Jae-Woong Nah

Mechanism of electromigration-induced failure in the 97Pb–3Sn and 37Pb–63Sn composite solder joints

The electromigration-induced failure in the composite solder joints consisting of 97Pb–3Sn on the chip side and 37Pb–63Sn on the substrate side was studied. The under-bump metallization (UBM) on the chip side was 5 μm thick electroplated Cu coated on sputtered TiW/Cu and on the substrate side was electroless Ni/Au. It was observed that failure occurred in joints in a downward electron flow (from chip to substrate), while those joints having the opposite current polarity showed only minor changes. During electromigration, in addition to the compositional change by the moving of Pb atoms in the same direction as the electrons, current crowding was observed inside the UBM and it enhanced the phase transformation of Cu to Cu3Sn and to Cu6Sn5 at the UBM/solder interface. Due to the growth of Cu6Sn5, the Cu UBM was consumed rapidly, resulting in void formation-induced failure at the cathode side. The Cu6Sn5 intermetallic compound and void were first initiated from the upper left-hand side corner of the contact ...

Electromigration in flip chip solder joints having a thick Cu column bump and a shallow solder interconnect

In advanced electronic products, current crowding induced electromigration failure is one of the serious problems in fine pitch flip chip solder joints. To explore a strong resistance against current crowding induced electromigration failure, a very thick Cu column bump combined with a shallow solder interconnect at 100μm pitch for flip chip applications has been studied in this paper. Results revealed that these interconnects do not fail after 720h of current stressing at 100°C with a current density of 1×104A∕cm2 based on the area of interface between Cu column bump and solder. The reduction of current crowding in the solder region by using thick Cu column bumps increased the reliability against electromigration induced failure. The current distribution in a flip chip joint of a Cu column bump combined with a shallow solder has been confirmed by simulation. However, Kirkendall void formation was found to be much serious and enhanced by electromigration at the Cu∕Cu3Sn interface due to the large Cu∕Sn ra...

Electromigration induced ductile-to-brittle transition in lead-free solder joints

The effect of electromigration on ductile-to-brittle transition in flip chip solder joints has been studied using one-dimensional bamboo-type samples of eutectic 95.5Sn–3.8Ag–0.7Cu solder joined by Cu wires at two ends. Both electrical current and tensile stress were applied to the samples either in serial or in parallel. In serial tests, the strain rate was 6×10−3. In parallel test, the creep stress was 7MPa. The current density applied was (1–5)×103A∕cm2. The working temperature was 100–150°C. In both tests, the authors observed the ductile-to-brittle transition in which the fracture migrates from the middle to the cathode interface of the joint with increasing current density and time. The transition is explained by the polarity effect of electromigration, especially the accumulation of vacancies at the cathode interface.

Effect of current crowding and Joule heating on electromigration-induced failure in flip chip composite solder joints tested at room temperature

The electromigration of flip chip solder joints consisting of 97Pb–3Sn and 37Pb–63Sn composite solders was studied under high current densities at room temperature. The mean time to failure and failure modes were found to be strongly dependent on the change in current density. The composite solder joints did not fail after 1month stressed at 4.07×104A∕cm2, but failed after just 10h of current stressing at 4.58×104A∕cm2. At a slightly higher current stressing of 5.00×104A∕cm2, the composite solder joints failed after only 0.6h due to melting. Precipitation and growth of Cu6Sn5 at the cathode caused the Cu under bump metallurgy to be quickly consumed and resulted in void formation at the contact area. The void reduced the contact area and displaced the electrical path, affecting the current crowding and Joule heating inside the solder bump. Significant Joule heating inside solder bumps can cause melting of the solder and quick failure. The effect of void propagation on current crowding and Joule heating was...

