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Moisture under chip capacitors



Moisture under chip capacitors | 11 September, 2003

I have come across a rash of failed assemblies that have 0805 capacitors and resistors, bottom side glued, wave soldered and washed.

Trouble shooter reported touching up the solder joints on a few areas and the boards would pass test. I had a look at one board which was reported to have a 0805 56K resistor with "drifting" values. I pulled the component off mechanically in order not to apply any heat and noticed several mini-voids within the adhesive body. I have seen solder bridge within the glue before but this was the first time I saw wet residue within the voids.

You can view a sample image from the following:

The image showed the bottom side of the cracked resistor after removal (white), with a portion of solder mask (green) and SMT adhesive (red). The wet residue has clear appearance.

I used a 8mil stencil. The thermal profile was verified and conformed to the slope/ curing specs. The adhesive was printed and cured at no more than 2deg/s and 150degC for 100secs.

We suspected that it was a material related issue - either the glue was mishandled and left too long on the line, or old/ expired batch of glue was used.

Has anyone seen this before? Any explanation as to how the void and moisture was created and suggestion on baking cycle to drive the moisture out from the assembled units? Does anyone re-use their glue on the line or throw away any left-over to avoid moisture in the glue?

Rgds, Peter

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Moisture under chip capacitors | 11 September, 2003

We've seen two source of this: * Poor cure recipe [lookup 'microcanyon' in the fine SMTnet Archives] * Moisture absorbing glue formulations [lookup psi or aguayo in the fine SMTnet Archives]

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Moisture under chip capacitors | 11 September, 2003

Epoxy is hydroscopic. A cavity will form when the cure furnace reaches the boiling point of water. The steam expansion creates the pocket. It can be large enough to be filled with molten solder from the wave machine and cause a hard short under the resistor / capacitor.

Epoxy is cheap. I discard after each shift. Does this assembly see a wash cycle after wave solder? IF yes, your washer may not be drying adequately.

Email me at I will give you a key piece to this puzzle.

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