What To Expect From Using A Household Iron:
The problem with using an iron is the difficulty factor to apply a lot of pressure during the transfer. Sure the iron can put out sufficient heat, but because the iron's foot-print is relatively large, it's difficult to get a lot of pounds per square inch delivered to the PCB when making the transfer. Even putting your entire body weight over the iron for the 30 second application time, doesn't equate to much more than a few "psi" delivered to the transfer. The only thing the iron has in its favor is, everybody has one!
To get repeatable results (to a degree) the iron has to be calibrated. It's not just a matter of applying a lot of heat to get the toner to stick to the copper surface of the board. This has been the biggest misconception for the past 20+ years since this technique first hit when laser printers were first introduced back in 1985. You have to understand one underlying principle at play here.
Toner has two temperatures to be aware of. The first is the point at which the toner gets tacky or sticky, and the other is the melting point of toner where the printed image looses its form and distorts like stepping on a peanut butter and jelly sandwich. (I hope that was graphic enough!) These two temperatures are only stable if the amount of pressure is constant. The objective is to be able to heat the toner only to the degree where it will become sticky, all the while maintaining its form so we get a perfect, undistorted transfer from the transfer paper to the copper board (or any other metal for that matter). The higher the pressure, the lower the state at which it first becomes tacky. This is called the FUSING temperature.
As you can see, if we can lower the "fusing" temperature, there becomes a larger spread away from the melting temperature of toner. This is where we want to be. Create a spread that is large enough so we don't accidentally go too hot and encroach into the melting/distorting temperature. It's all based on pressure. This why our introduction of the "pouch laminator" idea has become very popular. It removes the need to apply physical pressure over the handle of an iron. Instead of trying to apply pressure over a large area all at once (like that of a iron or a "T-Shirt" press) resulting in only a dozen or so PSI, the laminator approach uses pinch rollers that is only making contact with the board where the two rollers come together. Do the math and you arrive at HUNDREDS of PSI being delivered to the board! With the correct, relatively low temperature (about 270ºF) and very high pressure, toner becomes very stable for perfect repeatable results. Ok, enough of the plug for the lamintor... let's get the iron calibrated so you can try your hand at the least expensive method for making higher quality boards.
Calibrating An Iron:
Since all toners have a bit different “fusing” temperatures due to their specific formula, and there are no two irons alike, the iron needs to be set to provide the maximum temperature for the pressure being applied to the toner image without causing any image distortion. This means that regarless of how much TIME the iron is laid over the toner image and board, the combination of heat and pressure will never reach the melting point of the toner image. So with this understanding, we only need to apply the iron and pressure for the minum 30 seconds of time. Also of extreme importance is to have a good quality printout. Ideally you want to be using a quality toner (brand-name toner vs. after market and have your printer set to it's maximum printing density. (See instructions for that in the main menu listing). We can supply you a free sample "test" sheet of known good toner density for calibrating purposes if you request it at the time you place an order for any product.
An iron has definite limits to how big of a PC board can successfully made - keep in mind that the bigger the board, the higher the chance of having what we call an "oops" (sections of an image that don't transfer perfectly). A minor "oops" can be fixed using any indelible (waterproof) ink pen like the venerable "Sharpie", etc.) All of this is just a 'heads-up' to let you know the iron is not the ideal heat source, but by carefully calibrating your iron, you can make it work quite effectively.
Keep in mind that changing your printer or toner brands will require a re-calibration since there are a lot of different toners on the market and they all have their own separate manufacture's parameters.
What you will need to perform this simple calibration.
- Suitable desk-top that can be subjected to high heat without damage
- One sheet of "overhead transparency" film available from any office supply store
- Sample circuit image about 3" square printed on a sheet of TTS paper
- Set your iron to its "mid" range position
- Piece of copper-clad board the size of the iron's solid heel "no-hole" area
1) Lay the PCB copper-side up on a wood or suitable smooth surface
2) Lay the overhead transparency over the board
3) Lay the sample printed image face down over the clear transparency
4) Position the heel of the iron (no steam hole area) over the back of the transfer paper and apply your entire body weight over the handle of the iron. Your feet should be off the floor or as close to this as you can for 30 seconds. (It will seam like an eternity!) We're trying to make pressure a "constant".
5) Set the iron aside. Lift up the overhead transparency sheet and transfer paper for observation of the circuit image. (The transfer paper should be stuck to the mylar sheet - if not, raise the temperature a "notch" and do it again). Look through the clear overhead transparency and see if the image is distorted. If not, raise the temperature again another notch and apply pressure again. Keep doing this, increasing the temperature each time until you get the image to be visibly distorted. Look for fattening of traces, smears, etc.
NOTE: If you can't get the image to stick to the overhead transparency it probably means your printer isn't putting out high enough toner density and you may need to investigate using a different printer that will perform better. Ensure your "DENSITY" is max'd out. This can be a bit illusive to find sometimes with newer printers. In case you didn't read this before, no "Brother" brand printers! Their toner will not work with our system. Their toner is epoxy-based and requires much higher temps to fuse.
Once you have gotten the image to distort, simply reduce the temperature to the last prior test setting and mark your iron's heat setting for future use. You have successfully found the temperature at which the toner will NEVER transition to the melting state. Your iron is now set to the ideal temperature when using THIS toner, at THIS density setting, with YOUR body weight. If you cnange any of these parameters, you must recalibrate!
Congratulations! Calibraton is complete.
Now let's start making some boards!