Welcome to Duh's Lab

      The flip chip technology with the aid of BGA interconnection has attracted a great deal of attention in today’s electronics packaging. One of the challenging issues is the material selections for solder bump. Solder bump contains two parts. One is solder ball, and the other is under bump metallurgy (UBM).

 

      The conventional material for solder ball is eutectic Sn-Pb, with an eutectic temperature of 183°C. Due to the satisfactory mechanical properties and the detailed database, the eutectic Sn-Pb is still widely used in current microelectronic industry. However, owing to the environmental concern, the development of lead-free solders becomes a crucial issue. The candidate materials for solder are Sn-Ag-Cu based alloys. The melting point of Sn-Ag-Cu based alloy is around 217°C. It has been one of candidates because of its excellent mechanical properties and wettability. However, the drop performance of Sn-Ag-Cu solder alloy was poorer than that of Sn-Pb solder. Addition of 4th element doping in Sn-Ag-Cu solder alloy has been proposed in order to enhance the reliability of drop test.

 

      An under bump metallization (UBM) is generally a multi-layer combination and inhibits the interactions between the interconnection, either Al or Cu, and solder during reflow and field use. Nowadays, available UBM structure includes Cr/Cr-Cu/Cu, Ti/Ni(V)/Cu, Ti-W/Cu/Cu or Ni/Cu. The wettability of Cu-based UBM is good, yet it forms the brittle IMCs and Kirkendall voids with lead-free solder easily due to fast reaction between Sn and Cu. As a result, the reliability of solder joints with the Cu-based UBM would decrease. The Ni-based UBM is of interest owing to a slower chemical reaction with lead-free solders as compared to the Cu-based UBM. The Ni or electroless Ni UBM acts as a role of wetting layer and diffusion barrier. However, the wettability of Ni-based UBM was poor and the “black pad” issue may occur in ENIG (electroless Ni/immersion Au). In recent years, the alternative UBM has been proposed, e.g. ENEPIG (electroless Ni/electroless Pd/immersion Au) and ENEC (electroless Ni/electroless Cu).

 

      During the recent 20 plus years, research activities of our group have been focused on the material issues for solder joint reliability, including:

 

(1). To investigate the reaction mechanisms of conductor/substrate, dielectric/substrate, and solder/substrate by the material properties and the elemental distribution in the hybrid systems under different test conditions;

(2). To apply the electroless plating technology to the new substrate material of microelectronic package, i.e. electroless Cu and Ni plating;

(3). To manufacture and develop new lead-free solder alloys;

(4). To study the interfacial reaction, microstructure evolution, and the growth kinetics of intermetallic compounds in the solder joint systems;

(5). To evaluate the reliability of solder joints by micro-analysis and mechanical tests.

 

   In the following, the properties of the lead-free solder and the interfacial reaction, microstructure, as well as reliability of solder alloys with different substrates would be introduced, respectively.

 

 1. Present study

1.1. The properties of lead-free solders

      The properties of lead-free solders, such as melting point, density and mechanical properties have been investigated in our lab. Recently, the research focuses on the effect of minor doping on microstructure of lead-free solder and grain size and orientation of Sn. It is hoped that the results can provide material database for lead-free solders in future application.

 

1.1.1. The minor Ni doped effect on the microstructure and interfacial reaction of Sn-Ag-Cu solder joint

      The reliability of SnAgCu solder can be improved by doping minor element. The Ni doping in Sn-Ag-Cu solder can reduce the interfacial reaction and suppress the Kirkendall voids. In this study, the Ni re-distribution in Sn-Ag-Cu solder after aging was analyzed by FE-EPMA. The diffusion behavior of Ni and the Ni doping effect on the reliability of solder joints would be discussed. 

