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Investigation of Sn3.5Ag and Sn3.8Ag0.7Cu Pb-Free Alloys for BGA Application on Ni/Au Finish
Keywords: Pb-free , BGA , Ni/Au
The eutectic SnAg (Sn3.5Ag) and near eutectic SnAgCu (Sn3.9Ag0.6Cu, Sn3.0Ag0.5Cu) alloys are two major Pb-free alloys that have been studied extensively. Both of them show lower creep rate and higher resistance to thermo-mechanical deformations compared with eutectic SnPb. However, there is a significant disadvantage. Due to the lower creep rate and higher modulus, these Pb-free alloys will induce higher stress level at the solder/intermetallic interface in high deformation rate events such as impact or shock. Therefore, intermetallic (IMC) brittle failures are more likely to occur. At the package level, the direct result of this is increased missing ball yield loss due to IMC fracture in ball grid array (BGA) assembly, test, shipping and general handling. With SnAgCu, the situation becomes aggravated when it is attached to Ni/Au finishes. The previous work shows that the intermetallic formed between SnAgCu alloy and Ni are highly sensitive to the Cu content in the sphere. Dual IMC layers are formed with Cu content between (0.2~0.6%). The weak interface between the two layers is the primary failure location. As a comparison, SnAg will form only a single Sn-Ni IMC layer during ball attach and better mechanical strength is expected. This work was conducted to compare the overall performance of SnAg and SnAgCu on BGA solder pad with electrolytically plated Ni/Au finish. BGA packages with different configurations were assembled with both Sn3.5Ag and Sn3.8Ag0.7Cu spheres. Studies were performed at different levels. At the intrinsic material property level, the melting and wetting behavior of the solder alloys were examined by Differential Scanning Calorimetry (DSC) and wettability test. At the solder joint level, cold ball pull (CBP) were used to assess the mechanical strength of individual solder joint at different stress conditions (as-assemble, multiple reflow, high temperature storage, etc.). The brittle fracture failure rate from this comparative test is believed to have correlation to the missing ball rate in the field. At the same time, the geometry of the spheres such as ball height/diameter and positioning were also recorded and compared. At the package level, the tray drop and packing drop test were conducted to provide a close resemblance to real shipping and handling conditions to predict the missing ball rate. At the board level, air temperature cycling and mechanical bending test were carried out to evaluate the solder joint reliability. Microstructure analysis by SEM/EDX was performed to compare the morphology and composition of the intermetallic layers as well as the bulk solder. The above investigations reveal that the mechanical strength of SnAg is considerably higher than SnAgCu at the solder joint and package level. In CBP test as well as package drop test, SnAg shows significantly less IMC brittle fractures as compared to SnAgCu. The relationship between the IMC brittle fracture and the IMC microstructure are discussed to explain the differences between the two alloys. In contrast, solder joint reliability tests show that there is little difference between SnAg and SnAgCu. The different behavior of SnAg on package level and board level are also discussed. In the mean while, experimental results show that two alloys have similar melting point and wettability which indicate that they have similar manufacturability. The SnAgCu alloy is recommended by iNEMI and JEITA for BGA spheres. However, the frequent occurrence of brittle IMC fracture and resulting high missing ball failure rate of SnAgCu on Ni/Au pad has raised serious reliability concerns. The result of this study shows that SnAg can reduce the missing ball yield loss by at least an order of magnitude while maintaining the same level of manufacturability and solder joint reliability. Therefore, it is a viable candidate to replace the SnAgCu alloy. List of references a. T. Laurila, V. Vuorinen, J.K. Kivilahti, “Interfacial reactions between lead-free solders and common base materials,” Materials Science and Engineering , v49, 2005, pp. 1–60. b. K. Newman, “BGA Brittle Fracture – Alternative Solder Joint Integrity Test Methods,” Proc 55th Electronic Components &Technology Conf, Lake Buena Vista, FL, May 31- June 3, 2005, pp. 1194-1201.
Min Ding, Packaging Engineer
Freescale Semiconductor, Inc.
Austin, TX
USA


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