In message <[log in to unmask]> [log in to unmask] (Doug McKean) writes:
> We are currently having our board assembly house use a quick drying glue 
> to secure a 30AWG wire to the pcb for a modification.  

I assume this is a cyanoacrylate adhesive (CA), from your description.

> 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.

CA's cure on contact with alkaline surfaces (i.e. from surface moisture) - that 
is, they like to cure in small amounts between two surfaces. A large blob on a 
pcb will not cure easily on its own, which is why you have to use an activator. 
They are very sensitive to the ambient humidity when curing, and often you will 
see white residue around the adhesive, which is known as blooming, occur 
at higher humidity. I'm not sure what this "bloom" consists of, but probably 
some acrylic-type residues.

My guess is that the electrical problems could be caused by either the "bloom", 
or by excess activator. The latter seems unlikely, as the activator is made up 
of a very small proportion of active ingredient (amine?) in a volatile solvent 
like isopropanol or heptane. When the solvent evaporates, the active ingredient 
left would be small in volume. Still, you could test if this was the case by 
spraying (excess) activator onto the fine pitch, letting the solvent evaporate, 
and electrically test.

The "bloom" will often be less if you put the activator down onto the pcb 
surface, then drop the CA on top, but cure times go up. Still, if it is the 
"bloom" which gives electrical problems, you may have to consider this or other 
options.

I would be very interested to hear your conclusions in due course, as I have 
been researching some of this stuff myself (c.f. my paper at Nepcon West this 
year).

Regards,
Peter

I-I-I-I-I-I-I-I-I-I-I-I-I-I-I-I-I-I-I-I-I-I-I-I-I-I-I-I-I-I-I-I-I-I-I-I-I-I-I-I
Peter Swanson                                         Oxfordshire, England
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