Effect of Cu content on interfacial reactions between Sn(Cu) alloys and Ni/Ti thin-film metallization

Abstract

The effect of Cu content in Sn(Cu) alloys on the interfacial reaction between Ni thin film and Sn(Cu) alloys has been investigated. We have found that the variation of Cu content has a strong influence on the spalling of the Ni thin film. With small Cu additives in the Sn, spalling was deferred to longer reflowing time. When the Cu content increased to about 1.0 wt.%, a layer of Cu-Sn compound formed on the Ni thin film, and no spalling was observed after 20-min reflowing. The possible mechanism of spalling deferring is proposed. A Cu flux from the solder to the interface compensated the ripening flux of the semispherical compound grains; therefore, spalling was retarded. The driving force of the Cu flux was attributed to the reduction of Cu solubility caused by the presence of Ni at the interface of the Ni thin film. The Cu flux from solder to the interface is calculated to be in the same order with the ripening flux of the Cu 6 Sn 5 compound grains, which confirms the proposed mechanism of spalling deferring. For the Sn(Cu) alloys having Cu content over 1.0 wt.%, the Cu-Sn compound layer grew so fast that the surface of the interfacial compound layer was free of Ni. There was no Cu flux to compensate the ripening flux; therefore, the ripening flux dominated, and spalling occurred after a short reflowing time.

Formation of Intermetallic Compounds Between Liquid Sn and Various CuNi x Metallizations

“…For example, a detailed study of the microstructures of the regions next to the Sn/Cu interface revealed numerous tubes and bundles of Cu 6 Sn 5 fibers inside the solder matrix, the formation of which has not been clarified. 13 In a few recent publications [14][15][16][17][18][19][20] the formation of intermetallic layers between Cu-bearing lead-free solders and a Ni substrate or between Ni-bearing lead-free solders and a Cu substrate have been explained by making use of the diagram proposed by Lin et al 21 On the other hand, Hsu et al 22 and Wang and Liu 23 used an earlier preliminary version of the Sn-Cu-Ni isothermal section 24,25 to explain the formation of (Cu,Ni) 6 Sn 5 in soldering reactions between Cu-alloyed Sn-Ag solders and a Ni substrate as well as in the Ni/Sn-3.5Ag/Cu sandwich structure. However, the authors did not consider either the supersaturation of the solder with dissolved Cu and Ni atoms or the existence of the metastable solubility limit.…”

“…The published results show slightly different values related to the minimum amount of Cu in Sn-based solders which is required to change the primary intermetallic compound from (Ni,-Cu) 3 Sn 4 to (Cu,Ni) 6 Sn 5 between the solder and Ni metallization. [14][15][16][17][18][19][20][21][22][23] For example, Alam et al presented that after 20 min annealing of Sn-3.5Ag-0.5Cu (wt.%) on Ni/Au metallization at 240°C the Cu content of the liquid has decreased to 0.2 wt.%, and that was the reason why (Ni,Cu) 3 Sn 4 started to form between Ni and (Cu,Ni) 6 Sn 5 . 26 Hsu et al annealed the binary liquid SnCu solders on Ni/Ti thin-film metallization at 250°C for different periods of time up to 20 min.…”

“…26 Hsu et al annealed the binary liquid SnCu solders on Ni/Ti thin-film metallization at 250°C for different periods of time up to 20 min. 22 They reported that already after 30 s a layer of (Ni,Cu) 3 Sn 4 was formed at the interface when using Sn-0.6Cu (wt.%) solder and that the higher Cu-contents resulted in the formation of (Cu,Ni) 6 Sn 5 . 22 Wu et al reported the formation of (Cu,Ni) 6 Sn 5 in the reaction between Ni/Au metallization and Sn-3.0Ag-0.5Cu wt.% solder when a peak reflow temperature of 250°C and time of 75 s were used.…”

Electromigration-Induced Failure of Ni/Cu Bilayer Bond Pads Joined with Sn(Cu) Solders

“…We can clearly observe the appearance of the typical needle-shaped morphology of the (Cu,Ni) 6 Sn 5 compound layer. [7][8][9] After 15 h of current stressing, unlike for the pure Sn/Ni/Cu joint, we found that the interfacial (Cu,Ni) 6 Sn 5 compound layer grew with the current stressing time. After a prolonged 25 h of current stressing, the interfacial (Cu,Ni) 6 Sn 5 compound layer grew even thicker (over 6 lm), much thicker than could possibly form for the same thermal annealing conditions but without the EM effect ( Fig.…”

Mitigation of Sn Whisker Growth by Composite Ni/Sn Plating

“…[4][5][6][7][8] The Ni underlayer serves as a barrier for Cu diffusion into Sn, causes Cu to dissolve into Ni at a much lower rate than into Sn, allows a certain amount of Sn to diffuse into the Ni layer developing tensile stress in the Sn layer, and delays the growth of Sn whiskers. 9 In addition, the literature states that using a pulsed plating technique can provide Ni deposition with significantly improved properties with respect to those obtained with the traditional direct current (DC) plating procedure. 10,11 Based on the valuable properties of the Ni underlayer, specifically in pulsed plating mode, we have proposed a new composite pulsed plating method as a potential technique to enhance the mitigation of Sn whisker growth.…”