Smart tooling for assembly of thin flexible systems

Abstract

Emerging electronic assemblies are demanding lower cost, lighter weight, miniaturized packages mounted on thin flexible circuit boards and/or flex circuits. However, the compliant nature of the flex substrates poses new processing technology challenges for standard surface mount assembly equipment. A particular challenge is fixture tooling. The flexible substrate experiences significant transverse displacements under perpendicular assembly and/or fixturing forces during solder paste printing and component placement processes. The transverse displacements result in misregistration of the component leads and substrate bond pads, leading to severe assembly process defects. The solder reflow process further complicates the issue due to the thermo-mechanical warpage induced. Conventional assembly equipment utilizes dedicated tooling designed to handle rigid circuit board assemblies. As electronic assemblies move toward very fine pitch surface mount packages, chip scale packages, and flip chip attachment assembled to thin flexible double-sided circuit boards, reengineered and specialized dedicated tooling for fixturing flexible substrates in standard assembly equipment are becoming extremely important. This paper focuses on developing analysis methodologies and theories for implementing machine dedicated Smart Tooling. The primary goals being to determine the impact of fixturing on assembly process quality and to determine optimum fixturing configurations for thin flexible circuit board assemblies based on circuit design 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. Fixturing configurations based on a perimeter support technique of flex substrates is analyzed to minimize transverse displacements.