This article describes the process I used for applying solder paste using a stencil without the usual stencil printer.

 

 

When I get my boards from the fab house they come in panel sizes approx 8" x10".  They are v-scored have a 10mm tab around the outside of the panel. This is so the Yamaha YM-84 pick and place  machine does not interfere with components near the edge of the board. Also, in each corner of the panel is a 3mm hole that is used by locating pins on the machine to hold the board.

For paste application, I use stainless steel laser cut stencils. In the past I've used  brass stencils.  Previously to that I had a Mylar stencil made by these guys http://www.smtstencil.com/ They are cheaper but not as good as the brass ones which are not as good as the stainless. The brass are etched, whereas the stainless one are laser cut. The stencil thickness I've been using is 6mils.

The Key to my process is to get the stencil maker to include two of the 3mm  panel mounting holes in the stencil when made. If you have single boards not in a panel, you can always use the boards mounting holes instead. Most boards have these, so you just need to get a set of matching dowel pins to suit.

 

I then use  a piece of 20mm thick MDF as a base plate. Then using my CNC mill, drilled 2 x 2.6mm (tad under) holes to match the 2 holes on the top edge of the panel. I  then press 2 steel dowel pins into these holes. The extra holes are for other boards that use the same baseplate.

 

 

 

 

 

 

 

 

 

 

 

The PCB panel is placed onto the pins. The stencil is placed over the PCB panel and onto the pins. You can see the 2 pins on the left hand side.

 

 

 

 

 

 

 

 

 

 

 

 

 

This image shows the PCB panel in place. The other PCBs around the panel are to create a level surface to place the stencil on.

 

In the background you can see the Paste, gloves and plastic spatula.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

In this image you can see the 6mil brass stencil mounted onto the hardened dowel pins

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The stencil is perfectly aligned with the PCB. It is quite important to get the height of the pins right. They need to be around 1 mm above the stencil, as it is all a firm fit.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

This image shows the paste applied to the stencil.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

I then apply the paste with a plastic bog spatula. I do it in 2 passes. The first pass, the spatula is held at 30 -45 degrees so that a layer of paste covers the stencil holes. Sorry for the poor quality image, but you get the idea.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The 2nd pass, the spatula is held at 80 - 90 degrees to the panel. This pass removes all the excess paste. As you can see, there is very little paste left on the stencil.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The stencil is then lifted off the PCB from the non pinned edge. This is why it is important to get the pins at the right height. If they are too high the stencil gets jammed on the pins.

The PCB is lifted off the pins by slipping a spatula under the board near a pin to lift it off.

 

 

 

 

 

 

 

 

 

With this method I can apply paste to a board in under1 minute, basically as fast as a manual paste printer.  Initially I used 4 pin to locate the board and stencil but it made the PCB very difficult top remove off the 4  pins. I then found the 2 pin method to be as accurate.

As you can see in the image below, this process gives excellent results. The finest pitch components I have used this method on is  0.71mm SIOC chips. I see no reason why it could not work with 0.5mm pitch components as well. I'll be trying this in the near future.

I hope that this may help others in applying paste to their boards.