Electroless Nickel Immersion Gold Surface Finish Research Articles

Fatigue performance and microstructure of lead-free solder joints in BGA assembly at room temperature

One of the crucial factors in evaluating the reliability of an electronic appliance is fatigue failure of the interconnecting solder joints. In most situations, large bulk samples are used to investigate the fatigue characteristics of the solder materials. Only a few studies have examined the solder joints frequently observed in ball grid array (BGA) components. This study implemented a specialized test vehicle in BGA assembly connecting two substrates with nine identical solder joints. Four SAC-based solder alloys were tested using an Instron micromechanical tester in the stress-controlled and strain-controlled methods at room temperature and constant strain rate. At two distinctive stress and strain levels, the fatigue performance of alloys featuring organic solderability preservative (OSP) and electroless nickel-immersion gold (ENIG) surface finishes was investigated. The fatigue results obtained from the stress-controlled and strain-controlled tests were compared. Microstructural analysis was performed using scanning Electron Microscopy (SEM) and energy dispersive spectroscopy (EDS) to identify the precipitates in the bulk solder and the morphologies of IMC layers. The thickness of intermetallic compound (IMC) layers and the failure mode were examined in each case. The results demonstrated that the OSP surface finish significantly outperformed the ENIG surface finish regardless of the testing process and solder alloy. Various failure modes were observed, depending on the solder alloy and IMC layer.

Effects of Multiple Reflow on the Formation of Primary Crystals in Sn-3.5Ag and Solder Joint Strength: Experimental and Finite Element Analysis.

The growth and formation of primary intermetallics formed in Sn-3.5Ag soldered on copper organic solderability preservative (Cu-OSP) and electroless nickel immersion gold (ENIG) surface finish after multiple reflows were systematically investigated. Real-time synchrotron imaging was used to investigate the microstructure, focusing on the in situ growth behavior of primary intermetallics during the solid-liquid-solid interactions. The high-speed shear test was conducted to observe the correlation of microstructure formation to the solder joint strength. Subsequently, the experimental results were correlated with the numerical Finite Element (FE) modeling using ANSYS software to investigate the effects of primary intermetallics on the reliability of solder joints. In the Sn-3.5Ag/Cu-OSP solder joint, the well-known Cu6Sn5 interfacial intermetallic compounds (IMCs) layer was observed in each reflow, where the thickness of the IMC layer increases with an increasing number of reflows due to the Cu diffusion from the substrate. Meanwhile, for the Sn-3.5Ag/ENIG solder joints, the Ni3Sn4 interfacial IMC layer was formed first, followed by the (Cu, Ni)6Sn5 IMC layer, where the formation was detected after five cycles of reflow. The results obtained from real-time imaging prove that the Ni layer from the ENIG surface finish possessed an effective barrier to suppress and control the Cu dissolution from the substrates, as there is no sizeable primary phase observed up to four cycles of reflow. Thus, this resulted in a thinner IMC layer and smaller primary intermetallics, producing a stronger solder joint for Sn-3.5Ag/ENIG even after the repeated reflow process relative to the Sn-3.5Ag/Cu-OSP joints.

Interfacial Diffusion Mechanism between ALD Al2O3 passivation/Cu in Direct Bonded Copper Substrate for Power Electronics

Direct bonded copper(DBC) substrate requires surface treatments for oxidation prevention and adhesion enhancement to die-attachment. Conventional surface treatment, electroless nickel immersion gold (ENIG) has a cost and environmental problems, and immersion silver is sensitive to temperature and humidity. To solve these problems, this study presents the invention of aluminum oxide (Al2O3) passivation for DBC Cu pads, which meets the requirements of oxidation prevention and adhesion enhancement to die-attachment. Ultrathin Al2O3 passivation layers (~20nm) by atomic layer deposition(ALD) were coated on DBC Cu pads for replacing conventional ENIG and immersion silver surface finishes. This paper presents bondability and interfacial diffusion mechanism between ALD Al2O3 passivation/Cu in DBC substrate.

