John,
Here are some things to consider.
Your mold should have draft angles (sloped sides) to make finished
products easy to remove.
Make sure you use a release agent.
The mold should be made out of aluminum or steel for production. For
prototypes you can use a rubber mold to prove the concept. I have found
that rubber molds, for our applications, may give you a part that looks
like the model, but the potting material we use shrinks during cure and
the rubber does not prevent dimensional instability. This was noticed
most in a mold that consisted of two halves that were clamped together.
The clamping force required to prevent leakage reduced the side to side
dimension by up to 0.1". On an open faced mold about 14" long the
mounting inserts would "move" by up to 0.050" making assembly at the
next level almost impossible as mounting holes did not match up with the
threaded inserts. Both of these problems were eliminated by using
aluminum molds.
If you use aluminum and there are two parts screwed together or other
pieces screwed into the mold to make the final mold, I'd strongly
suggest adding Helicoil inserts to the mating threads.
Mold maintenance is critical. Over time release agent can build up on
the mold as can flashing left from the previous "pour". Look into what
will dissolve the cured epoxy. This can be nasty stuff, but essential
for keeping the molds in top condition.
We also find that a new or recently cleaned molds need to be "seasoned"
by spraying a coat of release on the mold and pre-baking the mold. With
the rubber molds we find that we MUST run a dummy of just epoxy prior to
running product. They come out filled with little bubble voids that I
can only assume come from the mold "out gassing" during the first
cure/bake cycle.
What is the connection method to the outside world? If it is "pigtail
wires" this is easy because you can have them come out of the top
surface of the mold where the encapsulating material was poured in. If
the wires must come through a strain relief, then it starts to get
tricky to hold the strain relief in position for encapsulating. If you
are using an edge connector (gold fingers on the board) or a connector
you have to figure out how to seal these to keep encapsulant out. This
is where a clever machinist is essential.
It will be important to remove any air trapped in the potting material.
This, of course, is determined by the material used for potting and the
final application of the potted part like our high voltage applications.
De-gassing in a vacuum during or after mixing is a good idea and if you
can pour in a vacuum even better.
And one final item. Make sure you mix too much epoxy. I hate it when
we pour 11 assemblies and one leaks or "burps out air" and then the
level is to low and we have to throw out the assembly. Ouch... my
profit sharing just got smaller.
Hope these tips help.
Phil Nutting
Manufacturing Engineer
Kaiser Systems, Inc.
-----Original Message-----
From: John Fahey [mailto:[log in to unmask]]
Sent: Thursday, August 16, 2001 10:57 AM
To: [log in to unmask]
Subject: [TN] ''Encapsulating/Potting PCB Assemblies"
I have recently come upon a requirement to encpasulate/pot a small PCB
assy
containing a T/H transformer. Size is approx .5"W x .5"L x .3" H
Can anyone who has experience in "Potting" or "encapsulating"
technologies ,especially potting PCB assemblies, give me some
hints/starting points, etc.
I would be very interested in any issues to watch out for, recommended
potting materials, equipment, standards to adhere to, tests, etc.
Basically anything that would be helpful.
Thanks in advance,
John
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