SMD Stencils on the cheap

I ran across a method of making SMD stencils at home, for next to nothing.

The original method (above) uses a Gerber viewer to modify (shrink) the size of the SMD pads, which isn’t a bad way to go. The trouble is: The program used is Windows only, and I get along with Windows about as well as my cats, which is to say, not well. Though I could certainly boot to Windows and use the tool, I searched for other ways to do it.

Problem #1: There are few Gerber readers for OS X. Certainly none that I could find that allowed me to easily grow/shrink the size of the pads...

Problem #2: There were certainly a lot of additional steps along the way to getting a printout (to heat-transfer to the metal)

Fortunately, a bit of digging, and I found that I was able to do all I wanted from Eagle PCB, which is where I generated my schematics and placed & routed my PCB’s... Which lends me to wonder why the additional work was suggested, given the original author used Eagle to generate his schematics & PCB designs as well.

The tip I found was: “With your Eagle board design, open the DRC (Design Rule Check). Click on the Masks tab, and set the Cream Min and Max to 0.05 mm or 2 mil (depending on the units you are using). Click Apply. Your board's cream layers will be updated.”

So, yeah... you shrink the pads by changing the design rule check... that’s just so intuitive. At least it makes the same sort of sense the rest of EAGLE does. which isn’t much.

Unfortunately, it seems that both tutorials are a tad shy of great... one mentions you want the stencil apertures ~30% smaller than the pad, which makes sense to avoid using too much solder paste and causing bridging.. The thing is, neither actually uses a 30% ratio, instead simply shrinking the pads by a fixed amount (ie. a few mils). Just select min size of 0 mils and max size of 10 mils, with a 30% shrink... boom.

From there, I create a CAM job to generate a PostScript file, and from there, to the printer... and off to etch...

So, here’s my full procedure, which is slightly modified from the original:

For all steps: Be sure you have adequate ventilation. Most of the steps involve something that will emit a gas you don’t want to breathe.

1 Drink a soda.
⁃ The original article mentioned that Coca-Cola and Dr. Pepper seemed to work a bit better. It’ll probably be a bit of trial and error for anybody involved
2 Cut open the can, and roll it flat, being careful to not create creases. I used a set of all-purpose snips from Stanley, whose primary virtue was they were $7.00
3 Heat the aluminum. I popped mine in my toaster oven at its maximum setting (450 ºF) for 15-20 minutes
⁃ The can is coated with a clear Epoxy resin. The heat helps break it down, turning it brownish
⁃ The baking process will produce vapors that can irritate your lungs
4 Pull the aluminum out of the oven, and wipe the ‘inside’ (clear) part of the can with a paper towel soaked with Acetone
⁃ It takes some elbow grease, but eventually the Epoxy will start to come off
⁃ Again, lots of ventilation; Acetone is not something you want to inhale
5 After removing the Epoxy resin, I decided to try polishing the bare aluminum. I used a dremel and a very fine polishing compound, and ended up with a almost mirror-smooth surface
⁃ Polishing alone didn’t appear to make much of a dent on a section of epoxy that remained on the board; dissolving the Epoxy with Acetone was a time-saver
6 I loaded EAGLE on my Mac, loaded the Board
⁃ I shrunk the mask size so it was 30% smaller than the pad
⁃ Some trial and error may be necessary in how much to shrink the pads. I probably should have shrunk the pads a bit more for the 64LQFP package I had
⁃ Run a CAM job, generating an “inverted” (black & white inverted), mirrored postscript job, on only the ‘tcream’ layer
7 Import the PostScript file in InkScape
⁃ Layout the transfer — you can batch multiple stencils, for example
8 I print the transfer onto Pulsar’s dextrose transfer paper
9 Tape the transfer paper piece onto the polished aluminum
10 Run the transfer paper & aluminum through a laminator to fuse the toner to the aluminum.
⁃ I use multiple runs through the laminator; 2 or 3 times.
11 After you’re finished with the last run through the laminator, dunk the mask into water.
⁃ The dextrose on the paper will dissolve, and you can gently lift the paper away from the aluminum, leaving you with a perfect transfer.
12 My printer isn’t 100% black, even with maximum toner density & quality. Even if it did, Toner is still porous, so it’s very helpful to use Pulsar’s Green transfer foil
⁃ This stuff goes on in the laminator again, but sticks to the plastic toner — but not to the aluminum. Neat.
⁃ Peel it away gently, and you may have to gently remove ‘loose’ foil that didn’t tear off, but didn’t adhere to the bare aluminum either.
13 Prepare the etchant:
⁃ Get some Muriatic (ie. Hydrochloric) acid from your home improvement store of choice. You want 20° Baume (31.45%) concentration
⁃ Get some 3% Hydrogen Peroxide (H2O2) from your local drug store/grocery store.
⁃ Pour in the Hydrogen Peroxide first. “Do as you oughtta, pour acid into watta”. Otherwise, an undesirable exothermic reaction can result, potentially involving acid being sprayed onto you.
⁃ Gloves and eye protection are especially necessary.
⁃ Mix 3 parts with H2O2 with 1 part HCl in a plastic container. I used a disposable plastic sandwich box.
⁃ Be careful! This stuff obviously eats aluminum, but it’ll also eat Copper, concrete, iron, steel, your skin, eyes, and hair.
⁃ The mixture (in its unused form) is also used to etch PC Boards, and many consider it better than the FeCl we’ve been using for ages, as it’s reusable, can be ‘recharged’, and has less waste.
⁃ It also emits fumes, so make sure there is plenty of ventilation.
14 Use clear tape on the backside of the Aluminum, and on the front — everywhere you don’t want to get etched.
⁃ Overlap the transfer image a little bit as well.
15 if your plastic container is light colored or clear, put something dark under it
⁃ So you know when etching is complete.
16 The next step is essentially Pulsar’s “DirectEtch” method:
17 Immerse the aluminum panel in the liquid.
⁃ Continually, and gently brush the transfer image with a foam/sponge brush.
⁃ Exposed metal will bubble; the brush helps ensure fresh etchant is circulated to the etching surface.
⁃ After a few minutes, it’ll etch through.
⁃ Pull the board out of the etchant, and flush it with lots of fresh water.
18 Store/save the remaining HCl + H2O2 etchant for later use or disposal.
⁃ Etching aluminum in this way produces hazardous waste, much like FeCl etchant. You have to store & dispose of it as appropriate with local laws.
19 At this point, remove the tape. Enjoy the nicely etched solder mask.