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In this work, we present ball impact test (BIT) responses and fractographies obtained at an impact velocity of 500 mm/s on Sn-4Ag-0.5Cu, Sn-1Ag-0.5Cu, Sn-1Ag-0.5Cu-0.05Ni, Sn-1.2Ag-0.5Cu-0.05Ni, and Sn-1Ag-0.5Cu-0.05Ge package-level solder joints. The solder joints are bonded on substrate pads of either immersion tin (IT) or direct solder on pad (DSOP) surface finishes. Differences of BIT results with respect to multi-reflow are also reported. Taking the impact energy as an indication of board-level drop reliability of the solder joints, the BIT results indicate that better reliability can be achieved by adopting Sn-Ag-Cu solder alloys with low Ag weight contents as well as IT substrate pad finish rather than DSOP. Moreover, the addition of Ni or Ge to the solder alloy provides a large improvement; Ni alters the interfacial intermetallic compound (IMC) structure while Ge enhances the mechanical behavior of the bulk solder.

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Following the strong demand of high-performance and miniaturized electronic devices with reduced process cycle time, the die attach film (DAF) has become a popular option for the die attach process of stacked-die packages in the semiconductor assembly industry. The working temperature of a die attach process is crucial to assembly yield and package reliability. However, as the number of die stacks increases, the die attach process with DAF may encounter a technical bottleneck since the working area is gradually away from the heat source. In this study, the thermal effect from bottom heating of plural-die-stack structures was investigated through transient thermal analysis as well as temperature measurements in an actual stacked-die process. It is clear from numerical and experimental results that as the number of dies in the die-stack structure increases, the time required to reach a working temperature for DAF increases significantly.

PurposeThe purpose of this paper is to report the effect of multiple reflow cycles on ball impact test (BIT) responses and fractographies obtained at an impact velocity of 500 mm/s on Sn‐4Ag‐0.5Cu solder joints.Design/methodology/approachSolder balls were mounted on copper substrate pads with immersion tin surface finish, supplied by two vendors. For these particular test vehicles and test conditions, fracture near the interface between the interfacial Cu6Sn5 intermetallic compound (IMC) and copper pad was identified as the only failure mode induced by BIT.FindingsMeasurement results indicate that BIT characteristics in general degrade as the number of reflow cycles increases. Furthermore, scanning electron microscopy observations show that the thickness and grain size of interfacial Cu6Sn5 increase with increasing number of reflow cycles. This correlation confirms the familiar notion that a thicker Cu6Sn5 degrades the interfacial strength.Originality/valueThere are few reports that can attribute failure directly to the IMC(s) at the interface. This paper, however, successfully correlates the weakening solder joints with the thickening and shape changes of IMC(s) in a direct way.

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