PCB Fuser for Toner Transfer Artwork
I've made quite a few printed circuit boards using various methods. I've concluded the laser printer toner transfer method is the best for prototype boards. You print your artwork (mirror image) on special paper in a laser printer, and then heat transfer the toner to the PCB. This method is well documented elsewhere so I won't repeat the details here.
One recent discovery is that Staples Picture Paper works absolutely great for this purpose. Its much cheaper and works better than the commercial offerings intended for making PCBs. Thomas P. Gootee has a most informative page on using this paper on his page Easy Printed Circuit Board Fabrication. Great stuff Thomas !
The hardest part of getting this method to work is getting consistent results with the heat transfer. Usually you use an iron and press on the paper & PCB for "a while". What's needed is a consistent, repeatable method for applying heat and pressure to the artwork and board. I first saw the fuser idea on the DynaArt site, the guys who make Toner Transfer paper (Staples Picture Paper works better IMHO). They have instructions on how to make a fuser out of a heat sealer on their web site, which is a lot simpler than my fuser but will cost you over $100. I don't know how well it would work with very thin PCB material, and it probably won't give you as much control over the fusing process as mine.
While you are at the DynaArt site check out their "direct etch" method using a sponge. They sell a special coating to protect the toner - with the Staples paper you don't need it. The sponge method works great - 1 oz copper boards etch in 2-3 minutes.
I had several old broken Laserjet printers lying around. I figured if the fuser can fuse laser toner to paper, why not use it to transfer the toner onto the PCB ? I quickly whipped up a computer controlled fuser that allows you to control the heat and speed that the artwork/PCB goes through the pressure rollers. You can't control the pressure since its fixed by the fuser assembly.
The good :
- it works beautifully on thin (.015") PCB material. Once you get the temp and speed right, excellent transfers with very little smearing or flaking of the toner. Detail is close to photographic PCB methods. I've only tried very small PCBs designed for surface mount parts.
- there are lots of broken LaserJets around and you should be able to find one free or very cheap
The not so good :
- heat control is manual using a lamp dimmer and its a little hard to adjust. You can get repeatable temps by measuring the thermistor in the fuser with an ohmmeter, but its a little hit and miss. With a bit more hardware to measure the thermistor and control the triac in the fuser supply the PC could control the heat very accurately.
- I've tried .064" board and it didn't work very well. There is too much thermal mass I guess and the fuser can't get the PCB hot enough. More experimentation is needed.
Making the PCB Fuser
I gutted the printer, keeping only the chassis, toner assembly, power supply, stepper motor and drive train. I used a very simple stepper driver board from another project and a dos program that drives the stepper motor forward or back at different speeds using the parallel port. This setup allows you to feed the artwork/PCB at a controlled speed thru the fuser.
Caution - this project requires hacking a 120volt power supply which can electrocute you ! The fuser has an overtemp switch that will pop if it gets too hot, but it could still cause a fire and you can get burned if you touch it. This is not a project for the inexperienced !
LaserJet Fuser stripped down to essentials.
I hacked the power supply to put a dimmer in series with the fuser heater. Normally the fuser heat is controlled by the Laser controller board which reads the fuser temp via a thermistor and turns the triac on and off to maintain a fixed temperature. I could have done the same thing using the PC as the temp controller but the lamp dimmer was much simpler.
Dimmer mounted on top of Laserjet power supply. The fuser supply has been modified to bypass the triac and safety cutoff relay. I removed the current sense transformer (largeish coil embedded in black plastic) and soldered the dimmer leads in its place.
Another view of the hacked power supply and gear train. If you look carefully you can see a spring that was added to keep the gear train engaged with the fuser gears. Normally closing the printer lid engages the gears.
Photo of the white and yellow thermistor wires which are on the right side of the fuser when you look from the back of the chassis. The resistance of the thermistor goes down as the fuser heats up. I found 3-5k ohms was about the right temp for .015" PCB material. You have to be VERY careful adjusting the dimmer - start with it off, turn it up slowly till it just starts to supply power (you'll hear the fuser heater humming a bit and see the while light on the ends) and then back it off. Keep measuring the thermistor till it stabilizes which takes 45 seconds or so. From there, small adjustments are all thats needed. Probably a good idea to mark the dimmer dial once you get it right, and double check by measuring the thermistor. Caution - if you overheat the fuser you will hear a loud ping which is the overtemp fuse - and it DOESN'T reset. If that happens you will have to get another fuser, or bypass the popped fuse inside the fuser assembly.
The Stepper driver board. This is a very simple board that uses TIP121 NPN darlington transistor drivers. The circuit is almost trivial - 1k resistor from the parallel port data line to the base of the transistor, the emitter is grounded, the collector goes to a stepper coil, and there is a reverse biased diode across the coil to supress inductive spikes. There are four identical drivers on the board. The stepper coil center taps go to a 5V power source which needs about 1-2A.
Fusing artwork to the PCB
Once the temperature is right (a drop of water will sizzle on the top roller) start the stepper driver program and adjust the speed and direction - I found it easier to feed the board from the back. Speed - for 1/32" PCB I used "1" which is about 1/10" per second. The artwork is taped to the board and you just feed it into the rollers. The rollers feed at an angle, so large PCBs might hit the printer chassis on exit. Its probably best to cut away the chassis for clearance but it hasn't been necessary for the small boards I've done.
A .015" PCB made with Staples Picture paper and the fuser. The board is 1/2" wide by 13/16" long, has surface mount parts including a 14 pin TSOP. It was tinned after etching. The photo doesn't really show the quality - its very good !
Download the stepper driver program step.zip. Runs in dos or a windows command box. Adjust the speed using keys 1 thru 9. "r" reverses the motor, "f" makes if go forward and "s" stops it. The program is hard coded for parallel port 1 @ 0x378. It uses port bits 0-3 to sequentially energize the stepper coils. I've included the Borland C source code so you can modify it to your needs.