Owing to the ban of lead, the conventional lead-bearing solder has been replaced by lead-free solder. Most lead-free solders are Tin-based. The drive for lead-free solders in the microelectronics industry presents some reliability challenges. Examples include package compatibility, creep , and Kirkendall void. Along the Cu 3 Sn/Cu interface, we can find a series of Kirkendall void. These Kirkendall void were the true culprit responsible for the weakening of the interface. It is widely accepted that the formation of these Kirkendall void is related to the growth of Cu 3 Sn. In order to promote the quality of lead-free solder, minor elements addition can reduce the Cu3Sn thickness. Recently, our research group showed that a 0.1 wt.% Ni addition to SnAg could reduce the Cu 3 Sn thickness during the solder/Cu reaction. We want to extend this past result to find out the minimum level of Ni addition that still retains this beneficial effect. In addition, we will also investigate whether the elements, Fe and Co will have a similar effect. The experimental solder alloys were fabricated from 99.999% purity Sn, Ag, Cu, Fe, Co, and Ni. The objective of this study is to investigate the effects of minor Fe, Co, and Ni on the soldering and aging reactions between lead-free solders and Cu. The experimental result shows that the presence of Ni can in fact reduce the growth rate of Cu 3 Sn but increase the formation of Cu 6 Sn 5 . Moreover, the presence of Fe and Co can have the some effect. We can find the Kirkendall void in the reaction between Sn2.5Ag-xNi (x=0~0.1wt. %) and electroplated Cu at 160 o C for excess 1000 hr. The observation of Kirkendall void formation near the Cu 3 Sn/Cu is direct evidence of Cu diffusion since we can use the voids to serve as diffusion markers. On the side, we didn't find voids in the reaction between Sn2.5Ag0.8Cu-xNi (x=0~0.1wt. %) and electroplated Cu. The growth of voids is complicated. We consider that the Cu concentration in the solders is the factor to control the void formation. In the Sn2.5Ag-xNi solders, the addition of Ni also produces two distinct Cu 6 Sn 5 regions at the interface. The outer region contains more Ni, and the inner region contains less Ni. Cooling conditions changed the Ni content of the Cu 6 Sn 5 formed at the interface. Besides, the Sn2.5Ag0.8Cu-xNi solders didn't have two different Ni content in the Cu 6 Sn 5 . This is because there are more Cu 6 Sn 5 precipitated in the Sn2.5Ag0.8Cu-xNi than in Sn2.5Ag-xNi solders. A part of Ni could be dissolved in the Cu 6 Sn 5 . Therefore, a few Ni could come back to interface.
A low-temperature, pressureless bonding process, referred to as the microfluidic electroless interconnection process, has been developed for chip-stacking applications, in which electroless Ni plating is employed to bond copper pillars in an attempt to reduce the risk of thermo-mechanical damage during the assembly process. Copper pillar joints with a low standoff height were used to assess the bonding performance of the microfluidic electroless interconnection process on a smaller scale. The results showed that a striking lamellar structure formed in the deposits when electroless Ni solution was fed into a microfluidic platform intermittently, and through this visible structure, the bonding mechanism of the process can be characterized fully. Preliminary results showed that a high level of plating uniformity across the die could be obtained using the microfluidic interconnection process, and that the copper pillars were joined completely by electroless Ni plating without voids or seams. Further, it was found that the process is able to compensate not only for non-uniform copper surfaces, but also for the misalignment of copper pillars, which provides a competitive edge over other bonding methods. In addition, the direct shear test showed that the bond strength of the electroless Ni bonds was quite strong.
The solders used for this study are Sn2.5Ag0.8Cu doped with 0.03 wt.% Fe, Co, or Ni and 0, 0.005, 0.01, 0.06, or 0.1 wt.% Ni. Reaction conditions included multiple reflows for up to 10 times and solid-state aging at 160degC for up to 2000 hrs. In multiple reflow study, Cu 6 Sn 5 was the only reaction product observed for all the different solders used. Reflows using the solder without doping produced a thin, dense layer of Cu 6 Sn 5 . The additions of Fe, Co, or Ni transformed this microstructure into a much thicker Cu 6 Sn 5 with many small trapped solder regions between the Cu 6 Sn 5 grains. In solid state aging study, both Cu 6 Sn 5 and Cu 3 Sn formed, but the additions of Fe, Co, or Ni produced a much thinner Cu 3 Sn layer. Specifically, Ni concentration higher than 0.01 wt.% could effectively retard the Cu 3 Sn growth even after 2000 hrs of aging, and accordingly 0.01 wt.% can be considered the minimum effective Ni addition. Because the Cu 3 Sn growth had been linked to the formation of micro voids, which in turn increased the potential for a brittle interfacial fracture, thinner Cu 3 Sn layers might translate into better solder joint strength.
The development of solder is from Pb-bearing to Pb-free solder because of toxin. Nowadays, Sn-based solder is usually chosen as the prime materials in electronic packaging. However, the melting point of Sn-based solder is higher than Pb-bearing solder about 40 °C. High process temperature causes the side effects on devices such as warpage, crack and damage. Requiring the melting point of solder is below 200 °C to overcome thermal budget. In is a good candidate to replace Sn owing to its good mechanical properties. Recently, a Cu/In structure is developed for 3D integration applications. Nevertheless, fewer researches investigate low temperature reaction between Cu and In. Previous literature of Cu-In interfacial reactions focused on thick In layer and high temperature reactions. Studying thin In layer and low temperature reactions are important for 3D IC integration. Microstructure evolution of Cu-In compounds at low-temperature are regarded as useful database for interfacial reaction. This study not only concentrates on Cu/In reaction but also discusses deeply the reaction between Cu, In and Ni. Electroplating In layer on Cu metallization and subsequently electroplating Ni on In. The electroplating procedure is one of the major challenges because of the rapid diffusion of Cu and In. Microstructure observation are from 100 °C to 140 °C. The samples were mounted in epoxy resin and ground with SiC paper and Al2O3 powders. The reaction zone was observed using an optical microscope and a scanning electron microscope (SEM). The compositions of Cu-In and Ni-In compounds were determined using an electron microprobe (FE-EPMA). The main results were as follows: (1) (Cu,Ni)In2 formation after electroplating, (2) (Cu,Ni)In2 is stable at 100 °C, (3) (Cu,Ni)11In9 is dominant phase at 140 °C, (4) Microvoids within (Cu,Ni)11In9 and (Ni,Cu)3In7 formation at 140 °C. The result provides detailed Cu-In and Ni-In compounds development and key findings related to the reliability of In-based solder. This study presents the growth of intermetallic compounds in Cu/In/Ni scheme resulting from aging at 100 °C and 140 °C.
Process temperature must be reduced to avoid the reliability degradation of 3D IC packages due to thermal damage of temperature-sensitive devices. Low-temperature bonding avoids thermal damages and bonding misalignment. Owing to the ban of lead, the lead-bearing solder has been replaced by lead-free solder. However, the melting point of lead-free solder is higher than lead-bearing solder. During high-temperature reflow process, the large package warpage is caused by the temperature. Requiring the use of a bonding process at low temperature below 200 °C. Low melting point solder can be considered as the interconnect material to reduce processes temperature. In is one of the potential materials because of its low melting point, ductility and anti-corrosion. The In-based solder is a potential candidate to replace Sn-based solder. However, there are only few previous literatures in the reaction between In and metallizations. It is of outmost important to realize the microstructure evolution of In-based solder. The interfacial reactions between In and Cu is studied by means of SEM, FIB, FE-EPMA and Nanoindention in this study.
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