William E. Coleman, Ph.D.
Photo Stencil, Colorado Springs, CO, USA
SMT Assembly is going through a challenging phase with the introduction of miniature components such as uBGA's, .3mm CSP's and 01005 passives in to the assembly process. Example assemblies are cell phones and other hand held devices driven by consumer demand for smaller devices with increased functionality.
Printing these miniature devices along with more conventional SMT devices like .5mm QFP's and 0603 and 0805 passives is a challenge. Whereas a 4mil (100 micron) or 5 mil (125 micron) thick stencil provides good paste transfer for the normal SMT devices, stencils with this thickness have very low Area Ratios for the miniature devices. For example a .3mm CSP with a 7.5 mil (190 micron) has a .47 Area Ratio for a 4 mil thick stencil.
This paper is divided into two parts outlining two different approaches to resolve this issue. Part 1 deals with a Two Print Step Stencil Process where small apertures are printed with a thin stencil thus providing acceptable Area Ratios. A second thicker stencil is used for normal SMT devices, RF Shields, and SMT connectors. This stencil has relief pockets formed on the board side of the stencils anywhere paste was printed with the first stencil.
Part 2 deals with different stencil types (technologies) and different aperture wall coatings. Stencil technologies include Laser and Electroform; Aperture Wall coatings include PTFE coatings. Aperture Wall pictures and paste print tests compare the performance of these different configurations and how they influence paste transfer for miniature devices with Area Ratios less than the standard recommended lower limit of .5. In the second case a step stencil is not necessary.
Dr. Coleman is a wealth of knowledge, that he so freely shares with the SMT community. He is to be applauded.
