(A) study on the reaction between lead-free solders and electroless Ni(P) metallization and its effect on mechanical reliability

Abstract

Reactions between electroless nickel-phosphorus (Ni(P)) metallization and lead-free solders were studied to investigate the mechanism of spalling of intermetallic compounds and its relationship with crystallization of the Ni(P) film. Based on the reaction study, mechanical testing was conducted to understand brittle fracture phenomena found when electroless Ni(P) metallization is used. The Ni(P) films with P 4.6, 9, and 13 wt.% were reacted with Sn to investigate the relationship between solder reaction at low temperature and self-crystallization at high temperature. $Ni_3Sn_4$ is detected in all samples heat-treated to 300 ℃ and some $Ni_3Sn_4$ change into $Ni_3Sn_2$ upon further heating to 450 ℃ when Sn is not sufficient. $Ni_3Sn_4$ starts to form around 190 ℃ well below the melting temperature of Sn and the heat of reaction of $Ni_3Sn_4$ is measured to be 241.2 J/g. A Sn layer on top of Ni(P) reduces the self-crystallization temperature by about 10 ℃. The heat of self-crystallization also decreases with an increased Sn thickness. The interfacial $Ni-Sn IMC(Ni_3Sn_4)s$ showed strong (111) texture when the P content of the underlying Ni(P) layer was low (4.6 wt.% P). The thickness and density of Ni-Sn IMCs formed on Ni(P) were strongly affected by the P content of the underlying Ni(P) layer; higher P content yields thinner and less densely packed IMCs. Growth of $Ni_3Sn_4$ intermetallic compound on the EP Ni and the electroless Ni(9P) metallization was studied during the reaction with Sn3.5Ag at 250 ℃. The IMCs on the Ni(9P) layer grew thicker than those on EP Ni, which means that EP Ni metallization is better diffusion barrier layer. The time exponents for the thickness and lateral size of the IMCs on EP Ni were 4.4 and 7.7, respectively. And the time exponents for the thickness and lateral size of the IMCs on EP Ni were 3.6 and 2.8, respectively. The IMCs formed on the Ni(9P) layer grew faster with increasing reaction time. The ratio of IMC thickness to l...