Understencil cleaning really is a process within a process (printing) and merits appropriate attention. I've written a lot about the elements of cleaning: tools that help improve cleaning speed, proper chemistry to ensure good solder paste removal and optimized software controls, among others. The other critical cleaning input is the fabric, or the understencil cleaning roll, and that is the emphasis of this month's column. For some perspective, one must understand the evolution of understencil fabrics. Historically, these cleaning materials were literally paper; they were pulp-based materials made from wood fiber. For early SMT processes, understencil paper was adequate and low cost. In fact, many manufacturers still employ understencil paper rolls today. There are, however, several challenges with pulp-based products when used in current SMT printing processes. First, the papers generally have poor wicking capability, which means the solvent will not flood onto them evenly, leaving areas of the stencil without solvent and proper cleaning action. Second, these products lack porosity so that the vacuum cannot deliver a good flow of air through them. Generally speaking, paper-based understencil rolls are not fantastic cleaning materials.
展开▼
