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CHAPTER 4 NOTES ON MOUNTING
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Figure 4-11 shows an example of what is observed to happen when “solder joint detachment” occurs between a BGA ball and the bonding pad. In this example, the solder ball has become rounded where it contacts the pad edge and seems to be falling off the pad edge. It also seems that, when looking at the entire ball array, the balls on the inside are more vulnerable to detachment. This phenomenon is often seen in cases where reflow is executed multiple times. From this, we can infer that in the second and subsequent reflow processes, warping of the device or printed circuit board occurs in the direction in which the mounting interval becomes wider, and at the same time, the remelting of the solder balls causes peeling between the solder and the intermetallic compound layer. A similar phenomenon may also occur when flow soldering is executed after reflow soldering. For details of this defect, see 4.2 Reflow + Flow Soldering. |
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Figure 4-11. Cross-section of Ball Falloff Defect and Results of Peel Check
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Figure 4-12. Mechanism for Inferring Cause of Ball Falloff
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The following measures can be implemented to prevent the problem of ball falloff.
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