Smart tooling for assembly of thin flexible systems

Emerging electronic assemblies demand lower cost, lightweight, miniaturized packages mounted on thin flexible circuit board and/or flex circuits. However, the flexible nature of these substrates poses new process challenges for standard SMT assembly equipment. One particular challenge is fixture tooling. The flex substrate suffers significant transverse displacements under perpendicular assembly/fixturing forces during solder paste printing and component placement. This displacement results in component lead and substrate bond pad mis-registration, leading to assembly process defects. Solder reflow further complicates the issue due to the thermo-mechanical warpage induced. Standard assembly equipment uses dedicated tooling designed to handle rigid PCB assemblies. As electronic assemblies move toward very fine pitch SMT packages, CSPs, and flip chip attach assembled to thin double-sided flex PCBs, reengineered and specialized dedicated tooling for fixturing flex substrates in standard assembly equipment are becoming very important. This paper focuses on developing analysis methodologies and theories for implementing smart tooling, the primary goals being to determine the impact of fixturing on assembly process quality and to determine optimum fixturing configurations for thin flex circuit boards based on circuit CAD data. A mathematical model to describe both transverse and perpendicular displacements of flex substrates is developed, and its closed-form solution for transverse displacements is obtained. Use of a near-optimum fixturing configuration to minimize transverse displacements is verified