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Solder joint dewetting or non wetting

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Solder joint dewetting or non wetting | 5 November, 2013

Hello All,

We have a problem with dewetting of solder paste. Problem is most visible on the pads without components (see attached pictures) and occurs randomly, 20% of PCB's. Pcb supplier sent us a cross-section of pads with thickness of Cu, Ni and Au (see attached picture). It's OK 1,33um gold thickness stated by the manufacturer? It will not affect the amount of molten gold joint quality?

We use: KOKI S3X58-M406 solder paste, PCB's with Chemical Ni/Au surface finish, Hotflow 2/14 profile:Time above 220°C is 60sec, max. temperature 234,4°C, preheat: 110°C-190°C is 128sec (see attached picture).

Does anyone have a solution for this dewetting, contaminations of PCB, solder paste, reflow profile, thickness of surface finish?

Thank you in advance. Peter


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Solder joint dewetting or non wetting | 5 November, 2013

For the reflow profile specs that you quoted, are those numbers on the pads that you're seeing dewetting on, or on other selected pads that were used for profiling?

In the picture you sent, the most obviously dewetted pad appears to be lying on a fairly substantial copper pour, with a number of thermal vias in it. It's conceivable that this location in particular is either not seeing enough heat in process, or, seeing too much heat in process (exceeding the 60s TAL that you're shooting for). It's also possible that this joint is not seeing enough time above the flux activation temperature.

From the reflow profile information you posted, it sounds like you're using a ramp-to-spike style. If the RTS isn't providing enough soak above the flux activation temp, due to thermal density issues in some areas of the board, you could run into these issues.

I'd start by profiling in the areas that are exhibiting the dewetting, to see what's going on thermally. If you're dealing with boards that have been sitting around for a little bit (>3-6 months, say, depending on the environment), I might consider a ramp-soak-spike profile to allow sufficient time for the flux to clean any potential contaminants.

Cheers, ..rob

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Solder joint dewetting or non wetting | 6 November, 2013

Hi rob,

Thank you very much for your response. Profiling was made exactly on the pad, which showed dewetting, so the values are mentioned for this pad. PCB's is about a week old. It is possible that the surface of pads is already oxidized? I will try to solder through the RSS profile.

And what do you say to a thickness of gold, is not it a lot? It will not be very brittle solder joints.

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Solder joint dewetting or non wetting | 6 November, 2013

Hi, Peter,

I'm not an authority on surface finish thickness, and it's effect on reflowing, so, I can't advise on that front.

If the boards are a week old, and ENIG, I would not suspect surface contamination. It's always a consideration, of course, but, typically, ENIG finishes would last longer than that. There's a potential that the finish was contaminated coming from the board house (ie. their plating process was contaminated), but, you'll need their help, and a chemical analysis of the plating to prove/disprove that. You could try an experiment where you paste the board, and reflow it, with no components, to test the solderability of the pads. You could also ask the board house to perform a solderability test for you. For what it's worth, I've always had issues with board houses doing these tests...they don't tend to admit any responsibility unless you prove it independently, and they have no choice but to admit it...but, it can still be worth trying that avenue.

cheers, ..rob

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