Effect of Interfacial Reactions on the Reliability of Lead-Free Assemblies after Board Level Drop Tests

The reliability of lead-free electronic assemblies after board level drop tests was investigated. Thin small outline package (TSOP) components with 42 FeNi alloy leads were reflow soldered on FR4 printed circuit boards (PCBs) with Sn3.0Ag0.5Cu (wt%) solder. The effects of different PCB finishes [organic solderability preservative (OSP) and electroless nickel immersion gold (ENIG)], multiple reflow (once and three times), and isothermal aging (500 h at 125°C after one time reflow) were studied. The ENIG finish showed better performance than its OSP counterparts. With the OSP finish, solder joints reflowed three times showed obvious improvement compared to those of the sample reflowed once, while aging led to apparent degradation. The results showed that intermetallic compound (IMC) types, IMC microstructure and solder microstructure compete with each other, all playing very important roles in the solder joint lifetime. The results also showed that it is important to specify adequate conditions for a given reliability assessment program, to allow meaningful comparison between results of different investigators.     

Drop impact reliability testing for lead-free and lead-based soldered IC packages

Board-level drop impact testing is a useful way to characterize the drop durability of the different soldered assemblies onto the printed circuit board (PCB). The characterization process is critical to the lead-free (Pb-free) solders that are replacing lead-based (Pb-based) solders. In this study, drop impact solder joint reliability for plastic ball grid array (PBGA), very-thin quad flat no-lead (VQFN) and plastic quad flat pack (PQFP) packages was investigated for Pb-based (62Sn–36Pb–2Ag) and Pb-free (Sn–4Ag–0.5Cu) soldered assemblies onto different PCB surface finishes of OSP (organic solderability preservative) and ENIG (electroless nickel immersion gold). The Pb-free solder joints on ENIG finish revealed weaker drop reliability performance than the OSP finish. The formation of the brittle intermetallic compound (IMC) Cu–Ni–Sn has led to detrimental interfacial fracture of the PBGA solder joints. For both Pb-based and Pb-free solders onto OSP coated copper pad, the formation of Cu₆Sn₅ IMC resulted in different failure sites and modes. The failures migrated to the PCB copper traces and resin layers instead. The VQFN package is the most resistant to drop impact failures due to its small size and weight. The compliant leads of the PQFP are more resistant to drop failures compared to the PBGA solder joints.     

Impact of Thermal Cycling on Sn-Ag-Cu Solder Joints and Board-Level Drop Reliability

The electronic packaging industry uses electroless nickel immersion gold (ENIG) or Cu-organic solderability preservative (Cu-OSP) as a bonding pad surface finish for solder joints. In portable electronic products, drop impact tests induce solder joint failures via the interfacial intermetallic, which is a serious reliability concern. The intermetallic compound (IMC) is subjected to thermal cycling, which negatively affects the drop impact reliability. In this work, the reliability of lead-free Sn-3.0Ag-0.5Cu (SAC) soldered fine-pitch ball grid array assemblies were investigated after being subjected to a combination of thermal cycling followed by board level drop tests. Drop impact tests conducted before and after thermal aging cycles (500, 1000, and 1500 thermal cycles) show a transition of failure modes and a significant reduction in drop durability for both SAC/ENIG and SAC/Cu-OSP soldered assemblies. Without thermal cycling aging, the boards with the Cu-OSP surface finish exhibit better drop impact reliability than those with ENIG. However, the reverse is true if thermal cycle (TC) aging is performed. For SAC/Cu-OSP soldered assemblies, a large number of Kirkendall voids were observed at the interface between the intermetallic and Cu pad after thermal cycling aging. The void formation resulted in weak bonding between the solder and Cu, leading to brittle interface fracture in the drop impact test, which resulted in significantly lower drop test lifetimes. For SAC/ENIG soldered assemblies, the consumption of Ni in the formation of NiCuSn intermetallics induced vertical voids in the Ni(P) layer.     

