Calibrating An Iron

PUTTING IT ALL TOGETHER:

There is just a little bit more to understand and you'll be on your way to successfully using an iron. The reason so many people fail when trying to use an iron for this "DirectEtch" process is that they don't understanding the four factors that makes the toner transfer process work properly. We've mentioned the first two, heat and pressure, but there are two more factors; time and toner density. Too much heat with too much pressure will cause the toner image to quickly exceed the "fusing" temperature and shoot right up to melting temperature causing the image to look like stepping on a peanut butter and jelly sandwich.

Pressure and temperature work inversly. The greater the pressure, the lower the temperature needed to reach fusing temperature. This is important because the melting temperature of toner is not a constant! It also varies based on the temperature AND pressure being applied. I know this sounds confusing - but hang in there for one more second. If, for example, the toner image printed on a piece of paper is subjected to high heat, say 450ºF, it will actually maintain its shape because there is no pressure being applied. However, put any pressure over the image and it immediatly flattens out loosing form. The important part about understanding toner is that we can control the "temperature spread" between fusing temperature and melting temperature by pressure alone! Obviously, the more we can stay away from the melting temperature the better. In other words, the actual temperature required to reach either state becomes wider with increased pressure and decreased temperature! This is why we want to keep temperature as the only initial variable when we start the "calibration" technique below.

The fourth and last factor to be aware is having a high enough toner DENSITY to start with (ie. the printed image on the transfer pepper.) All toner formulations are about 55% pulverized plastic and it's this plastic content that makes toner "fuse" or stick to anything it touches, when subjected to the proper heat and pressure. If you are not sure about your printer's density setting - which can be illusive to find and set, see the menu button "Set Toner Density" in this section's listing for more info on how to set it.)

CALIBRATING THE IRON:

To start this calibration you will need to obtain one sheet of regular "acetate" (the plastic sheet that is most commonly used for "overhead projecters", available at any stationary store. It is critical to have this in order to calibrate your iron otherwise it will be "hit 'n miss" wasting a lot of transfer paper in the works.

SETUP:

  1. Set the iron to it's mid-range position
  2. Print up some sample images onto a sheet of the transfer paper
  3. Place a blank circuit board over a safe wooden surface
  4. Lay the acetate sheet over the copper
  5. Lay a sample printed image toner down over the acetate sheet

PROCEDURE:

  1. Position the iron squarely over the back of the image
  2. Apply your entire body weight over the iron for 30 seconds
  3. Remove the iron, lift the acetate sheet and examine the image
  4. If the toner image look normal with no distortions increase temp
  5. Do again until you see the image has distored

Once the image has visibly distorted, reduce the iron's temp to the last setting and mark the iron heat position. The iron is now calibrated for that printer's toner density with your body weight. From this point on, you could actually hold the iron over the transfer paper indefinitely and you will never cause the toner to melt causing distortions in the image.

Your only limitation at this point is in how fine you can make traces. It takes much more pressure in a more controlled environment to be able to make super fine traces, however, you should now be able to transfer traces in the .015" to .010" range without any problem.

HOW THE IRON CAN BE A CONTROLLED ENVIRONMENT:

An iron can be made to work fine with our technique for making instant PCB's but it can still pose limitations as compared to what the "Toner Image Applicator" can do, however, the iron is free and everyone has one! The main problem with using an iron is that it is not a well controlled heating device and pressure applied can be a total variable from person to person, so we're going to have you calibrate your iron specifically for your weight! Yes, you are going to be applying your entire body weight to the iron and that will now change the this 'variable' into a 'constant'.

ABOUT PRESSURE BEING APPLIED:

Heat and pressure are the two of the most important factors effecting how to control toner to be able to make it re-fuse to the copper surface to make a reliable, good quality etch resist mask. Repeatability, however, will suffer if the controlled application of heat and pressure is out of range. You can't just use the iron as if you were ironing a shirt. One of the key things to understand when using an iron, is the application of a constant pressure more than anything else! By using your entire body weight, we now have a good 'constant'. We also have a simple 'constant' of time. We're use 30 seconds as our baseline. That sounds easy to do, to put your entire body weight over the handle of an iron but for many this get's to be a bit difficult. The older we get, the harder that is to do! We're talking about basically balancing your entire body over the handle of the iron, hopefully not breaking the iron in the process!

Once the correct tempeature is achieved (from the calibration procedure below) along with a constant pressure, you'll have a repeatable method for making good looking boards. This is in effect what our "Toner Image Applicator" (or "TIA") is doing without the hassle. The biggest difference between using your iron and the TIA applicator is that the applicator is putting hundreds of PSI over the image whereas even your entire body weight over an iron is at best putting only a dozen or so PSI distributed over the size of the iron's platten. This would then give you only a slight difference between the temperatures for making toner "fuse" vs. "melt", and this why your body weight is so important in order to establish pressure as 'constant' as much as possible!