Solder Bump Joints Research Articles

Bonding Property of Flip-Chip Chip-Scale Package with Vacuum Reflow and Thermal Compression Bonding Processes

The package-on-package technology has been developed to facilitate high-density integration of semiconductor packaging. As the solder bump pitch is decreased, the void content in solder bump joints increases, thereby decreasing its reliability. Consequently, vacuum reflow (VR) is used to increase void fluidity and decrease void content in micro bump joints. Generally, silicon chips in application processor modules are bonded using thermal compression (TC) bonding, followed by the attachment of passive components through hot air reflow soldering. However, the simultaneous bonding of Si chips and passive components using VR can improve productivity.In this study, we performed TC and VR bonding without additional solder pastes, between an Si chip with a Cu pillar/Sn-2.5Ag bump and a bismaleimide triazine substrate plated with electroless-nickel electroless-palladium immersion-gold. It was observed that the void content at the Si chip solder joints yielded by VR (10.8%) was similar to that yielded by TC (11.2%). Furthermore, the Si chip solder joint of the VR sample was thicker than that of the TC sample. This process also resulted in the formation of the intermetallic compounds Cu<sub>6</sub>Sn<sub>5</sub>, (Cu,Ni)<sub>6</sub>Sn<sub>5</sub>, Ag<sub>3</sub>Sn, and Ni<sub>3</sub>P, at the Si chip solder joints. The results of this study suggest that co-bonding of active and passive components is possible through VR soldering for semiconductor packaging.

Growth Behavior of Intermetallic Compounds in Various Solder Joints Induced by Electromigration

With the development of the fine pitch technology, the size of the solder bumps contacting the chip and the substrate has decreased, thereby significantly increasing the current density. As a result, intermetallic compounds (IMCs) are formed in solder bumps, and the formation/growth of Kirkendall voids is the main reason for failure, which is accelerated by electromigration. Therefore, various studies have been conducted to restrain the growth of IMCs and Kirkendall voids. This study summarizes the issues that should be considered when investigating the effects of electromigration on solder joints. The prediction and observation of the growth behavior of the IMCs is summarized for various solder bump joints. In addition, we provide an effective method for assisting the fine pitch technology. Key words: Electromigration, Solder bump joints, Intermetallic compounds, Kirkendall voids, Modeling

Moisture Effects on NCF Adhesion and Solder Joint Reliability of Chip-on-Board Assembly Using Cu Pillar/Sn–Ag Microbump

Electronic devices as well as electronic packaging technology have required higher speed, I/O capability, and density. To meet these requirements, flip-chip solder bumps interconnection combined with reflow assembly process has been a widely used. In spite of the many advantages of flip-chip solder bump joints, there is a limitation for less than 100 $\mu \text{m}$ fine pitch interconnection due to the solder bump bridging during the assembly process. Therefore, nonconductive films (NCFs) and Cu pillar/Sn–Ag microbump interconnection become a promising interconnection solution for fine pitch assembly. However, NCFs technology has some problems for flip-chip assembly. Epoxy-based NCF can easily absorb moisture, causing delamination reliability problem at moisture environment. In order to solve the interconnection, the NCF adhesion should be enhanced. Silane coupling agent (SCA) was added to NCFs to secure the microbump joint reliability for chip-on-board assembly by increasing the adhesion strength in the pressure cooker test. After the humidity test, the NCFs’ modulus and Tg were increased by adding SCA content. Moreover, the measured adhesion strength and energy showed similar results after the humidity test that higher SCA content showed high adhesion strength and energy than lower SCA content and unmodified NCF at the interface between NCF and solder resist of the printed circuit board substrate. The bump joint lifetime of 5wt% SCA NCF was longer than 1wt% SCA and unmodified NCF after the humidity test. In this paper, we report results of our investigations on effects of employing SCA in NCF composition for improved moisture resistance.

Aging resistance and mechanical properties of Sn3.0Ag0.5Cu solder bump joints with different bump shapes

The effects of bump shape on the aging resistance and mechanical properties of Sn3.0Ag0.5Cu (SAC) solder bump joints were investigated by high-temperature aging test and shear test. Three different SAC solder bump joints with barrel type, cylinder type, and hourglass type, respectively, were provided by controlling solder process. The results indicate that as the bump volume decreases from barrel bump to hourglass bump in the reflow, the thickness growth rate of intermetallic compounds (IMCs) between different solder joints and Cu substrates in the isothermal aging of 150 °C decreases. And the change of shear strength of hourglass interconnection solder joint with the increase in temperature is the minimum in the three interconnection solder joints. Thus, as the solder dosage decreases properly, hourglass interconnection solder joint is obtained, which is avail to enhance the solder joint reliability at the high-temperature aging and the ability to adapt to the change of environment temperature suddenly.

