Eutectic solder balls (63Sn-37Pb) joined to Cu pads with an Au/Ni metallization have been widely used in wafer-level chip-size package (WLCSP) technology for providing electrical and mechanical interconnections between components. However, some reliability issues must be addressed regarding the intermetallic compounds (IMCs). The formation of a brittle IMC layer between the solder/Cu pad interface impacts considerably upon the solder-ball shear strength. In addition, it will degrade the long-term operating reliability of the WLCSP. This study investigates, by means of experiments, the growth of the IMC layer under isothermal aging for the eutectic Sn-Pb solder reflowed on a Cu pad with an Au/Ni metallization. Forming the Cu pad with an Au/Ni metallization was achieved by a simple semiconductor-manufacturing process. The effects of the intermetallic layer on solder-ball shear strength were examined for various parameters, including the thickness of the Au layer, solder-ball size, and the diameter of the Cu pad. Experimental results indicate that two IMC layers, Au 0.5 Ni 0.5 Sn 4 and Ni 3 Sn 4 , form at the solder/Cu pad interface after aging. The Au 0.5 Ni 0.5 Sn 4 intermetallic layer dominates the total thickness of the IMC layer and grows with aging time while the solder-ball shear strength decreases after aging. The degradation of the solder-ball shear strength was found to be caused mainly by the formation of the Au 0.5 Ni 0.5 Sn 4 layer. The experimental results established that a thinner Au layer on Cu pad can effectively control the degradation of solder-ball shear strength, and this is especially true for smaller ball sizes.
Purpose -Integration of Cu/low-k interconnects into the next-generation integrated circuit chips, particularly for devices below the 90 nm technology node, has proved necessary to meet the urgent requirements of reducing RC time delay and low power consumption. Accordingly, establishment of feasible and robust packaging technology solutions in relation to the structural design, as well as material selection of the packaging components, has become increasingly important. Moreover, the nature of low-k materials and the use of lead-free solder greatly increases the complications in terms of ensuring enhanced packaging level reliability. The foregoing urgent issue needs to be quickly resolved while developing various advanced packages. This paper aims to focus on the issues. Design/methodology/approach -The prediction model, especially for the fatigue life of lead-free solder joints, combined with virtual design of experiment with factorial analysis was used to obtain the sensitivity information of selecting geometry/material parameters in the proposed low-k flipchip (FC) package. Moreover, a three-dimensional non-linear strip finite element model associated with the two levels of specified boundary condition of global-local technique was adopted to shorten the time of numerical calculation, as well as to give a highly accurate solution. Findings -The results of thermal cycling in experimental testing show good agreement with the simulated analysis. In addition, the sensitivity of analysis indicates that the type of underfill material has a significant effect on the lead-free solder joint reliability. Originality/value -A suitable combination of concerned designed factors is suggested in this research to enhance the reliability of low-k FC packaging with Pb-free solder joints.
This study investigates the reliability of flip chip ball grid array (FCBGA) components with three types of solder materials: eutectic solder with a composition Sn63Pb37 and the lead-free solders SnAg3.0Cu0.5 and SnAg4.0Cu0.5. Two substrate-side solder mask (S/M) opening sizes, 0.4 mm and 0.525 mm, were used. Both the monotonic and cyclic mechanical fourpoint bend tests are conducted for the reliability assessment. It is found that the FCBGA components with SnAg3.0Cu0.5 solder have the best durability during the cyclic bend test, yet the eutectic solder is the strongest during the monotonic bend test. Besides, the FCBGA components with 0.525-mm S/M opening have around 3 times more life cycles than those with the 0.4-mm S/M opening in the cyclic bend test. It is also noteworthy that the lead-free solder materials have much variation in the failed cycles during the cyclic test. Moreover, the failure locations for those components with 0.4-mm S/M openings are found to be at the interface between the package side metal pad and the solder ball, and those with an S/M opening of 0.525 mm are observed to be failed mostly at the interface between the printed circuit board (PCB) side metal pad and the solder ball.
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