Paul,
I am having trouble seeing how the presence of water in a closed container
would damage the container more than air in the container.
If I have three identical jars.
Jar A is filled 1/4 with water and 3/4 air.
Jar B is filled 3/4 with water and 1/4 air.
Jar C is filled 1/4 with water but no air. Only water vapor exists in jar C
I put all three jars in an oven ant start heating slowly so all jars are at
the same temp.
Which jar has the highest pressure reference to outside the jar? What are
the pressures?
Jar A and B immediately start to increase in pressure as the air expands.
Jar C is actually at a negative pressure until it hits 100C.
What I don't know is if water vapor follows the Ideal Gas Law above 100C.
If it does the then jar C pressure will never exceed the pressure in jars A
and B.
Further, except for the thermal expansion of water in Jars A and B (which
is very small) Jars A and B will be at the same pressures for all
temperatures.
Did I get that right?
Diffusion of water vapor VS pressure is important but that can be controlled
by a bake temperature and time. But neither will defuse through copper,
right?
Thanks,
Bob Kondner
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Paul Reid
Sent: Thursday, July 15, 2010 5:31 PM
To: [log in to unmask]
Subject: Re: [TN] BAKING OF PWB QUESTION
Hi Werner,
Yes, we have had this discussion.
I apologies for the use of the term "live steam" but that is an
understandable example of vapor pressure.
I believe that Dr. Rothschild is correct; the volume of water counts, a
small volume of water has limited ability to produce pressure compared to a
large volume of liquid water in the same container. At some point you run
out of water and the pressure is limited. The pressure still increases, with
increased temperature, but only as a function of increasing the speed of the
molecules, not adding molecules as we see with vapor pressure. After water
condensed in the voids in the PWB or absorbed in the epoxy has evaporated
there is no vapor pressure, just gas pressure, acting as per the ideal gas
laws (thank you Bob Kondner). You notice no one is talking about the
pressure exerted in the PWB by nitrogen. Nitrogen is not in a "vapor
pressure" relation within the board. I don't know if this is the correct way
to express myself.
"Vapor pressure or equilibrium vapor pressure are the pressure of a vapor in
thermodynamic equilibrium with its condensed phases in a closed bottle". You
notice the word "equilibrium" is used. This is what I think Wayne was
pointing out when he says there is a limit based on amount.
Other comments for all ...
We cannot objectively demonstrate an increase in reliability of the board
that is significant with or without a bake if the board is well manufactured
etc. We have improved reliability by baking if the board was under cured. We
have degraded the reliability by baking too long and too high a temperature.
I believe baking stress relieves the board and drives out moisture but
rarely is that enough to affect a significant reliability improvement in
either extending cycles to failure were copper is the failure mode, or
reducing material damage where material degradation is the dominant failure
mode.
Many times, this year we have baked coupons that are prone to delamination.
Baked and unbaked coupon showed the same tendency to delamination and failed
electrically at the same rate.
We measure material damage by changes in capacitance. Measuring picofarads,
a 4% change in capacitance on our test coupons reflect material damage that
is visible by microsectioning. Coupons with a 4% change in capacitance may
exhibit and artificial extension of cycles to failure. With this method we
find the material damage that is not visible by macroscopic examination.
Using capacitance allow us an increased acuity of material damage and
insight to PWB reliability.
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
[log in to unmask] <mailto:[log in to unmask]>
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
Sent: July 15, 2010 3:01 PM
To: [log in to unmask]
Subject: Re: [TN] BAKING OF PWB QUESTION
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
[log in to unmask] <mailto:[log in to unmask]>
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