Jim,

When you say "Hot Set Resins" I assume you mean mounting resin systems which
produce an abundant Exothermic reaction, and not Hot Press Mounted.  Hot
Press Mounting uses a mini press and a heat activated resin system to for
the mount, and unfortunately the press usually crushes the PWB sample making
for a "interesting" microsectional mount which in no way represents the
original PWB.

The choice of High Exothermic (Hot) resins v.s. Low Exothermic (Cold) resins
is a lesson in compromise.  In general the Faster the sample cures the
hotter it gets, and the more shrinkage and sample pull away the resin
mounting system will exhibit.  Unfortunately resin systems with low
shrinkage and heat of reaction can take from several hours to a day or more
to fully cure.  Most PWB manufacturers can't afford to wait that long, so a
compromise must be reached.

I have seen examples of overzealous technicians who felt that if "X" amount
of hardner cured the Mount in 40 minutes,  "3X" of hardner can cure it in 10
minutes.  This has resulted in smoke rise and solder reflow from the mounts.
As you can imagine, this is BAD for the PWB sample.

In my May, June & July  96 columns in Circuitree Magazine I discussed
Microsectional sample preparation, mounting techniques, and
Grinding/Polishing techniques.  I have enclosed an excerpt from the May 96
column:

 "Mounting materials are typically combinations of two or more parts, that
when mixed together begin to harden (cure).  The mounting compounds which
provide the lowest shrinkage, best edge adhesion, and lowest heat of
reaction typically take 24 hours or more to cure.  By contrast materials are
available which cure in 10-15 minutes, but have a high heat of reaction and
high shrinkage ratio (poor edge adhesion).  Each mounting systems also has a
different liquid viscosity which will change the way the material flows into
PTH’s.
  Acrylic mounting systems are usually a powder/liquid combination and are
quick curing (< 30 minutes).  Epoxy mounting systems are typically
liquid/liquid combinations, and range from quick cure (<30 minutes) to
overnight cure (24 hours).  Many epoxy resins have some type of inert filler
material in them to improve the mechanical properties of the mounting
compound.  Certain epoxy systems may also require a high temperature
environment (60°C to 80°C) for proper curing or to decrease curing time."


At 11:19 AM 9/27/96 +-100, you wrote:
>Does anyone have any comments relating to the advantages/disadvantages of
mounting microsections in hot or cold set resins
>
>Jim Douglas
>Kam Circuits
>
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Bob Neves
Director of Technical Services
MICROTEK LABORATORIES
(800) 878-6601
[log in to unmask]
http://www.TheTestLab.com

.


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