Chip Packing Experiment On Prototype VFD Driver PCB

While we were building a breadboard prototype for our VFD driver, we weren’t terribly concerned with chip layout as it was largely constrained by the practicalities of a bread board anyway. Once we start thinking about transferring it to a PCB, however, we had more flexibility to be creative. The prototype printed circuit board is a grid of through-holes that we can use in any way we like. How shall we abuse this power?

While looking at how wires were run on our bread board, I noticed that there were a few pins that were perfectly aligned from one chip to the next. Placing the decoder output on PIC pins RB4, RB5, and RB6 lines them up perfectly with the input pins of 74138 decoder. As for the decoder output, six out of eight pins were directly lined up with a ULN2003.

Chip pin alignment

This allows some fairly straightforward wiring solder as these wires will not cross over each other and won’t tangle up and make a big mess. By itself that is valuable, but we were tempted to go one step further: how about we eliminate the wires entirely and jam those chips together? If they share the same PCB through-hole, that would eliminate wire soldering entirely.

It sounded good in theory, but in practice the chips are just a little too large for us to fit them three-abreast. I could push my decoder up against my PIC, but couldn’t push the Darlington array alongside the decoder as well. This is probably for the best – there’s value in having the ULN2003 lined up with its siblings.

Chips tightly packed on PCB

We may learn in the near future why this is a bad idea, but it’s all part of the fun: trying things and seeing how they work. (Or not, as the case may be.)

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