Friday, January 13, 2012

DIY PCB fabrication; Breakout Board Toner Transfer Experiments

Using modern components in PCB circuit board design essentially requires the use of small surface mount packages.  Testing these components can be costly and time consuming if you need to have a break-out-board professionally fabricated for every component.    It is possible to hand-solder extremely thin magnet wire to individual leads of a SMT package but that is a very painstaking process.

It would be much easier if individual PCBs could be hand-fabricated for the task.  I dug around on the web and DIY PCB fabrication for very small parts seems quite tricky; most people use home PCBs to make larger circuits out of thru-hole parts.

However, even if multi-chip SMT designs are not feasible, it would be very valuable to be able to rapidly fabricate single chip breakout boards for SMT parts (for noobies: a breakout board is a PCB that simply brings all the pins in a SMT chip to a male header that can be plugged into a solderless breadboard).  Especially small parts; if you are putting a TQFP-144 on you board there are places where you can buy breakouts.  And there are fewer footprints so you could build up a "library" of breakouts.  But there are tons of different packages for components ranging from 2 to 32 pins!

Making these small-pin count PCBs might be feasible (where large SMT designs are not) for several reasons.  First, the PCB will be quite small, allowing you to panel multiples "tries" onto a single DIY board.  You only need one success!  Second, the routing will be pretty short and simple.  If the pins are placed on either side of the chip the routes will be short and on a single side (so you can try the same circuit on both sides!).  Finally, I think that right angle .1" headers could be used in place of straight pins, so the final board would not need to be drilled.  This will save a lot of time (and painstaking work).

Here are my first attempts (ever) to use the toner transfer method to put a mask on copper PCBs.  I panelized 6 SOIC-8 and 6 SOT-23/5 breakout boards onto a space about 2x3 inches:

And the back side attempts:

The basic methodology (for people who are reading this as an instructable) is to:
1. Remove all layers of your PCB except routes, pads and vias.  Export it as an image (monochrome, 600 dpi).  Load it up in a graphics editor (gimp), reverse the color, and horizontally mirror it.

2. Next, use a laser printer to print it onto some kind of paper.  You want a paper with unique, generally undesirable properties; essentially the toner should just barely stick and it should dissolve readily in water.  Four types are commonly suggested:
  A) glossy paper ripped from a cheap magazine/catalog
  B) glossy photo paper
  C) sticker backing paper (pull the stickers off and print on the shiny side)
  D) speciality PCB toner transfer paper.

3. "clean" a blank PCB with steel wool, and then iron the toner on by placing the iron at its highest setting on the paper (which is on the PCB) for about a minute.  You can't move the paper relative to the PCB so some masking tape on the corners might help.  Also don't push down hard, and don't move the iron around much.

4. Soak the PCB in water for about 5 minutes and then rub the paper off.  You can rub quite hard; if it toner comes off then it did not transfer.

The PCB on the upper left of image 1 was made with glossy photo paper.  This paper essentially did not work for me; it would glue itself to the iron and then separate from the PCB or pick it up as well.  The toner was still melted at this point so most of it stayed with the paper.

The upper right image 1 board shows what happens when you push down too hard.  Pressure on the melted toner caused it to spread out.  So don't do that!! :-)  The bottom left of image 1 was ironed on for several minutes.  It got so hot it changed the copper color -- additionally the paper was extremely hard to remove.  Oops!  The last photo on image 1 was again done with glossy photo paper.  The paper still lifted off with the iron but amazingly it left a perfect transfer in the middle!  Unfortunately I doubt its repeatable.

I left board on photo 2 clean to show how the PCB should look after it has been rubbed by the steel wool.   The other 3 boards on the bottom image show transfers with the sticker-backing paper with ironing times of 30seconds, 1 minute, and 1:30.  I'm sure you can see which is which since they show more toner sticking.  In fact, the last image has perfect transfers of 9 out of the 12 circuits!  The only miss transfers are on the top of the PCB.  Clearly I did not get that portion hot enough (in fact, I think that it may have been under the steam holes of the iron).  Removal of the paper was pretty quick under hot water; about 5 minutes (so clearly the boards were not under the iron for too long).  But the paper did not "pill" up like other web postings suggest; it mostly peeled off.  After ironing, the paper it was a little browned in spots but not significantly.

Like any other craft there is clearly a bit of skill and practice required.  And some experimentation is required to become familiar with your particular tools.  However, so far this looks like a pretty feasible method for breakout board fabrication.  In the next installment I will attempt to etch the boards and solder the parts on!


  1. Such a brilliant post you share with us. its help me to understand more better ideas about printed circuit board.

  2. There is a better product for this than paper.