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The Effect of Ni Thickness on Mechanical Strength of Pb-free BGA Spheres on Selectively Plated Ni/Au Finish
Keywords: Ni/Au Metallization, Ni Plating Thickness, Leadfree Solder Joint
In this study, Tape BGA substrates with different Ni thickness (2~8um) and solder mask opening were built and attached with Sn3.8Ag0.7Cu Pb-free spheres. Mechanical tests were performed on different levels to examine the strength of the solder joint. On solder joint level, cold ball pull (CBP) and high speed ball shear (HS-BS) were used to evaluate the adhesion strength of individual solder joint. The brittle fracture failure rate from these tests is believed to have correlation to the missing ball rate in the field. On package level, the tray drop and packing drop test were conducted to provide a close resemblance to real shipping and handling conditions. On board level, mechanical bending test were carried out to assess the solder joint strength after board assembly. Microstructure analysis was performed to have more fundamental understanding. Cross-sectioning, SEM and EDX investigation gave information on the morphology and composition of the plated Ni layer as well as the intermetallic layer. In parallel, an FEA model was established to estimate the stress applied on the interface during ball shear and its relationship to the geometry of the Ni layer. It is found that the composition, and hence the intrinsic strength, of the intermetallic layer can be affected by the Ni thickness. A possible cause is the leakage of Cu from the edge of the pad during reflow process. At the same time, the stress level at the ball/intermetallic interface during ball shear is also greatly influenced by the Ni thickness. With selective plating, the un-etched solder mask forms a tail on top of the pad finish. This tail will create a pre-crack at the ball/intermetallic interface which will facilitate the crack propagation and decrease the solder joint strength. With increased Ni thickness, the intermetallic layer is moved away from the solder mask tail and the stress level is reduced.
Eu Poh Leng, Senior Staff Engineer (Research & Development Engineer III)
Freescale Semiconductors Malaysia Sdn Bhd
Petaling Jaya, Selangor 47300,
Malaysia


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