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The Knowledge and Know-how Connection
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June 18, 2013
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Views: 11522
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Your presentation is very timely, as we are currently trying to figure out "what went wrong" with some recent board builds. My group builds only a few prototype boards per year, so we rely on the expertise of our bare board and assembly vendors to provide us with the latest processes and procedures when it comes to making boards.
I had never heard (from the bare board vendor nor the assembly vendor) that baking the bare boards was something to consider with "older" boards. We received our new bare boards in June of 2008 - they were not in sealed bags, and we stored them on a shelf in our lab. We had 2 boards assembled within about 5 weeks, and they both worked fine.
Then at about 8 months out, we had 4 more assembled, and three of them worked. At 11 months out, we had 4 more assembled and only 1 of them worked. These are RoSH boards, 16-layers, with almost 100% SMDs, including 3 1136-bump BGAs.
The same vendor did all the assembly, presumably using the same process each time. We DID have the components baked, but it never occurred to me that the PWBs should have been baked, nor did the vendor mention that fact (this was around May 2009). The assembly vendor has since x-ray'd the bad boards, and not found any soldering defects.
Does this sound like a PWB moisture problem to you? How long has this problem been known? Would the PWB moisture-induced problems be something that can be seen? Does delamination also cause bad electrical connections? Can moisture crack the barrels inside vias? Where can I get more info on the problems associated with moisture in PWBs?
Thanks Much!
Jim Shoots, Xerox Corporation
As mentioned earlier on this topic, a Humidity Indicator Card should be present in the packaging. I reiterate this point, because I have visited many PCB suppliers in the USA and find that storing desiccant bags in an open container is common. Sometimes it is "a day's worth" in a small cardboard box, other times it is the whole 5-gal pail with no lid. In certain climates, the desiccant is saturated before it goes in the bag with your PCB's.
JAH, Garmin International
There can be two issues in my opinion. First, moisture may be in the board before vacuum sealing and not enough desiccate. Second, we have seen issues with laminate that has not been completely cured. "Green laminate" so to speak.
When soldered or reflowed it degases out the drilled or punched holes, producing defects. This problem can be eliminated or reduced by baking. A potential risk of baking then would be degrading OSP coatings if that process is used.
Brad Frederick, Kem-tron Inc
I received boards in a vacuum seal with a desiccant pack but NOT with a humidity indicator card. Perhaps a corner was cut on the edge of the sealed package in transit.
Anyway, we assembled the boards within 24 hours and we had delamination issues. From now on, we bake everything. At 50% RH, I should have had 7 days. I assume my RH was much higher.
Jeff Lynn, Colt Tech
I have some questions: What is the correct vacuum pressure to seal a bag of a moisture sensible component? Is the level of vacuum related to the level of sensitivity of the components? Where I can find some specification on the matter?
Norberto Barrera, EPIC Technologies
If the boards received do not have a desiccate pack and a indicator card included to indicate its present level, I have always stayed on the safe side and baked out the boards prior to assembly. It is a small price to pay to insure that the boards do not delaminate or bubble. Same goes for Jedec components. If packaging is opened a new desiccate pack and indicator card should be put in the bag and it should then be resealed.
Jim Holton, Endicott Interconnect
You are correct and don't need to bake if the package is vacuum packed with the assumption that the board is dry coming from the PCB supplier. You may want to add this assumption to your statement.
Harold Kleinfeldt
When measuring TMA, producing the results can give "unstable" laminate. What are the main causes of this (epoxy cure?) and what affect does moisture have on these results?
Paul