Sunday, 28 June 2015

So what's with all these double-sided boards anyway?

We're not normally a fan of double-sided PCBs.
The problem with them is they're a pain to make - not if you're a cool PCB fab house, but if you're a bunch of nerds with nothing more than a laser printer and some ferric chloride, getting a decent double-sided PCB can be a real pain. Normally we stick to single sided designs, use manual routing (rather than some, let's be honest, pretty crappy auto-router) and every now and again stick a surface mount 0R resistor in, to act as a jumper over single trace tracks.

That works fine for most projects.
Even when we get PCBs manufactured professionally, we try to stick to single sided designs. It means we get a chance to prototype the board before committing the design and getting a batch made (Steve knows only too well how often we end up with a ground plane missing, or a single connection - usually to somewhere important like the regulated 5v supply - completely forgotten about, because on the prototype we just cobbled in a bit of wire).

Single-sided boards also mean that if the layouts are published here, anyone else can have a go at creating the same thing - not everyone is willing to waste hours and hours and quite a bit of relatively expensive material, trying to get a double-sided design to work (and who can blame them!)

But this time we're adamant on a double-sided design. So why is that?

Well, we're back to our electronic board game idea. We've tried any number of different manufacturing methods, including (but not exclusively limited to)

  • each board section being made up from smaller PCB sections (makes the squares on the board too small or close together, and altogether very expensive to make a decent sized playing surface)
  • one massive PCB per playing surface (very, very expensive!)
  • multiple PCBs connected on the underside of a large playing area (no less expensive than a massive PCB, with the added hassle of having to connect everything together)
  • using large membrane switches instead of PCBs for the playing surface (too expensive)
  • using multiple small membrane switches (where the connectors go to the underside of the board, the playing surface is distorted) 
  • eliminating PCBs and wiring hall sensors to a number of microcontrollers on the underside (not nearly as expensive as having PCBs or membranes manufactured, but very time-consuming to construct)

From these main approaches, the pcb-free idea (running wires up and down the board and attaching hall sensors to them) is the cheapest. It's also the most complicated and time-consuming to build.

So we're looking for a short of half-way house. We're trying to build the smallest possible PCB to hold the maximum number of hall sensors and a shift-register (so that the PCBs can be daisy-chained to a single, central microcontroller). This should reduce the build cost (since we don't have large, vast expanses of PCB "real estate" standing empty) as well as the shipping costs (for the same reason). But also simplify the construction.

Of course, each piece still needs to be wired to the next in the chain, so it's not completely work-free. But it's a compromise between the massive amount of work needed working with just wires and the massive cost of working with just one big PCB.

Perhaps the answer lies somewhere in the middle.........