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November 1999

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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 24 Nov 1999 16:32:08 +0200
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Hi!

Apart from what Bev correctly says, there are a zillion variables that
can enter into the equation. SIR testing, depending on your instruments,
equipment and how you use them can give you almost any answer you want.
This is why you must follow a standardised protocol.

However, are you really measuring the true SIR or are you trying to
detect electrochemical migration? There is an enormous amount of
confusion between the two, due mainly to a historical legacy of bad
terminology and the fact that the test methods are fairly similar. True
SIR testing is done without bias, using voltage gradients generally less
than 50 V/mm for the test voltage which is typically applied 30 seconds
before the measurement is applied and cut immediately afterwards. ECM
resistance testing is done with a bias of a value designed to provoke
failure mechanisms.

The mechanisms which cause the measured R value to rise are generally
either evaporation of condensate, as Bev suggests, or
sublimation/evaporation of contaminants. These may be flux residues
(e.g. adipic acid in some "no-clean" fluxes) or hygroscopic residues
from any of a number of processes, such as those using PEGs, derivatives
or similar which actually are ab/adsorbed into the laminate surface by
van der Waals' forces. For this reason, it may be advisable to use
temperatures <40°C in your test chamber.

Lowering R may be due to the e.g. the laminate itself being hygroscopic,
due to ionic species within the resin but I would suspect condensation
on the test coupons as being a prime possibility.

Test coupons must be held vertically with the comb pattern vertical, in
a position where nothing can drop onto them. They should be edge-on to
the air current flow. The test chamber humidity should be checked with a
psychrometer (W&D or dew point): do not rely on types which depend on
the electrical characteristics of an absorbent polymer, they are rarely
better than +/-20%! Connections should be made with a pure rosin flux at
the bottom of the test coupons and the flux residues must never be
allowed to bridge the spaces between the conductors. The wires should be
PTFE insulated solid conductor, for preference. The test chamber must be
organically sterile and clean (nothing like a microorganism growth to
bugger up a test!). Use only 18 megohm water for humidification. This
very non-exhaustive list is just an insight on a few items that can
affect the results.

Brian

wmlee1 wrote:

> Hello,Can anyone help to answer the following questions about Surface
> Insulation Test: 1) I found that all SIR test results showed
> dramastically decrease in resistance reading after placing the test
> coupons into the chamber.  However, i found some test results showed
> increase in resistance gradually after a period of time the coupons in
> chamber, may be after 24 hours or 2 days.  While some test results
> showed continuously decrease in resistance.  Can anyone answer
> this? 2) I have been advised that we must take afford to prevent
> condensation inside chamber during testing, why? 3) I had experienced
> that the flux used for wire bonding would degrade the test result, so
> sometimes, i would not use flux for bonding when it is solder finish.
> However, if it is bare copper, problem would happen, can anyone advise
> some method for this? Thanks Leecp

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