Re: SMT chip replacement


Craig
 

On the topic of SMT replacement the subject of moisture may deserve mention.

All plastic parts absorb moisture over time and some parts are very sensitive to delamination (the plastic package expands and breaks the wire bonds between the silicon die and package leads).

Delamination is also called pop-corning because sometimes you can actually hear the IC "pop" from the expanding moisture inside when it is heated.

All plastic ICs are rated with a MSL (moisture sensitivity level) by the manufacturer and can be found on the data sheet. This is the maximum time the part can be exposed to room atmosphere before assembly onto the circuit board.

IC customers don't like to buy ICs rated above 3 because they cause logistical problems on the assembly floor. But there are a lot of MSL3 parts out there, especially larger ICs.
MSL 6 – Mandatory Bake before use MSL 5A – 24 hours MSL 5 – 48 hours MSL 4 – 72 hours MSL 3 – 168 hours MSL 2A – 4 weeks MSL 2 – 1 year MSL 1 – Unlimited MSL is important if you are removing an IC that you need to reuse or keep alive.



If you need to remove an IC from a board for reuse, it is good practice to bake the board over night at 75C to 100C to remove the moisture, unless you know for sure that the IC is rated MSL 1.


Best to everyone,

Phil

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