Modify Rework Procedures for Assemblies Fabricated Using OSP?

Phil
Welcome to Board Talk with Jim Hall and Phil Zarrow of ITM Consulting, the Assembly Brothers. Coming to you today from the ITM Consulting failure analysis cave, high atop Mount Rialto.

We are here to talk about electronic assembly, materials, equipment, components, practices and procedures, among other things. Jim, let's see.

There is another thing, today we have a rework question. This is from J.N.

J.N. asks should we modify any rework or repair procedures for circuit board assemblies fabricated using OSP, organic solderability preservatives, bare boards? That is a good one.

Jim
First off, I want to say if you are working with a completed assembly then there should no longer be any OSP on that board because all of the surfaces covered with OSP should have been covered with solder at one step of the assembly process. They should not have finished boards that have any OSP exposed, like test points or something else.

OSP is not a long term protection for copper on finished boards. It is strictly protection between the PC fab and the time you solder it. Everything should have solder on it.

It should just be an ordinary board. Perhaps you are doing touch-up after the first side of a double-sided SMT assembly.

So you haven't gone through wave soldering, you haven't gone through second side, and you may have some OSP pads which are not soldered at that point. Understand that it is not a physically robust surface.

It should take soldering temperature, but you don't want to expose it to any extreme temperatures. But most importantly you don't want to scratch it. OSP is not like a metallic finish that is robust.

When you are in there with soldering irons and soldering wicks and so forth if there is an OSP through hole nearby that hasn't been soldered yet you want to make sure that you do not scratch that OSP. You can very easily go right through that surface, expose some copper and then have a soldering problem in your next soldering step.

Phil
Very good. I hope we answered your question and anyone else who has been pondering the same thing in the wild world of OSPs.

You have been listening to Board Talk, with Phil and Jim the Assembly Brothers. Although the SMTA denies any responsibility for our actions, we are the Assembly Brothers.

Also, whatever you do please don't solder like my brother.

Jim
And don't solder like my brother.

To answer Phuong's question; OSP and soldermask are two different things. On a PWB where OSP is planned to be used, the vias should be covered with soldermask where possible, except those that may be required to be exposed for flying probe testing.

Per Phil's comments on covering any exposed copper surface with solder on OSP board, there will be board vias not covered by solder. Would there be any long term functional negative effect caused by them?