Diffusion Barrier Properties of the Intermetallic Compound Layers Formed in the Pt Nanoparticles Alloyed Sn-58Bi Solder Joints Reacted with ENIG and ENEPIG Surface Finishes

Pt-nanoparticle (NP)-alloyed Sn-58Bi solders were reacted with electroless nickel-immersion gold (ENIG) and electroless nickel-electroless palladium-immersion gold (ENEPIG) surface finishes. We investigated formation of intermetallic compounds (IMCs) and their diffusion barrier properties at reaction interfaces as functions of Pt NP content in the composite solders and duration of solid-state aging at 100 °C. At Sn-58Bi-xPt/ENIG interfaces, typical Ni3Sn4/Ni3P(P-rich layer) microstructure was formed. With the large consumption of the Ni-P layer, the Ni-P and Cu layers were intermixed and Cu atoms spread over the composite solder after 500 h of aging. By contrast, a (Pd,Ni)Sn4/thin Ni3Sn4 microstructure was observed at the Sn-58Bi-xPt/ENEPIG interfaces. The (Pd,Ni)Sn4 IMC effectively suppressed the consumption of the Ni-P layer and Ni3Sn4 growth, functioning as a good diffusion barrier. Therefore, the Sn-58Bi-xPt/ENEPIG joint survived 500 h of aging without microstructural degradation. Based on the experimental results and analysis of this study, Sn-58Bi-0.05Pt/ENEPIG is suggested as the optimum combination for future low-temperature soldering systems.

The Effect of Micro-Alloying and Surface Finishes on the Thermal Cycling Reliability of Doped SAC Solder Alloys.

The surface finish (SF) becomes a part of the solder joint during assembly and improves the component’s reliability. Furthermore, the SF influences the solder joint’s reliability by affecting the thickness of the intermetallic compound (IMC) layer at the solder interface and copper pads. In this experiment, five different alloys are used and compared with the SAC305 alloy, two of which, Innolot and SAC-Bi, are bi-based solder alloys. This study includes three common SFs: electroless nickel immersion gold (ENIG), immersion silver (ImAg), and organic solderability preserve (OSP). The performance of three surface finishes is examined in terms of component characteristic life. All of the boards were isothermally aged for twelve months at 125 °C. The boards were then exposed to 5000 cycles of thermal cycling at temperatures ranging from −40–+125 °C. Most of the current research considers only one or two factors affecting the reliability of the electronic package. This study combines the effect of multiple factors, including solder paste content, SF, isothermal aging, and thermal cycling, to ensure that the test conditions represent real-world applications. In addition, the electronics packages are assembled using commercialized alloys. The current study focuses on a high-performance alloy already present in the electronic market. The failure data were analyzed statistically using the Weibull distribution and design of experiments (DOE) analysis of variance (ANOVA) techniques. The findings reveal that the micro and uniformly distributed precipitates in solder microstructures are critical for high-reliability solder joints. Re-crystallization of the thermally cycled solder joints promotes the local formation of numerous new grains in stress-concentrated zones. As the fracture spreads along these grain boundaries and eventually fails, these new grains participate in crack propagation. Aging significantly worsens this situation. Finally, although the ENIG surface finish with its Ni layer outperforms other SFs, this does not imply that ENIG is more reliable in all solder paste/sphere/finish combinations.

Effective (Pd,Ni)Sn4 diffusion barrier to suppress brittle fracture at Sn-58Bi-xAg solder joint with Ni(P)/Pd(P)/Au metallization pad

