Eva,

On which surface did the joints fail - the terminations or the HASL pads? My guess is the latter, as adhesion is typically suspect at this interface with this finish. You might also see electrical integrity deteriorate with time at a silver epoxy/solder interface, due to oxide formation. It would be better to have a noble finish on the pcb as well - gold or even clean copper.

I don't think I have heard of a silicone-based conductive adhesive used for SMD bonding, but there are thermoplastic formulations around. If you do want to use an epoxy-based formulation, then you should get better results using a thermal cure rather than just room temperature.

Regards,
Peter

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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Eva J
Sent: 25 April 2013 15:01
To: [log in to unmask]
Subject: [TN] Conductive epoxy to install SMT parts

I am looking for guidance on conductive epoxy selection for components on PCBs. I heard that there is a formulation of conductive epoxy with silicone as a filler that is more flexible. Do you know of any studies or evaluations available on conductive epoxy for electronics?

We attached a 4 pin LGA component that has gold finish terminations to a HSAL finished PCB. We syringe dispensed the conductive epoxy to the land pattern. Optically machine placed part and allowed the conductive epoxy to cure over the weekend.
During environmental stress test ( hot / cold cycling) one joint failed.
All of the LGAs were removed and what we found is the component came off way to easily.

I would like to continue using conductive epoxy to mount special application SMD's: however reliability is a concern. Any words of wisdom?


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