Making solder paste stencils

by Dan Goldwater  dgold@zblob.com  http://www.zblob.com

• This technote covers:
• Making your own solder paste stencil / mask from your PCB layout
  
• Using this stencil to apply solder paste to a printed circuit board
• Using a solder paste stencil to apply solder paste is a lot quicker and more accurate than manually applying solder paste.  After setup, it takes only a minute or two to mask each board.  With minimal effort I was able to apply solder paste to boards with 0402 parts and 0.5mm pitch BGA's, and get near 100% yield after reflow.
  
• Needed items
• Laser cutter (minimal wattage needed) - see notes at end if you don't have this
• Mylar sheet 0.002" thickness
• 1/16" polyethylene sheet (used for spreading paste, other materials may work)
• 1/16" acrylic sheet
  
• software to edit PCB layout
• Making the solder paste stencil mask file
• The printed stencil will be similar or identical to the "cream" layer of your PCB (the layer which specifies where tinned pads are).  If you are only using large SMT parts then minimal changes will be needed.  For small SMT parts the cream layer needs editing because the laser cutter beam is about 5 mil wide - if you cut along the existing cream layer the stencil holes will run together.  I have been getting the best results by shrinking the cream layer polygons by 10 mil in each dimension.  for large pads you can get away without bothering but for small pads you will likely get bridging when you reflow unless you do this.  for example, a solder pad which is 20 x 30 mil in the cream layer gets converted to a 10 x 20 mil polygon in the paste stencil.  for any polygon dimension less than 12 mil wide in the cream layer, convert it to a line or dot instead of a rectangle.  eg: a 7 x 20 mil pad converts to a 10 mil line.  using lines and dots is needed to get the smallest features from the laser cutter.
• Beyond the general rules above, you will need to tweak the paste stencil polygons for certain parts.  Notably, BGA's and other parts with underside pads require less paste to prevent bridging, so you will need to make the stencil cutouts smaller for these parts.  A few parts I have used required more paste and I had to increase the stencil cutout for those pads.  If you need more paste than will fit on the pad you can extend beyond the pad a little.  You can also make a single stencil polygon covering several small pads, if the gaps between pads are small you will not get bridging unless there is too much paste.  If your entire board needs more paste use thicker mylar.
• Printing the mask
• Use 2 mil mylar for the stencil.  tape it to a piece of cardboard before cutting.  Cut slowly with low power and maximum resolution (here I am using 1000ppi, 1% power, 1% speed on a 100W cutter).
• Also cut a piece of 1/16" acrylic with a cutout that exactly fits your PCB, and has 3-5" borders.  This will hold your board and stencil in place while you smear on the paste.
• Applying the paste
• Put your board into the acrylic sheet cutout.  Put thin pieces of card under your board until the top of the board is slightly raised from the surface of the acrylic (about 10-20 mil raised seems to work - this helps get the stencil tension right).  fit the stencil over the board until it is aligned, and tape the top edge of it to the acrylic sheet.  Getting proper tension on the mylar stencil while you apply paste is important and a little tricky.  I have gotten the best (most accurate) results by taping only the top edge and tensioning the stencil with one hand on the bottom edge.  You can try taping both sides, but it is tricky to get it right.  Apply an excess of solder paste in a line around the top and right sides of your board.  Use a suitable spreader - very straight edge but fairly soft material.  stiff rubber or polyethylene sheet work well.  spread the paste diagonally across the board from top right to bottom left.  Do not press hard.  Hold the spreader at a small angle to the stencil, maybe 30 degrees.  This part takes a little practice to get right.  Your first few will be a mess but with practice you can get very accurate registration and paste thickness.  You may need to spread the paste back and forth a few times, always go diagonally.
• With small SMT's inspect the paste under the microscope until you are happy with the results.  You can tell under the microscope if the paste is too thin or too thick - a thickness of about 5 mils is good.
• Notes
• Sam Kimery writes: "I read your page on making your own solder paste stencils. Being a DIY solder paste stecil maker, I had a few comments. I don't have ready access to a laser cutter, but I've come up with a couple of "hillbilly" stencil hacks: Doing very small boards (1x2 inches) of low-medium density, pretty good results can be had with just printing the "creme layer" (using Eagle layout software) on transparency stock, then going at it with a #70 drill in a flex-shaft tool (Dremel or Ryobi). This direct drill method is quick and cheap, and works well enough for onsie-twosie lot sizes for minimal time/money outlay. I've had the good results with etching 5 mil brass plates (K&S Metals from Ace Hardware) just like photo-etching a PCB. You can develop a real respect for the guys that cut plates for currency with these methods... I'd sure prefer to use a laser cutter, though."