It's frustrating but for the second time, we've ended up with some etched circuit boards that are too small. The first time was back in November last year, when making our LED charlie-plexed matrix and we couldn't work out back then what had gone wrong.
Here's a quick rundown of how we etch our boards:
Firstly, draw the schematic in ExpressPCB then transfer the design onto their PCB layout software.
Then print the PCB layout but instead of sending it to a printer, direct it to CutePDF virtual printer. This creates a full-scale PDF file which can be uploaded, emailed and generally shared around the place.
That's how we create all our circuit boards, generating PDFs then emailing them to our machine that's connected to a big Xerox Phaser for printing onto press-n-peel.
Tonight we wanted to knock up a quick SOIC-to-DIP breakout board. Then as an after-thought, we added some LEDs to one side of the board. As always, create PDF then email to the printer machine.
But this time, the circuit board came out about 90%-95% of full size. Notice how the pins "fan out" as they aren't properly lined up with the PCB. The holes on the board are closer together than the 0.1" pitch of the pin headers. Yet in the PDF file, the holes are exactly 0.1" apart.
Now the only difference we can see between our "normal" method of creating PCBs and what we did tonight is that instead of downloading the file to our library, opening with Adobe PDF reader and printing from there, we just clicked the link in the web-mail and the PDF opened in Google Chrome.
Which leads us to the conclusion that the PDF viewer in Google Chrome (or at least the printer support in it) is a bit flaky. In fact, we've never been impressed by Google Chrome's print preview (it used to get confused when the printer default layout was landscape) but in recent months it's got better. Or, perhaps after tonight, we can only conclude that it hasn't!
In short, always download your PDF layouts and print from Adobe's Acrobat Reader to make sure your circuit boards are printed at 100% scale.