Due to a careless selection of components (and you could say also bad layout) I ended up having a PCB with surface mount pads on both sides and on the same x,y coordinates.
I have already used my airgun for the TQFNs on one side but I have to solder some more on the other side and I am worried that by applying heat with the airgun, the already soldered chips will drop off from the bottom as I am heating up the top side.
Has anybody dealt with this issue before and if so, how do I ensure that what is already soldered will stay there despite the extra heat.
I've thought about putting Kapton tape or simple tape to hold the bottom components in place while I heat up the top side but then this could trap the heat around the components and make them desolder easier.
What I am planning on doing is simply place the PCB on a table so that the bottom components are sandwiched between the PCB and the table and they will hopefully not move out of place even if the solder melts while the top side is being heated up.
Best Answer
There are a few ways to do this:
1) Direct the heat flow on the PCB
There are a few ways to direct the heat flow, you'll have to get creative especially if the parts are near each other. You could put a heatsink on the components that you don't want to move around, or cool it down with a cool air flow on the bottom side (which might make soldering on the top side harder. It might be advantageous to keep the flow of heat in mind if you ever rev your board.
Another clever way to direct heat is using aluminum foil around SMT parts to keep the hot air away from other components and the board. The picture below is an example (although they are using an iron, but this also works for hot air)
2) Use solder with different melting points
The way two sided PCB's are manufactured are with solder with two different melting points, a higher melting point for the bottom side of the board (which you assemble first) and a lower one for the top side. Not a great option for prototyping