We at TBRM are all about eliminating rework popcorn hence this helpful article. Popcorn is a great thing, its great at the movies, its great at home, its even great on the road. Popcorn on your BGA chip well thats another story. During a rework procedure you may get small little bumps on the chip and sometimes but rarely on the board as well. These little bumps are referred to as popcorning and the reason is because they pop at high temperatures same as in the microwave. The main reason this happens is due to moisture. Their is trapped moisture inside the board and it is expelled at high temperatures. You may ask why didnt it come out sooner and the response would be, it was. As the board was being heated it slowly started to expel SAFELY. However when the board started to reach higher temperatures, the moisture needed to escape immediately and so it was forced out and the result is popcorn.
By prebaking the circuit board you can remove the moisture at a safe temperature and thus reduce the potential rework popcorn issue. The prebake times will vary with different types of boards as they all have a different MSL or moisture sensitivity level. If the board is not to be reworked directly after prebake it is recommended to store the board in moisture cabinet. These are designed to keep the moisture at a safe level.
Prebake is not the only reason for BGA rework popcorn, but it is the most common, the other scenario involves improper profiling typically with the top heat not working in conjunction with the bottom heat. If their is too rapid of a top heat while the bottom heat has not provided the appropriate amount of preheat chances of rework popcorn will be increased heavily. The typical rule is 70/ 30 with appropriate ramp rates for each rework zone.
In order to effectively reduce rework popcorning proper procedures should be set into place. Establishing a proper profile for each pcb and its appropriate chipset is necessary.
The use of moisture meters or analyzers can help evaluate the moisture levels in your environment to aid in developing a proper rework profile.
Article collaboration with XModdz