Reliability study and failure analysis of fine pitch solder-bumped flip chip on low-cost flexible substrate without using stiffener

In flip chip on flex (FCOF) technology, the flexible substrate and the assembly process are essential for the reliability of solder joints. In this research, fine-pitch solder-bumped chips with various chip sizes were assembled on flexible substrates using FCOF technology. The five types of layout design were evaluated for their effects on the assembly process and reliability of FCOF technology. It was found that appropriate substrate layout designs provided more accurate control of the wetting area because they minimized the misalignment between the copper pad and the solder mask opening. The concept of wetting area error ratio is introduced to evaluate the variation of the wetting area on the various layout designs. The shapes of solder joints formed are also related to the flex designs. Die shear test was performed for the samples of FCOF without underfill materials to study the adhesion strength of the copper pad, the solder mask and the base layer. The failed samples after thermal cycling and HAST were analyzed using SAM, an X-ray imaging system, cross-sectioning, and scanning electron microscopy (SEM).