 

1.1.2. Orientation Imaging Studies of Sn-3.0Ag-0.5Cu Solder Joint after Aging

      Sn-Ag-Cu solder has been widely used in the electronic package. However, the Sn grain size of Sn-Ag-Cu solder is easily grown with slow cooling rate and aging. Coefficient of thermal expansion (CTE) along the c-axis of Sn is two times larger than that along a-axis. Larger Sn grain increases CTE mismatch and induces more stress at the grain-boundary region. In order to understand the Sn grain size and orientation, the orientation images of solder balls were acquired by electron backscattered diffraction, and correlated with the microstructure and composition evaluated by FE-EPMA.

 

1.2. Interfacial reaction and microstructure evaluation between solder and different metalized substrates

      Another critical issue for solder joint reliability is the interfacial reaction and microstructure analysis between solders and metalized substrates. Besides, the novel UBM materials were proposed in order to enhance the reliability of solder joint in our lab.

 

1.2.1. Formation mechanism of Sn-patch between Sn-Ag-Cu solder and Ti/Ni(V)/Cu under bump metallization

      Sn-patch would form in Ni(V)-based under bump metallization during reflow and aging. In order to elucidate the formation mechanism, the composition and microstructure were analyzed by FE-EPMA and TEM, respectively. It was revealed that there existed concentration redistribution in Sn-patch. The microstructure in Sn-patch also varied with aging. A possible formation mechanism of Sn-patch was proposed. It was demonstrated that the evolution for composition and microstructure of Sn-patch was correlated with the formation mechanism.

 

1.2.2. Interfacial reaction and mechanical properties of Sn-Ag-Cu solder joint with Ni-xCu UBM after multiple reflows

      The Ni-Cu UBM can provide extra Cu to the solders to maintain the Cu6Sn5 IMC at the interface, which can thus significantly decrease the Ni dissolution rate. In this study, the Cu content of the sputtered Cu/Ni-xCu/Ti UBM was varied from 0 to 20 wt.%. Sn-3Ag-0.5Cu solder was reflowed with Cu/Ni-Cu/Ti UBM for one, three, and five times. Reflow and cooling conditions altered the morphology of the IMCs formed at the interface. The amount of (Cu,Ni)6Sn5 increased with Cu content in the Ni-Cu film. The Cu concentration of the intermetallic compound was strongly dependent on the composition of the Ni-Cu films. The results suggest that Cu-rich Ni-xCu UBM can be used to suppress interfacial spalling and to improve shear strength and pull strength of solder joints.

 

1.2.3. Interfacial Reaction between Sn and Cu-xZn Substrate after Reflow and Thermal Aging

      An alternative design was carried out to fabricate pure Sn as the solder and Cu-xZn (x=15 and 30 wt.%) as the UBM to form the reaction couple. As Zn increased from 15 to 30 wt.% in Sn/Cu-Zn system, the growth of both Cu3Sn and Cu6Sn5 could be suppressed. Besides, no Kirkendall void was observed at the interface in both Sn/Cu-Zn couples during heat treatment. After 40-day aging, different multilayered phases of [Cu6Sn5/Cu3Sn/Cu(Zn)] and [Cu6Sn5/Cu(Zn,Sn)/CuZn] formed at the interface of [Sn/Cu-15Zn] and [Sn/Cu-30Zn], respectively. Intermetallic compounds (IMCs) growth mechanism during aging was discussed and proposed on the basis of the composition variation in the joint assembly with the aid of electron microscopic characterization and Sn-Cu-Zn ternary phase diagram. According to these analyses of interfacial morphology and IMCs formation in Sn/Cu-Zn system, Cu-Zn may be a potential UBM for retarding the Cu pad consumption in solder joints.

 

1.2.4. Wettability and interfacial reaction at Pb-free solder/electroless Ni-Zn-P joint

      Since Ni-Zn-P film owns better corrosion resistance than that in Ni-P film, it has a potential to replace the expensive Au/Ni-P UBM. Ternary electroless Ni-Zn-P film are developed to replace the binary electroless Ni-P film. The first step is to study the relationship between wettability and Zn content in the Pb-free solder/Ni-Zn-P joints. The second step is to study the interfacial reaction after multi-reflow and aged treatment in the Pb-free solder/Ni-Zn-P joints. In addition, the diffusion behavior in the Sn-Zn solder/(Au)/Ni-P joint will be discussed.

