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Many chips have Moisture Sensirivity Level rating (MSL)

I am trying to understand how exactly moisture can get into a silicon packaged chip. Every document I read just states it can, but doesn't say how. E.g. this one from TI:

Once the dry pack bag is opened, moisture from the ambient air gets into the device. If the floor life is exceeded and too much moisture gets into the device during reflow, the moisture can expand and damage the device.

My guesses are:

  1. there are small holes between pins and silicon package where the water molecules can seep in
  2. pins oxidize too much and make room for water to build up inside the outer edges of the chip
  3. silicon is porous and soaks up water

Does anyone have an insight into any of these?

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    \$\begingroup\$ Moisture gets into the packaging which is where the problem shows up. \$\endgroup\$ Oct 19, 2020 at 8:53
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    \$\begingroup\$ Silicon isn't porous but the epoxy packaging is. High-rel electronics used to insist on glass or ceramic packaging. \$\endgroup\$
    – user16324
    Oct 19, 2020 at 14:03

2 Answers 2

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Silicon is not the problem, the problem is with the plastic packages. Plastic is slightly porous. If the absorbed water turns to steam during reflow soldering, bad things happen.

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  • \$\begingroup\$ Popcorn isn't always a bad thing ... though in PCB assembly it usually is! \$\endgroup\$
    – user16324
    Oct 20, 2020 at 13:25
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The plastic package moulded onto the chip and leads is permeable to water.

This water is not usually a problem to the IC itself. However, when the package is heated to solder it onto the board, it can physically explode, as the water boils.

When an IC is really required to exclude the environment totally, it is hermetically packaged in metal, with glass frit insulation. It's rather more expensive than plastic packaging.

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