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Fine pitch/fine feature solder paste printing in PCB assembly has become increasingly difficult as board geometries have become ever more compact. The printing process itself, traditionally the source of 70% of all assembly defects, finds its process window narrowing. The technology of metal blade squeegees, with the aid of new materials, understanding, and settings such as blade angle, has kept pace with all but the smallest applications.
Enclosed media print head technology has existed, and has been under increasing development, as an alternative to metal squeegee blade printing. Until recently, the performance of enclosed print heads had been comparable to the very best metal squeegees, but advances in enclosed print media technology have now made it a superior alternative to squeegee blades in virtually all applications.
Solder paste printing through stencils has long been achieved using metal blades, or squeegees, which replaced polymer squeegee blades due to performance issues years ago. The move from screens to stencils, and then to smaller apertures and fine pitch land patterns, necessitated the change to metal, which offered superior printing performance characteristics.
The evolution of PCBs in terms of the miniaturization of assemblies, components, and ever-finer feature print patterns has not slowed, and as a result continues to present ever-increasing challenges to the makers of assembly equipment and solder paste printing technology, narrowing the process window. Fine pitch and fine feature printing applications, e.g., 200μ–.50 area ratio (AR) and 150μ–.375 AR, have been pushing blade printing technology to its limits.
Skilled printer operators using superior metal blade alloys, varying angle of attack, etc., can achieve acceptable results against the most challenging printing applications, but there are shortcomings that include considerable material waste, which translates into significant cost, less fill volume as apertures become smaller, and unacceptable variation in the consistency of results. Newly developed enclosed print media technologies have been developed as an alternative to metal blade printing. Due to recent advances, enclosed media printing delivers better results overall when measured against ordinary- use metal blades, and they excel in material savings since the print head encloses the solder paste or other media from the surrounding atmosphere, providing steady and uniform results for fine feature apertures filling due to tight process control. For these and other reasons that will be outlined in this paper, they constitute an attractive alternative to metal blade printing.
Editor's Note: This article originally appeared in the May 2015 issue of SMT Magazine.
