研究了温度为150 ℃, 电流密度为5.0×103 A/cm2的条件下电迁移对Ni/Sn3.0Ag0.5Cu/Cu焊点界面反应的影响. 回流焊后在Sn3.0Ag0.5Cu/Ni和Sn3.0Ag0.5Cu/Cu的界面上均形成了(Cu, Ni)6Sn5型化合物. 时效过程中界面化合物随时效时间增加而增厚, 时效800 h后两端的化合物并没有发生转变, 仍为(Cu, Ni)6Sn5型. 电流方向对Cu基板的消耗起着决定作用. 当电子从基板端流向芯片端时, 电流导致基板端Cu焊盘发生局部快速溶解, 并导致裂纹在 Sn3.0Ag0.5Cu/(Cu, Ni)6Sn5界面产生, 溶解到钎料中的Cu原子在钎料中沿着电子运动的方向向阳极扩散, 并与钎料中的Sn原子发生反应生成大量的Cu6Sn5 化合物颗粒. 当电子从芯片端流向基板端时, 芯片端Ni UBM层没有发生明显的溶解, 在靠近阳极界面处的钎料中有少量的 Cu6Sn5化合物颗粒生成, 电迁移800 h后焊点仍保持完好. 电迁移过程中无论电子的运动方向如何, 均促进了阳极界面处(Cu, Ni)6Sn5的生长, 阳极界面IMC厚度明显大于阴极界面IMC的厚度. 与Ni相比, 当Cu作为阴极时焊点更容易在电迁移作用下失效.
The effect of electromigration (EM) on the interfacial reaction in Ni/Sn3.0Ag0.5Cu/Cu solder joints was investigated under a current density of 5.0×103 A/cm2 at 150 ℃. All solder joints were aged at 150 ℃ for comparison purpose. It has been found that the (Cu, Ni)6Sn5 intermetallic compounds (IMCs) form at both solder/Ni and solder/Cu interfaces in the as-reflowed state. During aging at 150 ℃, the thickness of interfacial IMC increases with increasing aging time, and no interfacial IMC transformation occurs even after aging for 800 h. The flowing direction of electrons plays an important role in Cu consumption. When electrons flow from printed circuit board (PCB) to chip, the current crowding effect induces a rapid and localized dissolution of Cu pad on PCB and a formation of microcrack at the Sn3.0Ag0.5Cu/(Cu, Ni)6Sn5 interface. The dissolved Cu atoms are driven towards anode by EM, and a large amount of Cu6Sn5 IMC particles form in solder matrix along the flowing direction of electrons. When electrons flow from chip to PCB, no obvious consumption of Ni underbump metallogy (UBM) has been observed and few Cu6Sn5 IMC particles form in solder matrix near the anode interface. There is no evidence of failure induced by EM in solder joints even after EM for 800 h. To sum up, EM enhances the growth of interfacial (Cu, Ni)6Sn5 at anode side, no matter how the direction of electrons is. The interfacial IMC at anode side is thicker than that at cathode side. The Ni/Sn3.0Ag0.5Cu/Cu solder joint is prone to fail when electrons flowing from Cu to Ni.
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