Make high-quality double-sided PCBs – at home

-December 02, 2014

Why make your own printed circuit boards when you can get them commercially made for low cost? For one, it can take one to four weeks to receive the boards. For prototyping, this can be a major hurdle. Each design iteration will then take a month or more, and a project may need many months to get done. The DIYer can fab the board and assemble everything in one evening. That advantage is really hard to beat.

Besides time, there are other reasons to make your own board. Commercial services charge by board size, not complexity. Larger boards will cost more even if they are completely blank. I once had to make an oversized PCB because the parts had to be spaced far apart. It was a very sparse board, but getting it made from even the cheapest commercial source would have been expensive.

Having said that, this is not really about saving time or money. If the phrase “building your own” does not excite you, then read no more. Building a board may not take two whole weeks, but it will take many hours. Like in everything, there is a learning curve. It’s not going to work the first time. Remember, we are talking about building a near-professional quality board, not something that is thrown together to check the functionality of a circuit. If the process itself is an unpleasant experience then it will not be worth any amount of time or money saved. The reason people build instead of buy is because they enjoy learning and perfecting new skills. If you are detail-oriented, have lots of patience, and take pride in craftsmanship, then this might be for you.

Now on to the details…

 

Laser printed toner transfer method

The laser printed toner transfer method is by far the easiest and the best quality for the home builder, even compared to UV exposure of pre-sensitized boards. Why this is so requires some explanation. PCB manufacturers use UV exposure of photoresist using a photoplotted (not laser printed) mask. The resolution of photoplots can exceed 50,000 DPI, whereas a laser printer tops out at about 2,400 DPI. In order to exploit the advantages of photoplotted masks, you need a very uniform UV source and alignment capability, which is far beyond the average home shop. Most home builders who use UV use a laser printer to make the transparency and use that as a mask to expose a photoresist coated board. This doesn’t make much sense. Why? Laser toner is not fully opaque to UV since it has a large number of pin holes. UV wavelength is only 365nm, so it is very difficult to seal these pinholes and make it opaque. Combined with optical diffraction and scatter in the film, it is not difficult to see that the resulting image will be worse than the original print. On the other hand, a direct fusion of the toner does not suffer from any of these effects. I can get 5mil lines with the toner transfer, which is about as good as any commercial PCB service.

Now back to the toner transfer method. This requires a special paper that is coated with a release agent. Apparently, some glossy paper can also work, but it is better to get the real thing from www.pcbfx.com, which is also available through major outlets like Mouser and Digikey. They are about a $1 per sheet. You also need the laminator, which is about $90 from Amazon. Some people have used a clothes iron, but the temperature and pressure from the laminator will produce far more consistent results.

The key to successful toner transfer is to find a good printer. This is the tricky part because there seems to be a huge variation from one brand to the next regardless of the advertised DPI resolution. You need to make a test pattern of fine lines and try it on different printers and examine under a microscope. We are looking for a high density of toner coverage in the lines, clear white spaces and proper spacing of the lines. The expensive HP printer in my office gave surprisingly poor results under a microscope. The images below show 5mil lines with 5mil spaces from this printer. Notice the specks of toner in the white spaces and the two merged lines.

Printing services at office supply stores are good places to experiment because those printers tend to be well maintained. After some trials, I found a printer at Officemax that worked reasonably well. It is a Xerox Workcentre 5775. For 10 cents a page, I was able to get some decent results.

Figure 1. HP Laserjet 600 M601 (5 mil lines and spaces – microscope image). Notice the specks of toner in the white spaces and the two merged lines. 

Figure 2. Xerox Workcentre 5775 (5mil lines and spaces – microscope image). This is a much better print.


Loading comments...

Write a Comment

To comment please Log In

FEATURED RESOURCES