Should I insulate between 2 PCB's when SMT joining them together?

Question

I have designed a circuit board (5v, .02 amp max draw, UART and I2C comms) for an electronics project with many surface mounted components, which I have built and SMT from a house in China. When the board with mounted components comes back to me, I then join a mass-produced SMD bluetooth module (BLE) to the board using hand-soldering technique. I have shown the back of the board below, the top has all of the factory-mounted components.

I have traces running underneath the BLE mount area on the back, both power and signal, as well as a significant portion of the ground plane.

It is at this point which my electrical "ghost" seems to always appear. The board seems to always short out at some point after this, where I can either smell ozone (sometimes even see the smoke), or serial output kookiness appears. Being that I have only done this 10-15 times because I only have so many boards to test, I know that randomness may be at play, as far as timing of the problem.

I am experienced at SMD hand-soldering, have all the right equipment, so I know that I'm not doing anything wrong on that front. For the past 2 days I have experimented with putting a piece of electrical tape insulation between the BLE module and the main board before I join them together, and it SEEMS like the problem has disappeared (again wary of randomness). I wrote some code to track serial output and have not had a SINGLE piece of noise in throughput in 2 days of runtime. The power supply has stayed completely stable too, as tracked by my code.

Before I reconfigure my board or add an insulation protocol to the build process, I would like to know if this community who has much more experience on this front thinks that the two boards were shorting each other (perhaps that is a common problem?), or have I just been lucky that the ghost has not appeared for this admittedly small sample size? I come from a world of large-scale engineering projects and this is my first circuit board design (self-taught) so I know I have a lot to learn about design and implementation.

If the community thinks that the problem is actually occurring between the two boards when joined together, is the best solution to A) remove traces from underneath the BLE module or B) add insulation (if so what is the best insulation solution for a professional product)? I searched high and low on Google for answers to this, but to no avail. Not much I can find about design considerations when SMT'ing boards together.

Answer 1

In general, solder mask should not be relied upon as insulation. So it's a bad construction even if it's not the source of your problem. A thin (maybe 0.1mm including adhesive) layer of Kapton (polyimide) tape would eliminate any possibility of shorting and would not affect it mechanically much. Your assembly house can also get die-cut Kapton stickers in various sizes. I would request this construction in any case.

Eg. (source as per watermark)

I doubt you smelled ozone unless you have a kV source hidden somewhere, maybe burning insulation.

It's also quite possible (maybe likely) that you are doing something to the module to cause it to latch up and the resulting high currents are causing your exciting side effects. Typical causes of that include forcing inputs high when power is not applied to the circuitry and test probes slipping.

If you add some moderate amount of resistance such as 100 ohms on signal lines between the main board and module you can prevent much of that potential issue. Or just make sure that the main board can never be hot plugged or powered without powering the module.