March 16, 2014 in REFLOW SOLDERING
Hi, this post is about stencil usage.
Stencils are the best way of applying the correct quantity of solder paste to the pcb pads before using an oven to solder components. Other methods include syringes or paste dispensing pneumatic systems, the problem is that they are much more slower and the quantity of paste is difficult to adjust.
So what is a stencil, a Stencil is a laser perforated thin sheet of material (usually stainless steel but you can find then in plastics also). The holes in the stencil have the same fooprint as the pads of the pcb so to apply paste you just have to put it in one side of the stencil and them apply it with a squeegee, It is easier to see in the following photographs.
The advantages of the stencil is that in some seconds you apply solder paste to all the pads so you don’t have to pass many hours applying paste pad per pad with a syringe, and the quantity of paste is always the correct one. Using a stencil will reduce the quantity of shorts between pads and other usual problems when using reflow soldering.
The main disadvantage is the cost. Stencils are only valid for one particular PCB so you have to order one for each different PCB you want to solder, it is not a problem if you are going to solder 100 times the same PCB but it is too expensive for only 1-2 prototypes. Also the alignment of the PCB with the stencil is really critical so you would need to use some sort of holder for both the stencil and the PCB which fix both together and don’t leave any gap. These are called “stencil printers” and are quite expensive, normally around 400 to 2000 euros depending on the quality. You can see an example from eurocircuits below.
Personally I use a low cost solution which I find really good and less expensive. It is the eurocircuits EC-Stencil fix instead of a stencil printer. When you order a pcb and stencil in eurocircuits you can order them EC-registration compatible, that means that it will be supplied with exterior centering holes you can use to center it in the Ec-stencil fix, a board that helps to keep the stencil in place while applying the solder paste.
Now we use the centering pins for centering the PCB and the stencil.
I center my PCB using the botton pins:
I center the stencil over the PCB using the pins:
Next phase would be to add the solder paste:
Now in a single pass with the squeegee I add apply the solder paste to the pads:
The results as you can see in the photos are quite impressive for something as simple as this.
You can find lots of videos in Youtube about this ec-stencil, how to use it, fix it… etc.
This is not the only low cost stencil system in the market but it is the one I use and I am pretty happy with it. I have seen others worth trying like fixing stencils with magnets (http://www.beta-estore.com/rkuk/order_product_details.html?p=64) . If you have experience with other methods please contact me so we can share it with everybody.