Hi everyone, I have kind of an interesting proposition and am looking for some input.
I have an existing board with a QFP-32 pad for a microcontroller. I would like to design a small PCB with a fairly simple circuit of discrete components on the top, with corresponding QFP-32 pads on the bottom. In essence, I want to create a pin-compatible PCB replacement for the microcontroller.
Can you think of any concerns or gotchas?
How do you propose to mount the PCB to the board below?
Solder terminations. We’re talking a micro PCB with the same footprint as a QFP-32 microcontroller. I have since done some more reading on the subject, and it appears to be not entirely rare, if not exactly common. The typical method is to use through-plated half-holes (basically through-plated vias with the board outline passing through the middle of them) to create castellated terminations. This is undesirable for space applications (J-STD frowns upon castellated terminations), so I’m still looking at just doing bottom termination only using pads on the underside of the uPCB.
Sounds pretty interesting.
If you’re hand-soldering it, you can save some space by doing a via-in-pad configuration. That’s when you put a small via (I like 0.4mm) on top of a pad so you can connect it directly to the pads on the opposite end of the board. This saves space (no traces needed!) and has less impedance/inductance overall than running a trace from a pad to a via, then running a trace from the via to the bottom pad.
This also gives you a way to solder a PCB to another PCB pad. Apply flux to the main PCB and inside the “micro-PCB” via, then apply solder through the via. Capillary action will pull the solder through to solder the two PCBs together. Problem is you won’t be able to get them off without a heat gun, or you won’t be able to get it off at all if the pads you’re soldering aren’t near the edges of the “micro-PCB”!
If you’re using solder paste or wave-soldering this thing, forget my post. Via-in-pad will cause you problems with those two methods.
Solder paste (applied with a mask), preheat, and hot hair (no wave soldering).
Okay, then you’d be fine just going the route you described (I’m assuming nothing too special with the design, like high-speed signals and such.
The problem with via-in-pad configurations and solder paste (+ wave soldering) is that the via can “wick” away the solder from the pad. But there’s no way to really ascertain it will wick enough to solder the two boards together.
Best of luck!
@Asphodel: Hi, it is very interesting idea to develop the mini PCB with QFB-32 pads for a microcontroller and then place it over the another PCB as it reduces the complexity of the main PCB but can you please tell me how you will connect these two PCB’s, will you use the connectors to fix it flexibly so that you can remove the small PCB whenever you want or will you fix it permanently with the main PCB?