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68 SMT Magazine • September 2014 by rachel miller-Short PhoTo STenCIl llC the ShOrt SCOOp ColuMn Improving Stencil printing results As a continuation of my July 2014 column, this month I am providing some possible an- swers to the frequently asked question, "Why am I getting poor printing results?" There are a myriad of causes of poor print performance. The problem may stem simply from an inferior or worn-out stencil, which is typically the first place people focus when troubleshooting. However, the issue may also be caused by an improper aperture design or stencil thickness. Additionally, poor print per- formance might not be caused by the stencil itself, but rather an improper printer set-up, a non-optimal squeegee blade, or the rheology of the solder paste being used during processing. To shed light on this month's topic, I have compiled a list of some of the problems our us- ers encounter and possible solutions.. To find the cause of the problem, it is help- ful to break the stencil printing process into two phases. The first is the fill phase, when the aperture is filled with solder paste. The second phase is the transfer process, during which the paste is released from the aperture and trans- ferred to the pad on the PCB. prObLem: Insufficient solder volume transfer Potential causes for insufficient solder trans- fer are often associated with: 1. Rough aperture walls that cause poor solder paste release, particularly as ap- erture sizes decrease This problem has become more prevalent as board densities have increased and component sizes have shrunk. It is one situation that is di- rectly related to the fabrication of the stencil. Different stencil fabrication techniques, such as laser cutting and electroforming, yield differ- ent leve ls of aperture roughness. Before select- ing the stencil, evaluate the type of layout and configuration of the board, the types of com- ponents you are working with, and the board application. Then find the type of stencil that will give the paste release to meet those needs. The easiest to find and least expensive is an off- the-shelf laser-cut stencil. However, for more stringent applications where components are close together and very small, you might have to get a chemetch, NiCut, or electroform sten- figure 1: Aperture wall comparisons.

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