Reliability Study of Bottom Terminated Components (Part 1)

Estimated reading time: 5 minutes

Results and Discussions

X-Ray Inspection

Pre-tinned BTC components generally had less voiding than non-pre-tinned BTC components. However, voiding varied depending on the component type. Most of the components had an average void % of less than 25% except the dual row QFN132 component and FQFP176 component (Figure 7a). Maximum void percentage was seen up to 30%, 40%, 50%, depending on the component type, its pad size and process condition (Figure 7b).

Figure 7: Solder Joint Voiding Comparison for Sample Built with Pre-tinned BTC vs. Regular BTC. a) Average Void% b) Maximum void %.

Pre-tinned components did not eliminate voids. However, the size of the voids in the pre-tinned samples was typically smaller than the voids of solder joints using non pre-tinned component (Figure 8). The shape of the voids for pre-tinned sample was more defined and circular as compared to the non-pre-tinned solder joint sample (Figure 9).

Figure 8: X-Ray Images of QFN 3550 Solder Joint Assembled with Non-Pre-tinned Component vs. Pre-tinned Component.

Figure 9: X-Ray Images of QFN132 Solder Joint Assembled with Non-Pre-Tinned Component vs. Pre-Tinned Component.

Solder Joint Microstructure – Time Zero Analysis

Cross sections of the BTC solder joints were performed to evaluate the solder joint microstructure and inter-metallic formation at time zero (prior to thermal cycle testing). Non-pre-tinned components had a standoff height of about 2mil to 3.5mil, depending on the component type. The pre-tinned component standoff height was between 4-7mil. It was approximately twice of the non-pre-tinned component standoff [Table 2]. The BGA196 component had a standoff height of around 11mil.

Table 2: Solder Joint Standoff Height of BTC Assembled Using Regular Process and Pre-tinned Components

Good solder joint wetting and normal IMC layers were observed for all the components. No cracked solder joints were found at time-zero for all the samples (Figure 10, Figure 11 and Figure 12).

Figure 10: Cross Section Images of a QFN component at t=0_ Non-Pre-tinned Component.

Figure 11: Cross Section Images of a QFN Component after Reflow_ Using Pre-tinned Components.

Figure 12: Cross Section Images of BGA Component after Reflow.

In Part 2 of this two-part article series, failure analysis after thermal cycle tests will be discussed, as well as the impact of solder voids in the thermal pad of the BTC.

Editor's Note: This paper has been published in the technical proceedings of IPC APEX EXPO.

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