Eva,

You should ask the formulator for this information.  Really attempt to nail
them down and get them to provide real, and detailed data - one real live
part types.

In general, it is quite difficult to get most adhesives to adhere to the
diverse surfaces finishes presented in SMD work, at temperatures that the
SMD assembler [and components] will tolerate.  Making it more flexible is
only one factor to consider.

Mechanical, thermal, or other environmental stresses typically quickly show
the weak links.  


Side note - if "cured" over the weekend, I suspect that it is more of a
humidity cure RTV than an "epoxy" based system.  Do you smell acetic acid
[vinegar] during cure?  = bad idea.  Or do you smell more of an alcohol?  =
okay.  Silicones have a nasty way of tending to migrate to places you don't
really want them to go... switches, connectors,

Can your production live with a 48-72 hr cure?

I would be sure to ask the formulator whether the material has an epoxy
backbone, or a silicone backbone so that I could get a better grasp of its
mechanical properties.  I am assuming that if it is a silicone, under normal
conditions, it will already be above its Tg, and therefore already in it
'high expansion' state.  I also make the generalization that it will remain
flexible until down in the -20 to 0°C range [subject to the specifics of the
material]

Steve C

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Eva J
Sent: Thursday, April 25, 2013 10:01 AM
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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