What methods might be feasible for attaching/stacking one PCB immediately on top of another PCB, with the following conditions:
- Zero spacing/gap between the two PCBs
- Electrical contacts are needed, not just physical attachment
- Assume that the top PCB is about a third the size of the bottom PCB
I'm at the early design stage of a project and am trying to survey the options first, so I'm open to recommendations of standard methods as well as any creative ideas.
Note: I'm already familiar with edge castellations (AKA "half vias"), so other suggestions would be of interest.
For instance, is it possible to design it such that the top PCB has pad-contacts only at the bottom (QFN/QFP style) which are somehow solderable onto pads on the bottom PCB?
EDIT: To answer @Andrew's question:
My purpose of stacking the two boards like this is that the Top PCB will be variable across variants of my device (in fact, variable not only in what the Top PCB contains but also size and number of contacts it has), hence the goal of having one constant Base PCB with pads onto which I can attach a variable Top PCB.