 

1.2.5. Interfacial Reactions of Sn-3.0Ag-0.5Cu Solder with Cu-Mn UBM during Aging

      Cu UBM has been widely used as surface finish in the flip chip technology. The major disadvantages of Cu UBM are fast consumption of copper, rapid growth of IMCs and formation of Kirkendall voids. Recently, minor element addition into Cu UBM has been observed to suppress the formation of Cu-Sn IMCs at the interface. In this study, different Mn content (1-20 at.%) were added into Cu UBM by sputtering technique. With higher Mn concentration in Cu-Mn UBM, a new phase, MnSn2, was formed between Cu6Sn5 and Cu-Mn UBM. MnSn2 may be a diffusion barrier to reduce the interfacial reaction and to suppress the formation of Kirkendall voids. The detailed mechanism of the IMC formation will be probed and discussed. 

 

1.2.6. Interfacial Reaction between Pure Sn and Cu Foil with Different Grain Sizes

      Cu6Sn5 and Cu3Sn were easily formed at the interface between Sn and Cu during reflow and aging process. Thick Cu-Sn compound and Kirkendall voids at the interface would reduce the mechanical strength of solder joints. The IMC growth rate in solder was related to the grain size of Cu foil. In this study, the pure Sn solder was reflowed with the Cu foils with different grain sizes at 250 oC for 1-20 min. Cu6Sn5 and Cu3Sn growth rates were calculated with respect to various Cu grain sizes. It was revealed that the Cu foil with a critical grain size can reduce the IMC growth rate and the consumption of Cu foil.

　

1.3. The reliability of solder joints in microelectronic package

1.3.1. High speed impact test of SnAgCu solder joint after aging

       Recently, drop test play an important role for the solder joint reliability due to the prominent development of portable electronic products. However, drop test needs much time and money to carry out. For such reasons, high speed shear and pull tests were investigated and discussed in many literatures to substitute or simulate the results of drop test. In our group, with the aids of XYZTEC Co. in Holland and other foreign and domestic cooperations, the high speed impact test will be employed to evaluate the reliability of the solder joints and to correlate with the drop test for the reduction of cost and time.

 

1.3.2. Evaluate the reliability of lead-free solder joints based on microstructure analysis and mechanical tests

       IMCs are the important to link the solders and the substrates. However, too much brittle IMCs formed at the interface of solder joints would cause the weak reliability. In addition, the formation of Kirkendall voids inside the IMCs usually degrades the strength of solder joints. Therefore, how to evaluate the reliability of solder joints by micro-analysis and mechanical tests is a critical issue.

　

2. Previous Research Issues

 

2.1. The fabrication and properties of lead-free solders 

2.1.1. Microstructural evaluation and mechanical characteristics for the eutectic Pn-Sn and the unleaded Cu-Sn-Ni solder joint in microelectronic package

      The growth mechanism of IMCs between solders and metalized substrates, and the mechanical properties of solder joint after thermal aging are investigated. The solder used are unleaded Cu-Sn-Ni solder and eutectic Sn-Pb solder. The Pt-Ag/Al2O3, Cu block and the electroless Cu/Pt-Ag/Al2O3 are used as the metalized substrates.

 

2.1.2. Reliability evaluation of eutectic 42Sn-58Bi and ternary Sn-Bi-1Cu solder joints in electronic package

      This study presents series of results on the binary eutectic Sn-Bi and ternary Sn-Bi-l wt.% Cu solder joints. Composition analysis, solidification behavior and wettability of the as-fabricated solder alloys are characterized. In addition, microstructure, adhesion strength, fracture surface and contact resistance of the solder joints are also evaluated.

