Stencil Printing Process Guidelines for 0.3mm Pitch Chip Scale Packages

With the form factor of electronic assemblies continuing to shrink, designers are being forced towards smaller, more complex components with decreasing interconnection pitches. As a consequence, the Surface Mount assembly process is becoming increasingly challenged. For the stencil printing process, todays accepted stencil area ratio rules, (which dictate what can or cannot be printed), need to be significantly pushed to extend the printing process for next generation ultra -fine pitch components. With aperture geometries shrinking, anything which can influence solder paste transfer efficiency has to be considered. New process technologies such as ultrasonic squeegees have emerged in recent years to assist the process with some degree of success. However, something which is often overlooked in terms of stencil design influence is that a square shaped aperture, size for size, has a volume which is 21.5% than its circular counterpart. In a process where quite literally every solder particle that can be printed is becoming significant then this fact can be utilized to the process engineer's advantage. In this paper, the merits of stencil aperture shape, in conjunction with ultrasonic squeegees are investigated with the purpose of developing stencil printing guidelines for ultra-fine pitch components such as 0.3mm pitch CSP's.