Introduction: SMT 'Solder Paste Stencil' Production Jig Using a Laser Cutter

Surface-mount technology is prevalent in smaller circuit boards and often uses reflow soldering (solder paste & controlled heating) to affix the components. In this Instructable, I will share with you how I made a simple, efficient, and helpful jig for soldering SMT circuit boards in bulk.  I built this to speed up my production time for one of our products at work. I will first share my design of the jig and it's construction. Following that will be a step-by-step demonstration of how my SMD production jig works.

- Since I made this for a custom printed circuit board, the jig will obviously require modifications to be appropriate for the specific PCB that you are using. My primary purpose of this is to share the general idea and basic design of my jig.
- An essential part of the jig is the stencil. My stencil was cut out of Kapton(R) and shipped to us by an online service. Although, I imagine you could make your own stencil using a Laser Cutter on a solid sheet of 2-3 mil polyimide. I am investigating this presently.
- Another essential part of my SMT soldering process is our DIY reflow system. I'll leave that for a different Instructable but it is essentially a PID temperature controller hooked up to a 1550 Watt Black & Decker toaster oven.  There are also several other ways to reflow solder paste such as using an infrared lamp or a hot air gun/pencil.

Step 1: What You'll Need:

- Thin sheet of wood, ~2-3 sq. ft and 0.125"-0.25" thick. (The size and thickness is completely dependent on the size and the thickness of your PCB and the quantity of PCBs you want to produce per batch. I used a 14.5" x 13.5" sheet of wood that was 0.2" thick.)
- Graphic design software that can make vectors. (I used CorelDRAW at my local makerspace Maker Works, Ann Arbor. Inkscape is an open source alternative.)
- Laser cutter. (I used the 50 W Epilog Laser at Maker Works, Ann Arbor. Check online for your nearest makerspace.)
- Wood glue
- 4 small clamps
- 2 large hardcover books (or similar large flat object that can sandwich the jig when clamping)

Step 2: Measure Everything Precisely!

Grab a pad of paper and a pencil to record your measurements. 

Use a caliper to measure:
- Thickness of your wood (so you can find the correct laser power/speed/freq)
- PCB thickness (Thickness will be important since the stencil needs to lay completely flat over the jig & the PCBs)

Use a ruler (scale) to measure:
- Length & width of your wood (need to be able to fit your design)
- All dimensions of your PCB (Length & width & any contours will be important to fit and hold a PCB in place. 
- Lendth & width of the stencil

Next you need to take precise measurements so that the jig holds the holes of the stencil in perfect alignment with their respective PCBs' pads. If you made your own stencil, you could use your laser file from that to determine the jig dimensions easily. This also is probably possible (and definitely easier) if you have the Gerber files of your PCB.  I had to do it the hard way by measuring the stencil because we ordered it and did not have dimensions of the spacing between the PCB traces.

To do this, first determine a reference point. I measured:
- The left edge of the stencil to the left edge of the lowest and leftmost SMD pad
- The bottom edge of the stencil to the bottom edge of the same pad

Next, find the:
- The left edge of the PCB to the left edge of the lowest and leftmost SMD pad
- The bottom edge of the PCB to the bottom edge of the same pad
With the last four measurements and the PCB dimensions you recorded earlier, you can determine the placement of the lower and leftmost PCB outline. 

After this, determine where the rest of the PCB outlines will go by measure the spacing on the stencil between a pad on each PCB and the identical pad on the PCBs above or to the right of the first one. 

Step 3: Open Your Graphic Editing Software and Begin Your Design

As I mentioned, I used CorelDRAW to make this project. You can use any graphic editing software as long as it uses vectors (which are necessary to tell the Laser to cut instead of etch.

- Open your software and change the page dimensions to the length and width of the wood you are using. 
- Using the measurements you recorded in the last step, make a rectangle the exact size of the stencil. 

Step 4: Design - Create the Base of the Jig

- Make another rectangle that is slightly larger than the stencil-sized rectangle (approximately 0.5" in each direction did the job for me). This will be the base of the jig. Center this around the stencil-sized rectangle. 

Step 5: Design - Layout Your PCB Outline Traces

- Within the stencil-sized rectangle use your precise measurements from the last step to distribute the PCB outline traces, so that they will hold the PCBs with their pads in alignment to the stencil's holes.

I added the PCB pads in the file for measuring the spacing and alignment. These do not need to get etched into or cut out of the wood, so I added them to a layer that I turned off before printing. 

