Optimal SMT Electronics Assembly Guidelines for Stencil Printing

摘要

The continuing miniaturization of personal electronics devices, such as mobile phones, personal music devices, and personal computing devices has driven the need for increasingly smaller active and passive electrical components. Not too long ago, 0402 (40 × 20mils) passives were seen as the ultimate in miniaturization, but recently 0201 and now 01005 passives have arrived, with predictions of even smaller sizes to come. For active components, the 30mil CSP (a chip-scale package with the solder balls on 30mil/0.75mm centers) has virtually become a requirement for enabling the many features required in modern portable electronics devices. This miniaturization trend, occurring simultaneously with the conversion to RoHS-compliant lead-free assembly, has put a considerable strain on the electronics assembly industry. This paper will discuss guidelines to optimize the electronics assembly process focusing primarily on stencil printing. With a smaller process window, each variable in the process contributes to the overall success of the assembly. Simple contributions such as storage and handling of solder paste can make or break a printing process. Many solder defects (some say 60-70%) can be attributed to the printing process. Therefore, stencil printing set-up is discussed, as well as solder paste measurement metrics, to determine the potential for success and assure a reliable solder joint.