First of all, let’s take a look at the video clip below (link) to find out how it is possible to reflow solder components without stencils.
I talked about DIY your own reflow oven in my previous article. People may wonder that only a functioning reflow oven won’t do the job as you must have stencil to dispense solder paste. Yes, stencils are usually considered as key piece of equipments for reflow soldering process. The purpose of stencil is to restrain the solder paste within the specific pad area. The thickness of stencil also decides the amount of the paste to apply for soldering. Stencil, however, is non-trivial to manufacture. It usually requires special material and laser-cutting. That’s why stencil is quite costly, sometimes even more expensive than your PCB board.
Figure 1. Left: Solder paste applied with stencil; Right: Solder paste applied manually (without stencil)
The difference is quite obvious: you get a clean and neat solder paste dispensing with the help of stencil. On the contrary, the PCB on the right side looks messy as the paste is simply dropped on to the pads without a stencil. Messiness might be acceptable considering this is not your final product. The “bridge” where paste (usually applied too much) connects to its neighbors across different pads seems to be a problem because it might cause short-circuit.
Figure 2. The sketch illustrates how paste is bridging across two pads.
With the help of “surface tension”, the PCB on the right side of figure 1, however, is still a useful board for reflow process. As we know when solder paste is heated to certain temperature, it melts and becomes “liquid”, which will flow in compliance with surface tension. The molten solder that was placed in between pads (also called solder mask) will be pulled away by its neighboring paste that was placed right on the pads. Further more, surface tension also helps the alignment of components. If a component is not accurately positioned over its footprint, the surface tension will help “move” it into place when the solder paste melts. The video above contains a clip towards the end shows how small SMT LEDs are “pushed” and “rotated” into place when the solder paste melts.
So, as long as you have a reflow oven, chances are that you can start soldering your small footprint ICs without stencils or automatic mounting machines!