Step 6: Design - Create an Inner Rectangle

- Now create another concentric rectangle smaller than the stencil-sized one (I made this rectangle ~0.5" smaller in each direction). Align the center of all three. Make sure that this new rectangle does not overlap any of the PCB outlines, a little leeway makes it easier.

Step 7: Design - Quadruple Your Rectangles

- Next, copy and paste the set of the three concentric rectangles (do not include the PCB traces), so that you have four sets. You can space them from each other however you would prefer for now.

Step 8: Design - Remove Extra Rectangles

- On the first set of rectangles (the one with the PCB traces), delete just the stencil-sized rectangle (the middle one). Choose one set to be the base (doesn't matter which one) and delete both inner rectangles, leaving just the base-sized rectangle. On the third set, delete just the stencil-sized rectangle (middle one). One the fourth, leave it with all three concentric rectangles.

Step 9: Design - Add Tabs (optional)

-  Add tabs for ease of use. The tabs were added to the layers that are lifted off (so all three except the base). I first made a 1" diameter circle and aligned its midpoint with the midpoint of the short edge of the rectangle. Then selected both that circle and the rectangle and used the "Weld" tool to make them one.  This is a helpful tool that CorelDRAW has but there are certainly other ways and styles of tabs you can add, just make sure that your vectors are all closed so that there is no discontinuity in the cutting.

Step 10: Design - Add Text/logos/art (optional)

- Add text or logos to etch.

Now your file should look roughly like mine. I had to overlap the vectors to fit them on my sheet of wood, you may not have to do that.

- Double check all of your vectors that will define the lines for the laser to cut to make sure they are less than 0.24 pts (in CorelDRAW, you can just set it to "Hairline"). If the vectors are larger than 0.24pts they will etch (raster) instead of cutting (vectoring).

Step 11: Fire the Laser!

- Turn on your Laser Cutter. Turn on the proper ventilation.

- Place your wood on the laser's table and set the proper home position. The Epilog 50W Laser I used is set up so the home position indicates the upper left corner of your document. So, appropriately, I set the home to the upper, left corner of my sheet of wood.

- Choose the proper settings to raster (etch) and vector (cut) your jig.  Recommendations can be found on the internet and will depend on the laser you are using and your material and its thickness. 

Setting I used: 
Raster: speed 100, power 70
Vector: speed 10, power 100, frequency 2500 Hz

- Print to the laser. Run the job on the laser and monitor to make sure nothing goes haywire. 

Step 12: Assembling and Gluing the Parts

I unfortunately did not take pictures during the construction, so hopefully you can understand the layers from these photos of the finished product. 

Be careful not to over-apply the glue, since we will be gluing and clamping it all at once. You do not want to accidentally glue together parts that should be able to be removable.

1) Glue the small rectangle with the PCB outline holes to the base. To keep this center, place the base-sized rectangle it was cut from around it.  Align this with the base layer.

2) Apply glue, then place the thin frame (inner dimensions = stencil size, outer dimensions = base size). Align with the base. 

3) Apply glue, then place the smaller thin frame (OD = stencil size) with glue facing up.

4) Place the final wider frame on top. 

5) Sandwich all four layers between two larger pieces of wood and clamp at each of the four corners.  This could also be done with any larger flat objects such as a table & heavy books. Wait until dry as per the instructions on glue container.

Step 13: Sand the Jig to Be Flush With the PCBs

The stencil should sit perfectly flat from above the PCBs to above the wood that holds them in place.  This is crucial so the solder paste can be applied in a smooth stencil-thick layer. 

So place your PCBs in their holes. If they are too low, too much solder paste could be applied.  If they are too high, the stencil will take a lot more wear and tear.  

To level them you can either 1) sand the wood to even with the PCB surface or 2) add a riser in the hole below the PCB.

Step 14: How It Works:

Here is how my jig functions:
Step 1: Place one face-up RoboRoach PCB in each of the 8 holes and then place the tray bottom.

Step 2: Place the stencil into the tray.

Step 3: Place the tray top.

Step 4: Apply a line of solder paste adjacent to the left side of each trace. (the photo below doesn't quite represent how much solder paste to use. I only had a syringe at the time, I would highly recommend jar paste for this process)

Step 5: Using an id (or some type of plastic card with a straight edge), squeegee the solder paste over the holes, making sure to cover them all evenly.

Step 6: Lift the tray (both tray bottom and tray top) up and off.

Step 7: Populate the PCB with its components.

Step 8: Reflow using modified toaster oven.

Ta Da!