How we fabricated prototype PCBs back in college
While there are plenty of online PCB fabrication facilities, sometimes you just have to get your hands dirty and DIY.
Back in college, going DIY meant an afternoon in the PCB lab, followed by a morning at the soldering bench.
Having a brand new PCB board was a luxury, so we would often have to salvage usable leftover corners from the scrap bin.
But why would you go DIY when you can outsource from a whole PCB industry?
Fabricating your own prototypes does allow for a quick design turnaround. And is highly educational for rookie EE students. Besides, not every class could afford the fabrication on top of the components for all the group projects. Students are free… Make them do it!
This resulted in a lot of ingenuity in cheap PCB manufacturing.
Our lab was equipped with a UV exposure machine, a CNC router, a laminator and a bubbly heated etching bath that might have its acid changed twice a year.
Most of this equipment was no match for our rookie hands, however.
We would curiously unwrap the photosensitive PCBs outside the dark room resulting in perfect copper plates for the CNC. However, the CNC bits would easily get stuck and break, and there were no replacements.
In the end, we adopted the toner transfer method.
Here, we use a laser printer to print our mirrored design onto cheap old magazine paper. The copper plate board is placed face down on top the printout, duct taped to the paper, and swept through a heated laminator a few times for the toner to be released from the paper and stuck to the copper surface. The paper is removed and off to the acid spa.
The toner transfer method is great for single layer PCB, but is actually doable for double layers.
During my final course project, a while ago, I fabricated several single and double layer prototypes before finally finishing the design and outsourcing the PCB.
Last week, a colleague needed help etching a double sided prototype. So, after a few years, I went back to the PCB lab and took a few photos to show for it.
First we printed the mirrored top and non-mirrored lower masks onto separate magazine sheets. Here we use old football sticker albums that work great!
We trim one of the sheets to slightly larger than the PCB plate we using. Here we have a 6×6 cm design and used an approximately 8×10 cm plate. We cut at ~1 cm margin around the PCB.
Bottom mask printed on a large magazine sheet and mirrored top mask cut out slightly bigger than the PCB plate.
On top of the light box, or against a window, we align both sheets to the holes of the design.
Aligning the top and bottom masks is the most important step in double layer prototyping using toner transfer.
We duct tape the aligned top and bottom sheets on three sides, making an envelope for the PCB to slide in. Preferably we would use paper tape, but here we used a regular transparent one.
The two sided copper plate is then slid into the envelope.
The top sheet is then stretched flat and the envelope is duct tape closed. This stretching allows alignment between both masks must be sustained during the toner transfer procedure.
Aligned top and bottom masks duct taped on three corners to allow double sided PCB plate to slide in from the side.
We now assume the alignment of the masks while we slide the sheets into the laminator.
The heat forces the toner to move from the magazine sheet onto the copper surface. It also shrinks the paper around the plate. The PCB is around 1.6 mm so it might seem that the masks have misaligned — but we assume they didn’t.
We repeat the lamination 4–6 times at around 150ºC
Duct taped PCB fixture goes through the heated laminator very slowly 4–6 times at 150ºC.
While the board is hot, after 5 sweeps through the laminator, we drown the board in water and let it soak for a few minutes. This will start to dissolve the paper and let the toner show on the copper surface. We rub the remaining paper with the smooth thumb skin, specially along the exposed tracks.
Toner transferred to double sided PCB after removing soaked paper. No way to verify that both sides are truly aligned.
We offer the board a nice warm ferric chloride bath. Our bath is already a few months old and highly saturated so the board takes around 25 minutes to etch.
Off to the acid bath it goes
We take the board off the bath when the desired design is etched. We take it out before removing the side copper to prevent over etching of the copper under the protective toner. Some unfortunate etching may still occur due to the grainy paper toner structure.
Finished board on light box. The old acid takes too long to remove large unprotected copper surfaces so we took out the board earlier to prevent over etching
The board is cut out to the correct size and the leftover toner removed by swabbing acetone on both sides.
Then it’s off to the drilling and via stitching!