 

2.1.3. Wettability evaluation and microstructure analysis of ternary Sn-Ag-Bi solder alloys

      The ternary Sn-Ag-Bi solder alloys are fabricated. The major function of Bi addition in eutectic Sn-Ag solder is to improve the wettability and to lower the melting point. Characteristics of as-fabricated solders including melting point, microstructure, coefficient of thermal expansion, and density are investigated.

 

2.1.4. Reliability evaluation of eutectic Sn-Ag and 95.5Sn-3.5Ag-1Zn solder joint in electronic package

      This study presents series of results on the binary eutectic Sn-Ag and ternary Sn-Ag-lwt% Zn solder joints. Composition analysis, solidification behavior and wettability of the as-fabricated solder alloys are characterized. In addition, microstructure, adhesion strength, fracture surface and contact resistance of the solder joints are also evaluated.

 

2.1.5. Microstructure evaluation of ternary Sn-Ag-Bi solder and effects of surface roughness and oxidation on the wettability of solders

      The microstructure of ternary Sn-Ag-Bi solder alloys before and after thermal aging test is evaluated and the effect of substrate surface roughness and oxidation on the wettability of solders is investigated. The ternary Sn-Ag-Bi solders are fabricated by different processes to evaluate its microstructure and composition. The homogeneity of the solder alloys is improved by the increase of molten time. Concerning the effect of surface, the time of solder to reach the static contact angle is shorter in the smooth surface. Besides, oxidation of both solders and substrates degrades the wettability of the solder.

 

2.1.6. Preparing lead-free solder paste by mechanical alloying (MA) process

      The application of soldering is well used in the electronic industry. However, development of alternative lead-free solders is required due to increased environmental and health concerns on the toxicity of lead. Powders of Sn-Ag-Bi with MA are derived. During MA process, the high-energy ball milling induces the large strain and crystal defects that increases the diffusion rate. Due to the excess energy, It allows the materials to be processed far from equilibrium. It also provides the means to overcome the drawback of forming new alloys by using a starting mixture of low and high melting temperature elements. It’s a convenient method to produce solder pastes by adding flux directly to the powders. In addition, the interface reaction of various lead-free solders derived by mechanical alloying on electroless Ni/Cu is also discussed here.

 

2.1.7. Interfacial reactions and compound formation of Sn-Ag-Cu solders by mechanical alloying on electroless Ni-P/Cu UBM

      The effect of Cu concentration in the ternary Sn-3.5Ag-xCu solder by MA was investigated. An effective approach was developed to reduce the particle size of the Sn-Ag-Cu solder down to less than 100 μm by doping the nano Cu6Sn5 particle during MA process. Besides, electroless Ni-P under bump metallization has been widely used in electronic interconnections due to the good diffusion barrier between Cu and solder. Solder joints after annealing at 240 oC for 15 min were employed to investigate the evolution of interfacial reaction between electroless Ni-P/Cu UBM and Sn-Ag-Cu solder with various Cu concentrations. After deliberately quantitative analysis with an electron probe microanalyzer, the effect of Cu content on the formation of IMCs at Sn-Ag-Cu solder/electroless Ni-P interface was investigated. With the aid of microstructure evolution, quantitative analysis and elemental distribution by x-ray color mapping, the presence of Ni-Sn-P phase and P-rich layer was revealed. SnAgCu composite solder reinforced with Cu6Sn5 nano dispersoids was studied for the related interfacial reactions. In addition, the contact angles of MA solder paste, less than 25o, ensured good wettability.

 

2.1.8. The influence of Ni3Sn4 nanoparticles and Ni concentration on morphology of Sn-Ag-Ni solders by mechanical alloying

      In this study, MA process was considered as an alternative method to produce the lead free solder pastes of Sn-3.5Ag-xNi. The particle size of pure-Ni doped solder was above 100 μm. With increasing Ni concentration, the particle size was reduced. To reduce the particle size of Sn-Ag-Ni alloys, Ni3Sn4 nanoparticles were doped into Sn and Ag powders to form Sn-Ag-Ni composite solder. The distinction of milling mechanism of Ni3Sn4-composite solder and the pure-Ni doped solder was discussed. Wettability test also revealed that the wetting angle of Ni3Sn4-composite solder was smaller than the pure-Ni doped solder.  In addition, the interfacial reaction between Sn-Ag-Ni composite solder and Cu substrate after various reflow cycles was also probed.

