The Interfacial Reaction Of Solder Joint For Flip Chip Assembly With Cu/OSP Pad And Its Reliability

As the requirement of electronic market, the electronic products with the multi-functonal, higher density and miniaturization have been developed. Meanwhile, how to improve the products function combined with the price reduction become the aim for the electronic company. The sillicon die can assembly into the electronic product through the solder joints as a result of the reduction of both assembly size and price, especially for the fine-pitch flip chip assembly. Ni\Au and OSP are common surface finish used in electronic industry. In typical assembly process, the solder joints as interconnection usually experienced several reflows, if OSP was used, it can make IMC much thicker as a result of nagative influence for solder joint reliability. While for Ni\Au, the interfacial IMC growth is slow, and thus suitable for milti-reflow process of typical electronic assembly. However, compared to Ni\Au, OSP can reduce the expense and and simplify the process, therefore OSP is a ideal choice as PCB Cu surface finish for the electronic assembly with one reflow process.The flip chip has everal advantages as mentioned above. However, due to the CTE difference between silicon die and PCB, the solder joints are easier to fail under the operation and thermal cycle, especially for the fine pitch solder joints. In addition, smaller size can cause the great microstructure change of solder joints under the operation and thermal cycle condition, therefore the solder joints of flip chip assembly have to face the new reliability problem.The aim of the project:the100μm pitch flip chip assembly with OSP on the substrate Cu pad and Ni/Au on the chip Ni pad was evaluated to be used on the real electronic product by the study of basic theory, reliability experiments and the comparison with the big pitch assembly. In this thesis,100μm and200μm pitch of flip chip assembly were used to investigate the microstructure evolution under reflow, thermal aging and thermal shock, analyze the size effect of microstructure evolution, and evaluate the influence on the solder joint reliability. The3-D element model was built to anslyze the mechanics behavior. And meanwhile, the failure mechanism of solder joint and the influence of size effect on failure were analyzed. Additionally, the theoretical model was used to evaluate the solder joint lifetime, and the lifetime model of solder joint was built according to the parameter of theoretical model. At last, the l00μm pitch flip chip assembly with Cu/OSP on substrate side was evaluated to be used in real electronic product。 The detail study result followed as below: The IMC growth and evolution were investigated on the single Ni pad surface for both sizes of solder bumps after bump making reflow, and they were still studied on both surfaces of Ni and Cu pad side. The results show that due to the size effect of solder joint, the different types of IMC formed on the Ni pad interface for200μm and100μm pitch, they are separately (Cu,Ni)6Sn5and (Ni,Cu)3Sn4, and the morphology of IMC are diamond shaped and needle shaped correspondingly. After interconnection reflow, based on the Cu inter-diffusion effect on the Cu pad, the (Ni,Cu)3Sn4transit to (Cu,Ni)6Sn5, and the morphology was exchanged to rod shaped, however the IMC thickness and IMC grain numbers were increased obviously. On the Cu pad interface, the (Cu,Ni)6Sn5was formed in both size of solder joints, however the rod shaped of (Cu,Ni)6Sn5is much longer for200μm solder joint, and there are a lot of micro-voids existed on the (Cu,Ni)6Sn5.The IMC growth and evolution were investigated on both side surfaces of both sizes of solder bumps under thermal aging. The results show that the stand-off height has a great influence on the Cu diffusion towards Ni pad interface as a result of all (Ni,Cu)3Sn4transition, while for200μm pitch solder joint, even aging to650h, the (Ni,Cu)3Sn4still existed on the Ni pad interface. Due to the effect suspession of Ni on Cu3Sn growth, the Cu3Sn growed slowly. Forthermore, the IMC growth rate become slower and slower as the aging time increase for both size of solder joints, and it is quicker on the Cu surface than Ni surface. The IMC growth dynamics shows that the IMC growth is controled by the interfacial atoms diffusion mechanism for two sizes of solder joints, and thermal activation energy of interfacial IMC are67.89kJ/mol and77.68kJ/mol separately.The IMC growth and evolution were investigated on both side surfaces of both size of solder bumps under thermal shock. The results show that the trend of IMC growth and evolution under thermal aging were similar with that under thermal shock, however the IMC on pad surface and solder matrix was bigger obviously, more stress promoted the IMC growth under thermal shock combined with the simulation analysis. Compared the IMC growth between the two size solder joints, the IMC growth rate of small solder joint was bigger obviously, the more stress small solder joint underwent is the reason as the simulation result. The IMC growth dynamics result shows that the IMC growth was controlled by interfacial atoms diffusion mechanism, the constants of IMC growth rate under thermal shock was bigger that that under thermal shock. The dynamics parameters of n and D were2.47and0.203μm/h1/2for100μm pitch solder joint,2.18and0.104μm/h1/2for200μm pitch solder joint. While for200μm pitch solder joints, its failure mechanism belong to premature brittle crack. The interfacial energy increased due to a great of particle shaped (Ni,Cu)3Sn4 gathered on the Ni surface, it provided the energy for crack growth; The impurities diffused on the interface as a result of interfacial adhesion reduction, which provided the situation for crack growth, while the higher stress concentration on the Ni interface caused by the CTE difference between the chip and PCB worked as driving force for crack growth. According to Weibull distribution, the characteristic lifetime of l00μm and200pm pitch assembly are4714and6217separately. Based on the Darveaux lifetime model, the life time of l00μm and200μm pitch solder joints are6171and7460. Darveaux lifetime model parameters of Ki, K2, K3and K4are1648.96.-0.2349.0.00479and-0.7004for l00μm pitch solder joint, while are551.6975.-1.1095.0.2362and1.5477for200μm pitch solder joint.As a result, the l00μm fine pitch solder joint of flip chip assembly with Cu/OSP and Ni/Au was feasible to be used in electronic assembly. However, this project was focused on the flip chip assembly with solder joints as interconection, this case is different with the typical electronic assembly. If the assembly need multi-reflow to complete assembly, the IMC should be much thicher to damage the solder joint on Cu/OSP surface finish structure, therefore this case should be evaluated in the future research.