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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