Making Copper PCB Stencils for SMT Reflow

Toner transfer over copper foil

Stencil after etching and cleaning

Solder paste deposited using the stencil

Assembled PCB after reflow

Over the weekend, I decided to try out making a stencil for solder paste from copper foil. I had thought about this previously, but after seeing an instructable on the same idea, I became more confident that it would work.

The process for making a copper stencil is basically the same as making a PCB. The resist image (i.e. paste layer, inverted) is transferred onto the copper foil using any method of your choice. For my tests, I did PnP Blue toner transfer onto 0.2mm-thick copper foil using a laminator. Afterwards, the back side of the foil is covered to prevent etchant from eating away all the copper. I used clear packing tape for this step. The one crucial thing here is to make sure no air bubbles remain in between the tape and the foil. Finally, etching is performed as usual and after cleaning and removal of the tape, one gets a nice looking stencil as shown in the photos above.

One thing to note is that, if one leaves the tape in place, this method can also be used to make simple flex PCBs. This could be quite useful for repair purposes, as well.

Although in my tests the stencil came out pretty well, the reflow (using a hotplate) did not go all that well, as the photos above show the bridges in between some of the legs. This can be due to several reasons: The paste itself wasn’t at very good condition to begin with, more flux may have been needed, the stencil may have been too thick (too much paste), and surely I could have done a better job when depositing the solder paste. Nevertheless, this is not too bad for a first try, and I am confident that I can tweak and improve the process over time for near perfect results for larger PCBs.

UFE New PCB Ready

UFE Rev A2 PCB Before Assembly

I haven’t posted in a while, but I was not lazy :). I was working on making the PCB for the next UFE hardware revision. I’ve tried a some new things and ran into a few problems in the process, and eventually, came up with the board in the photo.

With this PCB, the soldermask is noticeably stronger than the previous UFE revision, especially on the top side. On the bottom side, the soldermask still lifted in a couple of places. I plan to do some more tests with dummy boards soon and hopefully improve on the process.

One thing that still bugs me here the pitting of copper in large plane areas. Admittedly, this problem is more of a cosmetic nature (as double-sided boards are already limited in terms of signal integrity and speed). I also want to do some further tests to see how this situation can be remedied. Perhaps some Pulsar Green TRF can be useful, or I can even switch back to PnP Blue if I can get it to work with CNC drilled holes.

Built TFE+ Prototype Board

TFE+ Amiga Version Prototype Solder Side

TFE+ Amiga Version Prototype Component Side

I finally managed to build the TFE+ Amiga version prototype board. I fully assembled the board but did not install the ICs in their sockets yet.

Apparently, I did a rather sloppy job here. The toner transfer and etching went OK (done using glossy paper instead of the usual PnP Blue). I wanted to apply a soldermask afterwards and that’s where things went wrong. It seems I overcooked the board after applying the glass paint (that’s the reason for the discoloration of parts of the board) and I was not able to remove the masked areas after curing. So I removed the paint layer altogether, and just applied the protective flux layer instead. Well, it’s just a prototype, anyway.

Now, it’s time to get the firmware working on this board…

TFE+ Prototype Manufacturing Issues

It’s been a while since my last post. Most of this time I’ve been trying to deal with the problems I encountered making the TFE+ prototype PCB.

Since I use a CNC to drill the PCB holes, my process is that I get the holes drilled first and then apply the toner transfer. For this to work, it’s imperative that I get the laser printer printout to match the CNC-drilled holes. Previously, I had tested with some smaller boards, and I managed to obtain some fine results. The TFE+ board is quite a bit larger and this is where the problems started. It seemed that a non-linear error kept creeping into the printout and despite trying many different things, I never managed to get a good match.

The printer I’ve been using is a Samsung ML-1610. It’s a fine entry level printer and it has worked well with toner transfer during time I did manual drilling. Once I started having dimensional accuracy problems after the transition to CNC-drilling, I took a micrometer and measured the printout (on plain paper). It seemed that one side of the printout was smaller by almost a millimeter! The error was consistently there in subsequent printouts I took. I have no idea where this error comes from. I am sure that it was not in the source image. It could be a software/firmware error, or it might be that the some parts of the mechanism have worn-out (it’s more than 3 years old now). Or perhaps the printer was never intended to be very accurate (which is fine for a printer in its class).

Deciding that it is time to get a newer (and better) printer, I did some research and came up with the Kyocera Mita FS-1100. It is significantly more expensive than the ML-1610. But it’s built like a tank, it has a much higher speed and 1200dpi resolution, and it seems it may be more economical in the long run. In many ways it reminds me of the older Laserjet line of printers from HP. Last night I did some tests with laser printer transparencies (just to see). The printout from the FS-1100 still needed some scaling, but there was no non-linear error and I managed to get an almost perfect match. Far better than what I got with the ML-1610.

It still remains to be seen whether the FS-1100 output will work fine with toner transfer (e.g. with PnP blue paper). Over the weekend I’ll do some more tests, and if all goes well, I might even have a prototype PCB in my hands next week!

First Good PCB with the MF70 CNC

First Good PCB with the MF70 CNC

I’ve finally managed to manufacture a good-looking single-sided PCB using my CNC MF70 and toner transfer. I first drilled the holes using the CNC, then I applied toner transfer aligning the transper paper with the holes. I’ve etched the board afterwards but not cleaned the toner yet. The PCB is a parallel port breakout board. I’ll use it for the new parallel port-based stepper drives. Tracks are 20mil with 10/12mil clearance. This time, I’ve used ink jet transparency film, as it’s a bit easier to align with the board holes. The results are quite good (the photo does not do it justice), though I am not sure how it’ll perform with finer traces. The text at the bottom turned out all right so I guess it could work. Soon I’ll try out another board with PnP Blue.

The most significant problem I’ve faced so far was the alignment of the drilled holes with the toner transfer printout. It seems that the printout function of the PCB layout software I am using was inaccurate, as it introduced a kind of shearing transformation to the printed pattern. The effect was very slight but enough to cause mismatch. It would not be fair to fully blame the software though, as the printout function was mainly intended for documentation purposes and laser printers can be inaccurate anyway. The solution I found was to first export Gerber files, then use the excellent gerber2pdf.py utility by Joseph C. Chavez. Printing the resulting pdf file solved the shearing issue, but there was still a linear mismatch in the Y axis of about 1%. I’ve done some searching on the net and it seems that this kind of mismatch can be expected from laser printers. The gerber2pdf utility accepts a scale factor as input, so this was easily fixed, and the result is the board I mentioned earlier in this post. The alignment is now almost perfect, although the scale coefficient could probably use some finetuning.

Soldermask Tests

Just before heat-curing

After heat-curing and mask removal

 

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.

UFE Main PCB First Try

UFE PCB Toner Transfer First Try

 

Last night, I did a test to see how the PnP Blue toner transfer for the top layer of the UFE main PCB would work. As you can see from the photo, the results are quite promising. Except for the upper left corner area where I had to do a few touchups, the results are almost perfect. I wanted to go ahead with the manufacturing at this stage, but then I realised that I forgot to set the proper hole sizes and octagonal shape for the pads. So,  hopefully during the weekend I’ll do another transfer and get it perfect this time 🙂 .