Microelectronics wirebond pull and shear test simulations using finite element method

Microelectronics wirebonding is used in the vast majority of semiconductor assembly and packaging operations worldwide. In the last few years, much of the manufacturing that traditionally had been gold (Au) wirebond has switched to using copper (Cu) wire. Once a wirebond “recipe” is established in manufacturing, reliability is checked by Bond Pull Strength test (Mil-Std 883G method 2011) and Bond Shear test (JEDEC JESD22 B116A). Both industry standards were established based on gold (Au) wirebonds. Cu wire is stronger and stiffer, producing higher values than Au in both pull and shear tests when well-bonded, so companies are comfortably complying with the old Au reliability limits while using Cu wire. But in fact, Cu intermetallic (IMC) growth and bonding is quite different than for Au, and failure modes in the shear test can be different for Cu than for Au. A new committee is active to revise the JESD22 B116A method to cover Cu wirebond as well as Au. BYU-Idaho students have responded to recent interest in switching from Au to copper Cu wire, modeling both the bond pull strength and bond shear tests using finite element methods. Simulations include the various ball bond wire types (Au, Cu, and silver (Ag) alloys), ball sizes, pad aluminum (Al) thicknesses, and IC bond pad structures in silicon wafers. Results show how stresses imparted to the bond pad in these reliability tests change when switching wire types, etc., helping to explain the physical results as well as to indicate potential issues in the interpretation of failure modes.