Printed circuit boards typically have a pre-finished copper weight of 0.5 oz, 1.0 oz, or 2.0 oz on the outer layers. The PCB manufacturing process consists of etching the copper to produce the circuitry (traces, SMT pads, via pads, etc.). The copper etching process will produce a trapezoidal cross section (see figure below) where the top of the SMT pad will be smaller than the bottom.
In most cases, the bottom of the SMT pad will match the size in the electronic PCB files since the bottom dimension is critical in meeting impedance requirements for the circuitry. The top of the SMT will be smaller because of the copper etching process (the size reductions can vary between PCB suppliers, but the reductions shown in figure 3 are typical). The smaller top size must be used in determining the stencil surface area since the adhesion of the solder paste to the SMT pad is what pulls the solder paste from the stencil. As previously mentioned, the size of the SMT pad on the PCB determines the adhesion strength between the solder paste and SMT pad.
The most accurate method of determining potential stencil print performance is a modified surface area ratio formula that compares the surface area of the SMT pad (at the top and the surface area of the stencil aperture walls (see
figure A below).
This comparison takes into account changes in the PCB SMT pad sizes, based on copper weight and surface finish. Heavier copper weights will have a larger size difference between the top and bottom of the SMT pads. ENIG, OSP, immersion Ag, and immersion Sn surface finishes are flat and allow the solder paste to stick to the entire top surface of the SMT pad. A HASL surface finish has more of a domed finish and the semi-rounded surface makes it more difficult for the solder paste to stick to the entire surface. This will reduce the adhesion strength between the solder paste and SMT pad. Heavier copper boards, and those with HASL surface finishes, will present the most print challenges when miniature components are present. Stencil aperture sizes have been increased many times with the expectation that a larger stencil aperture size increases the surface area ratio. In many of those cases, solder paste release was not improved and this is due to the fact that the size of the SMT pad, and the corresponding adhesion strength between the solder paste and SMT pad, did not change.
However, there are times when increasing the stencil aperture size can help. As long as the modified surface area ratio passes, the benefit of a larger stencil aperture size is a wider alignment tolerance between the stencil and PCB at the printer. The wider alignment tolerance increases the process window at the printer and can increase assembly yields.
While the standard aspect ratio and surface area ratio calculations have been a very good determination of stencil print performance, they do not consider the printed circuit board. Components are getting small enough now that the printed circuit board, its copper weight and surface finish, has to be an important part of determining stencil print performance.