Oh. [Howabout if I smack you?]
If ahma connectin� wif whut yer sayin�, you: * Don�t like the appearance of the glue slobberin� out from under the component even though there is no apparent glue on solderable surfaces. * Concerned about the hydroscopic nature of glue, in that it could concern the LT reliability of your product.
APPEARANCE OF THE GLUE SLOBBERIN� So, we talkin� cosmetics here, eh? Why not try a different shaped glob of glue? Maybe: * Two smaller dots between the pads. * An oblong dot [head moving while dispensing]
... Or * Buy BIGGER caps. * Take a deep breath, look around, fix something that will make money. * Analog of the previous one: Pimp slip the one that has you chasing on this. Well, unless it's yer boss, yano. * It hurts to say this, replace your 3609.
HYDROSCOPIC NATURE OF GLUE How is this characteristic of your glue more concerning than the hydroscopic nature of your: * Solder mask? * FR-4 construction of your boards?
True. Epoxy is hydroscopic. It is used for many purposes in electronic manufacture. But the real issue isn�t moisture lovers. Electrochemical failures are the result of contaminants residing on the surface of the laminate and/or components of your assembly. To create the reverse plating cell that produces electromigration, required components are: * Bias voltage * Moisture * Active ionic material.
Increasing any one of the these required components will increase in electrochemical migration.
BIAS VOLTAGE. The circuit design dictates the bias voltage present, which leaves you with control of the active ionic material present and to some extent the moisture penetration through the conformal coating (type and thickness).
MOISTURE. Conformal coating does not completely stop moisture penetration, so any surface ionic material will react under bias to create the electromigration condition.
ACTIVE IONIC MATERIAL. Proper cleaning of the laminate surface during the fabrication processes, adequate flux activation during all thermal excursions, and subsequent cleaning (as required by flux vehicle used) in the assembly process will produce a finished assembly with low risk for electrochemical failure. Halide residues are usually the root cause of dendritic growth. Halide residue may be from fluxes in the assembly operation or from HASL fluxes used in PCB fabrication. You should investigate both areas, bare board and assembly, as either or both could be contributing to the problem.
Closure on your moisture sensitivity concern: Use surface insulation resistance [SIR] testing as the bottom line of whether a residue is harmful. If SIR testing can show no detrimental leakage currents under humid conditions, no corrosion, no metal migration, which are the big factors in figuring out electrochemical reliability, your product should be good in the field. People often have a greater comfort with passing SIR than they do with passing other tests [ie, ROSE, IC, etc]. Of course, SIR testing takes a while and so it is best to do both ion chromatography and SIR at the same time so you can determine the correlation between SIR performance and cleanliness levels for troubleshooting the process later.
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