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Heat Sink Spring Screw Thermal Interface Material Stand off Board Carrier Tray Spring-Loaded Solution. Heat Sink O-ring Screw Thermal Interface Material Stand off Board Carrier Tray O-Ring Solution. Heat Sink
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FIGURE 58.17 Heatsink attachment methods.
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TABLE 58.2
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Comparison of Different Heatsink Attachment Methods Advantage Useful for absorbing significant tolerance variations by varying spring constants. Less expensive. Less bulky. More versatile. Could be designed to absorb a wide range of expected loads. Disadvantage Relatively expensive. Bulky
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Attachment method Spring-loaded solution
O-Ring solution Integrated cantilever spring solution
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COMPONENT-TO-PWB RELIABILITY
Surface Finish Issues In recent years, the role of package surface finish in the robustness of BGA packages has gained more prominence. Several different surface finishes are used across the industry, and there are trade offs with the use of each one.The surface finish used could impact intermetallics formed in the solder joints, which in turn can impact the thermomechanical and mechanical reliability of the package. In recent years, a number of surface finishes have become more prevalent. The surface finishes with which the effect on package reliability is relatively well known, are outlined in this section: 58.3.11.1 Electroless Nickel Immersion Gold (ENIG). This has been the surface finish of choice for most high-end, flip-chip BGA packages. It enables the routability of fine pitch traces in high pin count packages. Historically, ENiG produces excellent board level solder joint reliability. However, there are two primary concerns with ENiG: 1. Brittle Fracture This failure mode is seen in high strain/strain rate conditions, such as shipping, testing, and handling. A typical signature of this failure mode is a clean separation in the intermetallic between the pad and the solder joint. This failure typically has a low defect rate in production, but the ensuing line stops, customer returns, and root-cause analyses are quite expensive. Moreover, the unpredictability of the failure and the susceptibility to mechanical handling/testing/shipping conditions makes it quite undesirable. There are a few failure mechanism theories proposed to explain brittle fracture,18 but no conclusive failure mechanism has been agreed upon in the industry. The predominant theory is based on the formation of Kirkendall voids during reflow, in the phosphorus-rich Ni layer and Sn-Ni intermetallic interface. Kirkendall voids are voids that form at the interface of two dissimilar materials. They form because the two materials diffuse into each other at different rates. Other factors such as the concentration of P in the Ni-P+ layer and the density of mudflat cracks in the Ni-P+ layer have also been shown to have an influence on the propensity of brittle fracture. The intermetallics formed in the joint during the assembly process are shown in Fig. 58.18. The presence of pervasive Kirkendall voids could reduce the strength of the solder joint. A mechanically applied strain could result in the sort of brittle solder joint failure shown in Figs. 58.19 and 58.20.
Ni-Cu-Sn Intermetallic Another phenomenon reported to cause a similar failure is based on the formation of a Ni-Cu-Sn ternary intermetallic in the solder joint.20 The Ni-Sn intermetallic forms on the component side during ball attach. When the part is mounted on the board, copper migrates from the board pad side, across the BGA ball, to form a Ni-Cu-Sn intermetallic on the component side. The thickness of this ternary intermetallic, typically 3 to 5 mm thick, could grow with additional reflows.20 Clearly, an important factor contributing to this phenomenon is the surface finish on the PWB side. If the surface finish has a barrier like Ni to prevent copper from migrating across the BGA ball, this ternary intermetallic is less likely to form on the component side. Strain/Strain Rate It has also been observed that the strain to failure of solder joints is a strong function of the strain rate. The higher the strain rate, the lower the strain-to-failure. One theory to explain this strain rate dependency is that the stiffness of bulk solder itself is strain rate dependent: at low strain rates, solder tends to deform and absorb some of the applied strain. Thus, less strain is actually transferred to the Kirkendall void-rich region. At high strain rate levels (typically above 5000 u /sec.), bulk solder behaves more like a linear elastic material and transfers more of the applied strain to the voids, causing brittle solder joint fracture. A graph illustrating the relationship between strain and strain rate for solder joint failure is given in Chap. 59.
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