In this study, we investigated the interfacial reactions and mechanical properties of Sn58Bi solders alloyed with Ag nanoparticles (NPs) after bonding with electroless nickel-immersion gold (ENIG) or electroless nickel-electroless palladium-immersion gold (ENEPIG) surface finish. On the ENIG surface finish, Ni3Sn4, Ni-Sn-P, and Ni3P layers were sequentially formed on the Ni(P) layer. On the ENEPIG surface finish, (Pd,Ni)Sn4 intermetallic compound was formed on thin Ni3Sn4/Ni3P on the Ni(P) layer, and it survived nine reflow cycles, unlike Sn-Ag-Cu solder joints in previous studies. This effectively stopped the formation of the Ni-Sn-P layer, which acted as an effective diffusion barrier to suppress the consumption of Ni in the Ni(P) layer. A mixed-mode fracture—partially through the bulk solder (ductile mode) and partially through Kirkendall voids in the Ni-Sn-P layer (brittle mode)—was observed with the Sn-58Bi-xAg/ENIG joint, whereas only ductile fracture was observed with the Sn-58Bi-xAg/ENEPIG joint. Alloying with 0.5 wt% Ag NPs strengthened the composite solder by refining the eutectic microstructure of the Sn58Bi solder and by inducing precipitation hardening with fine Ag3Sn particles. However, the addition of more Ag NPs led to mechanical degradation by re-coarsening of the eutectic microstructure and by providing an easy fracture path along the coarsened Ag3Sn/solder interface. The highest shear strength was obtained with Sn-58Bi-0.5Ag/ENEPIG, which was suggested to be the most reliable system for low-temperature soldering at 200 °C.

Microstructure, thermal behavior and joint strength of Sn-0.7Cu-1.5Bi/electroless nickel immersion gold (ENIG)

This paper details an investigation into the microstructure, thermal behaviors and joint strength of Sn-0.7Cu-1.5Bi solder alloy on electroless nickel immersion gold (ENIG) surface finish. Besides conventional techniques, the real-time synchrotron imaging was used to analyze the microstructure evolution in Sn-0.7Cu-1.5Bi/ENIG. This research investigated the growth behavior of the primary (Cu,Ni)6Sn5 intermetallic compounds (IMCs) in the solder joint with the Bi alloying. The elemental distribution analysis showed the Ni diffused from the ENIG surface finish and dissolved into the bulk solder during solidification and that the size of the primary (Cu,Ni)6Sn5 IMCs decreased due to the addition of 1.5 wt% Bi. The average kinetic growth rate of the primary (Cu,Ni)6Sn5 IMCs in Sn-0.7Cu-1.5Bi/ENIG was lower than that of the Sn-0.7Cu/ENIG. The thermal analysis revealed that the pasty range slightly increased and the undercooling degree decreased due to the addition of 1.5 wt% Bi for free-standing solder and soldering on ENIG surface finish. The shear strength of the Sn-0.7Cu-1.5Bi/ENIG was determined using a high-speed bond tester, and it increased by ~12% at bulk solder fracture of ~15% within the solder joint interfacial fracture due to the addition of 1.5 wt% Bi into the Sn-0.7Cu. These occurrences can be attributed to the solid solution strengthening effect at the bulk solder and the formation of finer interfacial (Cu,Ni)6Sn5 IMCs within the solder joints. The results indicated that the microstructural changes, especially the size reduction of IMCs, in Sn-0.7Cu-1.5Bi/ENIG impacted the joint strength.

Effects of Surface Finish on Sn-3.0Ag-0.5Cu Solder Joint Microstructure and Strength

The effects of copper organic solderability preservative (Cu-OSP) and electroless nickel immersion gold (ENIG) surface finish reflowed on Sn-3.0Ag-0.5Cu (SAC305) solder have been investigated in detail. Besides conventional cross-sectional microstructure observation, advanced characterization techniques such as synchrotron radiography imaging and synchrotron micro-x-ray fluorescence (µ-XRF) were utilized to elucidate the microstructural evolution in the solder joints during soldering. Additionally, high-speed shear testing was performed to understand the influence of the surface finish on the solder joint strength. The results indicated that the presence of nickel (Ni) from the ENIG surface finish decreased the growth rate but increased the amount of small Cu6Sn5 primary intermetallics, resulting in a slight reduction of the average interfacial intermetallic compound (IMC) thickness in the SAC305/ENIG solder joints. Due to the refined control of the solder joint microstructure, the average high-speed shear strength was higher for as-reflowed SAC305/ENIG versus SAC305/Cu-OSP solder joints. These results indicate a significant influence of the surface finish on SAC305 solder joint microstructure and strength and could provide a basis to improve solder joint strength.

