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I've purchased three WM8804 MSL 3 chips, which came in a sealed moisture-sensitive package with instructions to mount the chips on the board within 168 hours of the bag being opened. However, it is not possible for me to mount all three of them within that time (I'm doing prototyping right now, not production).

Basically, I know that the first chip will be fine (I'll mount it right after I open the bag). However, the other chips will not necessarilly be mounted 168 hours after opening (there may be problems, I might need to order new boards, etc.).

I don't have the equipment necessary to "bake" the chips, and I'm not doing reflow (just hand soldering). Are there any ways I can prolong the 168 hours (storage techniques, etc.) in order to prevent moisture damage to the unmounted chips? Or is it fine to simply reseal the bag after use?

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3 Answers 3

up vote 9 down vote accepted

Open the bag, take one IC out, place a teaspoon or two of white rice into the bag, and make sure to reseal airtight. The rice will take up most of the mosture that you've introduced with the atmosphere exchange when opening the bag. If you want to do it professionally, use moisture absorbing silica gel, but rice works perfectly (Don't cook rice or silica gel afterwards).

I've had a part which said it should be soldered within four weeks after unpacking, lying around for about a year in the open, and successfully put it to operation (hand soldering). Nothing popped off or cracked. So it can't be too critical, and maybe relevant only for reflow or wave soldering.

By the way: Why can't you bake the devices? An oven in the kitchen will probably do just fine.

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You can bake the silica gel to drive off the water, but it's quite inadvisable to eat it. Almost every electronic device that's shipped overseas comes with a packet of silica gel- you can save them in an air-tight container. You can also buy bulk silica gel (pay heed to handling warnings). –  Spehro Pefhany Sep 1 at 18:43
    
It just feels strange baking electronics in an oven designed for food - and I'm not allowed to do that anyways. I'll stick with the silica gel for now. –  d.free Sep 1 at 18:44
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@d.free: I regularly put a cheese sandwich next to my reflowing PCBs. (jk...) –  DerManu Sep 1 at 18:55
    
You can also find pure silica gel in the form of certain cat litters (usually marked as crystal cat litter). Calcium Chlorite would also work, it's sold here as a product called "Damprid". I've used the latter to bring water logged electronics back from the brink, it's very effective stuff, and it can easily be reused. You can find it in some supermarkets, and most home improvement stores. –  Assorted Trailmix Sep 1 at 20:30
    
Packing in dessicant might help, and might even entirely solve problems, but I don't believe it meets moisture sensitivity standards: en.wikipedia.org/wiki/Moisture_sensitivity_level. –  Scott Seidman Sep 1 at 20:44

My understanding is that IC's are moisture controlled because excessive moisture can do bad things during reflow, causing the chip to explode or just move prior to soldering being finished. If you are not planning on reflowing, you have nothing to worry about.

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"Explode" is a strong word for what actually happens... –  DerManu Sep 1 at 18:29
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@DerManu not all explosions take out buildings, sometimes they just split packages. The phrase is perfectly fine. –  placeholder Sep 1 at 20:29
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@placeholder: I know it's a valid technical description, but people might get unnecessarily worried about consequences of moist ICs. That's why I said it's a strong word, not a wrong word. –  DerManu Sep 1 at 20:43
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Having an assembly run come up with a high error rate is a pretty big worry -- MUCH bigger than an IC explosion in a reflow oven ;) –  Scott Seidman Sep 1 at 20:45
    
Be careful about being overly pedantic, or else you invite people to point out that moist is also a wrong choice of words as all you need is moisture absorbtion and not necessarily adsorbtion. ;) –  placeholder Sep 1 at 20:56

The moisture in the air does not damage the chip, static does! If you leave the remaining chips in the bag and reseal it, all you have to do prior to soldering them, is dry them with a hair drier. Five seconds with the hot hair drier should be more than enough (no need to "bake" them). I recommend using a heat resistant glove, to hold the chip.

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