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Grainy Solder Joints

tcp

#29254

Grainy Solder Joints | 25 June, 2004

We have had some field returns of a double sided SMT board manufactured in the February timeframe. The boards are encapsulated in an epoxy potting. When removing the potting some SSOP devices came off in the potting. The leads had pulled out of the solder joint reveling a grainy texture with voids internally. Externally the solder joints look good and meet IPC requirements. We are pulling our hair out to get to the root cause and haven't been able to duplicate. Any ideas?

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#29255

Grainy Solder Joints | 25 June, 2004

What are the specifications on the solder that you use? IE: Water soluble,RA,RMA, N/C? Also do you clean the boards and if so how and what detergent if any are you using?

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tcp

#29256

Grainy Solder Joints | 25 June, 2004

We use water soluble paste, Kester R560. These units are wave soldered and cleaned in an Electrovert Aquastorm water wash with a 3% Kester saponifier solution in the first stage.

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RDR

#29259

Grainy Solder Joints | 25 June, 2004

Sounds like the wrong reflow profile was possibly used on this batch, Or you have a lead solderability problem. Usually you will pull a pad off of the board instead of the lead out of the solder joint. When a lead is torn from the joint it will have a very rough appearance, But as for these voids you see, were they inside the remaining solder (as seen by Xray) or on the surface?

Russ

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tcp

#29260

Grainy Solder Joints | 25 June, 2004

We have beaten the reflow profile angle to death and haven't been able to duplicate the condition, so I don't think that is the problem. The voids were visible on the surface of the remaining solder. Where the lead tore out of the solder left indications of non-ductile fractures in most cases.

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Hoss

#29262

Grainy Solder Joints | 25 June, 2004

Any chance your potting compound was changed in the timeframe mentioned or the unit(s) were used under high heat stress conditions?

If significant thermal expansion of your potting material happened and it is a hard durometer, it could shear at your joint connection point. Heard of it happening but haven't seen it.

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tcp

#29263

Grainy Solder Joints | 25 June, 2004

The potting definitely is a factor in fracturing the grainy joints, but does not seem to damage a "good" solder joint.

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#29264

Grainy Solder Joints | 25 June, 2004

For what its worth. Even though you are potting the boards. If for whatever reason flux residue (OA) was left on the board prior to potting this flux could migrate and cause embrittlement problems with the solder joint. I would definately inspect for flux migration. Could be there was a cleaning issue back earlier this year.

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#29266

Grainy Solder Joints | 25 June, 2004

Nonductile fracture!!! Sounds peculiar. In that case, stop wasting you time messing with the thermal recipe. Either your: * Components have poor solderability [test this by dipping the leads of a couple of the SSOP in your solder pot and then assessing how well solder sticks] OR * Flux in your solder was not active enough [but since other components soldered well, this option doesn't lift our skirts] * Encapsulant cure cycle generated too much stress on component leads [this seems unlikely since multiple component types were not ripped loose from their moorings]

If the poor solderable in components proposition holds water, it would go a long way in explaining why you can't duplicate the problem, since this is often batch related. [It does raise some ugly recall / customer notification issues, though.]

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