STENCIL PRINTING OF SOLDER PASTE FOR FINE-PITCH SURFACE MOUNT ASSEMBLY

The increasing availability and use of fine pitch integrated circuit packaged places increased demands on the stencil printing process for solder paste deposition. To attain the resolution needed for high assembly yields, both materials and operating parameters must be optimized. The resultant deposits must be uniform and of the proper geometry to promote satisfactory device placement and reflow. This paper summarizes our findings in the following areas: solder paste-powder particle size distribution, rheology, slump, and solder ball formation: stencils-single versus dual thickness, brass or other materials, opening sizes, vendor capabilities; printed wiring boards-pad geometry, dimensional tolerance, pad surface finish; printing process-setup and alignment, squeegee materials, printing speed downward pressure; component placement-handling, placement accuracy, force limitations. In conclusion, some practical concerns including the current process operating window and future fineness of print limitations are discussed.