Formation Mechanism And Reliability Of Cu-Sn Intermetalic Compounds During Ultrasonic Solder Bonding

Miniaturization and high density integrated developmen t of electronic devices present the increasing challenges to the packaging industry. There are still many reliability problems though flip chip interconnect technology is widely applied. Ultrasonic flip chip bonding has received much attentions because of the advantages, such as the shorter welding time, smaller thermal damage to devices and lower interfacial stress mismatch owning to the local thermal generation. To obtain a reliable interconnected joint via ultrasonic bonding, the formation and growth of the interfacial Cu-Sn intermetallics(IMCs) is essential, and it is also the emphasis and difficulty of ultrasonic bonding realizaiton. In this article, experimental study and semi-quantitative calculation are used to analyze the interfacial morphologies, phase types, growth mechanism of the Cu-Sn IMCs, and their effects on joint reliability.Firstly, by considering the mechanical calculations and experimental exploration, a grooved square welding head and a fixture for the 17×17 mm2 BGA devices(256 solder bumps) are designed and successfully install on the ultrasonic bonding machine.Secondly, the Cu-Sn IMCs on the Cu pad side are divided into three regions according to the microstructures. The semi-quantitative analytical calculation model is established based on the element content data from EDS and AES, and the the distribution s of Cu6Sn5 and Cu3 Sn on the interface of Cu pad side are confirmed. Based on the experiment study, Cu3 Sn is the initial phase generated on the Cu pad and then some textures of Cu6Sn5 are formed on Cu3 Sn surfaces. The flux is helpful for the generation of Cu 3Sn layer. In addition, under the influence of ultrasonic cavitation and ther mal effects, the Cu6Sn5 phase will gradually spread from region I to region II and region III.Thirdly, there is no difference in the distribution of Cu-Sn IMCs on the monocrystalline and polycrystalline Cu pads. The growth of Cu-Sn IMCs is interpreted based on the growth kinetic theory. In addition, the relationships among the processes(i.e. the welding time, welding pressure and height difference between the lower electrode and acoustic horn), the IMC distribution and shear bond strength are also discussed in detail.