I’ve been experimenting with making PCBs. I’ve always wanted to, but it’s something that I’ve never actually had the chance to do. I went through many a lot of iterations trying to get the toner transfer method to work.
In retrospect my big mistake was grabbing Kodak’s Ultra High Quality inkjet glossy photo paper. Photo paper can work well, but not this 10.7mm thick stuff with a plastic back. I didn’t actually have an iron, so I tried everything I could think of and only wound up going from almost no toner transfer to getting around 80% of the toner transferred with lots of broken traces. I started with a ‘hobby iron,’ which didn’t have enough contact area or enough heat. I was going to use a hobby heat gun, but I couldn’t locate it. I went through probably around 20 sheets of the stuff. I tried heating the paper from the back lightly with a propane torch, which actually gave me some results but only near the center. I heated the copper layer with the torch first, and actually destroyed a couple boards that way by applying too much heat from the torch. I did wind up getting vastly improved transfer this way as I rolled a piece of ~1.5″ aluminum pipe over it and VERY gently heated the paper with a torch.. but I was still left with an unusable amount of transfer.
So I was going to buy baking sheets today (aka cooking parchment, apparently) as I read that had excellent qualities for laserjet toner transfer. But before I ran to Walmart I was in Walgreens which is much closer to me. I picked up their glossy ink jet photo paper thinking I couldn’t do much worse than what I had already done. I learned that there were many thicknesses of glossy photo paper (The More You Know..) and this material actually had a paper back so it should fall apart in water and require less work for removal. I bought a cheap $18 iron while I was there. I’m not sure if it was the iron or the paper (but I’m betting it was mostly the paper with the iron not exactly hurting matters) but I got usable transfer on my first try. There was one little area where a large power trace was broken. I had the foresight in Walgreens to pick up fine-tipped Sharpies, so I scribbled in the broken trace and dumped the board in some Ferric Chloride.
This was a pretty good choice for a first etched PCB, I went through a bunch of layout iterations for each module that would be duplicated over the board, and I finally got something that I liked — the trace layout looked better to me than some of the other options and it was efficient in terms in space usage from left to right. I couldn’t fit a 5th row on the PCB without significantly reducing how many columns I had. I intended on using PCB terminals from Radio Shack (hence the 0.3″ spacing next to the resistors), but they were too bulky so I just soldered some Male-Male jumper wire to hook into the Arduino and the 8-way relay boards I ordered before I decided to start making PCBs. I’ll probably wind up integrating the relay board onto the JFET tester with an ADC and a header to connect directly to the Arduino. At least making PCBs is a cheap hobby, and I’ve always wanted to make SO many PCBs — I’ve always resorted to wiring on standard perfboard with thin wire and a decent amount of solder to work as ‘traces’ for audio amplifiers. PCBs would make things SO much easier — if only I had a small CNC mill to drill all the holes for me!
As you can see, each DUT Cell has gotten smaller with time. The last one got slightly larger to accommodate the RS PCB Terminals, but they’d need even more space to properly fit. They hit the resistor on the cell it’s in and on the one before it.. then there’s little clearance even between the terminals so it’d be hard to insert wires.
I actually picked up an AVR ATMega2560 based board cheap at MicroCenter. I’ve been using AVRs forever, the Arduino IDE just makes things a little easier in some respects, even if it is a bit more limiting than I’d like. I’m used to using Atmel’s stuff, I’ve been using AVRs since the first ATMegas came out and there was a long lead time on the 16MHz variants, so at least I’m fairly familiar with them and their inner workings. I couldn’t resist buying the ATMega2560 board as it has *16* ADC inputs without using an external multiplexer or ADC. I have a few ADCs laying around, but the only one I could find is a TLC1543IN which wouldn’t work with the Arduino Due without a level shifter as this particular model of the 1543 (IN) is a 5v part, so it would blow up the Due’s 3.3v IO without bi-directional level shifters which I don’t have.. nor do I have signal MOSFETs, and IRF510s in a TO-220 would look ridiculous even in the interim. 🙂
I’ve only physically populated a single row, and I used 1% metal film resistors (again from Radio Shack since they’re right there and I don’t have to wait for things even if I pay through the nose and out the other side of it). Clearly I’m not an electronics engineer, and my circuit design is probably poor overall for some reason, but the JFETs will (hopefully) not oscillate and we’re only testing two parameters so it should suffice. I measured the resistors to make sure I was reading the bands correctly (it’s been a while and I don’t have a magnifying glass — and damn they’re harder to read than I remember), and my little Fluke thought they were all within +/- 1%. I didn’t want reading variations with 5% resistors.
The process is the same as before. The relays are flipped to measure IDSS or Vpp, and we use a single pin to flip the relays. We’ve just scaled it up some by taking it off of a breadboard. You can see the PCB I printed this time only does 36 FETs at a time. I was trying to minimize the area outside of the center of the board and these are just prototypes so there was no reason to not do so.
Speaking of, I have design decisions to make for my prototypes — either use ATTiny828s that have 4 ADCs and mux them out to 28 for a mere $2.24 each (in TQFP from DigiKey) and put them on board (say with 112 JFETs on a larger board) with serial uplinks or to a control processor either on or off the board. With only 512B of RAM they’d need a separate processor to offload readings as quickly as possible. At least it’s cheaper than a halfway decent ADC and leaves me room to update their firmware and possibly do things a bit differently.
Here’s an example of *90* JFETs in the same board space (4.5″x6.375″), up from 36. They’re packed in tight, but there’s enough clearance everywhere. This one uses 2 pin male headers for each JFET.. routing them to a central place for a ribbon cable would be nice, but would require a two sided layout which I’m not quite ready for yet..
So I’m going to wire the 12 JFETs up to a couple 8-relay boards and see what happens. Hopefully I didn’t screw up the circuit design from my breadboard layout and there’s nothing seriously wrong with the design..
Part 3 incoming, it’s either going to detail a massive failure and/or another upgrade.. hooray?