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Evaluation of Irregular Inter-Metallic Compound Growth in Gold Wire Bonds Encapsulated with Reliable, Green Epoxy Mold Compound
Keywords: irregular inter-metallic compound, gold wire bonding, green epoxy mold compound
The mechanical robustness of gold ball bonds after isothermal ageing during high temperature storage test is often used to assess the suitability of gold wires for package qualification[1-2]. The use of lead-free, Ag-Sn based solders require new integrated circuit (IC) packaging material that are thermally stable to withstand much higher reflow temperature. In new IC package qualifications, it is the intermetallic growth of the ballbonds encapsulated in epoxy mold compound that is of real interest. Additives in the molding compound coupled with moisture during reliability test is thought by some to cause acceleration of bond failure. The mechanisms of non-uniform intermetallic and void formations in bond failure is a combination of intermetallic formation, ingression and reaction of key elements around the gold wire bonds in EMC [3]. Thermogravimetric analysis reveal selected additives within green epoxy molding compound degrade and outgas at Pb-free reflow temperature as well as high temperature storage test conditions. The residue diffuse thru the ball bond-pad interface, deposit on the interface and hinder the vertical diffusion of Au-Al intermetallic which result in irregular intermetallic compound growth and void formations. Ball pull strength of the wire bonds (after acid etch removal of the EMC) also show that number of wire ball joints with low pull strength increases with increasing high temperature exposure at 175 C. Irregular IMC growth formation result in increasing pad lift failures within a package with increasing exposure to high temperature storage. Surface analysis using XPS as well as SEM-EDX of affected bond pads will be shared to reveal selected additives being the main source of bond pad contamination. Additional factors such as the acidicity of the mold compound as well as the presence of trace elements present in catalyst, adhesion promoter as well as ion scavengers that influences irregular IMC growth in gold wire bonds shall be shared in this paper. Reference: [1 Breach CD, Wulff F, Tok CW. An unusual mechanical failure mode in gold ball bonds at 50um pitch due to degradation at the Au-Au4Al interface during ageing in air at 175C. Microelectronics Reliability (2005). [2] Sutiono S, Breach CD, Calpito D, Stephan, Wulff F, Saraswati, Seah A and Chew S. Intermetallic Growth Behaviour of Gold Ball Bonds Encapsulated with Green Molding Compounds. Electronic Packaging Technology Conference EPTC 2005. [3] Ker-Chang Hsieh et al Bromine- and Chlorine-induced Degradation of Gold Aluminium Bonds. Journal of Electronic Materials, Vol.33, No.10, 2004.
Spencer Chew, R&D Director
Cookson Electronics
Singapore 628975,
Singapore


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