on Mon, 26 Aug 1996 09:34:19 -0400
[log in to unmask] (Doug McKean) asked:

>We are currently having our board assembly house use a quick drying glue 
>to secure a 30AWG wire to the pcb for a modification.  We are finding 
>the glue can cause electrical problems (not all the time) if it is 
>in the leads of FINE PITCH surface mount devices.  This is confirmed 
>when the glue is removed.  There is the telltale white residue on the 
>glue indicating the glue is applied first and the accelerater applied 
>second.  Something I've done myself.  It's my understanding that the 
>accelerator should applied first and the glue second.  There are no 
>controlled impedence issues and comparativley low frequency of 
>1 to 32 MHz digital signals.
>
>1. Has anyone had electrical problems with glue in the leads of 
>   fine pitch devices? 
>
>2. Can the glue be conductive for at least fine pitch devices? 
>
>I've seen this glue used tons of times without this effect. 
>Thanks in advance. 

Doug-

A few comments I'd add to the others:
- The white residue sounds likeing "blooming", outgas by-products of the
un-cured adhesive.  The residue could be your real problem, not the adhesive.
- Check the manufacturer's technical data sheet for cure schedules and
proper accelerant usage.
- The tech data sheet should also talk about electrical properties of the
material and its suitability for this application.  Of course, you can
always talk to the mfr's tech support people for more info.
- If you've used this adhesive previously without problems, I'd look into
whether you just have a bad container of it; or look at the application
process as you've hinted at (maybe not enough accelerant?  thicker adhesive
application?).  Maybe this application is very sensitive.
- Yes, I've had problems with adhesives/sealants causing electrical
problems.  Uncured/partially cured sealants/adhesives don't exhibit the same
properties as fully cured materials.  Some materials may take a week or
longer to reach full properties.  I had a vendor encapsulating partially
cured RTV which resulted in lowered insulation resistance and failure.

Regards,
Gregg
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