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Lead Contamination....Coplanarity???

Jeff Woodruff


Lead Contamination....Coplanarity??? | 9 February, 2000

I have seen an issue that escapes our ICT and functional testing or operates in an intermitent fashion. The lead is slightly separated from the PCB with a fully formed solder joint behind it at the heel an toe. The joint apears to have started to travel up the back of the lead and under the lead, but appears that it didn't stick. The joint is shiny/smooth on the sides and under the lead. We have done steam aging, x-ray florecence, and solderability tests on PCB's and parts and we have found nothing. It is a less than 1% issue, but we are dumbfounded on what will cause such a condition. Any ideas????

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Re: Lead Contamination....Coplanarity??? | 9 February, 2000

Jeff: Two things come to mind (if that's not stretching things too much ...):

1 You've got something unsolderable and not removeable with fluxes is on your component leads (or you don't have enough stomp in your flux). Do the leads that don't solder take solder, when soldered by hand? (If that's not too many "solders" in one sentance) 2 You're not putting enough solder on the pads

Good luck

Dave F

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Re: Lead Contamination....Coplanarity??? | 10 February, 2000

Jeff: We had a similar problem a couple of years ago with boards from one specific subcon. At first sight the joints looked well formed sometimes with a little bit more sometimes with a little bit less solder but still in limits not to be worried about. First electrical testing also worked fine but later under real conditions in field test the boards started to fail after a couple of days. Reworking the joints on the largest QFPs eliminated the malfunctions. Tests were made and it showed up that you could easily move the leads in the solder with almost no force applied. The subcon insisted that in his opinion nothing was wrong and everything was performed correct. A year later I met some guys from this subcon at a symposium talked to them and they said that our complaint led to an investigation of their processes and it was detected that either their reflowprofiling nor the paste used at that time were adequate for the boards with that complexity and finish. Especially Gnd and VCC connections to the planes were doomed to fail.

So how is profiling, are there special parts, leads involved ? Did you perform further tests of the solderjoints or only optical inspection ?

Even if it�s 1% now there is no guaranty that it stays like that if it�s a process related problem.



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Re: Lead Contamination....Coplanarity??? | 11 February, 2000


I guess my experience in such issues always took me back to the screen printing process and I only want to bring up that portion of the possible causes to this case.

When we talk about coplanarity, the component lead coplanarity first comes to mind. However, paste disposition coplanarity is also important since that's the plane the leads will be sitting on. So, if one pad is lower, the lead will not touch the paste there especially since the paste height starts decreasing once the solder paste starts melting.

I would look into the solder paste definition especially the height (and volume) variance after screen printing.

M2C Erhan

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