Opening screen apertures to cover the entire pad can overcome incomplete solder spread experienced with OSPs in lead-free assembly applications.
Organic solderabilty preservative (OSP) is a general term that describes a variety of water-based surface treatments containing organic compounds that are designed to serve the same function. OSPs coat the copper surface by forming an organo-metallic complex. Thickness and chemical composition may range from a molecular monolayer to complex films with thicknesses up to 0.5 microns. OSPs are copper-specific and are often designed specifically to minimize film formation on gold or other surfaces. The coatings protect the underlying copper from oxidation during storage and through the assembly process, thus preserving solderablilty.
Today, the industry is faced with smaller form factors, high aspect ratio vias or through-holes and lead-free (LF) solder. Because LF solders melt at temperatures of 20ËšC to 40ËšC higher than the standard eutectic solder, it is again time for the suppliers to come up with the next generation of OSPs that can withstand multiple thermal excursions at LF temperatures.
There are a variety of OSPs available in today’s market, having different capabilities and serving different functions. Some examples of classes of organic compounds used in OSPs are benzotriazoles, benzimidazoles, alkyl benzimidazoles and aryl-phenylbenzimidazoles. The ability of a particular OSP to withstand thermal excursions at assembly can be affected by a variety of formulation factors, including the decomposition temperature of the organic compound. The simpler types may only withstand a single eutectic thermal excursion, while the more sophisticated ones can withstand multiple lead-free thermal excursions, in air or in a nitrogen atmosphere.
The manufacturing process steps are: cleaner, rinse, micro etch, rinse, OSP, rinse and dry. The first two steps are pretreatments before the application of the coating. The dwell time in any of these steps is less than 5 minutes, and the temperature does not exceed 50o C. This makes the process very conducive to conveyorization. Unlike HASL, the application of an OSP coating does not entail any thermal shock to the board.
The thickness of the OSP film has traditionally been measured by dissolving a coating of known area into a known volume of diluted acid and then assaying the organic content using a spectrophotometer, in the UV spectral range, against a standard of known concentration. The method is accurate and manageable; however, it is destructive, as the OSP must be removed for analysis.
Oxford Instruments offers a non-destructive tester that works on the principle that the light reflected from the air/OSP interface exhibits phase differences that create intensity oscillation across a wavelength range of 400 nm to 700 nm. By analyzing the intensity of these oscillations, the film thickness of an OSP is determined.
OSP coatings ranging from 0.02 μm to 5μm can be determined using fundamental parameters and without the need for a calibration standard. The instrument is capable of measuring a spot as small as 1 μm by 1 μm and as large as 325 μm by 256 μm. The obvious benefit is that the OSP thickness can be measured in situ during production, without the need for controls.
When considering OSP as a surface finish, it is advisable to work with the manufacturing site to confirm that the assembly conditions, e.g., number and type of reflow cycles and in-process hold times, are compatible with the OSP. Some designs are not suitable for OSP. An example would be wave soldering a high aspect ratio (>12:1) through-hole in a multilayer board with large ground planes. The ground plane areas have a tendency to cool the molten solder as it wicks its way up the hole, resulting in incomplete hole fill.
OSPs, in general, have limited solder spread as compared to other surface finishes. This is particularly true for lead-free solders. Screen apertures for solder paste application should be opened to cover the entire pad, if complete reflow coverage is to be accomplished. The coating requires proper handling to avoid damaging the surface. OSPs are also hard to inspect, due to the transparency of the coating.
During assembly, the OSP coating is readily removed when it comes in contact with soldering flux. The intermetallic formed at the solder joint/pad interface is a Cu/Sn compound. The OSP coating itself does not become part of the solder joint.
Shelf life of OSP-coated PCBs may be as high as 12 months but will vary depending on the type of OSP, the packaging and the storage conditions. Boards that have exceeded their shelf life may be easily reworked by stripping and recoating.
The simpler formulations are very cost-effective; while the more complex formulations could conceivably approach the cost of some of the metallic surface finishes. Today, 25% to 30% of all boards manufactured worldwide use OSP as a surface finish when soldering is the only process required at assembly.
PCD&FGeorge Milad is the national accounts manager for technology with Uyemura, International Corp;
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