 

2.1.9. Synthesis and characterization of the lead-free solders with Sn-3.5Ag and Sn-3.5Ag-xCu nanoparticles by chemical reduction method

      Sn-Ag-Cu alloys are investigated as alternatives to lead-bearing alloys for solder bumps, yet the mainstream method of formation has been limited to screen printing technology, for this is a low-cost method for producing relatively large geometry bumps. Alternatively, the lead-free solders with Sn-3.5Ag and Sn-3.5Ag-xCu nanoparticles were synthesized by chemical precipitation with NaBH4 in this study. In the wettability test, good metallurgical bonding was revealed between solders and substrates after reflow. Nanoparticles derived by chemical reduction method can be further used as an appropriate solder powders in electronic packaging.

 

2.1.10. Synthesis of Ni3Sn4 and Cu6Sn5 nanoparticles in deriving lead-free composite solders

      Ni3Sn4 and Cu6Sn5 IMCs play important roles in the interfacial reaction between the lead-free solder and the Ni/Cu under-bump metallization during reflowing in electronics packaging. In this study, SnAgCu and Sn-Ag-Ni composite solders were produced by MA process with doping Cu6Sn5 and Ni3Sn4 nanoparticles, respectively. The nanoparticles of the Ni3Sn4 and Cu6Sn5 intermetallic compounds were synthesized by chemical precipitation with NaBH4 in aqueous solutions. The nanoparticles were prepared by the precursor reacting with NaBH4. The Sn-Ag-Cu composite solder joint doped with the nanoparticles of Cu6Sn5 IMC formed thinner (Cu, Ni)6Sn5 layers at the solder/electroless Ni-P interface than that formed in the commercial solder joint and MA solder joint. The nanoparticles of Ni3Sn4 intermetallic compound doped into Sn-Ag-Ni composite solder joint exhibited the similar effect.

 

2.2. Interfacial reaction and microstructure evaluation between solder and different substrates

2.2.1. Phase Formation and Microstructural Evolution in Sn-Pb Solder and Pd/Ag Thick Film Conductor Metallization

      Intermetallic compound formation between thick film mixed bonded conductor and Sn-Pb solder was investigated. Microstructural evolution of the interfacial morphology, elemental, and phase distribution were probed with the aid of electron microscopy and X-ray diffraction. A decrease in adhesion strength occurs when the sample was aged at 130 °C for long periods of time. Microstructural analysis revealed the formation of intermetallic compounds, i.e. Pd3Sn, Pd2Sn, Pd3Sn2, PdSn, Pd3Pb, Ag5Sn, and Ag3Sn. A possible mechanism of adhesion loss for the conductor was described. In the initial stage, the fracture interface was located where the Pd3Pb exists, which was near the solder. However, the fracture took place within the solder after aging. It was argued that volume change of the conductor film resulting from the intermetallic formation and incoherency between the compounds due to grain growth were major factors in the degradation of the peel strength.

 

2.2.2. Indirect Bonding of Ni-Electroless Plated AlN and Cu by Hot Pressing Method

      The electroless Ni (EN) plating method was employed to metallize the AlN ceramic substrates. The EN-plated AlN substrate was bonded with the Cu foil to form a sandwich-like AlN-EN/Cu/EN-AlN assembly by hot pressing in vacuum with a pressure. For the bonding temperature below the Ni-P eutectic temperature of EN at 880 °C, the samples were bonded through solid state diffusion. On the other hand, the samples were bonded via a liquid phase media through both wetting and diffusion if the bonding temperature was above 880 °C. An optimum adhesion strength around 10 MPa occurred within bonding temperature range 600-700 °C. The increasing temperature enhanced interdiffusion of Cu and EN to form a strong bond, yet resulted in a large residual thermal stress in the AlN/EN interface. Additionally, it is argued that the etched surface of AlN with micro-etched holes provides the anchor sites for interlocking with the EN film, which results in a good mechanical bonding in the joint.