Effect of Blast Wave on Intermetallics Formation, Hardness and Reduced Modulus Properties of Solder Joints

Tin-Silver-Copper (SnAgCu) lead-free solder on Electroless Nickel Immersion Gold (ENiG) and Immersion Tin (ImSn) surface finish printed circuit board was subjected to blast test. A variation of intermetallic compounds (IMC) layer, hardness and reduced modulus of soldered sample exposed to blast test were intensively investigated using optical microscope and nanoindentation machine. Formation of IMCs due to reaction between solder and substrate during blast test provided deleterious effect of metallurgical bond strength and reliability on the solder joint. Microstructural analysis was evaluated via Infinite Focused Microscope (IFM). The findings of these studies indicate that best surface finished for blast test performance was not necessarily the best surface finish for optimum reliability. ENiG and ImSn surface finish can be advantage or a disadvantage depending on the application, package and reliability requirements. As a result, most component assemblers are using ENiG and ImSn in order to improve solderability as well as the wettability between solder and the substrate and to meet various package requirements.

Effects of Surface Finish on the Shear Fatigue of SAC-Based Solder Alloys

Solder joints in electronic assemblies are subjected to mechanical cycling in normal operating conditions. Testing individual solder joint in mechanical cycling allows monitoring the evolution of hysteresis loop with time and provides the ability to study the factors that affect the reliability. In this article, the effect of surface finish on the fatigue behaviors of four doped lead-free solder alloys (CycloMax, Innolot, Ecolloy, and SAC-X-Plus) and SAC305 was investigated. Individual solder joints with three surface finishes [organic solderability preservative (OSP), immersion silver (ImAg), and electroless nickel immersion gold (ENIG)] were prepared and subjected to various cyclic shear stress loadings until complete failure. Hysteresis loop was systematically recorded using an Instron machine. Characteristic fatigue life and energy dissipation per cycle were analyzed, and the Morrow energy model was applied to empirically predict the fatigue life as a function of work dissipation. Failure mechanisms were investigated as well using SEM/EDS, and the results were correlated with the fatigue behavior of the solder joints. The results indicated that the fatigue resistance of the solder alloys decreased with the increase in stress amplitude. The fatigue resistance of the solder joints with OSP and ImAg surface finishes outperformed ENIG surface finish. CycloMax with OSP surface finish was associated with the best fatigue resistance, while SAC-X-Plus with ENIG surface finish was associated with the worst fatigue resistance. Three failure modes were detected in this study: ductile, brittle, and near-intermetallic compound (IMC) failure. It was also found that Innolot with ENIG surface finish is more susceptible to brittle failure because of the excessive brittleness associated with this alloy.

Lifetime Prediction of Electrochemical Ion Migration with Various Surface Finishes of Printed Circuit Boards

Electrochemical ion migration (ECM) can be generated by the electrochemical reaction between the anodic and cathodic electrodes of an electric circuit in the case of temperature, humidity and applied voltage. ECM can finally induce a malfunction of electronics due to precipitation of metallic ions in the cathode. In this work, we study the failure mechanism based on the identifying stress factor of ECM to occur and the accelerated life prediction of ECM occurrence. The modified Eyring model, which includes a stress model (temperature, humidity and voltage), is utilized to accelerate the life prediction of ECM. To obtain the temperature and humidity coefficient factors of ECM failure, an accelerated life test is conducted with a more than 50% failure of five types of test conditions, namely, 85°C/75% RH, 65°C/85% RH, 85°C/85% RH, 75°C/85% RH and 85°C/95% RH. The failure criterion of insulation resistance between the conductors is less than or equal to 107 Ω. In situ monitoring of surface insulation resistance is performed throughout the temperature-humidity-bias tests for over 2600 h. From these results, we deduce the temperature and humidity coefficients of the acceleration model for predicting ECM time-to-failure in electroless nickel-immersion gold (ENIG) surface finish conductors covered with a solder mask. In addition, the electrochemical oxidation and reduction mechanisms of ECM are examined by physics-of-failure. Finally, we predict the B10 life for ECM to occur on a FR-4 printed circuit board with an ENIG surface finish in use environment.

