New boards arrived!
Its a perfect chance to talk a bit about DFM using this small batch that showed up today as an example:
When Axiom -in its current form factor- started being prototyped I tried to keep all components in the same side. It is much easier for me to reflow soldering when they are only on one side of the board. Otherwise in the second reflow I always fear that some components in the underside will fall, its not a reasonable fear as I've done dual sided assembly a few times and it was never a problem... but the fear is there.
However the actual, tangible reason I avoid going dual side is because its a ton of extra work. You spend hours opening digikey bags, placing tiny components, dealing with your shaky hands, the solder paste mess, the hunger because once you start you can't stop, the backache, aligning the components to tolerances the human body isn't meant to resolve, and then you can sit and watch that satisfying reflow. When you have a 2 sided board, that soul crushing experience starts over again, and you didn't even get to see an LED turn on.
You need to go through that experience to really appreciate the *cost* of going 2 sided. You become aware of the steps that manufacturer needs to go through when you make that choice, exposing the board to 250°C twice, and it is really needed? It turns out that Axiom has enough real state available and a single sided assembly will do just fine, plus the underside of the board will be exposed to higher EMI and if the belly stays flat you can do some shielding and heatsinking. YMMV of course.
So single side it is, and in the picture above, you'll find that we only asked for SMT assembly, leaving out the through hole parts. There's 2 reasons behind this:
- Through Hole assembly is expensive. Or relatively expensive, we have an expensive BOM so it doesn't affect us that much, but is a place to save $.
- The TH parts pay import taxes in the country its assembled, plus import taxes in our country.
If we assemble TH locally we avoid both the assembler and a bit of foreign taxes, it makes sense for a small run.
Since this is our second small batch made for beta testers, our assembler has all the tooling in place, they won't charge you again for the stencil and pcb tooling, plus now they know the BOM, component location, they have your pictures, that amazing interactive assembly and several mails with instructions to assemble that first batch, so now its easy to produce more boards! You just say how many and ask how much, it's that easy. You've done all the heavy lifting optimizing your BOM cost, making sure you don't rely on hard to find parts, selecting different suppliers according to the price, etc.
And a quick tip: treat your manufacturer like a small child, be clear, go to town with as many drawings as you can and also listen to their suggestions. They are no childs, they are extremely competent and there are teams with technicians and engineers at your service, but you'll save time and money by just being graphical and verbose.
Its a long way to get to receive a well done batch arriving to your door, but that's barely the beginning! When they arrive you'll have to unpack the boards, and test them one by one. Axiom comes pre-programmed with a DFU bootloader in ROM that makes it possible to flash the MCU by just plugging the board over USB. In seconds we can check that:
- MCU is OK
- FPGA is OK
- Resolver chip is OK
- MCU supply rail is within tolerances
- Analog signals are reasonable (many of them should be at 1.65V for example)
- USB is OK
- Current sensor failure detection is OK
- Program vesc bootlader and the application so its ready to use
All that by just plugging it over USB!
Sooner than later we will need a jig to test that the analog signal path is within tolerances, test pwm outputs and comm interfaces, but for now, and since its a high end product we test each board directly with a powerstage. If the board can regulate 3A with those 425A sensors, your signal path is quite okay!
DFM does not end at the board level, Axiom design needs to take into consideration the mechanical assembly of the whole thing, including power stage, thermals, wiring, piping, bolt-screwing, torque-ing, the enclosure, and a long etc. Its not just a matter of avoiding collisions. But lets leave the mechanical side for another day, shall we?