Morphology and Growth of Intermetallics at the Interface of Sn-based Solders and Cu with Different Surface Finishes

Several types of surface finishes have been applied on Cu substrates in an effort to facilitate bonding and improve the reliability of lead-free solder joints. In the current research, the effects of printed circuit board surface finishes on the reliability of the solder joints were investigated by examining the morphology and growth behavior of the intermetallic compounds (IMCs) between Sn-based solders and different surface finishes on Cu. Three types of Cu substrates with different surface finishes were fabricated in this study: organic solderability preservative (OSP)/Cu, Ni/Cu, and electroless nickel immersion gold (ENIG)/Cu. Sn-3.5Ag and Sn-3.0Ag-0.5Cu were used as the solders. In the experiment, the solder joint specimens were aged isothermally at 150°C for up to 1000 h. Experimental results revealed that the OSP surface finish promoted the interdiffusion between Cu and Sn during soldering. The composition and morphology of the IMC layer at the solder/Ni/Cu interface were sensitive to the Cu concentration in the solder. Meanwhile, the solder joints with different morphological features of the IMCs exhibited significant differences in shear strengths. The Au-containing ENIG surface finish affected the shear strength of the solder joint significantly at the initial stage of isothermal aging.     

Failure mechanisms of lead-free chip scale package interconnections under fast mechanical loading

The reliability of chip scale package (CSP) components against mechanical shocks has been studied by employing statistical, fractographic, and microstructural research methods. The components having high tin (Sn0.2Ag0.4Cu) solder bumps were reflow soldered with the Sn3.8Ag0.7Cu (wt.%) solder paste on Ni(P)|Au- and organic solderability preservative (OSP)-coated multilayer printed wiring boards (PWBs), and the assemblies were subjected to the standard drop test procedure. The statistically significant difference in the reliability performance was observed: the components soldered on Cu|OSP were more reliable than those soldered on Ni(P)|Au. Solder interconnections on the Cu|OSP boards failed at the component side, where cracks propagated through the (Cu,Ni)₆Sn₅ reaction layer, whereas interconnections on the Ni(P)|Au boards failed at the PWB side exhibiting the brittle fracture known also as “black pad.” In the first failure mode, which is not normally observed in thermally cycled assemblies, cracks propagate along the intermetallic layers due to the strong strain-rate hardening of the solder interconnections in drop tests. Owing to strain-rate hardening, the stresses in the solder interconnections increase very rapidly in the corner regions of the interconnections above the fracture strength of the ternary (Cu,Ni)₆Sn₅ phase leading to intermetallic fracture. In addition, because of strain-rate hardening, the recrystallization of the as-soldered microstructure is hindered, and therefore the network of grain boundaries is not available in the bulk solder for cracks to propagate, as occurs during thermal cycling. In the black pad failure mode, cracks nucleate and propagate in the porous NiSnP layer between the columnar two-phase (Ni₃P+Sn) layer and the (Cu,Ni)₆Sn₅ intermetallic layer. The fact that the Ni(P)|Au interconnections fail at the PWB side, even though higher stresses are generated on the component side, underlines the brittle nature of the reaction layer.     

Study of immersion silver and tin printed-circuit-board surface finishes in lead-free solder applications