Effects of Surface Finish Material on Millimeter-Wave Antenna Performance

Recently, the popularity of millimeter-wave circuits has increased. One of the main reasons for this increase is offering a wide frequency band and high-speed data transfer. Due to the manufacturing requirements, these circuits are covered with surface finishes. Although surface finish protects the exposed copper circuitry, these additive layers on copper foil affect the electrical properties of the conductor. Unlike previous papers in the literature, this paper deals with the effects of surface finishing materials on millimeter-wave antennas instead of transmission lines. With this aim, a differential dual exponentially tapered slot antenna (DETSA) that has almost constant gain, beamwidth, and low reflection coefficient over the entire V-band is designed. A balun is designed for one-port measurement of the differential antenna. Then, two widely used surface finish solutions, namely, electroless nickel immersion gold (ENIG) and immersion silver (IAg), are applied to the copper profile of the differential integrated antenna combined with balun. The structures are simulated with a widely used full-wave analysis tool. The antenna without surface finish, with ENIG, and IAg surface finishes are fabricated and measured. A peak gain of 8.7–9 dBi is measured in the 57–64-GHz ISM band for the antenna without surface finish. Only 0.2-dB decrease in maximum gain is observed with 0.4- $\mu \text{m}$ silver. With ENIG surface finish, the gain decrease is observed as 1 dB. ENIG has higher loss due to flow of current in nickel material having lower conductivity compared to copper. The measurement results are consistent with the numerical results.

Effect of Epoxy on Mechanical Property of SAC305 Solder Joint with Various Surface Finishes Under 3-Point Bend Test.

Microstructures and mechanical property of Sn-3.0Ag-0.5Cu (SAC305) and epoxy Sn-3.0Ag-0.5Cu (epoxy SAC) solder joints were investigated with various surface finishes; organic solderability preservative (OSP), electroless nickel immersion gold (ENIG) and electroless nickel electroless palladium immersion gold (ENEPIG). Bending property of solder joints was evaluated by 3-point bend test method. Microstructure and chemical composition of solder joints was characterized by scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX), respectively. Epoxy did not effect on intermetallic compound (IMC) morphology. Scalloped shaped Cu6Sn5 IMC was observed at OSP surface finish. Chunky-like shaped and needle-like shaped (Ni,Cu)6Sn5 IMC were observed at the solder/ENIG joint and solder/ENEPIG joint, respectively. The bending cycles of SAC305/OSP joint, SAC305/ENIG joints and SAC305/ENEPIG joints were 720, 440 and 481 cycle numbers. The bending cycles of epoxy SAC and three types surface finished solder joints were over 1000 bending cycles. Under OSP surface finish, bending cycles of epoxy SAC solder was approximately 1.5 times higher than those of SAC305 solder joint. Bending cycles of epoxy SAC solder was over twice times higher than those of SAC305 solder with ENIG and ENEPIG surface finishes. The bending property of epoxy solder joint was enhanced due to epoxy fillet held the solder joint.

Electrochemical migration of Ni and ENIG surface finish during Environmental test contaminated by NaCl

The electrochemical migration (ECM) mechanism occurs at the presence of moisture in the case of operating circuits and results in shorts (dendrites) between adjacent conductor lines/traces. Dendrite growth occurs as a result of metal ions being dissolved into a solution from the anode and deposited at the cathode, thereby growing in tree-like formations. In this study the water condensation process and ECM behavior of Nickel (Ni), Electroless Nickel Immersion Gold (ENIG) and pure copper were investigated using Thermal Humidity and Bias (THB) test in NaCl environment. The THB results show that Cu has higher ECM resistance than Ni and ENIG surface finishes, which was an unexpected result. The main influencing factors of the water condensation (e.g.: surface roughness, thermal parameters) and the ECM processes (precipitates and dendrites) were investigated and discussed in details. Furthermore, a novel ECM model for Ni and NiAu surface finish was established using THB test in case NaCl contaminated samples.

