Back to the P170-DH replacement PCB

It's been months since I worked on the replacement PCB for the P170-DH calculator. Both personal and work life has conspired to keep me away, but also I've been frustrated trying to finish the routing. Sometimes I'd open the PCB layout and stare at it for a while, then close it again having made no changes.

Recently I've been reading through this blog, and the desire rose to finish up projects I've left dangling for too long. I tried to beat down that feeling, but I think I may have failed.

The upper 40% of the board is mostly routed. This is the area above the display, hosting the power supplies and printer drivers, and the area behind the slide switches, which is where I've put the display drivers. But the lower 60%, the area behind the keypad, has been driving me nuts.

Currently this is a four layer PCB. Of necessity, the top layer in the keypad area contains the contact pads for the conductive pill keypad. Some signal tracks can run through this area but no components can be placed on top. I've routed the tracks that connect the contacts that make up each of the five rows of the keypad together on this layer, which pretty much restricts it to horizontal tracks. There are a few are already routed there.

Since nothing can protrude from the top of the board in the keypad area, all the components in this area are surface-mounted on the bottom. Thus the bottom layer as many interconnections as possible, and they run in all directions. The inner layer closest to the bottom layer is a ground plane. That leaves the other inner layer for mostly vertical tracks.

I'm sure this can be routed with only four layers. Out of frustration with my lack of progress I gave some thought to switching to a six-layer board. It would double the cost of the board itself (from about $100 to about $200, though JLCPCB in China), but it would let me have a power plane along with the existing ground plane, and add yet another layer for routing. I'm not sure I'll need it though; having decided I could afford a six-layer board I seem to be making progress with the four layer design.