Ask the Experts
December 22, 2010 - Updated
December 20, 2010 - Originally Posted

Laminate Glass Transition Change

We are in the process of shifting our PCB assemblies to lead free. We have based our laminate specification with minimum Tg of 170 degrees C. There is a suggestion to use laminate with Tg of 150 degrees C. Do you agree?

G. C.

Expert Panel Responses

The answer is "it depends". Tg is the point at which the polymers become "less solid". Molecular movement increases above the Tg, as the polymers want to behave more like liquids (but still very much a solid). Among the factors which may change as Tg is approached are dimensional stability, and surface hardness/softness. Heat capacity of the polymer changes as well. In the case of thin films, the polymer may shrink or curl as Tg is approached. If the lead free process changes result in lower process temperatures, then a lower Tg might be a logical move to evaluate. However, my experience has been that lead free generally means higher temperatures, not lower. At the higher temperatures typical of RoHS process requirements, I would expect that any ill-effects due to changing Tg of the materials would be exacerbated with a lower Tg laminate. Again the two areas I would worry about the most are dimensional changes and surface hardness, especially if the lower Tg materials experience higher temperatures due to process changes. If the 170 degree Tg works satisfactorily, "why fix it if it's not broken?". So, my answer is not an unequivocal 'yes' or 'no'. Clearly, a change would mandate exhaustive testing under the conditions that the laminate would be used. For further discussions of Tg effects, please feel free to contact me directly.

Jim Williams
Polyonics, Inc.
Jim Willimas is a PhD Chemist in Polymers and Materials Science. He specialize in printing, cleaning, inks, and coatings used in electronics manufacturng operations. Williams has more than 30 years experience.

Depends on board size, board thickness, and reflow peak temperature. Cell phones can easily be assembled with 150 Tg laminate. Telecom struggles with 170C.

Dr. Craig D. Hillman
CEO & Managing Partner
DfR Solutions
Dr. Hillman's specialties include best practices in Design for Reliability, strategies for transitioning to Pb-free, supplier qualification, passive component technology and printed board failure mechanisms.
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