Jonghyun Chae

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.

Reliability of Au-Ag alloy wire bonding

Au-Ag alloy wire is a low cost wire bonding solution instead of gold wire for IC packaging. Comparing with copper wire, Au-Ag wire has better productivity because it does not need protective gas and it is softer. The main issue of Au-Ag alloy wire bonding is its reliability in humidity environment. In present study, the bond parameters effect on PCT reliability was investigated. The correlation between the IMC profile and PCT lifetime was established. The IMC characteristic length was defined to express the relationship quantitatively. Meanwhile, The Au-Ag-Al IMC growth equation was obtained through HTS. It is found that the IMC growth rate is slower one order of magnitude than 99.99% gold wire.

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.

Mechanism and process study for high performance COB development

In this paper, the stress distribution and weak point of Au bonding wire in Chip-On-Board (COB) package were analyzed via a stress released model. In order to find out root cause and factors of it, simulations were performed to explain TC failure mechanism and how package styling affected the stress wire suffered. It is found out the stress distribution is related to loop direction. Based on this, new methods were developed to redistribute stress which induced by loop adjustment, neck profile optimization, and encapsulation modification. What is more, the effect of different chip size on stress was also studied. Then, TC performance of each solution were obtained and compared to the normal situation. It demonstrated that TC performance can be obviously enhanced by lowering neck height, increasing neck angle, using small chip and optimizing encapsulation.

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.

COB 3wheel reliability simulation method development

in this paper, a COB (chip on board) 3wheel test simulation method for assessment banking card bending performance was fully studied and developed. To achieve high precision prediction, the dynamic 3 wheel test is divided into series of static analysis process in which the plastic work can be accumulated. And ANSYS sub-modeling displacement solution is adapt to fine-analysis due to the large dimensional scale from smart card to Au wire in the model. Besides, tie constraints and contact pairs are set up between the top & bottom layers to uniform the meshes. After post processing, the prediction results are correlated to the experiment results, and then getting the high accurate judgment criteria for COB 3wheel lifetime. With this simulation method, the COB structure factors, such as encapsulation thickness/size/shape, chip thickness, loop height and milling size are fully studied and defined. And it matches well with the experiment data. COB product developer using this guide to make the selection pertinently and optimum product design, and gets the high bending performance product with high development efficiency. It can be also used to clear the COB 3wheel test failure mechanism.