We should pay highly attention if there are some moisture-sensitive devices (MSDs) in electronics assembly. Since improper management of moisture-sensitive devices will directly lead to poor soldering, resulting in various solder defects such as pseudo soldering, solder bridges and insufficient solder. So how we can do for moisture-sensitive devices before PCB assembly ?
The management of moisture-sensitive devices is mainly carried out from two aspects of storage and feeding.
Each moisture-sensitive device has its own MSL (Moisture Sensitivity Level), which clearly stipulates the maximum exposure time and the feeding schedule. If their exposure has exceeded the rating, it must be strictly baked before PCB assembly. Moisture-sensitive devices are generally stored in moisture resistant cabinets with specific functions. Normally, a temperature and humidity sensor need to be installed around the moisture resistant cabinet to alarm the warehouse workers that once the humidity exceeds the standard, they have to take an action right now. When taking moisture-sensitive devices out from the cabinets, you must check the status of humidity control card, and record the time of taking it out and the return time of the remaining materials.
The definition of MSL (Moisture Sensitivity Level):
MSL 1 – Unlimited floor life (less than or equal to temperature 30°C / 85% RH)
MSL 2 – One year (less than or equal to 30°C / 60% RH)
MSL 2A – 4 weeks (less than or equal to 30°C / 60% RH)
MSL3 – 168 Hours (less than or equal to 30°C / 60% RH)
MSL 4 – 72 hours (less than or equal to 30°C / 60 % RH)
MSL 5 – 48 hours (less than or equal to 30°C / 60% RH)
MSL 5A – 24 hours (less than or equal to 30°C / 60% RH)
MSL 6 – Mandatory bake before use (less than or equal to 30°C / 60% RH)
The process of MSL testing as below:
1) SAT (scanning acoustic tomography) of the qualified IC to see if any delamination.
2) Bake the IC to completely remove moisture.
3) Humidification according to MSL level.
(4) Pass IR-Reflow 3times (imitate IC assembly, maintenance dismantling, maintenance and assembly).
(5) SAT test if there is delamination or if the IC can work well.
If all above testing passed, it means that the IC package meets the MSL level.
Generally speaking, PCBA processing will specify a strict form, requiring the operator to perform strict registration before each pick-and-place, inspection and feeding to ensure the effectiveness of the raw material. The management of moisture-sensitive device is an extremely critical operation requirement in the entire PCBA process, and must be strictly managed.
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