Friday, January 20, 2012

DIY PCB SMT Breakout board fabrication: Results

In the last posting I described how I made a few PCBs to act as SMT breakouts.  I've discovered that this seems to be a pretty ideal job for DIY PCB work!  To mount thru-hole stuff, you need the hole -- and that means hand drilling... but a small breakout board can be done on a single side, and its possible to use .1" Right Angle headers soldered onto an oval thru-hole pad so you don't even need to drill the hole for the breakout header.

In my last posting I finished with the circuit board ironed onto the blank PCB.  It turns out that the final steps are quite easy.  First you get some etchant.  I used ferric chloride from Radio Shack, but I wish I had used muriatic acid and hydrogen peroxide as it seems slightly better for the environment.  I poured the etchant in a leftover food container (now forever dedicated to PCB etching) and left the boards in for about 30 minutes.  I used a twisted wire hanger to keep my fingers out of the etchant:

Then rinse the board in lots of water, and leave the tap running to dilute any drips of etchant solution that go down your drain.   When you're done, put the top on the container (make sure it seals!) and store for next time.

To remove the laser printer toner, I tried a few paint removers, liquid sanders, etc.  Nothing really worked.  It turns out that Acetone (nail polish remover) is required.  Its available in the paint section of hardware stores, and is absolutely amazing at the job. 2 swipes with a rag daubed in Acetone cleans the toner like wiping milk off a table!

Next I soldered the parts on to the PCBs using a toaster oven.  I was concerned that the lack of a solder mask might make the solder bridge across PCB pads because its impossible to keep the solder paste onto pads.  In fact, I don't even bother, I just paint a thin line down across all the pads in a row (see my earlier post on DIY SMT soldering techniques here).  But in fact, the solder balled up nicely as you can see here:

These are tiny SOIC-8 and SOT-23/5 parts.  I left the chip off the rightmost SOT-23/5 to see how well the solder wicked onto the pads.  Before putting it in the oven, the space between all the pads was completely covered with solder paste -- the wicking works great!

I cut some of these boards with a hobbiest bandsaw, like you can get for $200 at Home Depot.  But its even easier to cut them by scoring the front an back with a ruler and then breaking them against the edge of  table (preferably metal).  Place something hard (like a metal or hardwood block) on top, grab the protruding board with pliers and then crack it by rotating the pliers downwards quickly.

One surprise was how fine a pitch may be possible.  I had originally "designed" this board for professional fabrication and I accidentally left some tiny lettering on the board.  Amazingly, it transferred to the PCB perfectly well!  Here is a close up:

As you can see, by the ballpoint pen I put in for scale, the height of these letters is less then one millimeter!  I will need to go back to the file to check, but I'll bet that the line width is just a few mils (thousandths of an inch) wide.  Many professional PCB houses won't do less then 8mils!  Of course, they require repeatability across hundreds of large boards -- my goal is to get a single small breakout board out of a panel with a half dozen attempts.

So basically, if the transfer works without smudging or distortion you can get amazingly fine detail.  And from my limited experience it does tend to do so reliably across portions of the board that can be 2 square inches in size.  This makes me believe that it may be possible to home fabricate chip-scale BGA breakout boards -- at least for small BGA packages.  I have not discovered anyone who has tried it yet; so I think that will be my next experiment (as soon as I find a "fun" BGA chip)!

EDIT:  Here's a photo of the SOIC8 installed on a PCB.  I think that its better to put all the pins in a single row, rather then the DIP configuration because that gives you the other side of the breakout board to use to make your circuit!

Also note that no drilling is needed!  Get right angle breakaway headers and then solder them to the (undrilled) thru hole pad.

EDIT 2:  Here is a photo of a SOT-363 chip with a 2 mil circle indicating pin 1.  To get an idea of the size, the circle is indicated by the lead of a standard 1/4 watt resistor. As you can see the 2 mil line width was etched perfectly!

1 comment:

  1. awesome. I gotta do this...was wondering if I should try the photo-resist method instead, since the prints are reusable...