This is a “how to” set for the hand solder paste printing of PCBs for the purpose of making an electronics assembly prototype. The scope of these instructions is for the eventual assembly of a few PCB for a first article prototype or to see if a section of a design is working properly.
The Materials for This Process are as Follows:
1. PCB needing to be assembled
2. Matching fabricated metal foil only stencil
3. Tape for holding both the board holders as well as the stencil in place
4. Solder paste-typically type 3 or 4 solder paste. A “no clean” (requires no cleaning after assembly) solder paste chemistry and a non lead-bearing paste is recommended.
5. Isopropyl alcohol for cleaning the stencil post printing
6. Wipes for cleaning of the stencil post printing
7. Squeegee for rolling the solder paste through the apertures of the stencil
8. PCB board holders
Step 1: Inspect the PCB
Inspect the PCB to make sure you are starting with the right design. If there are a lot of finger marks on the PCB wipe the surface down with a lint free cloth using isopropyl alcohol. Make sure the stencil is correct and lines up with the PCB pads which need to be printed. Affix the board holder in place using tape (either a 2” wide Kapton or blue painters tape).
Step 2: Place the Board
Place the board in the upper corner of the holder.
Step 3: Place the Other Board Holder in the Opposite Corner
Place the other board holder in the opposite corner to the board. Use tape to affix the hold in position when it lines up. Allow for some room to place the boards in and out of the holders.
Step 4: Align the Metal Foil Stencil
Align the metal foil stencil over the board aligning the apertures to the pads of the board. Affix the foil with tape so it will not move during the squeegeeing process. Sometime I only affix the stencil at one edge so it can be a flap and move up/down to “unload” and “reload” boards in to the fixture.
Step 5: Squeegee the Paste
After bringing the solder paste up to room temperature use a plastic stir stick to mix the solder paste such that the flux and the rest of constituents do not separate. Scoop some the solder paste ion to the top of the stencil. Squeegee the paste using a moderate amount of force and having the squeegee at a 45 degree angle to the board. The squeegee should be overlapping the print area about 1” on either side. Make sure you “finish” the squeegee pressure after you have passed all of the apertures. Do not finish this squeegeeing operation before passing the last of the apertures.
This is what it will look like when you are finished.
Step 7: Lift Stencil Off of the PCB
Carefully lift stencil off of the PCB. The lifting pressure should be consistent so as to not “smear” the solder paste underneath the stencil.
Step 8: Inspect the Solder Paste
Inspect the solder paste that has been deposited onto the PCB. Make sure the solder “bricks” are as consistent as possible and that there is no smearing of the solder paste underneath the stencil. You are now ready to place components.
Step 9: Clean the Stencil
If you need to print and fabricate numerous boards you will probably have to clean underneath eh stencil to prevent subsequent “smearing” of the solder paste. Use a lint free cloth and isopropyl alcohol to clean the stencil.