August 15, 2012, anysilicon
MSL stands for Moisture Sensitivity Level. It represent the amount of time an IC can be exposed to ambient conditions and still be assembled on a PCB without being damaged.
When the antistatic bag is opened and the ICs are exposed to ambient conditions, the moisture in the air is trapped inside the device. This means that during the PCB assembly process (e.g. reflow) this moisture expands and can damage the device.
The standard specification is: IPC/JEDEC’s J-STD-20
Why is this YOUR problem?
Well, the good news is — it’s not your problem. It’s the PCB fabrication house problem, and they need to specify the required MSL level because they know how long the device will be outside the ESD bag before it’s assembled. This is the floor life of a device.
The popcorn effect is when the IC “pops” because the moisture inside the package expands in the reflow process. As a result of this expansion the substrate, the die, or the wirebonds could be damaged. The damage is often invisible and requires X-ray equipment to conduct a proper analysis.
Here are the allowed exposure times for MSL categories:
To avoid the popcorn effect, simply bake the devices and seal them in a hermetically sealed antistatic bag. Use a moisture indication card to ensure you devices are not turning into popcorn.
If the devices were exposed to moisture, re-bake them and assemble the devices within the allowed explosure time. The baking is driving all the moisture out of the device.
Often your test partner will be able to offer bake and dry pack your ICs just after testing.