Enhancement of mean-time-to-failure of Sn3.0Ag0.5Cu solder bump joint under current stressing via controlling bump shape

In this research, two different Sn3.0Ag0.5Cu solder bump joints with barrel shape and hourglass shape respectively were fabricated by controlling solder process, and evolution of intermetallic compounds (IMC) between different solder joint and Cu substrate under 4 × 103 A/cm2 current stressing at 60 °C were investigated. The results indicate that both the decrease of IMC layer thickness in the cathode and the increase of IMC layer thickness in the anode of the hourglass-shaped solder bump joint were obviously slower than that of barrel-shaped solder bump joint, and the formation of the voids at cathode interface was observed in the barrel-shaped solder bump joint after current stressing for 240 h. The mean-time-to-failure (MTTF) of solder joint with hourglass-shaped bump is efficiently improved compared to that with barrel-shaped bump. Compared to that of the solder joint with barrel-shaped bump, enhancement of MTTF of the solder joint with hourglass-shaped bump is attributed to relieve current crowding in entrance and outlet of electric current.

Theoretical and experimental determination of Cu diffusivity in eutectic Sn–Ag system at 235–280 °C

This study evaluated the Cu diffusion and dissolution behaviors in eutectic Sn–Ag alloy with a mathematical model in combination with experimental data. The diffusion was performed at temperatures ranging from 235°C to 280°C with a Cu/Sn–3.5Ag solder bump joint, where the Cu substrate was a solid but the Sn–3.5Ag solder bump was a liquid at all of the temperatures examined. The Cu diffusion coefficient (DCu) with respect to temperature, the activation energy of the Cu diffusion (Q), and the average Cu concentration (Cavg) as a function of diffusion time (t) were investigated via this metallurgical system. The coefficients DCu were 1.2(±0.4)×10−9m2/s (235°C), 2.4(±0.6)×10−9m2/s (250°C), 2.6(±0.7)×10−9m2/s (265°C), and 3.9(±1.6)×10−9m2/s (280°C), which are of the same order as those acquired from the capillary-reservoir technique for the bulk Cu/Sn system by Ma and Swalin in 1969. The activation energy Q was estimated based on the Arrhenius relation and was approximately 13.3kJ/mol at 235–280°C, reflecting the insensitivity of DCu to temperature. A calculated Cavg–t profile predicted that Cavg approaches saturation after 70s (235°C), which agrees well with experiments. The modeling results were consistent with the experimental and literature data, indicating that the proposed mathematical model is an effective method to represent the Cu diffusion/dissolution behavior in a confined solder alloy.

Electrical behavior of Au–Ge eutectic solder under aging for solder bump application in high temperature Electronics

As deep earth oil exploration transits into deeper earth, it faces challenges in producing reliable microelectronics devices and sensors that will not degrade at high temperature, especially those related to packaging materials and assembly of the devices. It has been reported that the mechanical performance of a high temperature Au–Ge eutectic solder is able to fulfill the minimum interconnection properties specified by the oil and gas exploration industry. However, the impact of thermal aging to Au–Ge eutectic solder’s electrical property is not clear. In this work, it is observed that the bulk Au–Ge electrical resistivity decreased initially with thermal aging and then saturated. This effect is mainly contributed by the grain growth of Au, which results in a reduction in grain boundaries, as well as coarsening of the Ge phases. On the other hand, study on the microstructure and electrical resistance of Au–Ge solder bump joint after aging shows that the electrical resistance increased. The increase in resistance is contributed mainly by the NiGe and Ni5Ge3 intermetallic compounds growth, which also led to a degradation in the Ni(P) barrier layer.

Damage of lead-free solder joints in flip chip assemblies subjected to high-temperature thermal cycling

The damage mechanism of lead-free ball grid array (BGA) solder joints is studied whilst the magnitude of the damage was quantitatively evaluated to determine the impact of the thickness of inter-metallic compound (IMC) layer on the reliability of the joints. Five virtual test vehicle assemblies of flip chip (FC) which have 4–12μm thickness of IMC layer were subjected to the same accelerated high-temperature cycles utilising, in parts, the IEC standard 60749-25. The visco-plastic responses of the assemblies’ solder bump joints to the induced thermal load were simulated using Anand’s plasticity model. The results demonstrate that very thin and thick thickness of IMC layer impact the reliability of the FC solder joints. It was found that strain range at steady state joint damage is approximately 0.07–0.12. This finding indicates that strain controlled studies on bulk solder which are not up to these limits are inadequate to characterise damage behaviour and model damage evolution in the FC BGA solder bump joints. Further results show that the magnitude of the average accumulated visco-plastic energy density per cycle (wacc) depends on the volume percentage of solder and IMC in the joints. This dependency explains why the existing life prediction models based on wacc are not satisfactory in estimating the damage in solder joints. The authors propose the development of new prediction models which will be specific on the volume percentage of solder and IMC in the joints.

