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Subject:	Re: Surface Insulation Resistance, Flux
From:	Douglas Pauls <[log in to unmask]>
Reply To:	TechNet E-Mail Forum.
Date:	Wed, 10 Jan 2001 07:29:53 -0600
Content-Type:	text/plain
Parts/Attachments:	text/plain (50 lines)

Hi Technetters,
In IPC-TM-650 Number 2.6.3.3, the test method is to characterize
fluxes by determining the degradation of electrical insulation resistance
of PCB specimens after exposure to the specified flux. In the test method,
it also has two state---uncleaning and cleaning after exposure to
the specified flux and solder. My questions are as follows:

**Keep in mind that the two states are in there to address both fluxes
intended for cleaning (e.g. OA) and those intended for no cleaning (e.g.
low solids fluxes).

1. Is insulation resistance of PCB reduced after exposure to the flux and
solder?

*In general, yes. Exposing a laminate to temperatures above its glass
transition temperature degrades its insulation resistance. Simple fact of
physics. I have found that high solids rosin fluxes tend to encapsulate
surfaces, resulting in less degradation of IR than comparable OA or low
solids fluxes.

2. Is the magnitude of insulation resistance of cleaned specimen less
reduced than this of uncleaned specimen?

*I don't think that the flux itself is the major factor. How well you
clean, and the nature of the uncleaned residues will determine the IR
differences. I don't believe you can make a blanket statement.

3. Do we have method to avoid the reduction of insulation resistance after
solder?

*Not really. Soldering is tough on organic materials. Simple fact of
nature. You have to determine how much degradation you can live with for
your product in your end use environment. Burn in and life testing,
correlated with SIR or ionic cleanliness measurements can help in this
respect.

Doug Pauls
Rockwell Collins
Chair, SIR Task Group

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