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78 PCB007 MAGAZINE I MARCH 2019 Attributes and related questions include [1]: • What is the cost sensitivity of the product (how important is the surface finish cost)? • What are the product volume requirements (high, medium, or low)? • Is it an SnPb- or Pb-free process? • If Pb-free, is shock/drop a concern for your product? • Is fine pitch assembly required (how fine)? • What is the user environment (is corrosion a concern)? • Is wave solder required, and if so, how thick are the boards? • Is high yield in-circuit testing required? • Is wire bonding required? As you review the list of final finish desired attributes, there is concern that no one finish will meet all product segment requirements. So, where does OSP fit? OSPs As of this writing, over 45% of the PCB sur- face area produced annually is finished with OSPs. This is significantly more than any other final finish can claim [2] . However, certain con- ditions and end-use environments may nega- tively impact the use and implementation of OSPs. As an example, many EMS providers de- sire to perform in-circuit test (ICT) to measure electrical continuity of the assembled circuit before shipping to the end customer. This can be a challenge because the coating is often dif- ficult to probe through. OSP coatings are non- conductive. One procedure is to print solder paste on test points. However, flux residues can coat the probes and impact ICT. There is also a concern that multiple IR reflows will harden the OSP, further impacting ICT. Another potential drawback with OSPs is its compatibility with lead-free assembly. With the introduction of lead-free alloys, the higher temperatures and dwell times of assembly im - pact the solderability of any final finish. Oxi- dation is the enemy of good solderability. So, how does OSP fare under these conditions? What if moisture removal bakes are imple- mented? Other perceived downsides include solder flow-up through vias and solder paste spread- ability. Will OSP processes meet the minimum criteria for solder joint strength and overall reliability? In the past, there was documented evidence that solder flow-up is not successful with OSPs under extreme temperature condi- tions. Of course, this must be addressed. Finally, there are additional concerns with corrosion of final finishes. How do OSPs com- pare under harsh environments to other fin- ishes including ENIG, immersion silver, and hot air leveling? These and the other aforemen- tioned questions will be answered in a future column. Equipment Considerations Let's discuss equipment as the equipment design relates to effective OSP process perfor- mance. While OSPs can be effectively applied to the PCB by immersion or conveyorized pro- cessing, the latter provides a more controllable and higher through-put option. As stated ear- lier, the proper processing sequence is: • Acidic cleaning • Rinse • Microetch • Rinse • Pre-dip (optional) • OSP process • Rinse • Dry If persulphate microetches are used, it is highly recommended to utilize an acidic pre- As of this writing, over 45% of the PCB surface area produced annually is finished with OSPs. This is significantly more than any other final finish can claim.

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