George,

Good luck!!

This is a very serious complicated issue and as such the answer is always "it depends".
I will touch on some of the more interesting points.

At first glance the question seems to raise a paradox. Coating over no-clean
means there are some environmental concerns but
there are no infant mortality concerns with the manufacturing process.
This is "backwards" to standard reliability design logic.

Your first task is to define what is meant by "reliability". This term will vary
from case to case. Once defined then it becomes necessary to develop a method to measure it.
Because you are dealing with a 20 year period "wear-out" mechanisms become an issue.
Virtually all forms of testing concern themselves with infant mortality warranty failure modes.
They are typically stimulation type testing and can not be correlated to "long term" wear out failure modes.
Thus coupon testing and most chamber testing will not be of value.

At this point someone usually realizes they will not be around in 5 years let alone 20 and
signs off on the verification process because it won't be his or her problem!

If you really want answers you must clearly define all the terms used in the contract and develop
simulation type testing based on the expected worst case operating conditions and run proto-type/production
units through these tests (big bucks & very time consuming).

The other choices include;
1. go by a good book and read it (you won't be around for 20 years)
2. scrap the current process and use paralyne with a different cleanable flux process
3. beat your head against management trying to get the correct testing done (this results in becoming another
"consultant"!)

So George, just how close to retirement are you?

David A. Douthit
Manager
LoCan LLC

"Carroll, George" wrote:

> 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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