The wetting of I-Ag (immersion silver) and I-Sn (immersion tin) printed-circuit-board (PCB) finishes by Sn/Ag/Cu and eutectic Sn/Pb solders was studied in this work with Ni/Au (electroless nickel/immersion gold) and organic solderability preservative (OSP) finishes as baselines. Wetting tests were performed on fresh boards and boards subjected to different preconditioning treatments that simulated the effects of aging, storage, and multiple reflow cycles. When the boards are fresh, the wetting of the I-Sn and Ni/Au finishes is better than that on the I-Ag and OSP finishes. However, after the preconditioning treatments, the wetting of the I-Sn finish degrades the fastest, whereas the wetting of the I-Ag and OSP finishes degrade less through the different preconditioning treatments. The wetting of the Ni/Au finish remains excellent through all the preconditioning treatments. The chemical and microstructural changes in the finishes during aging treatments were evaluated using electron spectroscopy chemical analysis (ESCA), x-ray diffractometry (XRD), and cross-sectioning followed by scanning electron microscopy (SEM). The results indicate that a single lead-free reflow cycle consumes the I-Sn layer faster than a Sn/Pb reflow cycle because of the formation of the Sn/Cu intermetallic compound (IMC). Consequently, I-Sn finished boards having an original Sn thickness of ∼1 μm will not withstand multiple lead-free reflow cycles without significant degradation in wetting but up to two Sn/Pb reflow cycles are still feasible. The minimum thickness of I-Sn required for adequate wetting was evaluated by comparing the wetting after different aging treatments. The exposure of I-Sn samples to 85°C/85% relative humidity (RH) conditions increases the thickness of the Sn-oxide layer, which, above a certain thickness, can degrade wetting. Oxidized copper areas formed on top of the I-Ag surface after exposure to 85°C/85% RH treatment, and this was considered a major factor influencing wetting. The formation of sulfides on I-Ag was detected, but their overall quantity remained too small to have a detectable impact on the wetting.     

Intermetallics Characterization of Lead-Free Solder Joints under Isothermal Aging

Solder interconnect reliability is influenced by environmentally imposed loads, solder material properties, and the intermetallics formed within the solder and the metal surfaces to which the solder is bonded. Several lead-free metallurgies are being used for component terminal plating, board pad plating, and solder materials. These metallurgies react together and form intermetallic compounds (IMCs) that affect the metallurgical bond strength and the reliability of solder joint connections. This study evaluates the composition and extent of intermetallic growth in solder joints of ball grid array components for several printed circuit board pad finishes and solder materials. Intermetallic growth during solid state aging at 100°C and 125°C up to 1000 h for two solder alloys, Sn-3.5Ag and Sn-3.0Ag-0.5Cu, was investigated. For Sn-3.5Ag solder, the electroless nickel immersion gold (ENIG) pad finish was found to result in the lowest IMC thickness compared to immersion tin (ImSn), immersion silver (ImAg), and organic solderability preservative (OSP). Due to the brittle nature of the IMC, a lower IMC thickness is generally preferred for optimal solder joint reliability. A lower IMC thickness may make ENIG a desirable finish for long-life applications. Activation energies of IMC growth in solid-state aging were found to be 0.54 ± 0.1 eV for ENIG, 0.91 ± 0.12 eV for ImSn, and 1.03 ± 0.1 eV for ImAg. Cu₃Sn and Cu₆Sn₅ IMCs were found between the solder and the copper pad on boards with the ImSn and ImAg pad finishes. Ternary (Cu,Ni)₆Sn₅ intermetallics were found for the ENIG pad finish on the board side. On the component side, a ternary IMC layer composed of Ni-Cu-Sn was found. Along with intermetallics, microvoids were observed at the interface between the copper pad and solder, which presents some concern if devices are subject to shock and vibration loading.     

印制电路板铜面保护层对无铅焊点结构影响

采用扫描电镜与能谱测试,研究了两种不同印刷电路板铜面保护层,即有机保护层(Organic Solderability Preservatives,OSP)与浸银层(Immersion Ag,I—Ag),对无铅焊点结构的影响。结果显示,采用有机保护层的焊接界面金属间化合物层厚度明显超过了浸银层;在两种不同保护层中的焊点中,均出现薄片状或树枝状Ag3Sn金属间化合物,但在浸银层焊点中,薄片状Ag3sn主要在焊接界面层处非均匀形核长大,而有机保护层焊点中,薄片状Ag3Sn较少出现,代之以树枝状Ag3Sn近似均匀地分布在焊点中。断口分析显示,采用有机保护层的焊点中出现了较多的气孔,而且气孔主要出现在靠近铜面焊点中,这明显降低了焊点的强度。     

