An overview of how to apply solder paste and reduce rework and post-assembly problems
Solder paste placement is probably the most critical stage of the PCB assembly process, and most assembly problems can be traced back to poor paste placement. If you want to avoid time-consuming manual rework on your assembled boards, it's essential that you have a clean, precisely positioned layer of solder paste. A few extra minutes ensuring that your paste layer is correct can easily save hours of rework with a hot air gun and a soldering iron later. Thankfully, there's nothing magic about it.
What you'll need
- A laser-cut stencil (preferrably metal)
- Fresh solder paste (brought to room-temperature)
- A stencil holder (or some extra PCB of the same thickness to hold the stencil and PCB in place)
- A thin metallic blade to 'wipe' the solder paste across the stencil and onto the PCB pads
- Optional: Some IPA (Isopropanol) wipes to clean the stencil afterwards or clean any botched PCBs
The most important item when applying solder paste with a stencil is obviously the stencil itself, which range in price from under 50€ for plastic laser-cut stencils (mylar, etc.), to 75-100€ for unframed metal stencils (which we will be using in this tutorial), to 200€+ for a framed metal stencil (assuming you have a stencil holder that is capable of using them).
Metal or Plastic Stencils
While plastic stencils are economical, they have a limited life expectancy and aren't nearly as sturdy as a metal stencil. They're fine for instances where you will only be making a few prototypes, but it's worth spending a little bit extra for a metal stencil that will last for hundreds of boards if you think you may use the stencil more than once or twice.
Metal stencils are available from a variety of manufacturers. Some PCB manufacturers like PCB Pool (available in both the US and Europe) give you the option of adding a free stainless steel, laser-cut stencil to your order, but generally you can purchase flat, non-framed stencils for well under 100€. (Note: We have no association with PCB Pool, but we do sometimes use them for PCB prototypes and for quick, unframed laser stencils, and are always happy to point people towards services that have worked well for us.)
Before starting, take your solder paste out of the fridge. It needs to be kept in a cool place to maximise shelf life, but let it get to room temperature before using it (about 30 minutes should do). It will spread much more naturally if it isn't 5°C.
Setup a small work area, ideally with some newpaper or something underneath to keep the solder paste from making a mess anywhere since it can be hard to wipe off of fabric, etc. Ideally, the area should be well lit, and you may want to have a small magnifying glass on hand to inspect the pads when trying to align the stencil.
Step One: Fasten the PCB in Place
The PCB needs to remain firmly in place, since any left-to-right movement, etc., will almost certainly result in poor paste placement. In the photo below we are using a relatively inexpensive stencil holder where the metal stencil is attached to a hinge using rare-earth magnets. The PCB is held in place with magnetic strips that are cut to size (the white thing in the upper-left-hand corner of the PCB).
If you don't have a stencil holder, though, you can also use a handful of old PCBs of the same thickness arranged 'around' your target PCB, taping them firmly to a surface.
Step Two: Line up the Stencil
This is the most critical stage. You need to make sure that the stencil is perfectly aligned on top of the PCB, and that each pad is fully visible in every hole. It may take some trial-and-error to get the hang of this, but make sure that the stencil is perfectly aligned before proceeding or you'll guarantee yourself a lot more lost time later when the boards come out of the reflow oven.
Step Three: Apply some Solder Paste to the Stencil
Add a strip of room-temperature solder paste at the bottom edge of the board, as seen below.
Step Four: Apply the Paste with a Metal Wiper
Now you can take a metal wiper, and slowly drag it across the surface to fill all the holes with solder paste. The way that you do this is highly personal, and you may find different methods that work better for you (different types of blades, different speeds or motions, etc.). Personally, I'm in the habit of making slow right-to-left motions making sure every pad is filled. This allows me to carefully go back a bit if a pad isn't properly filled (which is the case towards the end of the video below), though that's something you want to avoid if possible. One nice, clean swipe it definately what you're aiming for and a second pass will almost certainly cause more problems. If you mess something up, you'll be better off wiping the board clean and starting over again.
Step Five: Inspect the Pads
Once a good paste layer has been applied, you can carefully lift the stencil up and inspect the pads. Particularly on the fine-pitch parts (QFN, QFN, etc.) you want to make sure that the paste on the individual pads isn't touching any neighbouring pads, since this can lead to bridges. You also need to make sure that the paste is position on the metallics pads, and not on the solder mask (the white part of the PCBs in this case).
Here is an example of good paste placement. The paste has nice, clean edges meaning that there is almost no risk of bridges forming during reflow, and the paste itself is perfectly centered on the pads, meaning that the stencil was properly positioned and remained firmly in place while the paste was being applied:
Step Six: Clean the Stencil and Repeat
If you are making more than one board, it's important to clean the stencil between each use. This is time consuming, but every time you use a stencil you leave some residue inside the stencil holes which can result in poor paste placement on the next board. An extra 30 seconds cleaning the stencils with an IPA wipe can save 30 minutes later in the assembly process.
IMPORTANT: When cleaning the stencils with IPA wipes (or cleaning PCBs if you wish to remove the paste layer and try again), be sure to wipe them dry afterwards with a lint-free cloth. If there is any wet residue left on the board or the stencil when you apply a new layer of solder paste, the liquid from the IPA wipes will cause the flux in the solder paste to thin, resulting in poor paste placement.