Effect of Solder Paste Volume and Reflow Parameters on Solder Paste Wicking and Joint Shear Strength of Ni–Au-Coated Cu Spheres

Abstract

Ball grid array (BGA) architectures are being widely used in the semiconductor industry in surface mount technology (SMT) applications to satisfy the miniaturization needs of emerging microelectronics systems. However, these architectures are not compatible with socketing applications as the mechanical contact between the gold paddle and the solder sphere leads to undesired reactions, increasing the contact resistance and degrading the reworkability overtime. To address this challenge, surface modification of BGA spheres with multilayered thin-film metallic coatings such as Ni-Au is proposed to maintain a nonreactive noble metal interface when used in a socket. This article focuses on the process of attaching the surface-modified spheres on a package using two solder pastes—Sn57.6Bi0.4Ag (SBA) and Sn3.0Ag0.5Cu (SAC305) with liquidus temperatures of 138 °C and 219 °C, respectively. The challenge of solder paste wicking on the entire surface of the Au-coated sphere was addressed by controlling the volume of the solder paste, and the joint shear strength was optimized by controlling the time above liquidus (TAL) during reflow. A 30–40-s window for TAL was established to achieve a strong solder joint with controlled paste wicking for both solder pastes. The strength of the joint was measured by a single-ball shear test, followed by microstructural analysis to identify the failure modes. Brittle fractures were observed for both solder pastes with an optimum joint shear strength ranging from 55 to 70 MPa for the SBA paste, and 72 to 75 MPa for the SAC305 paste.