Popcorning

07 February 2005

Popcorning is now a well known phenomena having existed during the early introduction of SMT. The name was probably coined due to the noise that can be heard when a component cracks during rework. It is surprising how the "pop" can easily be heard when you are concentrating intently on your rework process.

Cracking occurs during heating and this can become a greater risk as we start to implement lead-free, as peak processing temperatures increase from 220°C to 245°C. Moisture accumulates where there are any voids or where the bonding between the plastic and the lead frame is weak, allowing a moisture trap to exist. Moisture can also be present in the die mounting compound. As the moisture turns to steam it expands and the failure can be found at the point of least resistance. Cracking can occur on the base of a QFP or large SOIC device below the die. It can also be seen on the top of the package or at the lead plastic interface. Normally, though not desirable, failure does not occur when the cracks are on the base of the device and you don't see them. Cracks on the top of the device are visible; they can be detected using a fingernail and lead to intermittent bond wires. As most parts have the die mounted on the top of the lead frame the bond wires are damaged when the plastic expands. Popcorning can also occur on small parts like electrolytic capacitors when the electrolyte overheats as shown in the example image.

In the case of Plastic Ball Grid Array (PBGA) devices, cracking can normally be seen between the top of the moulding compound and the fibreglass substrate. When considering all surface mount parts plastic PBGAs are probably the most prone to damage of this type. In the last five years there has been a move in the industry to classify all vulnerable parts so that design and process engineers can take precautions during handling, storage and soldering. The controls and preventative measures necessary to avoid cracking are documented in IPC/JEDEC J-STD 033A and also in IPC 020B . These documents also help to outline the drying requirements and timescales


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