In the process of solder paste printing for a Printed Circuit Board, the main objective is to consistently deposit the proper volume of solder paste in the right location. Over time, it has become apparent that manufacturing as well as design variables are the cause of nearly all printing defects and it has become critical to stop these printing defects. In the past, the self correcting nature of tin or lead solder during re-flow helped eradicate some of the printing shortcomings. Laser Cut Stencils and electro formed stencils are the two per-dominant fabrication stencil types. However, stencil design changes have helped greatly in correcting some of the problems in printing.
The way in which the aperture is made controls both the smoothness and accuracy of the aperture walls and it is therefore important to get this right in order to ensure no defects occur. When making stencils, it is important to note that there are two main processes by which stenciling can be done and these are; laser cutting and the additive process.
When using Laser Cut Stencils, laser cutting is a subtractive process in which the apertures being made are tapered slightly. The Laser Cut Stencils are produced directly from Gerber data in order to ensure that aperture accuracy is achieved. The data can then be adjusted to change the dimensions if necessary or when required.
Another benefit of using stencils that have been laser cut is that the walls can be tapered. A tapered aperture is extremely desirable. These tapered apertures usually have an opening slightly larger on the board side than on the squeegee side as this is the ideal size desirable for easier pate release.
Laser Cut Stencils are also suitable for ultra fine pitch component printing. Stencils that are laser cut also produce ragged edges due to the fact that the vaporized metal is transformed into metal slag during the cutting process. This can unfortunately cause paste clogging but this can be corrected by using micro-etching to smooth out the walls.
It is, however, important to note that Laser Cut stencils cannot make stepped multilevel stencils without pre-chemical etching of the areas that need to be thinner.
