Corrosion of Copper Wire bonded Packages by Chlorine Containing Foreign Particles

Abstract

Corrosive failures for wirebonded packages due to halide ions (especially chloride (Cl-) uniformly distributed in the mold compound matrix are well understood and measures have been taken by the industry to address the related reliability issues. However, the impact of localized Cl sources within the mold compound matrix from foreign materials (Cl-FM) not intended to be part of the bill of materials (BOM) has been overlooked and rarely reported. There are multiple possible sources of Cl-FM ranging from the raw materials suppliers to the mold compound manufacture to the package assembly site. To understand the impact of a localized Cl source on corrosion in a typical copper wirebond package, test package samples were created by molding die with Cu wirebonds on Al pads with deliberately added polyvinyl chloride (PVC) particles to simulate Cl-FM contamination. Failures due to Cl-FM occur only when the Cl containing particles are in proximity to a copper ball bonded to an aluminum (Al) pad. Neighboring ball bonds were unaffected due to the limited mobility of Cl ions through the mold compound. Factors affecting the mobility of chloride ions through mold compound is important, so experimental studies were conducted to identify key factors impacting Cl- ion movement through the epoxy mold compound matrix. Moisture content, voltage/electric field strength and Epoxy Mold Compound (EMC) cross-link density were some of the key factors that affect chloride ion movement through EMC. Analytical methods, such as ion chromatography (IC) and secondary-ion mass spectrometry (SIMS) analysis, were used to evaluate chloride diffusion through EMCs under various conditions. The presence of moisture content and electric field can affect Cl diffusion by orders of magnitude. EMC structure and cure conditions, such as post mold cure (PMC) time, can also impact the diffusion of chloride. Understanding the factors that impact ionic Cl movement through mold compound can be directly applied to develop estimates of field life for packages affected by Cl-FM.