Effect of Cu stud microstructure and electroplating process onintermetallic compounds growth and reliability of flip-chip solder bump

In electroplating-based flip-chip technology, the Cu stud and solder deposition processes are two of the most important factors affecting the reliability of solder joints. The growth of Cu-Sn intermetallic compounds (IMC) also plays a critical role. In this paper, the effect of Cu stud surface roughness and microstructures on the reliability of solder joint was studied. The surface roughness of the Cu stud was increased as the Cu electroplating current density increased. The microstructural morphology of the Cu-Sn IMC layer was affected by Cu stud surface structure. We found the growth rate of IMC layer increased with the increasing of Cu stud grain size and surface roughness during aging test. The growth kinetics of Cu-Sn intermetallic compound formation for 63Sn/37Pb solder followed the Arrhenius equation with activation energy varied from 0.78 eV to 1.14 eV. The ratios of Cu₃ Sn layer thickness to the total Cu-Sn IMC layer thickness was in the range of 0.5 to 0.15 for various Cu microstructures at 150°C during thermal aging test. The shear strength of solder bump was measured after thermal aging and temperature/humidity tests. The relationship between electroplating process and reliability of solder joints was established. The failure mode of solder joints was also analyzed     

Evaluation on Influencing Factors of Board-Level Drop Reliability for Chip Scale Packages (Fine-Pitch Ball Grid Array)

Board-level drop testing is an effective method to characterize the solder joint reliability performance of miniature handheld products. In this study, drop test of printed circuit boards (PCBs) with a four-screw support condition was conducted for a 15 mm times 15 mm fine-pitch ball grid array (FBGA) package assembly with solder ball compositions of 36Pb-62Sn-2Ag and Sn-4Ag-0.5Cu on printed circuit board (PCB) surface finishes of organic solderability preservative, electroless nickel immersion gold, and immersion tin. Finite element modeling of the FBGA assembly was performed to study the stress-strain behavior of the solder joints during drop test. The drop test results revealed a strong influence of different intermetallic compound formation on soldered assemblies drop durability. The lead-based solder supersedes the lead-free composition regardless of the types of surface finish. Joints on organic solderability preservative were found to be strongest for each solder type. Other factors affecting drop reliability such as component location on the board and thermal cycling aging effects are reported. Finite element modeling results showed that a solder joint is more prone to failure on the PCB side, and the predicted solder joint stresses are location dependent. Predicted failure sites based on simulation results are consistent with experimental observations.     

Isothermal and thermal cycling aging on IMC growth rate in lead-free and lead-based solder interface

The growth of interfacial intermetallic compounds (IMC) between Pb-free and Pb-based solders with different surface finish (Cu and Ni/Au) metallization is a major concern for long-term solder joint reliability performance in electronic assemblies. The growth rate of the IMC layer can affect the solder joint reliability. Analysis of solid-state diffusion mechanism for the growth of IMC between solder-to-substrate interface for Pb-free and Pb-based solders subject to isothermal and thermal cycling aging were conducted. Experimental study of IMC layer growth between Sn3.8Ag0.7Cu and Ni/Au surface finish by isothermal aging versus thermal cycling (TC) aging was investigated to develop a framework for correlating IMC layer growth behavior. An integrated model for IMC growth was derived to describe the Ni-Cu-Sn IMC growth behavior subject to TC aging. Comparison of modeling and test results showed that IMC layer growth rate under TC aging was accelerated. It is noted that IMC layer growth study from various references showed different experimental data and growth kinetic parameters for both liquid-state and solid-state reactions.     

