Combined Effects of Cu Concentration, Solder Volume, and Temperature on SnAgCu/Ni Interfacial Reaction
Date Issued
2010
Date
2010
Author(s)
Yang, Su-Chun
Abstract
Soldering is the most important joining technology in the electronic industry and the Sn-Ag-Cu serious of lead-free solders, around the Sn(3.5Ag±0.3)Ag(0.9±0.2)Cu (wt.%) ternary eutectic composition, are the most promising alternates. With the miniaturization of modern electronic products and devices, the packaging density of components such as chips significantly increases, and the scale of interconnections becomes smaller and smaller. Consequently, the reliability of solder joints plays key roles. And understanding the volume effect of SAC solder on interfacial reaction will be critical in solder joint reliability. Furthermore, temperature is also an important factor in the reaction between solder and substrate. In this study, the effects of Cu concentration, solder volume and temperature are studies concurrently.
In this study, three different sizes of solder spheres (300, 500, and 760 micrometer diameter) with different Cu content (0.3 - 0.7 wt.%) were used to investigate the effect of Cu concentration and solder volume on interfacial reaction between SnAgCu and Ni. The samples were reflowed and subsequent aged at 160oC and 180oC.
In our previous studies, the “Cu concentration effect” reported in the literatures were established in bulk reactions where the supply of Cu approached infinite. As a result, the Cu concentration could be assumed to be constant during the study. In those studies, the interface had reached local equilibrium and the Cu-Ni-Sn isotherm could be used to rationalize the formation of the reaction product(s). For example, when the Cu concentration was high, (Cu,Ni)6Sn5 was predicted to form next to the (Sn) phase. When the Cu concentration was low, (Ni,Cu)3Sn4 was predicted to form next to the (Sn) phase. When the Cu concentration was in-between, the diffusion path would pass through the three-phase field of (Sn) + (Cu,Ni)6Sn5 + (Ni,Cu)3Sn4 first. This study reveals both the Cu concentration and the solder volume had a strong effect on the type of the reaction products formed. The well-known “Cu concentration effect” refers to the sensitivity on the Cu concentration for the reactions between Ni and Cu-bearing solders.
In a solder joint, the supply of Cu is limited because Cu is a minor constituent in solders. During the reaction, Cu in solder is incorporated into the reaction product(s), and as the amount of the product(s) increase, Cu in solder is gradually consumed. Consequently, the Cu concentration in solder might not be a constant during soldering. This change in Cu concentration may then makes the reaction product at the interface change from one compound to another. In the case that the Cu concentration does decrease from a concentration higher than 0.4 wt.% to less than 0.4 wt.% during reflow, the (Cu,Ni)6Sn5 which is original at the interface will spall massively because now Ni prefers to be in contact with (Ni,Cu)3Sn4.
This massive spalling phenomenon is also observed in the other three systems, SnZn/Cu, high-Pb PbSn/Cu, and high-Pb PbSn/Ni and a unified thermodynamic argument is presented to rationalize the occurrence of the massive spalling in four different solder/substrate systems. The massive spalling occurs because during reaction the original reaction product at the interface is no longer in local thermodynamic equilibrium with the solder, and this compound is driving away to make room for the nucleation and growth of the equilibrium phase. Two necessary conditions for the occurrence of massive spalling are identified. The number one condition is that the reactive constituent must be present in a limited amount, and the second is that the soldering reaction must be very sensitive to the concentration of this element. However, in SnAgCu/Ni system, massive spalling phenomenon didn’t occur on solid-solid reaction as the equilibrium at interface changed. On solid-state reaction, (Ni,Cu)3Sn4 grows and (Cu,Ni)6Sn5 shrinks as the equilibrium compound changes from (Cu,Ni)6Sn5 to (Ni,Cu)3Sn4.
Finally, the critical Cu concentration was a strong function of temperature. The relationship between critical Cu concentration and temperature has been calculated by Yu et al. [58] and Chen et al. [40]. Our study confirmed the critical Cu concentration for the formation of (Cu,Ni)6Sn5 or (Ni,Cu)3Sn4 decreases as temperature drops.
Subjects
Lead-free solders
Cu concentration effect
Solder volume effect
Temperature effect
Thermodynamics equilibrium
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