Effects of Joining Sequence on the Interfacial Reactions and Substrate Dissolution Behaviors in Ni/Solder/Cu Joints

Abstract

The effects of the joining sequence on the interfacial reactions and substrate dissolution behaviors in Ni/solder/Cu joints were studied by using 500-lm (diameter) Sn-3.5Ag solder balls and substrates with a 375-lm (diameter) opening. Three distinct paths for the joining sequence were studied. In path I, a solder ball was first joined to the Cu substrate and then to the Ni substrate. In path II, a solder ball was joined to both the Cu and Ni substrates simultaneously. Path III had the opposite joining sequence to path I. The results of this study indicated that (Cu,Ni) 6 Sn 5 was the predominant reaction product at both the Ni/solder and solder/Cu interfaces regardless of the joining sequence. However, the composition, morphology, and thickness of the (Cu,Ni) 6 Sn 5 varied considerably with the different paths. The dissolution behaviors of Cu and Ni were also different. Dybkov's dissolution kinetics and Cu-Ni-Sn isotherm data were utilized to rationalize these differences.

Theoretical and experimental determination of Cu diffusivity in eutectic Sn–Ag system at 235–280°C

“…The Cu 6 Sn 5 in the solder matrix exhibited a rod-like morphology rather than the scallop-like structure at the interface. Recent studies showed that 9 μm of Cu was depleted after a soldering reaction with the Sn-3.5Ag alloy, resulting in a Cu concentration in the solder greater than 1 wt.% [3,8,9]. The dissolved Cu precipitated out as the primary Cu 6 Sn 5 phase in the solder matrix during solder solidification due to the solubility limit in the solid solder.…”

“…They concluded that D Cu is one of the major factors, dominating the dissolution of the solid Cu into the molten solders [7]. However, unlike the scenarios examined in the previous works [1,7], Cu might behave in a very different diffusion/dissolution manner in modern microelectronic packaging because the solder volume of such a packaging scale is quite limited (typically less than 500 μm in diameter) and the entire solder enables saturation with Cu very quickly [3,8,9] compared to the bulk reactive couple [1,7]. The out diffusion of Cu driven by the Cu concentration gradient (chemical potential gradient) is expected to gradually decrease as a function of time and to eventually cease once the Cu concentration in the solder reaches the solubility limit [3,8,9].…”

Formation mechanism of pillar-shaped intermetallic compounds dispersed lead-free solder joint

“…Formation of the IMC pillar is considered as follows. ① to ②: Cu dissolves in molten solder until the solubility limit of Cu in solder at the joining temperature [7,8]. ② to ③: When the temperature of solder reduces below the liquidus line, crystallization of Cu 6 Sn 5 IMCs starts.…”

Solid–Solid Reaction Between Sn-3Ag-0.5Cu Alloy and Au/Pd(P)/Ni(P) Metallization Pad with Various Pd(P) Thicknesses

“…The driving force for Cu 6 Sn 5 migration is to seek Ni from the Ni(P) substrate, which allows the phase to turn into an even more stable Ni-containing phase, i.e., (Cu,Ni) 6 Sn 5 . [17][18][19] However, a dense layer of (Cu,Ni) 6 Sn 5 behaved as an effective diffusion barrier between the solder and the Ni; 17,38 in this case, the migration of Cu 6 Sn 5 was retarded because the Ni diffusion was blocked by the structure, thereby decelerating the growth of (Cu,Ni) 6 Sn 5 . In contrast, when the (Cu,Ni) 6 Sn 5 was discontinuous at the interface [e.g., the 0.22-lmPd(P) case after reflow; Figs.…”

“…38 However, the composition of the (Cu,Ni) 6 Sn 5 domains varied with their location in the solder matrix. It was found that the Ni content of (Cu,Ni) 6 matrix dissolved with much less than 8 at.% Ni after reflow, which is not sufficient to stabilize Cu 6 Sn 5 in the solder region of the joint aged at 180°C.…”