Interfacial reaction study on a solder joint with Sn-4Ag-0.5Cu solder ball and Sn-7Zn-Al (30 ppm) solder paste in a lead-free wafer level chip scale package

The interfacial reactions of solder joints between the Sn-4Ag-0.5Cu solder ball and the Sn-7Zn-Al (30 ppm) presoldered paste were investigated in a wafer level chip scale package (WLCSP). After appropriate surface mount technology (SMT) reflow process on the printed circuit board (PCB) with organic solderability preservative (Cu/OSP) and Cu/Ni/Au surface finish, samples were subjected to 150°C high-temperature storage (HTS), 1,000 h aging. Sequentially, the cross-sectional analysis is scrutinized using a scanning electron microscope (SEM)/energy-dispersive spectrometer (EDS) and energy probe microanalysis (EPMA) to observe the metallurgical evolution in the interface and solder buck itself. It was found that Zn-enriched intermetallic compounds (IMCs) without Sn were formed and migrated from the presolder paste region into the solder after reflow and 150°C HTS test.     

Interfacial microstructure and bonding strength of Sn-3Ag-0.5Cu and Sn-3Ag-0.5Cu-0.5Ce-xZn solder BGA packages with immersion Ag surface finish

Although it has been verified that tin whiskers can be prevented by the addition of 0.5 wt.% Zn into a Sn-3Ag-0.5Cu-0.5Ce solder, no detailed studies have been conducted on interfacial reactions and mechanical properties of Sn-3Ag-0.5Cu-0.5Ce-xZn solder joints with an immersion Ag surface finish. The intermetallic compounds formed during the reflow and aging of Sn-3Ag-0.5Cu and Sn-3Ag-0.5Cu-0.5Ce-xZn solder ball grid array (BGA) packages were investigated. Because more heterogeneous nucleation sites, provided by CeSn₃ intermetallics and Zn atoms, formed in the Sn-3Ag-0.5Cu-0.5Ce-xZn solder matrix, and Cu and Zn have a stronger affinity than Cu and Sn, the Cu-Sn intermetallics growth in Sn-3Ag-0.5Cu-0.5Ce-xZn solder joints with Ag/Cu pads was suppressed. The 0.2% Zn addition for inhibiting rapid whisker growth in RE-doped Sn-Ag-Cu solder joints is more appropriate than 0.5 wt.% additions, as excess Zn addition causes poor oxidation resistance and inferior bonding strength.     

Effects of Surface Finishes and Current Stressing on Interfacial Reaction Characteristics of Sn-3.0Ag-0.5Cu Solder Bumps

The effects of surface finishes on the in situ interfacial reaction characteristics of ball grid array (BGA) Sn-3.0Ag-0.5Cu lead-free solder bumps were investigated under annealing and electromigration (EM) test conditions of 130°C to 175°C with 5.0 × 10³ A/cm². During reflow and annealing, (Cu,Ni)₆Sn₅ intermetallic compound (IMC) formed at the interface of electroless nickel immersion gold (ENIG) finish. In the case of both immersion Sn and organic solderability preservative (OSP) finishes, Cu₆Sn₅ and Cu₃Sn IMCs formed. Overall, the IMC growth velocity of ENIG was much lower than that of the other finishes. The activation energies of total IMCs were found to be 0.52 eV for ENIG, 0.78 eV for immersion Sn, and 0.72 eV for OSP. The ENIG finish appeared to present an effective diffusion barrier between the Cu substrate and the solder, which leads to better EM reliability in comparison with Cu-based pad systems. The failure mechanisms were explored in detail via in situ EM tests.     

