SMTA International Conference Proceedings

Reliability Issues Caused by Bending HDI Flexible Circuits

Authors: John Dzarnoski and Kexia Sun
Company: Starkey Laboratories
Date Published: 10/24/2010   Conference: SMTA International

Abstract: There has been enormous worldwide effort to increase the volumetric efficiency of electronic packaging. Much of this effort has been driven by the telecommunications industry that has succeeded in reducing cell phone size while simultaneously increasing functionality. The hearing aid business has always had the need to use extremely small electronic packaging because hearing aids pack electronics into the ear canal. In recent years hearing aid microelectronic packaging has moved from ceramic hybrid based packaging to a flexible circuit based technology. Due to the size and shape of all types of hearing aids, flexible circuits need to be folding and bent to fit inside hearing aid cases. In order to reduce space, bends and folds are pushing conventional wisdom on the limits of mechanical integrity.

There is little published information regarding bendability of multilayer HDI flexible circuits for medical device applications. This paper will examine bending and folding effects on a Polyimide/Adhesive/Copper/Polyimide/Copper/Adhesive/ Polyimide structural stack-up on test coupons and a HDI flexible circuit intended for production use. A forming tool that controls all aspects of the bend process will be discussed. A variety of factors were investigated including bend angle, thickness of the circuit, bend radius, foil weight, copper materials, construction, and frequency of bending. Circuit electrical resistance, substrate microstructure and crack initiation were monitored and will be examined. Failure mechanisms will be discussed as well as their impact on design.

Key Words: 

Folding, Reliability, Bending and Radius

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