Electronic – How does moisture get into silicon chips

humidity

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?

Best Answer

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.