 

2.2.3. Interfacial reactions and wetting property between electroless Ni in the under bump metallurgy (UBM) and Sn-37Pb solder

      The electroless-nickel (EN) has been deposited for the application of pad in the microelectronic industry. The wettability and interfacial reaction between solder and EN on the underlying substrate is thus a critical issue. EN deposited with various pH values is employed on the Al2O3 and Cu/ Al2O3 substrate, and both the wettability and interfacial reactions between EN and eutectic Sn-Pb solder were investigated.

 

2.2.4. Wetting behavior and interfacial reactions between lead-free solder and EN in UBM

      EN acts as a wetting layer and diffusion barrier. Study concerning wetting behavior and interfacial reaction between EN and recently developing lead-free solders are essential for solder joints reliability. The first part of this study investigates the wettability of several lead-free solders, such as Sn, Sn-Ag, and Sn-Bi, on EN with various phosphorus contents. The role of phosphorus in the wettability is probed. The second part discusses the interfacial reaction between Sn-Bi solder and EN. The EN and electroplated Ni were deposited on Cu/Al2O3 substrates to confirm the performance of diffusion barrier. Joints of 42Sn-58Bi/Au/EN, Ni/Cu/Al2O3 were annealed and aged to investigate the interfacial reaction.

 

2.2.5. Interfacial reactions between Ni/Cu UBM and eutectic Sn-Pb solder in flip chip technology

      Interfacial reactions between Ni/Cu UBM and eutectic Sn-Pb solder in the 63Sn-37Pb/Ni/Cu/Ti/Si3N4/Si multiplayer structure for the flip chip technology were investigated. The types of interfacial reaction products varied with different reflowed times. The diffusion path for solid-liquid reaction during periods of reflow could be determined with the aid of ternary Sn-Cu-Ni phase diagram. The correlation between IMC growth and shear strength is also investigated.

 

2.2.6. Diffusion behavior between Cu and Ni-P

      This study investigates the diffusion behavior between Cu and EN. Two diffusion couples (EN/Cu and solder/EN/Cu) were set up to discuss the effect on soldering reaction. Moreover, the diffusion behavior of Cu in EN could also be evaluated. The interfacial reaction products might affect the Cu diffusion, thus, the interfacial reaction between solder and EN is another issue of study. Evaluate the best operation parameter for electroplating Ni with 3 μm from the view of microstructure is another issue to focus.

 

2.2.7. Wettability of electroplating Ni-P in under bump metallurgy with Sn-Ag-Cu solder

      The nickel plating has been used as the under bump metallurgy (UBM) in the microelectronic industry. In this study, the electroplating process was demonstrated to be a good alternative approach to produce the Ni-P layer as UBM. The wettability of several lead-free solders, such as Sn-3.5Ag, and Sn-3.5Ag-xCu solder, on electroplated Ni-P with various phosphorous contents was investigated. The role of phosphorus in the wettability was also probed. The correlation between wettability and phosphorus contents in electroplating Ni-P was evaluated. As the phosphorous contents increased, the nodule size of the Ni-P deposit reduced and surface roughness of Ni-P became smaller. The improvement of surface morphology and surface roughness enhanced the wettability of electroplated Ni-P. The interfacial reaction between lead free solder and electroplated Ni-P UBM was also investigated.

