osp process

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anic Solderability Preservative (OSP) OSP process selectively applies a flat, anti-oxidation film onto the exposed copper aces of the PWB to preserve the solderability of the copper. This coating reacts with the per in an acid and water mixture to form the nearly invisible protective organic coating. OSP esses can be based on benzimidazole chemistries that deposit thicker coatings, or on otriazoles and imidazoles chemistries which deposit thinner coatings. The thicker OSP ings, which are evaluated in this CTSA, can withstand a mini mum of three and up to as many ve thermal excursions while still maintaining coating integrity. Coating thicknesses of 0.1 to microns (4 to 20 microinches) are typical for the thicker coatings, as opposed to the omolecular layer formed by the thinner OSPs. process is typically operated in a horizontal, conveyorized mode but can be modified un in a vertical, non-conveyorized mode. OSP processes are compatible with SMT, flip chip, BGA technologies, as well as with typical through hole components. The OSP surface finish ot be wirebonded. OSP surfaces are compatible with all solder masks, can withstand 3 to 4 mal excursions during assembly, and have a shelf life of up to one year; extended shelf life s may result in a degradation of the coating. ow diagram of the process baths in a typical OSP process is presented in Figure 2-5, wed by a brief description of each of the process steps.

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Page 1: OSP process

Organic Solderability Preservative (OSP)

The OSP process selectively applies a flat, anti-oxidation film onto the exposed copper

surfaces of the PWB to preserve the solderability of the copper. This coating reacts with the

copper in an acid and water mixture to form the nearly invisible protective organic coating. OSP

processes can be based on benzimidazole chemistries that deposit thicker coatings, or on

benzotriazoles and imidazoles chemistries which deposit thinner coatings. The thicker OSP

coatings, which are evaluated in this CTSA, can withstand a minimum of three and up to as many

as five thermal excursions while still maintaining coating integrity. Coating thicknesses of 0.1 to

0.5 microns (4 to 20 microinches) are typical for the thicker coatings, as opposed to the

monomolecular layer formed by the thinner OSPs.

The process is typically operated in a horizontal, conveyorized mode but can be modified

to run in a vertical, non-conveyorized mode. OSP processes are compatible with SMT, flip chip,

and BGA technologies, as well as with typical through hole components. The OSP surface finish

cannot be wirebonded. OSP surfaces are compatible with all solder masks, can withstand 3 to 4

thermal excursions during assembly, and have a shelf life of up to one year; extended shelf life

times may result in a degradation of the coating.

A flow diagram of the process baths in a typical OSP process is presented in Figure 2-5,

followed by a brief description of each of the process steps.

Page 2: OSP process
Page 3: OSP process

Step 1: Cleaner: Surface oils and solder mask residues are removed from the exposed

copper surfaces in a cleaner solution. The acidic solution prepares the surface to

ensure the controlled, uniform etching in subsequent steps.

Step 2: Microetch: The microetch solution, typically consisting of dilute hydrochloric or

sulfuric acid, etches the existing copper surfaces to further remove any remaining

contaminants and to chemically roughen the surface of the copper to promote

coating adhesion.

Step 3: Air Knife: An air knife removes excess water from the panel to limit oxidation

formation on the copper surfaces prior to coating application. This step also

minimizes drag-in of sulfates, which are harmful to the OSP bath.

Page 4: OSP process

Step 4: OSP: A protective layer is formed selectively on the exposed copper surfaces by

the OSP in an acidic aqueous bath. The deposited protective layer chemically

bonds to the copper, forming an organometallic layer that preserves the

solderability of the copper surface for future assembly (Mouton, 1997).

Step 5: Air Knife: An air knife removes excess OSP from the panel and promotes even

coating across the entire PWB surface. The air knife also minimizes the chemical

losses through drag-out from the OSP bath.

Step 6: Dry: A warm-air drying stage cures the OSP coating and helps to remove any

residual moisture from the board.

Page 5: OSP process

OSP reaction mechanismOSP reaction mechanismOrganic film is formed by azole compound in OSP material.

