Thermal aging reliability of socketable, surface-modified solder BGAs with and without polymer collars

Abstract

Ball Grid Array (BGA) package designs are increasingly used in surface mount applications while Land Grid Array (LGA) designs are predominantly used in socketing. The need to converge to a single package design has been driving the need to enable socketable BGAs. BGA spheres with a noble metal surface provide a stable mechanical contact interface with the socket paddles. These noble contact interfaces, however, have to remain intact through the socketing life of the product, considering accelerated testing temperatures of 100-120 °C. Under such conditions, it has been reported that the solder from the ball-attach joints undergoes solid-state wicking along the surface of the ball, leading to complete Au dissolution and potential undesirable intermetallic formation with the socket paddles, along with a drop in ball shear strength due to depletion of the solder from the joints. This paper discusses the use of polymer collars to address this challenge and improve the thermal aging reliability of packages with surface-modified BGA interconnections. The polymer collars were spin-coated on the package, which was aged alongside a reference package with no collars, at accelerated test temperatures of 100 °C and 120 °C, respectively. XPS studies showed that after 650 h of aging, no Au signal but a strong Sn signal was observed in the package without collars, which confirmed complete solder wicking to the top of the ball, while the Au signal remained for the package with the collars, confirming that polymer collars are effective in inhibiting solid-state solder wicking from the ball-attach joints. The joints with polymer collars also showed mechanical stability throughout thermal aging with a 3X improvement in joint shear strength.