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Tg and reflow

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#38549

Tg and reflow | 16 December, 2005

Hi. I am trying to understand Tg and how it relates to reflow. With the higher temperatures of lead free, we have been looking toward laminates with a Tg of 180 instead of the current 140. My question is a little more general. In either lead free or leaded reflow, these temperatures are greatly exceeded. What happens when the Tg is surpassed? Is there a point when it is greatly surpassed that we are concerned about?

Sorry for my ignorance ...

Thanks, Mike.

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JAX

#38582

Tg and reflow | 19 December, 2005

Tg is just a way to say when the board goes limp... or a quick way to rank laminates... who cares.

For lead free you want to look at Td, Decomposition Temperatures. ( Temp at which the material weight changes by 5% )

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#38583

Tg and reflow | 19 December, 2005

We agree with Jax. Tg of resin does not necessarily mean better thermal performance at temperatures above the Tg. Tg is an indication of the degree of cure for that resin system based on an expected value. Since lead-free soldering temperatures are well in excess of Tg, the most important condition is the decomposition temperature of the base material. Specifying a Td of >=330*C is good advice.

Search the fine SMTnet Archives for earlier discussions on Td, like http://www.smtnet.com/forums/Index.cfm?CFApp=1&Message_ID=32430

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Amol Kane

#38592

Tg and reflow | 19 December, 2005

from what i know, Tg is important from thermal stresses point of view. board warpage increases greately when Tg is exceed

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