I’ve been manufacturing my own PCBs in my basement now for more than two years. It is amazing what you can achieve these days with relatively inexpensive tools.
For my PCB designs I use KICAD, which is an open source CAD software (IMHO equivalent to Eagle), which is aimed to generate gerber files, ready for professional PCB manufacture. I have never submitted a PCB design myself, but all the dangerousprototypes.com projects have been designed in KICAD and they are mass produced and available for international shipping (see? it works!).
If you (like me) use your CAD software to produce toner transfer designs in particular, you run into some problems that the CAD programmers obviously didn’t care about.
In this article I do not want to give yet another explanation of the toner transfer method, but give you an overview over the techniques (or can you already call them RL hacks?) that proved to be of value for me. Especially if you just want to get started with toner transfer PCB production, take some minutes to look through my tips, maybe it will save you some pondering later.
Mandatory tools for (SMD) PCB projects
- A fine tip temperature controlled soldering iron station (invest ~100€, it’s really worth it)
- Fine wire cutters
- Precisely closing pointy tweezers
- Fine solder (0.8-1mm diameter), the standard stuff with lead. While lead free solder works for soldering standard through hole components, trust me, it will not work for you in the realm of fine pitched ICs.
- Desoldering wick
- A soldering flux pen
- A (small) bench drill, or a drilling machine + drill rig
- PCB/metal drills with the following gauges
- 0.6mm (for via holes)
- 0.8mm (for standard through hole components like restistors, capacitors, IC pins)
- 0.9mm (for pin header drill holes)
Track width, via size, part dimensions
What path widths can be realized with the toner method, or better, what makes sense?
What usually works for me:
- Projects with through hole components or “macroscopic” SMD components (SOIC packages, etc …)
- Track width: 16 mil
- Track clearance: 10 mil
- Projects with fine pitched SMD components (TSSOP packages, etc …)
- Track width: 10 mil
- Track clearance: 8 mil
- Via drill hole diameter: 0.6 mm
- Whole via diameter: 60 mil
What components can I use for my toner transfer project?
- Resistors and capacitors with form factor down to 0603 are fine (would not go smaller)
- ICs: if it still has legs and you can reach each leg with 10 mil wide traces, you can solder it. In the worst case you would have to use the “solder blob + desoldering wick” technique.
DIY wire vias
As you might have seen, you can produce double sided PCBs with reasonable effort. What really is annoying is dealing with vias. You think: Vias are easy, you just put a wire through the hole in your PCB and solder it from both sides. You will find that this is a nasty task. You have to fix the wire relative to the PCB. Due to heat conduction, the solder on the first solder joint will melt, as soon as you solder the second side. Then if you’re unlucky the surface tension of the solder will pull out the whole wire when you remove your soldering iron. Here is what made wire vias feasible for me:
The pictures speak for themselves. The wire I use are cut-off through hole resistor leads because they just fit so neatly in 0.6 mm holes. Use pliers to squeeze one end of the wire, push it through the via hole. Cut the other end of the wire with a wire cutter. Squeeze the second wire end, too. Hooray, the via is locked in place and ready to be painlessly soldered from both sides :D
Pins / pin headers as vias
When you design a project with pin headers you probably already realized: The pin header is a through hole component and has pads on both sides of the PCB. Can’t I use the pin also as a via? But then how do I solder the top side that is covered by the black plastic part of the header?
Do the following:
- Solder your pin header from the bottom. If the header has two rows, use two single row headers of the same length in parallel.
- Use your tweezers as a lever to lift up the black plastic ribbon. Lift them one by one if you have a “compound header” as described above.
- Solder the pins from the top
- Use your tweezers to push the black ribbon back down or remove it completely (if the pins are too short)
Don’t bet on the pin being electrically connected to the top pad without soldering just by being pushed through!
Avoiding vias / avoiding a second layer
Very often you route a PCB project and everything works just fine for you. You realize you need just one layer because all the traces just fall into place. And then, like always, you have two or three lines left unconnected that you just can’t seem to get squeezed on your board. Here’s two things you can do:
Coated copper wire
Go ahead, introduce a second layer but put as few traces there as possible. Produce your PCB but without the second layer. Don’t drill through the via pads, use thin (0.2mm diameter) coated copper wire to make point to point connections over medium distances. Using 0.3 mm or thicker wire makes a huge difference. The cross-section area is much bigger (remember the square law), so the wire is much stiffer and conducts heat much better. This can lead to involuntary melting of the first solder joint when soldering the second.
Cut the wire to the proper size, then burn away the coating at both ends with a hot soldering iron and a tiny solder blob on its tip. Now you can solder the copper wire to the corresponding via pads.
Zero ohms resistors
You can use zero ohms SMD resistors to bridge short distance, i.e. just “hopping” over another path. Don’t place an extra resistor in the schematic file, just draw a short path on the second layer with a length of 70 mil/1.8 mm (via diameter = 60 mil). Just like that you have a place to put a 0603 zero ohms resistor.
Toner transfer tips
The best results with the toner transfer technique I get with the following steps:
- Print out the layout to a catalog page using a laser printer (found out I have to use the brand toner, not the cheap refill that doesn’t melt properly)
- Use a modified laminator to transfer the toner to the copper plated epoxy board, results are way better than an electric iron
- Let the board cool a bit, then let the catalog paper soak in water (dish washing liquid optional)
- Carefully peel of the catalog paper, dry off the board
- Take a hot air gun and heat up the toner again. The toner melts again and closes fine pores in the toner layer.
- Let the board cool again, don’t smear the hot toner
- Put the board in the etching bath
- When etching finished, remove the board from the bath and rinse with water.
- Use brush cleaner (or nitro-cellulose combination thinner) to remove the toner from the board. Works like a charm. In the past I had quite some trouble getting the toner off with acetone or sand paper.
- Spray the board with solder lacquer, let the board dry until the surface isn’t sticky any more.