Estimation and visualization of the fatigue life of Pb-free SAC solder bump joints under thermal cycling

A method for estimating the fatigue life of Pb-free solder (Sn–3.0Ag–0.5Cu) bump joints under thermal cycling is described. The test specimens were 4, 6 and 8mm chip size packages mounted on printed circuit boards. To develop a semi-analytical model for estimating the thermal fatigue life, thermal cycle tests were carried out on the test specimens and the thermal fatigue life obtained experimentally was compared with the corresponding result from nonlinear, elastic–plastic finite element analysis. It was found that the chip size affected the thermal fatigue life and the strain accumulated in the solder bumps during thermal cycling. This was due to the difference in mechanical boundary conditions around the Si chip. Based on the Coffin–Manson law, an equation for estimating the thermal fatigue life was derived and the predicted fatigue life for various chip sizes and under different rises in temperature were found to be in good agreement with the experimental results. Moreover, a measurable parameter that correlates with the accumulated strain in the solder bumps was introduced for estimating the thermal fatigue life of the test specimens.

Effect of Bump Shape on Current Density and Temperature Distributions in Solder Bump Joints under Electromigration

Three dimensional thermo-electrical finite element analysis was employed to simulate the current density and temperature distributions for solder bump joints with different bump shapes. Mean-time-to-failure (MTTF) of electromigration was discussed. It was found that as the bump volume increased from hourglass bump to barrel bump, the maximum current density increased but the maximum temperature decreased. Hourglass bump with waist radius of 240 μm has the longest MTTF.

Effect of Interfacial Reaction Layer Thickness on Fracture Morphology and Impact Resistance of Sn-Ag-Cu Solder Bump Joint

Effect of Interfacial Reaction Layer Thickness on Fracture Morphology and Impact Resistance of Sn-Ag-Cu Solder Bump Joint

Use of Modified Accumulated Damage Model to Predict Fatigue Failure Lives of Sn-Ag-Cu-based Solder Joints in Ball-Grid-Array-Type Packages

We have developed a modified accumulated damage model that can be used to predict fatigue failure lives of solder joints in electronic devices. Our model calculates the fatigue failure life of solder on the basis of the damage that accumulates during crack propagation by using a finite element method and corrects for the dependence of element size on the calculated life by using the Hutchinson-Rice-Rosengren singularity theory. We predicted the fatigue lives of conventional and copper-core solder bump joints in ball-grid-array packages in thermal cycling tests. The good agreement between these predictions and experimental results indicates that our model can effectively predict fatigue failure life in solder joints.

Advanced Bump Structure for Improving the Board Level Characteristics of WLCSP

In this study, to increase solder joint reliability, we developed the epoxy reinforced bump structure surrounding the solder bump joint. The thermal-mechanical reliability of the epoxy reinforced solder bumped wafer level package was carried out during the thermal cycle and drop shock test of the package. The reliability of solder bump joint was evaluated by means of the thermal cycle in the range -40° to 125°. The experimental results revealed that the thermo-mechanical fatigue properties of the epoxy reinforced bump structure was better than that of the conventional solder bump structure. Epoxy reinforced bump structures improved the WLP characteristic life by at least 2 times. Also, the drop shock reliability of epoxy reinforced bump structure was enhanced 3 times better drop characteristic life than that of conventional single bump structure. The life time of epoxy reinforced solder bump joint was longer than that of the conventional WLCSP solder joint.

Solder joint reliability in flip chip package with surface treatment of ENIG under thermal shock test

This study investigated the microstructural evolution and evaluated the thermo-mechanical reliability of a Sn-3.0Ag-0.5Cu (in wt.%) flip chip package during a thermal shock test. The reliability of the flip chip bonded packages was evaluated by means of the thermal shock in the temperature range of 233 K to 398 K. After 250 thermal shock cycles, cracks finally occurred at the corner solder bump joint on the chip side interfacial regions. In that, the crack initiated at the solder region near the intermetallic compound (IMC) layers. For the interpretation of the failure mechanism of the package, finite element analyses were conducted. From the finite element analysis or computational simulation result, the distribution of the plastic work in the corner solder bump was found to be most considerable. This means that the corner solder bump has the highest potential for first crack initiation and growth, because the thermo-mechanical failure of the solder joints is mainly caused by the accumulation of the plastic work. This matched well with the experimental results.

