Stencil design

As with most processes, numerous variables affect the final product. For solder paste deposition using stencils, Chouta and Heck identified 39 variables relating to materials, 32 variables to processes and equipment, and 10 variables to personnel and environment. Among variables critical for the successful stencil dispensing of adhesives are stencil design, the adhesive material and its rheological properties, and the printing process. 

Coleman, W.E., Stencil Design and Application for SMD, Through-Hole, BCA, and Flip Chips, Advancing Microelectronics, January/February 1996. 

Coleman W.E., Jean, D., and Bradbury-Bennett, J.R., Stencil design for mixed technology through-hole/SMT placement and reflow. Soldering and Surface Mount Technology, Vol. 12, No. 3, 8-12, 2000. 

Stencil designs that preclude solder paste from entering the PTH also result in a significant reduction of solder paste on the top surface and in the PTH. These reductions may make the pin-in-paste technique impractical for some boards. If the solder volume is too low, the available solder distributes unevenly around the PTH pin and the PTH barrel, resulting in voids... 

IPC-7351 Generic Requirements for Surface Mount Design and Land Pattern Standard IPC-7525 Guidelines for Stencil Design... 

With lead-free alloys, wetting generally is less than that of Sn/Pb systems. This may leave exposed comers or edges on the pads. If full coverage is required, then a change in stencil design is needed so that the stencil aperture covers 100% of the pad. 

In the solder paste printing process, defects typically are caused by poor alignment between the substrate and stencil, incorrect material selection (substrate, paste type and stencil design), or variations in the amount of paste deposited. Defect elimination relies on the engineer and operator to address these variables and monitor the process. 

FIG. 19 Land and stencil design patterns used for 0603 chip bridging and mechanical joint strength testing. (Courtesy of Panasonic.)... 

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