Keeping Count: Accurately Tracking Stencil Life

摘要

OUTSIDE OF SHEER printing machine capability, the stencil is arguably the next most important element of the printing process. Stencil material, thickness, aperture integrity, sidewall smoothness (or lack thereof), and tension all play a role in the quality of the solder paste deposit. And, like all consumables, metal stencils have a lifetime: They do not last forever. Unless a stencil is damaged, tension loss is the factor that most often determines when a stencil has run its course. A properly tensioned stencil enables a good, solid release of the paste deposits onto the board. Alternatively, a stencil that has lost tension and has begun to "sag" may result in defects such as "dog ears"1, bridges, or insufficient paste on pad, to name a few. Today, stencil tension is more important than ever. Historically, when stencil thicknesses averaged 200μm, one was far more likely to retire a stencil from damage than from wear. Now, however, with the exceptionally thin 60pm foils required for miniaturized designs, tension loss can occur sooner, as repeated stencil pressure during the print stroke eventually reduces stencil elasticity. As has been addressed in this column, there is a proven correlation to changing tension and the output of the printing process.2