Interfacial Reaction of Electroless Ni-P Plating with Sn-3.5Ag (-Co) Solder

The reaction between Sn-Ag (-Co) solder and electroless Ni-P plating was investigated in order to clarify the effect of the addition of Co to Sn-Ag solder on the formation of intermetallic compound (IMC) at the interface and the joint strength at the interface. Sn-Ag-Co solder was specially prepared. The results show that there is little effect of the addition of Co to the Sn-Ag solder on the IMC formation and the thickness of the IMC at the interface. For the pull strength of the solder bump joint, the addition of Co to the solder didn’t strongly affect the pull strength of the solder joints, but it affected the fracture mode of the solder joints.

4328 銅バンプ接続の高信頼化検討(J05-3 実装信頼性,J05 電子情報機器,電子デバイスの熱制御と強度・信頼性評価)

The structural reliability for Cu-bump joint was studied analytically by using a design of experiments. Moreover, the influence of mechanical property of underfill resin on the fatigue life of Cu-bump interconnect under the thermal cycle test was also studied. In conclusion, it was found that the thermal stress at the interface between the Cu-bump and the UBM increases as the increase in thickness and length of Cu-bump. The interface stress especially depends on the length of Cu-bump. The length of Cu-bump was thus important design factor to be compatible both the joint reliability and the highly heat dissipation. It was also found that the existing mechanical property of underfill resin for solder-bump joint was appropriate for Cu-bump joint connected to the substrate using solder.

Aging treatment characteristics of solder bump joint for high reliability optical module

The joint strength and fracture surfaces of Sn–37 mass% Pb and Au stud bumps for photo diode packages after isothermal aging testing were studied experimentally. Al/Au stud bumps and Cu/Sn–37 mass% Pb solders were adopted, and aged for up to 900 h to analyze the effect of intermetallic compound (IMC). The joint strength decreased with aging time. The diffraction patterns of Cu 6Sn 5, scallop-shaped IMCs, and planar-shaped Cu 3Sn were characterized using transmission electron microscopy (TEM). The formation of Kirkendall voids and the growth of IMCs at the solder were found to be a possible mechanism for joint strength reduction.

Shear Strength in Solder Bump Joints for High Reliability Photodiode Packages

The shear strength and fracture surfaces of Sn-Pb bump and Au stud bump for photodiode packages after isothermal aging testing were studied experimentally. Al/Au stud bumps and Cu/Sn-37 mass%Pb solders were adopted, and aged at 373 K and 423 K for up to 900 hours to analyze the effect of intermetallic compound (IMC). The shear strength decreased with aging time. The diffraction patterns of Cu6Sn5, scallopshaped IMCs, and planar-shape Cu3Sn were observed by transmission electron microscopy (TEM). The formation of Kirkendall voids with the growth of IMCs at the solder was found to be a possible mechanism for shear strength reduction. IMCs between Au stud bumps and Al pads were identified as AlAu2.

Studies on the Interfacial Reaction of Screen-Printed Sn-37Pb,Sn-3.5Ag and Sn-3.8Ag-0.7Cu Solder Bumps on Ni/Au and OSP finished PCB

In this study, three solders, Sn-37Pb, Sn-3.5Ag, and Sn-3.8Ag-0.7Cu were screen printed on both electroless Ni/Au and OSP metal finished micro-via PCBs (Printed Circuit Boards). The interfacial reaction between PCB metal pad finish materials and solder materials, and its effects on the solder bump joint mechanical reliability were investigated. The lead free solders formed a large amount of intermetallic compounds (IMC) than Sn-37Pb on both electroless Ni/Au and OSP (Organic Solderabilty Preservatives) finished PCBs during solder reflows because of the higher Sn content and higher reflow temperature. For OSP finish, scallop-like <TEX>$Cu_{6}$</TEX> /<TEX>$Sn_{5}$</TEX> and planar <TEX>$Cu_3$</TEX>Sn intermetallic compounds (IMC) were formed, and fracture occurred 100% within the solder regardless of reflow numbers and solder materials. Bump shear strength of lead free solders showed higher value than that of Sn-37Pb solder, because lead free solders are usually harder than eutectic Sn-37Pb solder. For Ni/Au finish, polygonal shaped <TEX>$Ni_3$</TEX><TEX>$Sn_4$</TEX> IMC and P-rich Ni layer were formed, and a brittle fracture at the Ni-Sn IMC layer or the interface between Ni-Sn intermetallic and P-rich Ni layer was observed after several reflows. Therefore, bump shear strength values of the Ni/Au finish are relatively lower than those of OSP finish. Especially, spalled IMCs at Sn-3.5Ag interface was observed after several reflow times. And, for the Sn-3.8Ag-0.7Cu solder case, the ternary Sn-Ni-Cu IMCs were observed. As a result, it was found that OSP finished PCB was a better choice for solders on PCB in terms of flip chip mechanical reliability.

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.