George: There are reams of information on this as you know and I'm sure
there will be many responses. So let me try to be short and direct with an
answer.
Even though you may have coated "tens of thousands" in the past without
adhesion problems. It is not just a matter of adhesion causing problems. It
is also not just an issue of flux residue and bare board cleanliness.
As my good friend Carl Tautscher and I used to discuss, it is the unknown.
The lack of consistency of your incoming materials, components and operator
practices. All bare boards are not the same level of cleanliness. Your
components have varying degrees of contaminates, residues, handling oils,
salts. The flux you use can have raw material or formulation changes. The
operator doing rework, touch-up and hand soldering can influence how benign
the flux residues are by their techniques or using a different roll of
solder or soldering iron. Handling and personal cleanliness practices are
different person to person.
A bi-polar wash process is your insurance step that you have a more
consistent product to be conformal coated. You are taking a calculated risk
that the cost saved by not washing will not be exceeded by the cost of
product failure in the field and the ill will it creates.
Regards
Michael Barmuta
Staff Engineer
Fluke Corp.
Everett WA
425-446-6076
-----Original Message-----
From: Carroll, George [mailto:[log in to unmask]]
Sent: Friday, November 15, 2002 2:31 PM
To: [log in to unmask]
Subject: [TN] To wash or not to wash (again)
I've read through the archives, but I'll ask again anyhow.
Washing no-clean boards before conformal coating: Is it necessary for
extended reliability? The polyurethane conformal coated, mixed technology
board in question will be in a housing but not sealed from the outside
environment of daily and seasonal temp and humidity variations (SE US).
Customer is looking for 20 years of nearly continuous service.
First, adhesion is not an issue. Our process has coated tens of thousands
of no-clean assemblies without coating adhesion issues. We use a ORL0 flux
and REL1 type paste. I've Omegameter tested incoming bare boards and they
are <2 micro g. / sq.in. Our assembly operators are instructed to handle
boards with gloved hands. The question is - is washing prior to conformal
coat required for long term or, for that matter, does it even improve (or
degrade) the long term prospects.
At this point it would appear that if board washing were mandated, it would
probably be with saponifier followed by D.I. rinses and a bake to dry the
assemblies. Our conformal coating always measures (flat coupon) > 1 mil and
averages 1.8.
My own opinion from lack failures of coating over no-clean in the field is
that coated no-clean boards will survive and function as needed. However
when a customer asks, "20 years?", it's hard not to waiver. Do I really
need a belt AND suspenders?
I will be coating and having B-25A's tested to 830B in our lab in a month
(and setting up accelerated aging tests of these boards later on) to look at
this and other questions but, for the moment, I'm looking for the opinion of
the experts.
George Carroll
Process Engineer, Siemens Energy & Automation
P.O. Box 1255
3000 Bill Garland Road
Johnson City, TN 37605
(423) 461-2948
[log in to unmask]
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