Hi Graham,
Thanks for the reply. I've noticed some resistors we've removed have a sort of Swiss cheese look to the chip bonder remaining on the PWB. It this what you'd expect to see or is this exactly what you are talking about?
Thanks,
Ken
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Collins Graham
Sent: Tuesday, April 27, 2004 10:49 AM
To: [log in to unmask]
Subject: Re: [TN] Glue Dots (chip bonding)
Hi Kenny
Based on an issue found at a previous employer, I offer this advice -
check a couple of parts where you are seeing the shorts, remove the part
and see if there are voids in the glue.
Screen printed glue suffers from the possible issue of moisture
absorption. As it sits on the screen printer, if the glue is
hygroscopic it will absorb moisture. If the glue cure profile is hot
enough the moisture can popcorn, resulting in voids in the glue. These
voids can trap flux, leading to the sort of symptom you are seeing.
regards,
Graham Collins
Process Engineer,
Northrop Grumman Canada Corporation
Halifax
(902) 873-2000 ext 6215
>>> [log in to unmask] 04/27/04 02:20PM >>>
This is one of those questions to resolve a discussion we are having
regarding the size of glue dots.
We have had several failures in the past due to flux contamination
under some 0805 resistors causing a high impedance short. We found some
issues with the cleaning process that when corrected, I believe, fixed
the problem. We also changed the aperture of the stencils to a smaller
oval pattern.
My question is, does a glue dot that bridges under a part from pad to
pad (no chip bonder on the actual pads) make it any harder to clean than
a glue dot that does not bridge from pad to pad?
My feeling is that the cleaning process should clean off the flux in
either condition.
Thanks in advance for your expert opinions.
KennyB
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