I've been chatting a bit with Jim Hisert of Indium Corporation about various issues, challenges and changes we in the assembly industry face. Sometimes it's us service providers, sometimes it's OEM manufacturers and sometimes it's material suppliers like Indium that face the challenges.
POP (Package on Package) is becoming a more common practice these days. In general, when we see them here at Screaming Circuits, the sandwich has already been assembled so we can just treat it as a single part. Myself, I'm not that familiar with the techniques for building it up in-process, but my understanding is that the bottom part is placed on the pasted PCB like any other BGA. Then the top part is dipped in dipping paste and placed on top of the bottom part. That way, you don't have to try and screen paste on to the bottom BGA.
Here's Jim's comment on that:
"When I started working with dipping paste quite some time ago, I hated it. Why couldn’t everyone just use flux anyway? Flux dipping is easy. Easy to apply, easy to clean up - easy to make work. Why mess with a paste that can cause bridging, insufficient transfer, and has a shorter shelf life than flux alone? A lot has changed since then.
Warpage is a problem we can’t escape - we have to deal with it. Sometimes added solder volume is a necessity for PoP components and BGA rework. PoP components often need solder volume to compensate for the thinned-down intermediate packages that tend to warp. BGA rework needs extra solder to mimic the volume that is present when the initial BGA was attach in the SMT printing process. During rework, we can’t fit that 29” x 29” stencil over the board anymore, but it’s not too difficult to place a paste-loaded component down in between the other components on the board.
The good news is that solder suppliers are designing solder paste more like flux. Some of the new pastes actually look and feel like gray flux. With low viscosity and high tack, modern PoP pastes can transfer up to 300% - 400% more solder than modified SMT dipping pastes.
Paste still isn’t as easy to clean up as flux alone, but 2 out of 3’s not bad, right?