WPF106A and WPF15 have benz-imidazole , KESTER markets the two compounds, namely imidazole and triazole.

Reaction mechanism

azole compounds adsorption

on exposed copper surface

NH

N

R

NH

N

R

NH

N

N

benz-imidazole imidazole triazole

*note: R means branched chain, and is different between each OSP materials

copper land

NH NH NH

N

R

N

R

N

R

(2) Hydrogen(proton) elimination

and dehydration

copper land

N-

N

R

Cu+N-

N

R

Cu+N-

N

R

Cu+

+ H2O

Page 6: OSP process

OSP reaction mechanismOSP reaction mechanism

chelate formation

copper land

N-

R

Cu+

N:

N-

R

Cu+

N:

N-

R

Cu+

N:

copper land

N

R

Cu+

N:

N

R

Cu+

N:

N

R

Cu+

N:

N

R

Cu++

N:

N

R

N:

N

R

N:

Cu++ Cu++

(4) Organic film formation

Copper chelate is formed by Cu2+ in OSP material or Copper surface on PWB

Page 7: OSP process

Organic Solderability Preservatives (OSP, Anti-tarnish)

Of the OSP compounds, the two that are predominant in the industry are benzotriazole and substituted benzimidazole. The

former of these has been utilized both in the metal finishing industry as a true anti-tarnish for hardware plating, and to a

lesser degree, in the electronics industry for limited heat cycle solder applications on bare copper. The more recently

Developed substituted benzimidazole compounds have the advantage of withstanding multiple heat cycles typically found in

mixed technology PCB assembly operations (surface mount and through hole soldering). This advantage is primarily due to the

coating thickness that is achieved with the substituted compounds, ranging from 5 to 20 microinches1 (0.1 - 0.5 microns), as

compared to only a monomolecular layer formed with the benzotriazole materials.

In general, OSPs are considered the low cost, high volume alternative, particularly when applied to surface mount technology

due to the excellent surface coplanarity of pads. The cost advantage of the process becomes quite apparent to the board

fabricator. For example, facilities and maintenance expenses are only a fraction of the operating cost for the anti-tarnish

process as compared to HASL. The process does not involve heating and delivering the high volume air or maintaining a

molten pot of solder. Losses due to rework and rejected product from the HASL process are substantial and prevent a

decrease in the cost of manufacturing. An OSP application system eliminates this waste and allows for flexibility to pass the

savings on to the customer.

Page 8: OSP process

The cost advantage to the assembler lies primarily in the yield improvements of the component attachment process. Surface

mount pad planarity is a technical hurdle that HASL will never overcome, especially for fine pitch devices due to the extreme

variation in coating thickness. This non-uniform solder thickness can range from 30-1500 microinches (0.75 - 37.5 microns)

depending upon pad geometry and orientation within the panel. In contrast, an OSP coated board provides a surface finish

topography equivalent to the plated copper. This allows for improved component placement; furthermore, it will help move

the assembler towards finer pitch devices and denser circuits as the limiting factor for fine pitch assembly now becomes the

ability to deposit the solder paste.

It must be emphasized that implementing an OSP process may require a re-characterization of the assembly operation; a

number of issues must be addressed. For example, the number of thermal cycles a product may see in the assembly process

can reduce the operating window for soldering. If excessive, it may be determined that soldering in an inert atmosphere is

required in the reflow process, wave solder, or both. Typically, OSP suppliers advertise up to five thermal cycles can be

achieved in open air without detrimental effects to solderability. Developing a partnership with the board shop and supplier is

an important consideration during the implementation phase.

A second assembly process consideration when using OSP technology surrounds the type and activity of the flux used in

each of the assembly operations. Table 1 illustrates the results of an engineering evaluation used to benchmark OSPs in

comparison to HASL coated PCBs at one customer site. It was apparent in the evaluation that the OSPs performed differently

depending upon the type of flux used. A wide range of responses were found spanning from excellent surface mount

solderability with OA or RMA type solder pastes, to relatively poor filling of holes with a low solids (2%) no clean wave

solder flux that was not designed for soldering to OSP coated pads.

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