Seth

I think that right now there is the tendency to use solder paste in near
future rather than conductive adhesives. However in future some things
might change. I still think so even if this has been told for many years
now and nothing changed. There are several influencing factors that I know
but this view might be very subjective. As a result of the following
considerations it is difficult to say where the train goes to ( as we say
in Switzerland ). I personally believe that soldering will be THE process
for many years to come. Especially because the component manufacturers
don't care at all for the poor chaps that must fix the components on a
board and therefore it will be very difficult to find components with
terminations made for gluing.

- Leadless solder
A lot of research has been done with leadless solder because, especially in
the US, one wants to get rid of the lead. However, there hasn't been any
commercial success so far. Conductive adhesive might be a alternative. But,
as far as I know, the enthusiasm about this leadless attachment issue has
slowed down considerably in the last two years. I think, this is quite
sensible, because compared to all the other poisonous stuff found on and in
a PCB one may forget about the lead.

- Reliability
An organic compound shows another failure mechanism than solder. From
comparative tests one can assume that glued connections are more reliable
than soldered ones ( failure = open circuit ). But, to my knowledge there
are no profound investigation about the reliability of glued connections in
terms of the deformation- time and damage-time behaviour over a large
numbers of cycles and varying cycle parameters on FR4. Such a research is
also quite demanding, since epoxy is not epoxy and depending on the
conductive filler and of the amount of the filler the behaviour of the
adhesive changes considerably.

- Production
With adhesives the production of PCB's takes place at lower temperatures
than with solder. However, this is not a big deal in a normal production.
Where the thermal stress is an issue for the components adhesives are
already used with success.

- Recycling
This is a European issue. We are now in a project where we investigate the
reliability of reused components and repaired PCB's. From this point of
view adhesives might have some advantages since removing of the components
and reattaching them might be easier with adhesives than with solder or a
adhesive shows less influence of the removal action on the resulting
connection than solder ( intermetallics ).

- Mechanical properties
Tests we did showed, that glued connections do not like bending of the PCB
at all. In fact, when we moved the glued and cured PCB's ( 270x210mm ) in a
rack on wheels over the floor, which was a bit rough, all QFP's and PLCC's
fell of.

Best regards

Guenter Grossmann




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