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November 2001

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From:
Earl Moon <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 5 Nov 2001 08:17:53 -0600
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Though still looking for an alternative test method and acceptance/rejection
criteria to determine more accurately voiding in adhesives, I now need to
get a refresher on the adhesive application process. I?m looking both at
epoxies and films.

I am first looking at two part epoxies, that are thermally conductive, to
bond RF amps to a cold wall. There are many and suggestions are appreciated.
The process itself, using these epoxies, is pretty straightforward as I see
and have experienced it, but I am seeking process improvement suggestions.

Believing it is true that two part types promote an exothermic reaction, is
it not true that much voiding is prevented if most of the outgassing is
effected just by letting "nature" take its course? This is provided, of
course, the bonding surface areas are small enough, well cleaned, and the
process is well managed.

In a well managed process, isn?t application the primary factor promoting a
void free bond? This is true, if both surfaces are well ?coated? during the
process whether using syringes or screens/stencils (I?m looking at this much
the same way as in SMT and hybrid circuit assembly).

Using a film type adhesive, the bonding process takes place as an
endothermic reaction. I relate this to using prepregs, as in rigid MLB?s, or
acrylics, as in flex circuitry, with bonding taking place in a lamination
press under temperature and pressure over time. I haven?t quite figured out
how to use this process, if practical, on parts being bonded to the cold
wall inside a box about two inches on a side. Here, I?m thinking of small
clamps used to pressure the part to the wall. I?m also thinking of managing
this process inside an autoclave, or ?turkey? bag, with many bondings being
effected simultaneously, while applying heat and pressure over time while
pulling enough vacuum to get the gas out and minimize voiding. Any thoughts
here much appreciated as well.

I?m also thinking of using the autoclave/turkey bag thing with the two part
epoxies to aid in outgassing and void minimization. Wouldn?t this be much
like potted or molded assembly?

My biggest concern for the two part system is squeeze out and the amount
pushed outside the bonding area. Any suggestions about minimizing this also
greatly appreciated as well. I think the experiment here is to measure the
amount as a result of the amount of adhesive placed and where. Do I need to
be concerned with determining a ?low pressure? area, its shape, and how it
moves under pressure?

Again, I must come up with a process, hopefully not too far outside the
realms indicated above, that promotes, rather than inhibits, thermal
conductivity. First, it has to work thermally. Second, it has to be
consistent as a process and a product. Third, because of second, quality
must be verifiable and must be capable of being correlated to the process.
Fourth, third means there must be a good way to measure voiding. Fifth, an
experiment must be run to ensure the amount voiding correlates directly to
thermal effectiveness and efficiency. Fifth is the easy part.

I?m throwing a lot of stuff out to you folks, from my perspective and off
the top of my head, but I know many of you have already solved these
problems. Sure would like to benefit from your experiences and knowledge.

Earl

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