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try putting a soft fast cure rubber under the epoxy. The rubber should
take some of the strain.
______________________________ Reply Separator _________________________________
Subject: ASSY: Security coating of SMT leads.
Author: [log in to unmask] at internet
Date: 11/14/96 8:37 AM
Another try, hopefully this time with the text itself.
We will be building a product for a customer that has three
devices that need protecting from electrical access for security
reasons. The customer wants a coating over the leads that will
prevent people from probing them to find electronic access codes
used in the system. Obviously this can only be a deterrent, as
someone sufficiently determined could get past any coating or
barrier.
The coating needs to cover the leads only, so potting the
entire assembly isn't on. It needs to be a fast in line process. I
would like to limit any off line processing, like oven curing, to
under one hour. The cured material needs to be resistant to
soldering iron heat, the sort of solvents and acids that the PCB
will withstand and the sort of mechanical attack that won't wreck
the leads.
After discussion with material suppliers I can see two ways
of doing this. One is a filled rigid epoxy material of the type
used for glob topping bare die, that will encapsulate the leads.
This will be very resistant to attempts to remove it. The Cte will
be closely matched to the package, lead and board materials, but
the elastic modulus will be high. The other group of materials are
of lower modulus. These however tend to be easier to remove.
Processing will probably be dispensing from a barrel via an X,Y,Z
dispensing system. Curing will either be heat, or UV.
My principal concern is compromising the fatigue life of the
solder joints by having a high modulus encapsulation material that
constrains the movement of the gull wing leads and puts more
stress on the solder joints during temperature changes. The
product is a relatively high value consumer item for use in the
home. Has anyone done something similar? What materials did you
use? What were the results? What was the processing like? I will
be doing some reliability tests, but the time constraints mean
that the material choice needs to be right first time.
One device is a 28mm body 0.5mm pitch PQFP. The other two
devices are smaller in size. They will be conventionally attached
to an FR4 board.
Jeremy Drake.
Design to Distribution Ltd. Kidsgrove, Stoke on Trent, England.
+44 (0)1782 771000.
[log in to unmask]
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