Lead-free chip scale packages: assembly and drop test reliability

Three underfill options compatible with lead-free assembly have been evaluated: capillary underfill, fluxing underfill, and corner bond underfill. Chip scale packages (CSPs) with eutectic Sn/Pb solder were used for control samples. Without underfill, lead-free and Sn/Pb eutectic drop test results were comparable. Capillary flow underfills, dispensed and cured after reflow, are commonly used in CSP assembly with eutectic Sn/Pb solder. With capillary flow underfill, the drop test results were significantly better with lead-free solder assembly than with Sn/Pb eutectic. Fluxing underfill is dispensed at the CSP site prior to CSP placement. No solder paste is printed at the site. The CSP is placed and reflowed in a standard reflow cycle. A new fluxing underfill developed for compatibility with the higher lead-free solder reflow profiles was investigated. The fluxing underfill with lead-free solder yielded the best drop test results. Corner bond underfill is dispensed as four dots corresponding to the four corners of the CSP after solder paste print, but before CSP placement. The corner bond material cures during the reflow cycle. It is a simpler process compared to capillary or fluxing underfill. The drop test results with corner bond were intermediate between no underfill and capillary underfill and similar for both lead-free and Sn/Pb eutectic solder assembly. The effect of aging on the drop test results with lead-free solder and either no underfill or corner bond underfill was studied. Tin/lead solder with no underfill was used for control. This test was to simulate drop performance after the product has been placed in service for some period of time. There was degradation in the drop test results in all cases after 100 and 250 h of storage at 125/spl deg/C prior to the drop test. The worst degradation occurred with the lead-free solder with no underfill.     

Effect of (Au, Ni)Sn 4 Evolution on Sn-37Pb/ENIG Solder Joint Reliability Under Isothermal and Temperature-Cycled Conditions

The ternary intermetallic compound Au_0.5Ni_0.5Sn₄ forms at the Sn-37Pb/ENIG solder interface during aging and temperature cycling, leading to increased interfacial cracking and a corresponding decrease in solder joint reliability for 15 mm ball grid array (BGA) structures. (Au,Ni)Sn ₄ forms at both the board finish (bottom) and component side (top) of the solder joint for isothermally aged, temperature-cycled, and (aged + cycled) joints. For control specimens (reflow only), no cracks or interfacial Au are observed. For isothermally aged joints (170 and 340 h at 125degC), a broken, discontinuous layer of (Au,Ni)Sn₄ is present, but no cracking. For temperature-cycled joints, lowered reliability and interfacial cracking occurs along a continuous (Au,Ni)Sn₄ intermetallic layer on the solder side of the interface after ~450 h of cycling. Aging + cycling did little to inhibit cracking or formation of (Au,Ni)Sn₄. Development of a continuous (Au,Ni)Sn₄ film at the interface is the key failure mechanism. At low cycle numbers where high joint reliability is observed, the (Au,Ni)Sn₄ layer is discontinuous and not fully developed. At higher cycle numbers and longer aging times, the (Au,Ni)Sn ₄ layer becomes continuous and encourages crack growth along the intermetallic interface and consequent lower reliability. The correlation of interfacial smoothness with lowered reliability is consistent with recent work showing that, when intermetallic compounds form smoothly at the solder interface, the mechanical properties are degraded (compared to a rough intermetallic) due to the decreased resistance to shear along the interface     

Correlation between interfacial reactions and mechanical strengths of Sn(Cu)/Ni(P) solder bumps

The correlation between interfacial reactions and mechanical strengths of Sn(Cu)/Ni(P) solder bumps has been studied. Upon solid-state aging, a diffusion-controlled process was observed for the interfacial Ni-Sn compound formation of the Sn/Ni(P) reaction couple and the activation energy is calculated to be 42 KJ/mol. For the Sn0.7Cu/Ni(P), in the initial aging, a needle-shaped Ni-Sn compound layer formed on Ni(P). Then, it was gradually covered by a layer of the Cu-Sn compound in the later aging process. Hence, a mixture layer of Ni-Sn and Cu-Sn compounds formed at the interface. For the Sn3.0Cu/Ni(P), a thick Cu-Sn compound layer quickly formed on Ni(P), which retarded the Ni-Sn compound formation and resulted in a distinct Cu-Sn compound/Ni(P) interface. The shear test results show that the mixture interface of Sn0.7Cu bumps have fair shear strengths against the aging process. In contrast, the distinct Cu-Sn/Ni(P) interface of Sn3.0Cu solder bumps is relatively weak and exhibits poor resistance against the aging process. Upon the reflowing process, the gap formation at the Ni(P)/Cu interface caused a fast degradation in the interfacial strength for Sn solder bumps. For Sn0.7Cu and Sn3.0Cu solder bumps, Ni₃P formation was greatly retarded by the self-formed Cu-Sn compound layer. Therefore, Sn(Cu) solder bumps show better shear strengths over the Sn solder bump.     

