SMT007 Magazine

SMT-Sept2014

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72 SMT Magazine • September 2014 Potential Cures • Improve stencil and/or stencil design • Improve blade wiping • Adjust pick-and-place pressure The problems described so far relate to is- sues with the stencil and the equipment, but not the process. If you believe the problems are associated with the process itself, here are some general guidelines you can follow: Squeegee pressure or printing Speed • The pressure on the squeegee blade should be adjusted to the smallest pressure you can have and still wipe the stencil surface clean dur- ing printing. While you may see a layer of flux on the top of the stencil after the blade passes over, you should not see solder paste. If you see streaking, take a look at the blade to make sure it is not damaged or worn, and think about re- placing it rather than turning up the pressure. • The printing speed should usually be ad- justed to between ½ and six inches per second. The print speed must be set so the solder paste rolls perfectly on top of the stencil. Using too high of a speed will cause insufficient fill of the stencil apertures resulting in substandard print quality. Summary If you are not getting the results you want, try to determine if the problem is occurring when you apply the paste or when the paste is released. Make sure your stencil is an optimal one for your application and is designed with the apertures and the area ratios that you need. Check the equipment and the board support, the squeegee blades, the printing process, and don't forget to consider the solder paste you're using. Smt figure 3: Adjust your print pressure down to benefit from improved stencil life. rachel Short is vice president of sales and marketing at Photo Stencil llC. To read past col- umns or to contact the author, click here. figure 4: BGA/lCC/Qfn/QfP package repair tooling can be used when spacing is limited between components already on the board. The ShorT SCooP ImprOvING SteNCIL prINtING reSULtS continues

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