Fabrication and Assembly of Ultra Thin Flexible Active Printed Circuits

Abstract

Tremendous attention has been focused on flexible printed circuits for their light weight, minimized dimension and three dimensional packaging capabilities. However, the traditional flexible circuit fabrication and assembly process can not meet the demands for the continuous reducing of flexible circuit dimensions, while increasing the packaging density.

In this study, an ultra thin polyimide and liquid crystal polymer (LCP) flexible printed circuit fabrication process was developed. The circuits had 8 mil line pitch pads and 3 – 4 mil diameter vias, which is comparable to flexible substrates for high density interconnection applications.

Thinned silicon flip chip were then assembled on the polyimide and LCP flexible printed circuit with a backside assembly scheme, which not only lowered the assembly profile but also eliminated the need for a solder mask. With the development of an immersion flip chip solder bumping process, the overall assembly profile was lowered further and an hourglass shaped solder joint was obtained which has potential to improve the thermomechanical reliability.