Influence of Second Reflow on the Intermetallic Compound Growth with Different Surface Finish

The formation and growth of the intermetallic were frequently discussed since lead free solder took place replacing the lead solder. However, the effect of multiple reflow process on the intermetallic morphology that was subjected to aging still needs further investigation. Thus, this study aimed to investigate the effect of second reflow towards the intermetallic compound formation and growth. Two types of surface finishes were used such as Immersion Tin (ImSn) and Electroless Nickel Immersion Gold (ENIG). Both test boards were reflowed once with Sn-3Ag-0.5Cu at the temperature of 225 °C and soaking for 8 seconds. Then, they were reflowed again at the same temperature for 25 minutes prior to an isothermal aging process for 250, 500, 1000 and 2000 hours at the temperature of 150 °C. The ProgRes C3 IM7200 Optical Microscope and ImageJ were used for the microstructural study, which includes morphology and thickness. Results indicated that IMC thickness formed between solder and ImSn surface finish increased significantly with 1.28 µm incremental when exposed to the second reflow. Whereas the IMC thickness of ENIG surface finish was increased for up to 0.15 µm. In addition, ENIG showed higher activation energy as compared to ImSn.

Drop Reliability of Epoxy-contained Sn-58 wt.%Bi Solder Joint with ENIG and ENEPIG Surface Finish Under Temperature and Humidity Test

The influence of two kinds of surface finish, namely electroless nickel immersion gold (ENIG) and electroless nickel electroless palladium immersion gold (ENEPIG), on the interfacial reactions and drop reliability of epoxy-enhanced Sn-58 wt.%Bi solder has been investigated after temperature–humidity storage tests. The chemical composition and morphology of intermetallic compounds (IMCs) were characterized by scanning electron microscopy, energy-dispersive x-ray spectroscopy, and electron probe microanalysis. Also, the mechanical reliability of solder joints was evaluated using board-level drop tests. The Sn-Bi epoxy solder/ENEPIG joint exhibited higher IMC growth rate than the Sn-Bi epoxy solder/ENIG joint. After 500 h at 85°C/85% RH storage condition, new IMCs were formed on the Ni3Sn4 layer in samples with both surface finishes. The results of board-level drop tests showed that the number of drops was higher for the ENIG than the ENEPIG surface finish. Solder joint fracture occurred along the interface between the solder and IMC layer for the ENIG surface finish. However, with the ENEPIG surface finish, the crack propagated between the IMCs.

Board Level Drop Reliability of Epoxy-Containing Sn-58 mass% Bi Solder Joints with Various Surface Finishes

Board-level drop reliability testing under JEDEC (Joint Electron Device Engineering Council) drop conditions was conducted for low-temperature Sn-58 mass%Bi solder joints. The effects of surface finish and the number of reflow processes (one, three, or five) on the drop reliability of the Sn-58Bi joints were evaluated. Three types of surface finishes were used including electroless nickel immersion gold (ENIG), electroless nickel electroless palladium immersion gold (ENEPIG), and organic solderability preservatives (OSP). We successfully fabricated six types of drop test specimens with various surface finishes and bonding materials, and then performed the board-level drop tests. The board-level drop reliability was improved when using the composite Sn-58Bi solder with epoxy as compared to Sn-58Bi solder without epoxy. In addition, the reliability of the Sn-58Bi epoxy solder/OSP joint was better than both the ENIG and ENEPIG surface finishes. When the number of reflow processes increased, the drop reliability showed two different behaviors; specifically, the reliability of the ENIG and ENEPIG surface finishes decreased while the OSP surface finish improved. The existence of epoxy in the solder paste improved the drop reliability of the solder joints. After drop testing, cracks propagated differently with different surface finishes, namely between Ni3Sn4 IMCs and the Ni-P layer for the ENIG surface finish, between Ni3Sn4 IMCs and the solder bulk for the ENEPIG surface finish, and within the solder bulk for the OSP surface finish. These data show that, when it is necessary to perform repeated reflow processes for electronic components, solder joints with an OSP surface finish provide better drop test reliability results.

