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Title: Microstructural analysis of lead-free solders with under bump metallization
Other Titles: Microstructural analysis of lead-free solders with under bump metallization
Wu qian han liao yu di bu jin shu du ceng zhi jian jie mian fan ying de wei guan jie gou fen xi
Authors: Islam, Md. Nazrul
Department: Dept. of Electronic Engineering
Degree: Doctor of Philosophy
Issue Date: 2006
Publisher: City University of Hong Kong
Subjects: Electronic packaging
Lead-free electronics manufacturing processes
Solder and soldering
Notes: CityU Call Number: TK7870.15.I75 2006
Includes bibliographical references.
Thesis (Ph.D.)--City University of Hong Kong, 2006
xxii, 243 leaves : ill. ; 30 cm.
Type: Thesis
Abstract: The concurrent developments in emerging technology to realize increasingly miniaturized electronic products with multifunctional capabilities have driven electronic packaging to a high density, with the solder joints constantly shrinking towards much smaller scales. Therefore, selection of appropriate combinations of under bump metallization (UBM) of ball grid array (BGA) substrate and lead-free solder play an important role in developing a reliable electronic product. Sn-Ag, Sn-Ag-Cu, Sn-Ag- Cu-In, Sn-Ag-Cu-Bi, Sn-Cu, Sn-Zn and Sn-Zn-Bi solders have been selected for soldering and aging with Cu, Ni/Au and NiP/Cu UBM. The interface between the solder and the UBM affect the reliability of the solder joints. Microscopic analysis of these solder joints have been conducted by scanning electron microscopy (SEM) equipped with an energy dispersed X-ray (EDX) facility to investigate the composition and properties of intermetallic compounds formed as a function of time and temperature. An investigation has been carried out to compare the interfacial reactions of the UBM of a BGA substrate with molten eutectic Sn-3.5%Ag-0.5%Cu solder having different volumes. The Cu consumption was much higher for the Cu/Sn-Ag-Cu solder system with a higher solder volume. The interfacial reaction rate for the Cu/solder system is higher than that for the Ni/solder system. A change in solder volumes had no significant effect on the dissolution rate of the Ni layer. However, for both cases, the interfacial IMC thickness was higher with smaller solder balls. Higher spalling of medium-Cu containing TIMC and resettlement of Au are observed in the smaller volume solder ball/Ni system. Solder joints made with larger balls are more reliable than those made with smaller solder balls. The changes in the thickness of the Ni layer have a significant effect on the growth rate of IMC and the dissolution rate of the Ni layer. Copper atoms can diffuse through the thin Ni layer and TIMC layer, which change the microstructure and morphology of the interface. However, a higher consumption rate of the Au/Ni/Cu UBM in the thin Ni/solder system affects the reliability of the solder joint. For long-term reliability of the lead-free solder joint, a thin Ni layer should be avoided. During reflow, the formation of Sn-Cu-Ni TIMCs on the Ni layer is much quicker than the formation of Ni-Sn BIMCs. However, in the case of electroless NiP, the growth rates of IMCs are almost the same. The dissolution of NiP layer and P-rich Ni layer thickness in Sn-Ag-Cu solder joint are lower during 30 minutes reflow than those of the Sn-Ag solder. Sn-Ag-Cu solders have higher shear loads with Ni layer but show a relatively lower shear load with a NiP layer. Cu-containing solder alloys (Sn-Cu, Sn-Ag-Cu, and Sn-Ag-Cu-In) have been used to identify their interfacial reactions with NiP UBM. Some In-Sn-Au IMCs have entrapped into the interfacial IMCs due to lower diffusion of Au in the In-containing solder than that of the Sn-Cu and Sn-Ag-Cu solder. During extended reflow, high-Cu containing TIMCs have a lower growth rate and consume less of the NiP layer. The spalling of medium-Cu containing TIMCs in the Sn-Ag-Cu solder increases both the growth rate of TIMCs and the consumption rate of the NiP layer. Low-Cu containing QIMCs in the Sn-Ag-Cu-In solder are stable on P-rich Ni and reduce the dissolution rate of the NiP layer. Consumption of the NiP layer can be reduced by adding Cu or In, because of the changes of the interfacial IMCs phases which are stable and adhered well to the P-rich Ni layer during reflow. The addition of 1 wt% Bi into the Sn-2.8Ag-0.5Cu solder inhibits the excessive formation of IMCs during the soldering reaction and thereafter during aging. A significant increase of IMC layer thickness was observed for both solders where the increasing tendency was lower for the Bi-containing solder. The formation of IMCs during aging for both solders follows the diffusion control mechanism and the diffusion of Cu is more pronounced for the Sn-2.8Ag-0.5Cu solder. The IMC growth rate constants for Sn-2.8Ag-0.5Cu and Sn-2.8Ag-0.5Cu-1.0Bi solders were calculated as 2.21 x 10-17 m2/s and 1.91 x 10-17 m2/s respectively, which had a significant effect on the growth behavior of IMCs during aging. In the case of Ni/Sn0.7Cu solder system, Cu prevents the resettlement of Au to the interface. The shear load of solder joints is relatively stable from 1.98 to 1.86 kgf during long time reflow with high Ni and negligible amount of Au in the TIMCs at the interface. It is seen that shear load does not depend on the thickness of IMCs and reflow time. The percentage of Au into the high Ni-containing ternary and quaternary compounds is found to be low due to preferential diffusion of Au. The shear load during aging is not stable due to the formation of more Au-containing compounds at the interface. Cu and Ni play a significant role for interfacial reactions and formation of different type of IMCs which affect the reliability of solder joint The interfacial reactions of Sn-Zn based solders and a Sn-Ag-Cu solder have been compared with an eutectic Sn-Pb solder. The morphologies of the IMCs are quite different for different solder compositions. As-reflowed, the growth rates of IMCs in the Sn-Zn based solder are higher than in the Sn-Ag-Cu and Sn-Pb solders. Bi offers significant effects on the wetting, the growth rate of IMCs as well as on the size and distribution of Zn-ULFK_SKDVHV_LQ_WKH__-Sn matrix. No Cu-Sn IMCs are found in the Sn- Zn based solder during 20 minutes reflow. The consumption of Cu by the solders are ranked as Sn-Zn-Bi>Sn-Ag-Cu>Sn-Zn>Sn-Pb. Despite the higher Cu-consumption rate, Bi-containing solder may be a promising candidate for a lead-free solder in modern electronic packaging, taking into account its lower soldering temperature and material costs.
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