Adding Cu and Ni to Solders on the Mechanism of Retarding Micro Voids and Cu3Sn
Date Issued
2010
Date
2010
Author(s)
Wang, Yi-Wun
Abstract
The drive for solders in the microelectronics industry presents some reliability challenges. Examples include package compatibility, creep and micro voids. Along the Cu3Sn/Cu interface, we can find a series of micro voids. These micro voids were the true culprit responsible for the weakening of the interface. It is of importance for the reliability to have better understanding and control of the solder/metallization interactions during soldering. In the reactions between solders and Cu substrate, the formation of micro voids within the Cu3Sn layer had been report by many research groups. Because the Cu3Sn growth had been linked to the formation of micro voids, which in turn increased the potential for a brittle interfacial fracture, thinner Cu3Sn layers might translate into better solder joint strength. It is widely accepted that the formation of these micro voids is related to the growth of Cu3Sn. Main thrust of solder development has shifted to the minor alloy additions to retard Cu3Sn growth. However, the mechanism explaining how adding minor alloys can reduce the Cu3Sn thickness is still lacking.
The main objectives were to investigate the effect of Cu concentration and the effect of Ni addition on the formation of micro voids and the mechanism of Cu3Sn growth. This study divides two parts. In first part, we discuss the relation between apparent Cu concentration in solder and micro voids. In second part, we discuss the relation between Ni addition and Cu3Sn growth mechanism. The experiment parameters and results are as follows:
First part experiment:
The interfacial reaction between Cu and Sn-xCu (x= 0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8 wt.%) solders were examined. Emphasis was placed on the effect of Cu composition on the formation of micro voids in the Cu3Sn phase. Reaction conditions included one reflows and subsequent aging at 160 oC for up to 2000 h. After aging, both Cu6Sn5 and Cu3Sn formed at interface, and Cu3Sn was void-free, and remained void-free after 500 h of aging for all of the compositions. The micro voids appeared in Cu3Sn after 1000 h of aging, but only in those solder joints with their apparent Cu concentrations that were 0.46 wt.% and lower. After 2000 h of aging, micro voids formed only in solder joints with their apparent Cu concentrations that were 0.50 wt.% and lower. This observation suggests that the micro voids in Cu3Sn can be effectively inhibited during aging at 160 oC for 2000 h by maintaining a high Cu concentration in solder (e.g ≧ 0.58 wt.%). Rationalization for this Cu concentration effect is presented.
Second part experiment:
In the reactions between solders and Cu, minor alloy additions, such as Fe, Co, or Ni, to solders often reduce the Cu3Sn growth rate. Nevertheless, the mechanism for this effect remains unresolved. In order to investigate the effects of Ni on Cu3Sn, the solders used for this study are 10Sn90Pb and 5Sn95Pb doped with 0, 0.03, 0.06, 0.1, and 0.2 wt.% Ni. Reaction conditions included one reflow at 350oC for 2 min and solid-state aging at 160 oC for 500, 1000 and 2000 h. In reflow study, Cu3Sn was the only reaction product observed for all the different solders used. In solid state aging study, both Cu3Sn and Cu6Sn5 formed in 10Sn90Pb-xNi solders, but only Cu3Sn formed in 5Sn95Pb-xNi solders. The growth order of intermediate component can clearly know by using the ternary Cu-Sn-Pb phase diagrams. The key observation to be made in this study objective is to see whether the growth rate of Cu3Sn can be reduced by Ni additions under these situations. The experimental results show that minor Ni addition to high-lead solder can’t retard Cu3Sn thickness. However, the Ni addition to lead-free solder can retard Cu3Sn. This is because adding Ni to lead-free solder transformed the microstructure into a much loose (Cu,Ni)6Sn5. The Cu atoms prefer to react with Sn atoms of Cu6Sn5 interstice. They tend to form Cu6Sn5. In addition, the morphology of (Cu,Ni)6Sn5 gradually become layer-type (Cu,Ni)6Sn5 after aging. Thicker Cu6Sn5 layer-type morphology becomes a good diffusion barrier stopping the necessary atomic flux necessary for the growth of Cu3Sn. Therefore, Ni retards the growth of Cu3Sn through Cu6Sn5.
The objective of this study is to investigate individually the influences of Cu concentration on micro voids and Ni addition on Cu3Sn growth. Emphasis is placed on a systematic comparison study on the effects of Cu and Ni addition.
Subjects
minor elements
micro voids
Cu concentration
Type
thesis
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