Experimental studies of board-level reliability of chip-scale packages subjected to JEDEC drop test condition

We investigate in this paper board-level drop reliability of chip-scale packages subjected to JEDEC drop test condition B, which features an impact pulse profile with a peak acceleration of 1500G and a pulse duration of 0.5 ms. Effects of Sn–Ag–Cu or Sn–Pb solder joint compositions, fluxes, and substrate pads with Ni/Au surface finish or OSP coating on the drop reliability of the board-level test vehicle are compared. Locations and modes of the failed solder joints are examined using the dye stain test. The results indicate that solder joints with a low Ag weight content and substrate pads with OSP coating both enhance the drop resistance of the board-level test vehicle.     

Study of interaction between Cu-Sn and Ni-Sn interfacial reactions by Ni-Sn3.5Ag-Cu sandwich structure

The interaction between Cu-Sn and Ni-Sn interfacial reactions in a soldering system has been studied using a Ni-Sn3.5Ag-Cu sandwich structure. A layer of Cu-Sn intermetallic compound was observed at the interface of the Ni foil after 30 sec of reflowing. Two stages of the Cu-Sn compound growth on the Ni side were observed: (1) in the first minute of reflow, the fast Cu-Sn compound formation was rate-limited by Cu diffusivity in the Cu-Sn compound layer of the opposite Cu side; and (2) after 1 min of reflow, the Cu-Sn compound growth was very sluggish and depended on the Ni diffusion in the Cu-Sn compound of the Ni side. Very little Ni can be detected in the Cu side. This implies that Cu diffused and dissolved in the molten Sn3.5Ag solder much faster than Ni. When the dissolved Cu arrived at the interface of the Ni foil, a Cu-Sn compound layer formed on the Ni interface to prevent the Ni foil from reacting with the solder. The driving force of the dissolved Cu atoms toward the Ni side attributed to the Cu solubility difference across the molten solder, which was established due to the reduction of the Cu solubility near the Ni interface. The reduction of Cu solubility was caused by the presence of dissolved Ni near the Ni interface. Knowing the experimental value of the Cu flux toward the Ni side and assuming the diffusion of Cu atoms in the molten solder following Fick’s first law, the diffusivity of Cu is found to be 10⁻⁵ cm²/s.     

Intermetallic reactions in Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder BGA packages with Au/Ni surface finishes

The intermetallic compounds (IMCs) formed during the reflow and aging of Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder BGA packages with Au/Ni surface finishes were investigated. After reflow, the thickness of (Cu, Ni, Au)₆Sn₅ interfacial IMCs in Sn3Ag0.5Cu0.06Ni0.01Ge was similar to that in the Sn3Ag0.5Cu specimen. The interiors of the solder balls in both packages contained Ag₃Sn precipitates and brick-shaped AuSn₄ IMCs. After aging at 150°C, the growth thickness of the interfacial (Ni, Cu, Au)₃Sn₄ intermetallic layers and the consumption of the Ni surface-finished layer on Cu the pads in Sn3Ag0.5Cu0.06Ni0.01Ge solder joints were both slightly less than those in Sn3Ag0.5Cu. In addition, a coarsening phenomenon for AuSn₄ IMCs could be observed in the solder matrix of Sn3Ag0.5Cu, yet this phenomenon did not occur in the case of Sn3Ag0.5Cu0.06Ni0.01Ge. Ball shear tests revealed that the reflowed Sn3Ag0.5Cu0.06Ni0.01Ge packages possessed bonding strengths similar to those of the Sn3Ag0.5Cu. However, aging treatment caused the ball shear strength in the Sn3Ag0.5Cu packages to degrade more than that in the Sn3Ag0.5Cu0.06Ni0.01Ge packages.