Posted by retromaster on October 8, 2009
Over the last week I’ve been doing a lot of tests with the glass paint soldermask technique I mentioned in an earlier post. As the photos show, the results are quite promising!
At first, I was not able to get the Pebeo Vitrea 160 glass paint. Instead, I found another “heat-cureable glass paint” in a local hobby shop. It is of a local brand (called Rich) and it is quite a bit cheaper than the Pebeo, so I decided to give it a try. I did quite a few tests with it, but in the end I decided that it is not very suitable for the soldermask application. Although it is non-conductive and somewhat resistant to solder, it softens easily under the heat of the soldering iron. It is also not solvent-resistant enough to withstand the final mask-removal stage of the process. Also, it does not adhere well to the copper surfaces of the PCB.
Once I decided that the local brand paint is not suitable, I looked harder for the Vitrea 160. Luckily, I found it at a large bookstore that sells art supplies (actually right under my nose!). The photos above show the results of my very first tests. It seems that I rubbed a little too agressively during the mask removal stage, and this caused some scratches, but apart from that, I think the result looks quite fine! Although it is much more expensive than the local brand glass-paint, it seems that the Pebeo Vitrea 160 is superior in all respects, at least when it comes to the soldermask application. It covers the surface of the PCB well, it endures the heat of the soldering iron and the mask-removal process, and it is non-conductive.
Here are some details: I transferred the toner on the etched board using inkjet transparency film (easy to align) and an office laminator. I thinned the Pebeo Vitra 160 with water, with a ratio of roughly 1-to-1. It was applied using an airbrush, but I think that for those who do not have the means, sponge application could also work well. It was left to dry for about a day, and then it was heat-cured in a cheap toaster oven for about 45 minutes, with the temperature knob set to roughly 140 degrees Celcius. Despite the knob setting, it is difficult to know what the actual oven temperature was, since I strongly doubt that this oven is properly calibrated. I intend to obtain an oven thermometer to finetune this part of the process. After heat-curing, I removed the masked sections by rubbing with paint thinner and acetone.
I am still not sure how the process works, but I am guessing that the toner somehow interferes with the glass paint and reduces its resistance against solvents. If this is the case, then using PnP Blue paper for the solder image toner transfer may be a bad idea, since the blue backing may prevent the toner from interacting with the paint. In any case, inkjet transparencies work very well, they are cheaper and much easier to align.
Now, the next step is to figure out how this will work in double-sided boards, especially those I build using my “via press” (see previous posts). I plan to build a simple double-sided board help to test this and finetune the process.