Development of rigid-flex and multilayer flex for electronic packaging

Abstract

Recent development work on flex joining using different pre-pregs is highlighted, particularly with respect to their integration in laminate chip carrier substrates, and the reliability of the joints formed between the rigid and flex surfaces. A variety of rigid-flex structures were fabricated, with 1 to 3 flex layers laminated into printed wiring board substrates. Photographs and optical microscopy were used to investigate the joining, bending, and failure mechanism. Flexibility decreased with increasing number of metal layers. The flexibility of the various flexes was characterized by roll diameter and bend angle. Flex substrates exhibited roll diameter with polyimide dielectric as low as 180 mils for 2 metal layers, and as high as 1300 mils for 6 metal layers. Similarly, bending for 12 metal layers flex with thin and thick dielectric were <1 inch and >1 inch, respectively. Reliability of the rigid-flex was ascertained by IR-reflow, thermal cycling, pressure cooker test (PCT), and solder shock. There was no delamination for Resin coated copper (rigid)-polyimide (flex) samples after IR-reflow, PCT, and solder shock. The paper also describes a novel approach for the fabrication of flexible electronics on PDMS substrates. It was found that with increasing thickness, the flexibility of the polydimethylsiloxane (PDMS) based substrate decreased less due to stretching property of PDMS. The present process evaluates the fabrication of PDMS substrates using different circuit lines and spaces.