What Causes Non-wetting of OSP Test Pads?

Phil
Welcome to Board Talk. This is Jim Hall and Phil Zarrow, the Assembly Brothers, who by day go as ITM Consulting. Jim, today's question is from P.M.

Jim
The question is about non-wetting test pads. "What are all the probable causes of non-wetting for OSP board test pads when the circuit boards are run through our wave solder bath?"

Do you want all the reasons? We'll give you a few.

Jim
Since we're talking about wave soldering the obvious one is are you fluxing them?

If you're using a spray fluxer, are you programming exactly where the flux is being applied, are you remembering to program the spray fluxer to put flux on test pads?

The non-wetting, or poor wetting, with OSP has always been a touchy subject and a very subjective one.

An obvious problem with wave soldering is previous wave cycles. Have these assemblies seen one or two previous reflow cycles, which could have degraded the OSP coating, causing them to have poor solderability when you get to the wave, typically being the final soldering operation?

We just received our copy of IPC Specification 1601 on handling of printed circuit boards. We've touted it before because it addresses the issues of moisture absorption and baking.

Within the recommendations they do not bake OSP boards. So, a question for P.M. would be, had you baked these boards before assembly?

Obviously, we would like more data; are the through-holes soldering and not the test pads? Because if it's baking or previous heat cycles, then you shouldn't be filling your through-holes, either.

Phil
Handling being a touchy subject, you always want to exercise care. As with any kind of surface finish, keep the human contamination out. So, there are some of the reasons, if not all of them, and hope that helps.

And aside from that, it's Phil and Jim, the Assembly Brothers, saying whatever you do ...

Jim
Don't solder like my brother.

Phil
And don't solder like my brother, whether it's wave or reflow.

In the early 1980s I experienced nonwetting on test pads. The coating was not called OSP at that time, but the copper was coated with a flux coating. I did an experiment were I drilled a small hole in each of the test pads. I then ran the boards through the standers wave solder process. The solder skips went away, all of them.

I concluded that the solder was not making contact with the copper pad and the hole in the pad let the liquid flux and or gas escape through the hole. The solder could then contact the copper and start the wetting process. Without the hole in the test pad the solder mask around each test pad would hold the trapped flux and gas in place over the test pads.

You could try moving the solder mask away from the test pad, increasing the angle of the conveyor on the wave solder machine and drilling holes in the test pads. Drilling holes will add cost.

If you control the atmosphere quality in the reflow oven (low residual oxygen level), before the wave soldering process, you may have less oxidation on the PCB metal contacts and then better wetting in the wave soldering process.