Investigation of flip chip under bump metallization systems of Cu pads

In this study, UBM material systems for flip chip solder bumps on Cu pads were investigated using the electroless copper (E-Cu) and electroless nickel (E-Ni) plating methods; and the effects of the interfacial reaction between UBMs and Sn-36Pb-2Ag solders on the solder bump joint reliability were also investigated to optimize UBM materials for flip chip on Cu pads. For the E-Cu UBM, scallop-like Cu/sub 6/Sn/sub 5/, intermetallic compound (IMC) forms at the solder/E-Cu interface, and bump fracture occurred along this interface under a relatively small load. In contrast, at the E-Ni/E-Cu UBM, E-Ni serves as a good diffusion-barrier layer. The E-Ni effectively limited the growth of the IMC at the interface, and the polygonal-shape Ni/sub 3/Sn/sub 4/ IMC resulted in a relatively higher adhesion strength compared with the E-Cu UBM. As a result, electroless deposited UBM systems were successfully demonstrated as low cost UBM alternatives on Cu pads. It was found that the E-Ni/E-Cu UBM material system was a better choice for solder flip chip interconnection on Cu pads than the E-Cu UBM.

Electromigration in Pb-free flip chip solder joints on flexible substrates

Flip chip on flexible substrate technology has attracted much attention in the electronics industry for portable applications because the reduction of packaging volume coupled with long-term reliability is a challenging issue. In this study, the electromigration behavior of flip chip Sn96Ag3.5Cu0.5 solder joints on flexible substrates has been investigated under a constant current density of 4×104A∕cm2 at room temperature. The in situ resistance change of the circuit during the electromigration test was measured and found to show stepwise changes before the final failure. Scanning electron microscopic images of the cross section of samples showed the existence of voids in the bulk of the solder joints before electromigration and the movement of these voids after electromigration. The void movement was matched with the resistance change during the electromigration test, and voids moved from within the bulk of solder bump to the cathode interface between Al interconnection and under bump metallurgy in the d...

Electromigration Study in Flip Chip Solder Joints

We have studied the effect of thickness of Cu under bump metallization (UBM) from 5 mum, 10 mum to 50 mum on electromigration induced failure mechanism in flip chip solder joints. In the case of 5 mum Cu UBM, due to the direct current crowding effect at the UBM/solder interface, the failure mode induced by electromigration was the loss of UBM and the interfacial void formation at the cathode contact interface between the interconnect line and the solder bump. The current crowding effect in flip chip solder joints were reduced when the Cu UBM thickness was increased to 10 mum, and the flip chip joint with 10 mum Cu UBM showed much longer mean-time-to-failure than that with 5 mum Cu UBM because the 10 mum Cu UBM was enough to contain the current crowding inside the UBM. However, even when the current distribution was uniform in the solder area in this case, the final failure mode was the same as the case of 5 mum UBM. The failure occurred by a two-stage consumption of the 10 mum thick Cu UBM in the joint where electrons flowed from the chip to the substrate. In the first stage, the 10 mum Cu UBM dissolved layer by layer at the entire Cu UBM/solder interface. After half of Cu UBM was dissolved, the asymmetrical dissolution of Cu UBM was concentrated at the corner where electrons entered from Al interconnect to Cu UBM. A small number of Kirkendall voids were found in the Cu3Sn layer during the electromigration testing, but they were not a serious concern in this case. When the Cu UBM was 50 mum thick, more uniform distribution of current density was obtained in the solder bump, and the flip chip joint showed a very strong resistance against electromigration-induced failure. In the case of a 50 mum thick Cu UBM combined with a 20 mum height shallow solder interconnect, the flip chip joint did not fail after 720 hours of current stressing at 100degC with a current density of 6.75 x 104 A/cm2 calculated at the pad opening on the Si chip side. The reduction of current crowding in the solder region by using thick Cu UBM and the small ratio of Sn to Cu in the bump structure improved the reliability against electromigration-induced failure. The effect of Cu thickness on reducing current crowding during electromigration has been confirmed by simulation. However, in the case of 50 mum thick Cu UBM, Kirkendall void formation was found to be much more serious and the formation was enhanced by electromigration at the Cu/Cu3Sn interface due to the large Cu/Sn ratio. Furthermore, a very large temperature gradient exists across the shallow solder interconnects, leading to thermomigration. Electromigration accompanied by thermomigration could replace current crowding as a serious reliability issue in using 50 mum thick Cu UBM based interconnects. To provide a comparison for the microstructure evolution caused by electromigration and by thermal annealing alone, a flip chip joint with 50 mum thick Cu UBM and with 20 mum height shallow solder bump was investigated without any current stressing but with aging at 150degC for 720 hours. In the case of thermal aging, thick Intermetallic Compounds (IMCs) were observed not only under the Cu column bumps but also on the sidewall finish of the substrate because there was no electromigration and no temperature gradient for thermomigration between the chip and the substrate. The Kirkendall void formation was not very serious in the case of thermal aging as compared to the case of electromigration because Sn moved to both sides to form IMCs during thermal aging.

