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February 25, 2008

How much Board Baking is too much?

Is there a guideline for up to how many times a populated board can be subjected to baking?

We are baking a board at 90C for 24 hrs, and need to rebake again with the same temp and time, we are doing this in order to rework a LGA.



K. K.

Experts Comments

This is a good question but there are missing items. Why is the board being baked in the first place? Is it being prepared for a subsequent operation and it is being baked to reduce thermal shock? Is it being baked because of experiences with delamination? This must be defined.

Any thermal excursion the board is subjected to will impact its material properties, such as the Tg, Glass transition temperature, Td thermal decomposition characteristics. Placing the board in an oven and baking it will also impact its solderability, as it will increase the tin/lead to copper or tin to copper intermetallic growth, which will be by solid state diffusion if the solder or the tin does not get molten. Baking the board will also impact stress on the laminate material in three directions, x y and z, which could impact the smaller vias. As the board expands in the z direction, forces will be applied to the plated through hole which could cause it to fail through cracking.

So please define the reasons for baking, find out what kind of laminate material you have, understand what its physical characteristics are, then make the decision as to what temperature to use and how long to bake the boards.

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Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.
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