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Pre-bake of Polyimide flex ? ?



Pre-bake of Polyimide flex ? ? | 22 December, 2004

At a previous job I had been taught that when reflow soldering polyimide flex circuits, it was necessary to pre-bake them before reflow soldering to prevent delamination. Our procedures said that there must be a 1hr bake prior to reflow, and the flex had to be reflowed within 2 hours of removal from the bake oven, or it had to be re-baked prior to running in the reflow oven.

I am now at another job/assembler and looking at a new flex circuit project. I can't seem to find guidlines documented anywhere on the requirments of baking polyimide flex circuits prior to reflow. I searched the archives of SMTnet, but didn't find anything with the detail I'm looking for.

Can anyone point me towards an industry standard that details best practice for pre-bake of polyimide flex prior to reflow soldering, or if pre-bake is even necessary?

Can other's share what practices they are using for pre-baking polyimide flex?


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Pre-bake of Polyimide flex ? ? | 22 December, 2004

Very strange...I just logged into the internet to come post this very question for all the lurking experts.

We have a 16 layer rigid-flex that we have never baked. Our board vendor is expressing concern that we do not perform a pre-bake before assy. These boards are not sealed w/ desiccant. We've seen no delamination issues to-date. Are there any other problems I should be aware of? I'm told it is "industry standard" to bake any polyimide, regardless of its packaging. Does anyone else use a rigid-flex polyimide without baking? If so, why don't you bake?


Happy Holidays!

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Pre-bake of Polyimide flex ? ? | 22 December, 2004

IPC-FA-251 recommends 121*C for 30-60 minutes for 1 and 2 flex layer circuits. A rigid flex can take easily an hour per layer depending on the construction.

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Pre-bake of Polyimide flex ? ? | 22 December, 2004

Check with Dupont's web site. They have a wealth of information on the various polyimide products they manufacture. I was quite surrised at the differences in materials and in particular the dimensional stability and hydroscopy.

Good stuff maynard!

Our current practices are bake out, seal + desecant. No exceptions. Some of our stuff is used in medical devices (non-implantables) but are potted. The life expectancy is a mere year but that is due to the intense radiation environment. The last thing I want is an "autopsey" to reveal mfg. defect because I was too cheap (or eager) to use a proper handling procedure. Especially, considering the rate at which polyimide absorbs H2O. Its a no-brainer here...

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Pre-bake of Polyimide flex ? ? | 5 January, 2005

We had a contract in house that included a flex board which required an 18 hour bake at 105 degrees. The other brds got along fine with 2 hours, but the flex board would delaminate until we went to this long bake. I don't know how the engineer came up with this bake time, and maybe it was over kill, but after having several wrecked boards, this is how it was resolved.

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