What Are the Benefits and Drawbacks of OSP Finish?

Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting. Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes

Phil Zarrow With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes. Jim Hall A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.

Transcript

Phil
Welcome to Board Talk this is Phil Zarrow and Jim Hall, the Assembly Brothers. Our special guest today Joe Belmonte.

Jim
We have a question about surface finishes on printed circuit boards. This comes from S.B. What are the advantages of using an OSP (Organic Solder Preservative) surface finish? What precautions should be taken when using circuit boards supplied with OSP finish?

Phil
The most obvious advantage is that it's cheap.

Jim
It provides an extremely flat finish for placing your components. Under the proper conditions can give you very reliable solder joints because your actually forming the inter-metallic during the soldering process. On the other hand it has some limitations. If the finish is not properly maintained and properly handled, the solderability may be reduced.

Phil
So there are a number of storage considerations to take into account. And questions in terms of the number of thermal excursions.

Jim
The feeling many people have with OSP on a complex board with double-sided reflow, being separate reflow cycles followed by a wave soldering operation, is that they do not get adequate hole fill.

The argument is that during the first two reflow cycles the heat on the barrels of the hole breaks down the OSP. You get minor oxidation so when you wave solder, you don't get good hole fill.

Joe
We have to remember there are many OPS products and suppliers. There are different numbers of thermal excursions so you need to balance that with the best OPS product.

Another difficult wave soldering problem is a think board with heavy grounds planes and OSP finish. The challenging is getting proper top side hole fill.

Phil
No matter what surface finish you use, don't solder like my brother ...

Jim
And don't sound like my brother.

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