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Numerical prediction and experimental validation of flip chip solder joint geometry for MEMS applications

Authors Lo, Chi Chuen
Issue Date 2008
Summary Solder bumped flip chips are one of the superior interconnection methods for advanced microelectronics. The solder joints of flip chips provide both mechanical support and electrical connection. There are several well-known advantages associated with solder bumped flip chips. In particular, the self-alignment capability of solder joints can benefit many high end products. In some micro-electro-mechanical systems (MEMS) applications, the alignment between the MEMS device and other components can affect the overall performance of the system. In such cases, solder bumped flip chips may be applied as a low cost solution to avoid other interconnection methods involving expensive and time consuming active alignment. The geometry of the solder joint is very critical. It not only affects the solder joint reliability, but also determines the position of the component. The lateral position of the component can be defined by the bond pads locations on the component side and the substrate side. In some occasions, a relatively thick copper layer is used on the MEMS device to increase the mechanical rigidity. A thick copper layer may also be required for some intended electrical functions. Due to the thickness of the copper layer, it is not possible to deposit a passivation layer which is used to define the bond pads locations. As a result, the molten solder will flow along the copper trace to a certain extent during the reflow process. Such solder flow may change the final solder bump height and affect the yield of the assembly process. In this study, test vehicles were fabricated to investigate the effects of different parameters on the solder flow characteristics. The standoff height of the solder joint is also an important factor. There existed several proposed models in the literature to predict the standoff height of the solder joint. However, the proposed models were lacking of experimental validations. Furthermore, the previous models could not clearly identify the effects of various parameters on the standoff height. In the present study, a more effective model based on a second order polynomial was proposed. This model could clearly exhibit the effects of the surface tension, bond pad size, bond pad geometry and solder joint volume on the final standoff height. Comprehensive experiments were carried out and the results were used to validate the current model. The proposed model provides a very useful tool for solder joint standoff height prediction during the package design stage. An innovative package with 3D chip-on-chip configuration was proposed for packaging a MEMS based microphone. In this package structure, a certain gap space between the MEMS microphone chip and the package substrate was required to ensure the overall performance. The corresponding gap space was controlled by the standoff height of flip chip solder joints. A comprehensive analysis was performed using the newly developed model at the design stage. The prediction of standoff height of solder joints was successfully verified by the prototype. As a result, the MEMS based microphone could achieve the desired geometry to perform the intended function.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2008
Language English
Format Thesis
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