Evaluation of the Bondability of the Epoxy-Enhanced Sn-58Bi Solder with ENIG and ENEPIG Surface Finishes

The effect of different surface finishes, electroless nickel immersion gold (ENIG) and electroless nickel electroless palladium immersion gold (ENEPIG), on the mechanical properties of Sn-58Bi bumps made with solder paste enhanced with epoxy were investigated. The microstructure and fracture surfaces were observed with scanning electron microscopy, and the compositions of the IMC and solder were measured using energy dispersive spectrometry and an electron probe micro-analyzer (EPMA). To evaluate the mechanical properties, low-speed shear tests and board-level drop tests were performed. The result of the shear tests showed that the bonding strength of the epoxy-enhanced Sn-58Bi solder bumps was higher than that of Sn-58Bi solder for all surface finishes, because of the epoxy surrounding the solder, and the fracture surfaces of epoxy-enhanced Sn-58Bi indicated ductile fracture in the solder joint. However, the result of the drop tests showed that samples with the ENIG and ENEPIG surface finishes had lower drop numbers compared to the sample without these surface finishes. The lower performance resulted from insufficient ejection of epoxy from the ENIG and ENEPIG surface finishes during reflow, which reduced the interfacial bonding area.

Interfacial reaction and failure mode analysis of the solder joints for flip-chip LED on ENIG and Cu-OSP surface finishes

The interfacial reactions and failure modes of the solder joints for flip-chip light emitting diode (LED) on electroless nickel/immersion gold (ENIG) and Cu with organic solderability preservatives (Cu-OSP) surface finishes were investigated in this study. The experimental results demonstrate that the interfacial reactions in the Au/Sn–Ag–Cu(SAC)/ENIG and Au/SAC/Cu systems are different but the failure mechanisms of the two types of solder joints are similar during the shear test. For the Au/SAC/ENIG system, the Au layer on the surface finish of the diodes dissolved into the molten solder and transformed into a continuous (Au, Ni)Sn4 IMC layer at the diode/solder interface during reflow and the interfacial IMC at the solder/ENIG interface is dendritic Ni3Sn4 IMC grains which are surrounded by (Au, Ni)Sn4. For the Au/SAC/Cu system, however, no IMC layers can be observed at the diode/solder interface. The interfacial IMC at the solder/Cu interface is (Cu, Au)6Sn5 and a Cu3Sn IMC layer at the (Cu, Au)6Sn5/Cu interface. Tiny (Au, Cu)Sn4 IMC grains distribute in the solder layer and surround the (Cu, Au)6Sn5 grains. For the two types of systems, the primary failure mode for the cathode is due to the broken of the Si-based insulation layer which led to a high residue stress and poor connection between the Si-based layer and the solder layer. Meanwhile, the failure of the solder joint for the anode is mainly because of the failure of the solder layer under the conductive via. The crack generally forms at this area and then propagated along the diode or the diode/solder interface.

Effects of Microstructure and Loading on Fracture of Sn-3.8Ag-0.7Cu Joints on Cu Substrates with ENIG Surface Finish

When dropped, electronic packages often undergo failure by propagation of an interfacial crack in solder joints under a combination of tensile and shear loading. Hence, it is crucial to understand and predict the fracture behavior of solder joints under mixed-mode high-rate loading conditions. In this work, the effects of the loading conditions (strain rate and loading angle) and microstructure interfacial intermetallic compound (IMC) morphology and solder yield strength] on the mixed-mode fracture toughness of Sn-3.8 wt.%Ag-0.7 wt.%Cu solder joints sandwiched between two Cu substrates with electroless nickel immersion gold (ENIG) metallization have been studied, and compared with the fracture behavior of joints attached to bare Cu. Irrespective of the surface finish, the fracture toughness of the solder joints decreased monotonically with strain rate and mode-mixity, both resulting in increased fracture proportion through the interfacial IMC layer. Furthermore, the proportion of crack propagation through the interfacial IMC layer increased with increase in the thickness and the roughness of the interfacial IMC layer and the yield strength of the solder, resulting in a decrease in the fracture toughness of the joint. However, under most conditions, solder joints with ENIG finish showed higher resistance to fracture than joints attached directly to Cu substrates without ENIG metallization. Based on the experimental observations, a fracture mechanism map is constructed correlating the yield strength of the solder, the morphology and thickness of the interfacial IMC, and the fracture mechanisms as well as the fracture toughness values for different solder joints under mode I loading.