Many factors play a role in the overall outcome of PCB assembly, especially when very small components are involved. Broadly speaking, tombstoning is a problem with very small 2-pin components (0201 caps and such) and we've filtered out the following challenges:
- proper paste application
- component placement
- designing for "good" thermal properties of the PCB with regard to tombstoning
- moon phase
It seems that the effect of moon phase is least understood and researched, so I'm now designing a small PCB that is cheap, not useless (if the assembly process was successful), and - most importantly - contains some 0201 components.
Sorry that this took a year to figure out, but I've finally come up with a design that might not be totally useless, but easy to build and rather inexpensive: An LED tester and polarity finder.
When I had to handle 0201 LEDs, I quickly learned that they don't really come with useful polarity markings. Yes, there's something on the bottom, but when I try to flip them around again even with my best tweezers, chances are that they end up in any orientation. So I need to find polarity while the LED is already facing up.
It's also a bit hard to decide on a current limiting resistor before knowing how an LED will look when a given current is flowing.
So here's a polarity tester that allows to find the right orientation AND the right current limiting resistor:
multiple pads to place the LED on, with the large ground pad to the left. The right row of pads is supplied through a set of resistors
resistor values are chosen such that the current even from 2 or 3 pads won't destroy an LED rated for 20 mA
pads are close enough to test packages as small as 0201, but large enough to test up to 1206
supply through pin header (3...5 V), USB (5 V obviously), or a 2032 cell on the bottom through a diode to avoid reverse current when another source (pins or USB) is plugged in
A break-away tab to align the stencil. I'm not going to align this by hand, and a stencil is required to get reproducible results. If I didn't intend to make a lot of these, I'd just pre-tin all pads with a soldering iron.
just place an LED somewhere near the highest current limiting resistor (4k3),
rotate until it lights up,
optionally: move to lower value resistors until it's bright enough.
The resistors are in 0201 packages to fulfill the actual purpose of this design: see if the moon phase has an effect on tombstoning. 4 land patterns are stock KiCad, 4 are a custom (hopefully better) land pattern.
So in order to assemble one thing per week to compare results based on moon phase, I need to figure out what that thing would be. It should be cheap, contain a couple of 0201s - but not too much more to keep it cheap because I expect a lot of fail here - and it shouldn't be useless.
Feel free to make suggestions in the comments!
Once the design is decided on, I'll get a couple of PCBs made, order (or scrounge) components and build my dataset.