Intermetallic Compounds at the Interfaces of Ag–Pd Alloy Stud Bumps With Al Pads

Abstract

Stud bumping using a traditional wire bonding process followed by the severing of the wire material from the ball bond has been widely applied in flip chip packages. Traditionally, Au stud bumps were employed because of the physical properties of gold, but the mediocre reliability of Au with Al pads and the cost of gold make this material unfavorable for industrial needs. Copper stud bumps were also considered, but the intrinsic features of Cu, such as its high hardness and ease of oxidation, present no advantages over gold stud bumps. However, Ag-alloy stud bumps exhibit satisfactory bondability and have been proposed as the optimal substitute for Au stud bumps to ensure better reliability in soldering and better intermetallic growth kinetics with Al bond pads. In this article, the intermetallic growth of Ag stud bumps alloyed with 2%, 10%, and 12% Pd after high-temperature storage tests (HTSTs) is investigated. The results indicated that the Pd atoms remained immobile in the bump-side matrix without diffusing into the aluminum pad side. Alloying with higher Pd content enhanced the formation of the favorable Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Al intermetallic compound (IMC), whereas lower Pd content resulted in severe cracking due to the formation of Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al IMC. Compositional analysis showed that continuous layers of Ag-Al IMCs doped with large amounts of Pd formed at the interfaces close to the Ag stud bumps, but no Pd was detected on the Al pad side. The bonding strength increased after aging at 150 °C and 200 °C for 200 h, but it decayed drastically after aging at 250 °C for 200 h. The relationship between intermetallic growth and failure mode was established.