Identification and prevention of “Black Pad” in Sn/Pb soldering
Article publication date: 23 August 2011
The use of an electroless nickel/immersion gold (ENIG) surface finish comes with the inherent potential risk of Black Pad failures that can cause fracture embrittlement at the interface between the solder and the metal pad. As yet, there is no conclusive agreed solution to effectively eliminate Black Pad failures. The case studies presented are intended to add to the understanding of the Black Pad failure mechanism and to identify both the plating and the subsequent assembly processes and conditions that can help to prevent the likelihood of Black Pad occurring.
Scanning electron microscope (SEM) analysis of exposed pad surfaces on failed PCBs demonstrated a “mud‐crack” appearance, which is a characteristic of the Black Pad phenomenon. In addition, energy dispersive X‐ray (EDX) analysis was used to identify the elemental composition of the fractured layer between the Ni3P and Ni3Sn4 inter‐metallic compound, confirming the presence of Black Pad.
Grain boundaries or “mud‐cracks” that can be clearly seen in a top view of the failed pad surface and corrosion spikes in the failed pad surface, as evident from the cross‐section sample, should be used as a guideline to confirm Black Pad failures. Maintaining an optimum and well‐controlled EN and immersion gold bath, in addition to good process control prior to nickel‐gold deposition is recommended as the best approach for minimizing the occurrence of Black Pad failures.
Only Sn/Pb soldering processes using ENIG PCBs or package substrates were evaluated and discussed. Thus, the current case studies do not encompass Black Pad failures with lead‐free soldering.
The work reported provides guidelines that can be used to identify Black Pad occurrence. It also proposes relevant approaches for minimizing the possible occurrence of Black Pad.
The findings of these studies provide a basic understanding of the Black Pad failure mechanism. Subsequently, both the plating and the ensuing assembly processes and conditions that can help to prevent the likelihood of Black Pad occurrence were identified.
Hui Lee, K. (2011), "Identification and prevention of “Black Pad” in Sn/Pb soldering", Circuit World, Vol. 37 No. 3, pp. 10-15. https://doi.org/10.1108/03056121111155620
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