 

2.2.8. Characterizing metallurgical reaction of Sn-Ag-Cu composite solder by mechanical alloying with electroless Ni-P/Cu under bump metallization after various reflow cycles

      The Sn-Ag-Cu composite solder pastes were produced by MA process with doping Cu6Sn5 nanoparticle. In this study, the interfacial reaction of Sn-Ag-Cu composite solders with electroless Ni/Cu UBM was investigated with different reflow cycles. The intermetallic compound (IMC) formed at the interface between the Sn-Ag-Cu composite solders and electroless Ni/Cu UBM during reflowing were mainly (Ni1-x,Cux)3Sn4 and (Cu1-y,Niy)6Sn5. The reaction mechanism of interfacial phase transformation between Sn-Ag-Cu composite solders and electroless Ni-P/Cu UBM with different reflow cycles was proposed.

 

2.2.9. Interfacial reaction in the flip chip 63Sn-37Pb and 5Sn-95Pb solder joints

      Interfacial reaction for the flip chip solder bump of 63Sn-37Pb/63Sn-37Pb and 5Sn-95Pb/63Sn-37Pb combination structure after aging at 150°C was investigated in this study. The under bump metallization for the 63Sn-37Pb and 5Sn-95Pb solders on the chip side was Ni/Cu/Ti/Si3N4/Si, while the bond pad for 63Sn-37Pb solder on the plastic substrate side was Au/electroless Ni/Cu. The types of interfacial reaction products varied with the time of aging. The diffusion path for solid reaction during periods of aging could be determined with the aid of ternary Sn-Cu-Ni phase diagram. The role of Pb content on compound formation in solder joints after aging was also investigated.

 

2.2.10. The role of Cu content on compound formation near chip and substrate side for the flip chip Sn-3.0Ag-xCu solder bump after aging and thermal cycling

      In this study, interfacial reaction for the flip chip solder bump of Sn-2.3Ag/Sn-3.0Ag-0.5Cu and Sn-3.0Ag-xCu/Sn-3.0Ag-0.5Cu combination structure was investigated after aging at 150°C and thermal cycling between -55 and 125°C. The under bump metallization for the Sn-2.3Ag and Sn-3.0Ag-xCu solders on the chip side was SiO2/Cu/Al/Ni, while the bond pad for Sn-3.0Ag-0.5Cu solder on the plastic substrate side was Cu/electroless Ni/Au. The interfacial reaction in the solder joints could be related to the Cu concentration in the solder joint. In addition, the detailed microstructure evolution of solder joints after aging and thermal cycling was also discussed.

 

2.2.11. Application of NiTi shape memory alloy as under bump metallurgy to promote solder joint reliability

      After the crystallized NiTi SMA film was prepared, a ball grid array component with silicon chip, deposited multi layer thin film UBM, and the Sn-3.0Ag-0.5Cu solder ball was prepared and sent for thermal shock test, thermal cycling test and bend test. The cross section observation and solder ball shear and pull test was utilized to provide the reliability evidence. Meanwhile, a finite element model of the exact component was also set up for simulation of stress and strain distribution in the solder joint under different temperatures at which SMA possesses different material properties. Finite element analysis was able to discuss some geometry and material property issues in the simulation. At the same time, a mathematical model has been induced to calculate the thermal force and stress between solder/UBM/Si. The simulation result and mathematical calculation of stress and strain distribution in model under different temperatures revealed the mechanism of SMA reinforcement in solder joint reliability. 

 

2.3. Solid state reaction in the Au-Cu wire connection with Al-Cu pad during aging

      In integrated-circuit packages, wire bonding technique is the preferred method for making electrical connections between the chip and lead frame. The influence of aging at 150 °C for interfacial reaction of the Au-Cu wire bonded with Al-Cu pad was investigated in this study. The wire-bond were first cold mounted in epoxy and then sectioned by using a slow speed diamond saw. Cross-sectional samples were ground and polished. To observe various IMCs and Kirkendall voids with FE-SEM, polished samples were ion milled with precision etching and coating system (PECS). The types of interfacial reaction products varied with the time of aging treatment. With the aid of microstructure evolution, quantitative analysis, the interfacial phase transformation between Au-Cu wire and Al-Cu pad could be probed and revealed.