Hello,
The polyimide material is somewhat hygroscopic and brittle.
Consequently, you will need to bake it if you are going to thermally
process it after a storage period. Failure to do so will result in
delamination. The questions, as you mentioned, are when baking is
necessary and what baking conditions to use.
I have not tested polyimide personally, but I have considerable
experience testing cyanate ester, which exhibits similar behavior
regarding moisture retention to polyimide. I found that after 60 days
of storage in a typical air conditioned environment, it is best to
pre-bake the pcb's prior to thermal processing. The "typical" storage
environment is 40-50% RH and 22-25 deg C.
Your environment is a bit more severe, so I would suggest baking after
30 to 45 days. Certainly, any boards coming in from the field for
repair or rework should be baked.
A typical bake profile is 110 deg C for 8 hrs. The boards can be
stacked flat in the oven, but no more than 5 or 6 high. It is
preferable to have the boards vertically oriented and separated in a
convection oven for the best efficiency. The bake time can be
shortened somewhat by using a higher temperature, e.g.., you may be
able to bake for 4 hrs at 130C. I'm not sure if vacuum will help
because the limiting factor is diffusion of the water out of the
board, which is strongly influenced by temperature (T^3/2).
The high Tg materials like polyimide and cyanate ester are not as
forgiving to moisture as FR-4. While FR-4 absorbs moisture almost as
readily as the high Tg materials, it gets soft during thermal
processing allowing the vaporized moisture to escape. The brittle,
high-Tg materials do not easily allow moisture to escape resulting in
delamination or stress fracturing.
Regards,
Denis Mori
Hewlett-Packard
______________________________ Reply Separator _________________________________
Subject: GEN- H2O Bake out
Author: Non-HP-Gino-Cochella ([log in to unmask]) at
HP-Roseville,shargw3
Date: 5/16/97 12:42 PM
5/16/97 GEN: H2O Bake out
12:40 PM
Hello everybody:
We are trying to determine if a moisture removal bake is necessary for hand (sol
dering iron) and/or automated (screen print/vapor phase) multilayer PWB assembly
.
If baking is required, then what conditions should be used (time, temp, vacuum,
etc.)?
What influence does extended storage under ambient conditions (60%R.H. 70oF) hav
e? Does anyone use a drybox for storage?
Once boards entered the assembly area (and baked - if necessary), how long befor
e a re-bake is required?
We primarily use Type GIN glass/polyimide materials for passive chip SMT. Board
s that get assembled within a few days after fabrication would probably NOT requ
ire a bake, but we have to allow for extended storage - up to 5 years from date
of manufacture.
Thank you in advance for any help
Gino Cochella
(310) 814-3955
TRW Inc.
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