Hi Paul,
I think we went over the water vapor/steam issue once before.
It simply is not so that one needs liquid water to have an increase in pressure.
Consider a closed vessel with some water in it. As you heat the vessel the liquid water will vaporize until all of it has vaporized. Just because you no longer have liquid H2O does not mean that the pressure does not increase with increasing T—you now have superheated steam that follows the equation P= RT/V—so the higher the temperature the higher the pressure for a constant volume. V certainly can increase with delamination and some of the superheated steam will escape. The term 'live steam' has no meaning in this context—its primary use is for model railroading, and refers to steam actually doing some work in driving the locomotive. Inside the PCB, liquid water per se does hardly ever exist, the steam does do 'work' only in terms of increasing the internal vapor pressure, and you certainly do not need liquid water for that.
Werner
-----Original Message-----
From: Paul Reid <[log in to unmask]>
To: [log in to unmask]
Sent: Thu, Jul 15, 2010 11:37 am
Subject: Re: [TN] BAKING OF PWB QUESTION
This is a good thread.
Off-line Ian Hanna from Circuit Tech comments:
"Hey Paul, I would expect that the biggest threat from trapped moisture is the
vapour pressures generated during the initial shock of assembly...would this be
visible during cyclic reliability testing? Regards - Ian"
The answer to Ian's question is, yes, the greatest "threat" (or at least
pressure) from trapped moisture is during the initial thermal excursion and yes
it is visible during reliability testing. But no we do not see delamination
during the first thermal cycle and if the material delaminates the effect is
extended cycles to failure. The lesson is you have to test both the copper and
the material when doing reliability testing for lead/free applications incase
the material degrades and you get false positives from thermal cycle testing. We
measure material damage by means of changes in capacitance in representative
coupons in order to capture material damage.
Basically we are saying that many studies demonstrate limited impact, positive
or negative, on reliability due to baking. Dr. Taube shows "moisture in" or
"moisture out" has a limited reliability affect. The effect of baking is not
compelling for extending ability to survive assembly and rework.
The idea is that vapor pressure from trapped water "blows" the material apart.
It is my contention that if the adhesion between laminated surfaces is weak, by
that I mean, between B-stage and C-stage or B-stage and copper, then vapor
pressure generated from trapped water may cause frank delamination. But there is
more to this story.
I expect the greatest pressure from water vapor would occur during heating on
the first thermal excursion because the board has the greatest amount of water
at the beginning of assembly. After the first cycle moisture must have been out
gassing during the thermal excursion. The longer at temperature the less water
in the board so by the second thermal excursion there should be less pressure
because some of the water has been lost. Our findings are consistent with Dr.
Taube's findings. Delamination does not typically occur on the first thermal
cycle. Delamination frequently occurs after the third thermal excursion. Let's
let that stand for a second.
Dr. Wayne Rothschild points out that the vapor pressure is limited by the amount
of water available to the system. He points out that the gas laws take into
account the amount of water in the system.
Pressure increases rapidly with "live steam" but, when the water is all
vaporized, the pressure increase is reduced. "Live steam" requires liquid water
as a source to replace the steam. A steam train stops when it runs out of water.
The vapor pressure goes to zero once the water escapes out of the board.
So bringing Dr. Taube's and Dr. Rothschild's observations, our experience in
reliability testing, and how samples fail in T260 testing on a TMA, together, I
feel that the damage to material is from vapor pressure, z-axis expansion and
also material degradation. The epoxy system is failing and volatiles are
providing a force as is z-axis expansion of the epoxy system, to a point were
force is greater than the strength of the epoxy and the board fails. Epoxy
degradation is a major component of the failures and aggressive baking could
degrade the epoxy reducing reliability in a lead/free application.
All this stands given that the copper is not being degraded by 260°C, which I,
and others are beginning expect it is changing, when the copper is poorly plated
and of a lower quality. But that is another story.
Sincerely,
Paul Reid
Program Coordinator
PWB Interconnect Solutions Inc.
235 Stafford Rd., West, Unit 103
Nepean, Ontario
Canada, K2H 9C1
613 596 4244 ext. 229
Skype paul_reid_pwb
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