Thermo-mechanical behavior of elastomer for CSP and reliability

Reliability and work-ability are the more important issues in the field of chip size package (CSP). As a solution of these issues, mu BGA((R)), a package where a compliant layer is introduced, was proposed by Tessera Inc. The compliant layer, i.e. elastomer, enhances solder joint reliability significantly because the elastomer layer can relieve the strain induced by the coefficient of thermal expansion (CTE) mismatch among chip, flex substrate, and printed circuit board (PCB). As a result of introduction of elastomer, the reliability goes up to 4775 cycles during thermal cycling (TC) test. Currently, two types of elastomer, nubbin and dry pad, are used for mu BGA((R)) package. The former is superior in reliability and the latter has a good nature for workability. To utilize these advantages of each elastomer, a new type of dry pad one was developed by the technique of phase control in the base resin. In addition, the reliability of the three different types of elastomers was compared through a method of FEM calculation.