Myung-Kee Chung

Reliability of copper wire bonding in humidity environment

Copper wire bonding is being developed rapidly in recent years to replace expensive gold wire for electronic packaging. However, one issue about reliability in humidity environment causes risk in its application. The copper wire-aluminum pad interface degradation usually leads to electrical open failure. Present paper studied all the 5 factors including pad finish of chip, molding compound, wire bonding parameters, copper wire type and protective gas type in wire bonding process that affect this issue. Through bond pull test at different stage of uHAST, the influence and significance degree of the factors were obtained. The pad finish was found has the most significant effect. No degradation occurred at copper-gold and copper-palladium interface. For aluminum finish, mold compound and wire bonding parameters have significant effect. Green EMC, high USG and low Search Force in wire bonding process is benefit to enhance reliability of copper wire bonding in humidity environment. Concerning the palladium coated copper wire and bare copper wire, forming protective gas of 5%H2+95%N2 and pure N2, there was no significant effect founded. So for the safe copper wire bonding application, noble metal such as gold and palladium pad finish is the best choice. If the aluminum pad is used, green EMC and optimized wire bonding parameters are necessary to guarantee the reliability.

Mechanical properties and joint reliability improvement of Sn-Bi alloy

Sn-58Bi is a eutectic alloy that melts at 139°C, it is lead-free and strong, but brittle. Also, its fatigue resistance is questionable [1]. The aim of this research is to study the properties of Sn-58Bi low melting temperature solder ball and the method to improve its reliability. 78 ball grid array (BGA) with ball size 0.45mm and 0.8 mm pitch was selected as the target device. Joint protect flux (JPF) was used to enhance the component side joint reliability. The result shows that JPF can solve the ball off problem and improve the ball shear, thermal cycling, drop life time and bending performance of Sn-Bi solder. Simulation work was succeeding to confirm the experiment results. The drop result is good if the Sn-Bi solder ball using Sn-Ag-Cu (SAC) paste, but the result is bad if the SAC solder ball using SnBi paste.

Investigation of various pad structure influence for copper wire bondability

As an alternative bonding interconnect material to gold wire, copper wire technology is getting more attention in assembly processes for its excellent electrical and thermal performance. Copper wire bonding meets lots of challenges because of its free air ball hardness, such as Al extrusion, pad crack/damage, reliability and low yield issue. General speaking, there are two ways to improve the issue for copper wire bonding, one is wire properties improvement, such as softer and high reliability wire research; another one is copper friendly pad design and make bond pads suits to copper wire very well. In order to design the bond pad for copper, we should figure out which are important factors in pad to copper first. The purpose of this paper is to study the effects of various bond pad structures on copper wire bonding. In our study, not only the pad total thickness but also the pad structure was considered. As a result, pad total thickness, final Al thickness and VIA design play an important role in bonding. Theoretical analysis and theoretical prediction of a state-of-art pad structures for copper wire bonding application was presented.

Improvement of smart card mechanical properties

Smart cards are becoming more and more popular in our daily life for its characteristics of portability and intelligence. Since they are always subject to mechanical pressure during delivering or using, its mechanical performance is very important to guarantee good quality. In this paper, three mechanical pressure tests were introduced to characterize the mechanical performance of COB (Chip on board) of smart card. The test data corresponding to four main factors such as chip thickness, mold thickness, EMC (Epoxy molding compound) types and PMC (Post Mod Cure) was analyzed, it is found that EMC type is the key factor to influence smart card mechanical properties. And it is concluded that the best method to improve smart card mechanical performance is to use the EMC type with high mechanical strength and modulus.

Characterization of Wire Bondability on Overhang Structured Chip in Multi Chip Package

Characteristics of wire bonding on the overhang structured chip were investigated in this study. During the wire bonding process, chip crack and bondability were severe factors for thin wafer bonding. Chip crack was investigated by chip strength test and parametric study was performed by changing bonding parameters, constant velocity, force, and power. Bondability was measured for 500, 700, and 1000 mum overhang length. Bonding strength was tested by ball shear test method. 700 mum overhang showed better performance than 500 mum. 1000 mum overhang showed very low bondability. Bonding characteristics were observed by numerical FEM method. Nonlinear dynamic analysis was performed. Unstable contact pressure and shear stress were observed between ball and metal pad. Noise of reaction force was increased with overhang length.

Solder constitutive models and failure criteria selection in board level cyclic bending simulation

Board level bending testing is an effective method to evaluate the solder joint reliability of handheld products. In this work, the solder joints failure mode during cyclic bending was characterized by a board level bending test system with respect to strain range and strain rate. And then the finite element modeling and simulation was conducted to investigate the solder joints reliability by considering different solder constitutive models and different failure criteria. The Elastic-plastic, Creep and Viscoplastic models of solder alloy were measured and input to finite element model to compare their effect on the simulation results of solder joint performance. In addition to solder constitutive model selection, failure criteria for solder joint reliability were discussed in this paper. Stress, plastic strain and plastic work were calculated in simulation and discussed. Base on comparing the actual experimental results and simulation results, it was concluded that the proposed solder constitutive model, Viscoplastic model is the appropriate model which can be used successfully to simulate the failure mechanism of solder joints under cyclic bending condition. The plastic strain was selected as the failure criteria for evaluating the solder joints reliability during cyclic bending simulation.