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<p>This study investigates the influence of P content in electroplated Co–P diffusion barriers on interfacial reactions with Sn under solid-state aging at 160&#xa0;°C, 180&#xa0;°C, and 200&#xa0;°C. Two Co–P coatings, Co-2.5 wt.%P, and Co-3.2 wt.%P were fabricated by electroplating with varying concentrations of sodium hypophosphite. The interfacial reactions resulted in the formation of two distinct layers: a mixture of CoSn<sub>3</sub>/CoSn<sub>4</sub> layer and a nanograined Co–Sn–P phase. The Co-2.5 wt.%P coating exhibited a notable suppression effect on intermetallic compound (IMC) growth, especially in the early stage. In contrast, the Co-3.2 wt.%P coating exhibited accelerated IMC formation. At 200&#xa0;°C, the suppression effect was no longer effective, and the IMC growth rate surpassed that of the Sn/Co system, indicating a critical threshold in P content that significantly influences IMC growth behavior. To ensure effective suppression of IMC growth in solder joints, the P content in Co–P diffusion barriers should be maintained below 2.5 wt.%. Growth kinetics analysis indicated that IMC growth in the Co-2.5 wt.%P system initially followed a parabolic trend (diffusion-controlled), transitioning to linear behavior after reaching ~ 8&#xa0;μm thickness. Structural characterization by XRD and TEM identified the CoSn<sub>3</sub> phase as a tetragonal <i>d</i>-CoSn<sub>3</sub> with disordered stacking, while CoSn<sub>4</sub> was determined to possess a tetragonal IrSn<sub>4</sub>-type structure. The nanograined Co–Sn–P phase consisted of fine CoSn<sub>3</sub>, Co<sub>2</sub>P, and Sn<sub>4</sub>P<sub>3</sub> crystallites, originating from localized P accumulation in the original deposit zone.</p>

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Influence of critical phosphorus content on interfacial reactions in solid-state soldering with electroplated Co–P diffusion barriers

  • 期刊论文

  • 发表日期：

    2026年1月11日

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  • Chao-hong Wang,
  • Che-yang Lin

摘要

This study investigates the influence of P content in electroplated Co–P diffusion barriers on interfacial reactions with Sn under solid-state aging at 160 °C, 180 °C, and 200 °C. Two Co–P coatings, Co-2.5 wt.%P, and Co-3.2 wt.%P were fabricated by electroplating with varying concentrations of sodium hypophosphite. The interfacial reactions resulted in the formation of two distinct layers: a mixture of CoSn3/CoSn4 layer and a nanograined Co–Sn–P phase. The Co-2.5 wt.%P coating exhibited a notable suppression effect on intermetallic compound (IMC) growth, especially in the early stage. In contrast, the Co-3.2 wt.%P coating exhibited accelerated IMC formation. At 200 °C, the suppression effect was no longer effective, and the IMC growth rate surpassed that of the Sn/Co system, indicating a critical threshold in P content that significantly influences IMC growth behavior. To ensure effective suppression of IMC growth in solder joints, the P content in Co–P diffusion barriers should be maintained below 2.5 wt.%. Growth kinetics analysis indicated that IMC growth in the Co-2.5 wt.%P system initially followed a parabolic trend (diffusion-controlled), transitioning to linear behavior after reaching ~ 8 μm thickness. Structural characterization by XRD and TEM identified the CoSn3 phase as a tetragonal d-CoSn3 with disordered stacking, while CoSn4 was determined to possess a tetragonal IrSn4-type structure. The nanograined Co–Sn–P phase consisted of fine CoSn3, Co2P, and Sn4P3 crystallites, originating from localized P accumulation in the original deposit zone.