| Any info on the pros and cons of FR4 or Polyamide ref the use of soldering PGAs would be helpful. ie. temp co-efficient diffs, baking, wave soldering, Convection reflow etc | I'm sure you know all the basic differences as Tg, X, Y, and Z expansion characteristics. You know polyimide is capable of being relaminated to a Tg of up to 270 degrees C. You must know it should not be produced this way or certain problems will be effected as lessened innerlaminar and foil/pad/trace bond strength.
Polyimide, as MLB's provided from your suppliers, should be cured to a Tg of no more than 250 degrees C. to ensure more cycles to failure and better bond strengths. It should be noted that this resin system has about the same Z axis expansion rate of from 50 - 60 ppm/C. as FR-4. However, the time before the cookie crumbles is extended because of its high Tg.
Concerning any type soldering operation, polyimide meets or exceeds MIL-P-5510 (now IPC-6012) thermal stress and shock testing requirements. Therefore, it will survive all well managed soldering operations.
It should be noted that this material has a lower foil/pad/trace bond strength right out of the bag. It ranges from 6 - 7 lbs versus most epoxy resin system ratings of from 7 - 9 lbs. Also, over time and temperature excursions, this number drops significantly if a higher Tg cure is sought during relamination. This means it drops to less than you would ever want after soldering and performance operations when exposed to high delta thermal excursions - as in flight hardware. Also, it is more expensive, as you already know and is produceable by only a few of the best board houses.
Polyimide is more hygroscopic than epoxies. I approaches .5% hygroscopisity when exposed. An adequate bake (250 degrees F. for 3 hours within 24 hours of processing) is required often before soldering operations.
Personally, I always seek a good epoxy system with a Tg about 180 degrees C. If more is needed, bismaleimide triazine works. Polyimide is for those knowing the need and recognizing what it takes to process it and under what conditions it is to be subjected.
There's more if only I could rember it all - oh yes, don't forget to ensure the design is resin rich on all layers. No 7628 glass styles please. Keep your construction all together with 1080, 2113, and 2116 glass styles with higher resin to glass ratios (70 to 53% resin content, respectively).
Earl Moon
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Any info on the pros and cons of FR4 or Polyamide ref the ...
- Feb 15, 1999
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