| | | | We had some failures on an IC package that the manufacturer has put down to moisture inside the package, leading to popcorning. | | | | This seems a bit odd, the package is a PLCC 84 which is quite thick. I have only seen this problem on thin packages. | | | | How do we dry ? | | | | what are the alternatives - Nitrogen was a favorite medium in the past - any good ? | | | If you are buying new IC's fresh out of the dry pack you will not need to bake. But since that is not always the case the chip manufacturer should have a recommendation for a bake profile. | | | I have never baked PLCC84, have never had to, consider sending the package out to a lab to determine the failure mode. If it was indeed a moisture caused failure there should be evidence of a crack, albeit maybe small. | | | We use a N2 tank to store our BGAs after baking (if required). N2 alone will not Remove the moisture from the component. | | | But I have a question, Is there a danger of moisture intrusion into a component during a wash cycle. We run some moisture sensitive devices on double sided reflow boards. And we will wash the first side before flipping the board to run the other. | | | Any input? | | Mike, | | I ran the process you speak of for a while. I wondered the same thing myself but I never saw anything that would lead me to believe this was a dangerous process. Doesn't make much sense, does it? I suspect that the part isn't in the washing environment long enough to incurr a significant amount of moisture. There is also the drying phase at the end of the aqueous cleaner. This may be enough to counteract the "wet" portion of the wash cycle and make the entire process inert. Maybe somebody will help us out... | | Regards, | | Justin | I agree it's currious. What's more printed circuit boards, which are hydroscopic, don't seem to absorb much moisture during washing between reflow cycles either. | TTYL | Dave F Dave, Dave, I was able to attend a PCB technology course offered by a group called DMG Engineering (led by a former VP of Manufacturing of Hadco. It was their opinion that one should never have to bake a PCB for moisture removal purposes. Keeping in mind the guy is looking at things from a PCB shop perspective, and not an assemblers point of view. I work with a medium sized Electronic Assembly CM, and we have not baked a board in 3 years. As far as components go, I have read some interesting material on the subject. One book in particular "Ball Grid Array Technology" by John Lau. The book shows some moisture absorption charts, and also some bakeout curves. He makes note of a safe zone, a delamination zone, and a popcorn zone. At any rate the safe zone is situated at about .15% to .20% moisture weight vs component weight. The bakeout chart featured 3 sets of components, one at .475% moisture, one at .425% moisture, and one at .2% moisture. At the 4 hour mark of baking at 125C, all 3 sets of components were below .05% moisture, in fact the the set that absorbed moisture at 23C & 55% humidity for 150 hours to reach .2% moisture content, was at 0% moisture in 4 hours. We preach proper handling and drypacking of components, and if bake is necessary, bake in a nitrogen enviromnent to reduce oxidation. But the 25 hour bakeout at 125C is not necessary. This is especially importand if one is baking in an oxygen rich environment. The sample set exposed to 85C at 85% humidity took 50 hours to absorb .475% moisture, but the moisture was baked out to .04% at 4 hours, yet took an additional 21 hours to remove the last small percentage of moisture. Very interesting info, and keep in mind the focus was on PBGA's, but I feel the principals apply to plastic molded components. Keep in mind the percentage absorbtion rate per package weight would be greater with the greatest amount of surface area per weight such as TSOP and SSOP components versus SOJ and large SOIC components Obviously if you are dealing with tape and reeled components and cannot bake at 125C, then a longer bake at a lower temp is necessary. Good Luck Steve Abrahamson
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