choice of electrode material depends

on its machinability and coast as well as the desired MRR, surface finish, and tool wear

The dielectric fluid has four main functions◦Electrical insulation◦Spark conductor◦Flushing medium◦Coolant

Table 19-5 Melting Temperatures for Selected EDM Table 19-5 Melting Temperatures for Selected EDM Workpiece MaterialsWorkpiece Materials

Advantages and Disadvantages Advantages and Disadvantages of EDMof EDM

AdvantagesApplicable to all

materials that are fairly good electrical conductors

Hardness, toughness, or brittleness of the material imposes no limitations

Fragile and delicate parts

DisadvantagesProduces a hard

recast surfaceSurface may

contain fine cracks caused by thermal stress

Fumes can be toxic

HW for Chapter 19 (due date 11/30/2010)HW for Chapter 19 (due date 11/30/2010)Review Questions:7, 17, 18 (page 521, 5 points for each question )