Formation of Pb/63Sn solder bumps using a solder droplet jetting method

Formation processes of Pb/63Sn solder droplets using a solder droplet jetting have not been sufficiently reported. Solving problems such as satellite droplets and position errors are very important for a uniform bump size and reliable flip-chip solder bump formation process. First, this paper presents the optimization of jet conditions of Pb/63Sn solder droplets and the formation process of Pb/63Sn solder bumps using a solder droplet jetting method. Second, interfacial reactions and mechanical strength of jetted Pb/63Sn solder bumps and electroless Ni-P/Au UBM joints have been investigated. Interfacial reactions have been investigated after the second solder reflow and aging, and results were compared with those of solder bumps formed by a solder screen-printing method. Third, jetted solder bumps with variable bump sizes have been demonstrated by a multiple jetting method and the control of waveform induced to a jet nozzle. Multiple droplets jetting method can control various height and size of solder bumps. Finally, real applications of jetted Pb/63Sn solder bumps have been successfully demonstrated on conventional DRAM chips and integrated passive devices (IPDs).

Effect of electromigration on mechanical behavior of solder joints

The combination of electromigration effect and stress effect was investigated in lead free solder joints with a diameter of 300/spl mu/m. One dimensional structures, metal (wire)-solder (ball)-metal (wire) was developed. Mechanical force and current could be applied serially or simultaneously. The current density of electromigration was 1/spl sim/5/spl times/10/sup 3/ A/cm/sup 2/. The working temperature was 100-150/spl deg/C. Tensile test and shear test were taken before and after electromigration to make the comparison. The tensile strain rate was 3/spl mu/m/min. The experiment results show that, the samples broke at the middle of solder without applying current. However, after applying current of electromigration for 1 day, 2 days or longer, the failure always occurred at cathode interface. And the tensile strength was lower with longer electromigration time or higher current density. Shear test also illustrates the electromigration effect on mechanical property in composite solder joints.

A study on coining processes of solder bumps on organic substrates

Solder flip chip bumping and subsequent coining processes on printed circuit board (PCB) were investigated to solve the warpage problem of organic substrates for high pin count flip chip assembly by providing good co-planarity. Coining of solder bumps on PCBs has been successfully demonstrated using a modified tension/compression tester with height, coining rate and coining temperature variables. It was observed that applied loads as a function of coined height showed three stages as coining deformation; region of elastic deformation; region of linearly increase of applied loads; region of rapidly increase of applied loads. In order to reduce applied loads for coining solder bumps on a PCB, the effects of coining process parameters were investigated. Coining loads for solder bump deformation strongly depended on coining rates and coining temperatures. As coining rates decreased and process temperature increased, coining loads decreased. Lower coining loads were needed to prevent potential substrate damages such as micro-via failure and build-up dielectric layer thickness change during applying coining loads. It was found that coining process temperature had more significant effect to reduce applied coining loads during the coining process.