michael chiklis wrote:
What is the ball diameter of these mcu?
Do you get this balls on bottle or do you use solder paste?
Usually solder paste is prefered to use when is needed to make very tiny balls.
This PCBs are very fragile because are very thin, heat might kill inside tracks while unsoldering/soldering mcu.
Do you use prehating oven under this boards and low temperature hot air on top?
The stencil kit comes with 0.03mm 213C solderballs which I ended up using. The paste method we did try but something didn't work out so not sure why that was which is why I reballed it with the balls that came with the kit. They are tiny, even under microscope and they go everywhere. You could technically use 183C solderpaste which would make it safer.
Yes, heat was a real worry. I did practice a lot on other boards first before attempting it on customer board. Just taking off, cleaning with wick, reballing and putting back on and checking if board was still good and working on donors. Did this about 4 times.
We removed using flux starting at 250C 50Air to preheat, rising to about 310-350C 50Air till the chip can be lifted. The chip is not underfilled with glue which makes it easier. The surrounding chips should be masked off heat resistant tape foil or Kapton.
Just a warning, the temp depends on your ambient temp and your station, not sure what the MCU thermal limit was but glad we didn't cook it. I would definitely practice on a non customer board.
I don't use a preheater, just warmed up the area first with air from a distance. I have been practicing with working with safe removing, chip quick or low melt solder to clean pads on board and MCU chip before reballing back. If you use 183C solderpaste or balls, it will make it a lot safer.