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TechNet E-Mail Forum <[log in to unmask]>, Werner Engelmaier <[log in to unmask]>
Date:
Thu, 15 Jul 2010 12:21:52 -0700
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TechNet E-Mail Forum <[log in to unmask]>, Paul Edwards <[log in to unmask]>
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Paul Edwards <[log in to unmask]>
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I'm with Werner on this...

I am concerned with the general statements made that:

"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."

I can tell you that numerous assemblies and reworks have been done successfully due to the pre-baking of components, PCBs and assemblies...

We have run both no bake and pre-baked processes and found consistently that the pre-baked materials always yields higher finished product...

If baking out moisture did not increase reliability then there would be no detrimental effect to the unlimited exposure of non-ceramic BGAs to moisture...After all most BGAs use a PCB-like substrate...

Paul

Paul Edwards
Surface Art Engineering


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
Sent: Thursday, July 15, 2010 12:01 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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