One of the causes of shorts in PCB is solder paste. It spreads over a large board surface area and forms bridges with ease. The solder paste is not the major shorting reason. During every manufacturing stage, defects have easy access, such as the solder paste stencil, during the actual solder application and during reflow when the board goes into the oven.
Soldier Paste Instructions for a Neat PCB Layout
Your stencil is a major source of problems in applying solder paste. The stencils consist of masks fashioned from metal, and in some instances made from Kapton or Polyimide for lower quantity PCBs. The stencil pinpoints where you should apply the paste and it is reusable. A polyimide goes for less than five runs while a metal stencil can go for thousands of runs.
A solder paste stencil also has shortage problems
You may assume that a solder paste stencil design has no problems. Problems can interfere with your board in two major ways:
Too much Paste
Pads with excess solder paste are more likely to form bridges across adjacent pads. With excess paste application on a continual basis, across one board or various boards, you must decrease the aperture measurements or the thickness of your stencil.
Resolution of the stencil
Poor stencil resolution and uneven edges are also a problem. Dirty stencil edges are likely to cause too much application, lack of evenness in application or poor definition around joint ends. These situations are possible causes of solder bridges causing pad shorts.
You may end up with paste in places you did not intend it to be. The final board product is how you handle the materials after application. The temperature of the solder after reflow needs to be right for the best results.
Smears and Slumps
Another issue though not as common is solder application that slumps or smears on the board surface. A smear makes your edges untidy even if your stencil is sharp. Smearing also involves solder application in the wrong areas. This isn’t always a problem, but it makes the joint weak and easily shorts.
The temperature for solder paste reflow is not right; the paste might not melt properly. This is a problem when you need the paste to bond with the components. If too hot, the solder paste ends up flowing further away from the pads it should.
The best method to rectify solder paste shortages remains to check on the quality of the stencil and reflow much earlier in the production process. This prevents you from redoing the surface areas when you use too much solder. It also removes the likelihood of damage and surface and joint weakening. You may reheat and scrape off the excess paste, but this may also cause damage to adjacent joints and surfaces.
You should make sure you make the most of your equipment by keeping to the exact specifications to avoid damage and excesses.