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 (<a href="/scripts/wa.exe?LOGON=A3%3Dind9705%26L%3DTECHNET%26E%3D7bit%26P%3D966379%26B%3D--%26T%3Dtext%252Fplain%3B%2520charset%3DUS-ASCII%3B%2520name%3D%2522GEN-%2522%26N%3DGEN-" target="_parent" >[log in to unmask]</a>) 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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