Miscellaneous notes

PCB design advice

• The imperial unit standard on PCBs and metric standard on ICs is a pain to bridge and requires constant attention. A "mil" is 25.4 um.
• It's easy to design an IC landing/footprint that's impossible to wirebond. Bondwires can launch from, at most, a 45º angle from normal.
• It's easy to design a PCB that can't be assembled after wirebonding, or wirebonded after assembly. Tall components need to be well clear of the IC landing. The definitions of "tall" and "well clear" is something that needs to be specified by the wirebonding house and will probably be at least several cm.
• It's easy to design a PCB that uses components that will be out of stock for over a year or have been discontinued. Pay close attention to what Digikey, Mouser, Arrow, etc. have in plentiful (100s or 1000s) supply.
• Bondwires can be nominally modeled as 2 nH/mm and, as a result, provide a healthy impedance at highish frequencies. We got 2 stages of buffer to turn into an oscillator via power supply feedback once. The behavior showed up clearly in simulation once a few nH got added to the supply. (I think this was SCM1D.)
• Stick to 2 layer boards with components on one side if at all possible. A 3 mil trace and space is typically in the "budget" class these days, but many PCB houses start charging more for anything narrower than 6 mil.
• Avoid through-hole components besides the usual 0.1" headers. Solder them yourself to avoid damage to the IC/bondwire/epoxy matrix.
• Program the PCB design package with design rules from your PCB fabricator long before you start layout.
• In the layout side, the autorouter is never good but it can do rote work if you give it very simple routes.
• The biggest headaches we've had in PCB design/troubleshooting have been from not understanding the voltage generation tree and the fastest way to solving them have been to draw the tree out. By "tree" I mean first identify the external power supply (5V USB, 3.7 V batt, etc.), identify what voltage need to be generated from it and their current requirements, identify where those voltages need to end up, etc.
• For every power supply into the chip, include a jumper in series so you can insert a current meter.
• The second biggest headaches have been from connectors: either fiddly and fragile, or badly labeled on the silkscreen layer.
• Every externally-connected power or signal connection needs and adjacent ground pin.
• Put lots of extra ground pins strewn around for stuff like oscilloscope grounds to clip to.
• Include screw holes in 3-4 corners so you can anchor the board to something heavy at test time. Scope leads, SMA cables, etc. will try to throw the board around.