Tuesday 8 July 2014

Laser Cut Your Own Solder Stencils, and Matching PCB Jigs Too

Laser cutting solder stencils from Mylar sheets is effective and economical - especially if you have access to a laser cutter.  But just the stencil is only part of the solution.  If you have more than one or two boards to do,  you'll find that as soon as you do one PCB you'll find it hard to line up the next one because you can no longer see through the stencil.  Also, it's harder to get an even paste layer because you need to press the stencil down evenly on the board while squeegeeing - which gets harder to do with your fingers the more the stencil gets pasted up.

The industry standard solution is a simple jig to hold the PCB and stencil all aligned and in place.  Like this:
(Picture from Bill Shaw's page - You too can do 0402!)
Might not be worth it for one or two boards but it's needed for any kind of quantity.

This gets much more challenging if your PCB doesn't have straight edges and can't be fixed into a simple jig, like these which I recently made:

But if you already have a laser cutter involved, why not laser cut a jig to fit your board exactly?  We can take advantage of the laser cutter's precision and strong points to get two things at once.

I cut a jig out of acrylic the same thickness as my PCB.  The PCB sits inside and acts as a frame to both hold the PCB as well as support the stencil.  While I was at it, I cut the mylar stencil to have an outline that exactly matches the jig's frame - making alignment a snap.

It's small, it requires no set-up... it's worth its weight in gold.

First: Why use Laser-cut Mylar?

The industry standard for solder stencils is laser-cut steel.  These are expensive, even in "prototype" quality.  The first (and last) commercially-made steel stencil I purchased was the stencil itself (prototype quality), two angle pieces to make a jig, and a roughly-cut scrap of steel with tape over the sharp edges as a solder applicator.  That was $200, please.

Mylar stencils - while not as durable as steel - are far more economical.  And if you have access to a laser cutter then an added bonus is that doing it yourself is faster than any RUSH service out there.

Making the Stencil

Pololu has a service for laser-cutting stencils, but if you have access to a laser cutter you can make them yourself.

Hack a Day made a post featuring this fellow's step-by-step process of the software steps needed to make a stencil design from Eagle CAD files.

Briefly, this is the process:

  1. Export the "tCream" layer of the PCB (which is specifically for a solder stencil) to a gerber file (.GTP) from Eagle.
  2. Open that .GTP file in Viewmate Gerber viewing program.
  3. Use Viewmate to shrink each pad in the .GTP by 0.008 inches (this is to account for the kerf of the laser - the "width" of the beam - and also the slight contraction of the material away from the cut.)
  4. Either save the changes and export as a .DXF or some other format (if you have a paid version of Viewmate) or worst case just print to a PDF.
  5. Use the export of the adjusted pad sizes to laser-cut into a sheet of Mylar (this step depends entirely on what laser you have what formats it likes, etc.)  
The link I mentioned earlier has some good tips on laser settings and method for cutting Mylar cleanly.

A Word About Mylar

"Mylar" is actually a trade name for a product from DuPont.  You can purchase Mylar in various places but your best bet for making your own stencils is Dura-lar, made by Grafix.  It is conveniently sold in handy 9x12 sheets and looks like this:

If you don't order online, go to artist or drafting supply stores (not stationary stores).  Art or drafting supply places will know what it is.

You want it .005 inches thick.  If you have a choice go for clear film, but matte finish (which is not clear, but translucent) will work just as well.

Making the Jig and Frame

You'll need a sheet of material (such as acrylic) that is roughly the same thickness as your PCB.  I have found that 1/16" acrylic (such as from Inventables) is a close enough fit to "standard" 1.8mm boards to be workable.

We need to do one thing differently from the earlier instructions on making a stencil: instead of exporting only the tCream layer, we need to also export the board outline.  

Then we resize the pads but leave the board outline alone.  When it comes time to process the job on the laser cutter, the board outline needs to be cut through the acrylic and the pads need to be cut only out of the Mylar.

After having exported the board outline and the pads, and shrinking the pads as described earlier, this is the new process:

  1. Cut the board outline out of the acrylic.
  2. Pause to let you lay the Mylar on top of the acrylic (without moving the acrylic underneath) and anchor it there.
  3. Cut the stencil out of the Mylar - it will be aligned correctly with the hole for the PCB underneath.
  4. Cut a "frame" out around the whole thing, cutting through both the mylar and the acrylic in one go.
  5. The result is a solder stencil with that is perfectly fitted to the frame of the jig, which will itself perfectly fit your PCB.

The exact method will depend on your laser software and hardware.  Depending on your laser and CAD software it may make more sense to pause the job, or to simply make two jobs - one for cutting the PCB shape, the other for cutting the stencil and outside border.  But those are the right steps.

We're left with a square of acrylic with a form-fitting hole for our PCB, and a piece of Mylar the same size as the square with matching solder paste holes in it to match the PCB.

Now insert a PCB (file or trim any spots that don't quite fit) and fit the Mylar on top, making sure the stencil holes match the PCB.  Either tape or glue down the Mylar sheet to the acrylic, and enjoy applying paste to board after board - regardless of weird board shapes that don't fit into normal jigs.

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