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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Brian Ellis <[log in to unmask]>
Date:
Thu, 7 May 2009 17:51:23 +0300
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To understand this problem, you also need to understand what happens
when synthetic polymers are heated.  What I am about to say applies
equally to solder masks and to the substrates under them.

When a polymer is heated, it reaches a temperature where its
characteristics change, called the glass transition temperature.  Above
this temperature, the surface of a polymer becomes very porous and opens
up like a sponge.  To make things worse many of the chemicals that the
polymer comes in contact with contain surfactants or other molecules
which have a hydrophilic and lipophilic ends.  If the molecule is
linear, or nearly so, such as most of the common detergents and
surfactants, another phenomenon occurs and the lipophilic end of
hundreds of molecules rush into the pores and are trapped there when the
polymer cools again.  If, for example, the product is a flux, then some
activators will probably become co-trapped and these can be released,
not only with subsequent heating, but also at ambient temperatures over
a long period of time.

This is obviously a danger for the reliability of any printed circuit in
critical applications.  In addition, just to make things worse, many
thermosetting polymers have ionic salts in their molecular makeup.  For
electrical applications, these salts are practically eliminated but
there is always a risk of small quantities remaining and these can also
leach out.

Other posts have specifically mentioned HASL.  This is because the
fluxes used for this process are not only highly activated but nearly
all the other chemicals in them are closely related to surfactants!
You've got the worst of both worlds!

What can be done about them?  Not a great deal if you have a batch of
circuits that leach enough to compromise the functionality.  Perhaps a
good wash in DI water after having baked the circuits may improve the
situation if the problem comes from the fabricator.  If it comes from
your own workshop, then a systematic tracing will find the culprit.

Hope this helps!

Brian

Kane, Joseph E (US SSA) wrote:
> A cleaned bare PB is exposed to bake or reflow, and comes out with
> measurable ionic contamination.  The board is dirtier at the surface
> after 
> a heat excursion.
> 
> The contamination isn't coming from the oven, handling, or anything
> external.
> Heat causes something to move from the inside of the board to the
> outside.
> 
> We haven't done any ion chromatography or other testing yet to
> run this down, but has anyone heard of or experienced anything similar?
> 
> In our normal process, we clean everything multiple times, but if the 
> contaminant builds up under solder mask, or in other areas that are 
> difficult to clean, we could have SIR or corrosion problems around fine 
> surface features or in high voltage areas.
> 
> And if this ionic contamination doesn't just move up to the surface,
> but is somehow moving around inside the board, could it collect in 
> some places and lead to CAF or other problems?
> 
> Joe Kane
> BAE Systems
> Johnson City, NY 
> 
> 
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