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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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.
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.
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.
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
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