Amen to that and a Merry Christmas to techies everywhere (Oops non-PC
again)

Ian Fox
Goodrich ECS

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of paul reid
Sent: 22 December 2005 14:13
To: [log in to unmask]
Subject: Re: [TN] ] Qualification of the Pb-free capable manufacturers

I have to agree with Valerie.  Solder float has been demonstrated to be
a very much hit or miss test.  My main concern is the number of holes
examined are small and the ability to find interconnect failure or
delamination is severely limited by the random nature of
microsectioning.

My experience is that known unreliable boards can pass the solder float
tests.  We have had coupons that fail thermal cycling in just tens of
cycles and the fabricator argues the same board passed solder float with
no problem.  Those confrontations digress into a game of spec-man-ship
while the boards fail in assembly.


It is the nature of the solder float test that limits it's usefulness.
This test method was developed in the early 1960s when the industry was
attempting to simulate the stresses associated with wave and hand
soldering assembly techniques.  The times and temperatures now
experienced during surface mount assembly have little to no relationship
to the immediate intense thermal stress experienced during a solder
float test.

As processes and materials evolved the response has been to add the
number of solder float cycles or increase the temperatures.  What we
need is to get away from the "gerry-rigging mentality" and evolve to a
more representative and realistic test methodology.

To understand the stresses associated with today's assembly we need to
create a temperature profile representative of what the product
experiences during surface mount assembly and rework procedure.  Not
that we have to create and exact thermal profile but rather a standard,
reproducible profile that does not change.

Understand that the damage done in assembly and rework is accumulative
and not always expressed at the time of assembly and rework.  There are
plenty of times when the failure occurs during the thermal cycle but
frequently the failure occur during burn-in or in the first few days of
use.

All testing needs to take into account the interactions of the
fabrication process results (copper thickness etc.) in the light of the
materials capabilities.  It is not just each individual element in the
circuit board it is the sum total a all elements coming together,
affecting reliability.

Testing raw material is useful but cannot replace testing the material
in the fabricate condition.

Our company frequently tests different materials for our customers and
we have found the same material can have two different results processed
at two different fabricators.  Material "A" may work well in one
fabricator process and fair poorly with a different fabricator.  The
difference appears to be that fabricators tweak their process to
accommodate a given material.  They make subtle changes to lamination
cycles, drill feeds and speeds and hole preparation to accommodate a
material.  Competent fabricators will not accept a job with a new
material without running tests to understand how that material will fair
in their shop.

Some materials work well in single lamination processes only.  Exposed
to sequential lamination they fair poorly.

So the correct answer is everything, the fabrication process, the
assembly process, the end use environment counts in establishing a
materials'
reliability.  That means you need a lot of open kimonos for a fair
evaluation.

I think the best bet is to invoke reliability testing using coupons
fabricated with the materials of interest by fabricators of interest.
Then you "baseline" your fabricators and select the materials that meets
your needs best.  Of course since we just released this new capability,
we recommend the coupon should have delamination detection circuits.

Every build is an adventure but, are going into it prepared, or an
unwitting member of "Survivor - PCB"?


Sincerely,

Paul Reid


Program Coordinator
PWB Interconnect Solutions Inc.

Tel:  613-596-4244 Ext. 229
Fax: 613-596-2200

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