Sn 8Zn 3Bi Research Articles

A comparison of impression, indentation and impression-relaxation creep of lead-free Sn–9Zn and Sn–8Zn–3Bi solders at room temperature

Creep behavior of Sn–9% Zn and Sn–8% Zn–3% Bi solder alloys was studied by impression, indentation, and impression-relaxation tests at room temperature (T > 0.6Tm) in order to evaluate the correspondence of the creep results obtained by different testing techniques, and to evaluate the effect of Bi on the creep response of the eutectic Sn–9Zn alloy. Stress exponent values were determined through these methods and in all cases the calculated exponents were in good agreement. The average stress exponents of 8.6 and 9.9, found respectively for the binary and ternary alloys, are close to those determined by room temperature conventional creep testing of the same materials reported in the literature. These exponents imply that dislocation creep is the possible mechanism during room temperature creep deformation of these alloys. The introduction of 3% Bi into the binary alloy enhanced the creep resistance due to both solid solutioning effect and sparse precipitation of Bi in the Sn matrix.

Characteristics of Sn8Zn3Bi solder joints and crack resistance with various PCB and lead coatings

Reliability of QFP (quad flat package) solder joints after thermal shock was investigated for PCB’s and connecting leads plated with several different alloy coatings before soldering. Sn–8 wt%Zn–3 wt%Bi (hereafter, Sn–8Zn–3Bi) was selected as a solder, and FR-4 PCB’s finished with Cu/Sn, Cu/OSP and Cu/Ni/Au were used as substrates. The leads of the QFP were Cu plated with Sn–10 wt%Pb, or Sn, or Sn–3 wt%Bi. The QFP chips were mounted on the substrates using a Sn–8Zn–3Bi solder paste and reflowed in air atmosphere. The pull strength and microstructure for the soldered leads of QFP were evaluated before and after thermal shock testing. The leads plated with Sn or Sn–3Bi showed approximately 40–50% higher pull strength than the reference value of a Sn–37%Pb solder joint for all PCB-finishes. However, in the case of leads coated with Sn–10Pb, the pull strength of the leads soldered to a Sn-finished PCB was 21% lower than the reference value. In microstructure analysis of the joints with Sn–10Pb-plated leads, cracks were found along the bonded interface for Sn-finished PCB. The cracks were believed to start from the low melting temperature phase, 49.38 wt% Pb–32.58 wt%Sn–18.03 wt%Bi, found around the crack, and then propagated through Cu–Zn intermetallic compound. Meanwhile, even when using Sn–10Pb-plated leads, the PCB’s finished with Cu/Ni/Au coating had about 30% higher strength than the reference value, and cracks were hardly found on the soldered joint. Thus, even with Sn–10Pb-plated leads the Cu/Ni/Au-finished PCB’s were evaluated to be as reliable as the reference joint.

Effect of cooling rate on the room-temperature impression: creep of lead-free Sn–9Zn and Sn–8Zn–3Bi solders

The influence of cooling rate on the creep behavior of Sn–9%Zn and Sn–8%Zn–3%Bi solder alloys was studied by impression testing. The tests were carried out under constant impression stress in the range 60–190 MPa at room temperature. The alloys were prepared at two different cooling rates of 0.01 K s −1 and 8 K s −1. This affected the microstructure and thus, the creep behavior of the materials. In the binary Sn–9Zn alloy, the fast cooled (FC) microstructure, characterized by a uniform distribution of fine Zn precipitates, was much more creep resistant than the slowly cooled (SC) condition containing coarse needle-like Zn phase. The introduction of 3% Bi into the binary alloy enhanced the creep resistance and decreased the difference between SC and FC conditions. This is due mainly to the strong solid solutioning effect of Bi in Sn, which decreases the steady state creep rate, especially in the FC condition. The stress exponents, depending on the cooling rate, were found to be 6.2 and 8.5 for Sn–9Zn, and 9.3 and 9.8 for Sn–8Zn–3Bi, which are in agreement with those determined by room temperature conventional creep testing of the same materials reported in the literature.

Interfacial microstructures and solder joint strengths of the Sn–8Zn–3Bi and Sn-9Zn–lAl Pb–free solder pastes on OSP finished printed circuit boards

Two kinds of lead-free solders, Sn–8Zn–3Bi and Sn–9Zn–lAl, were used to mount passive components onto printed circuit boards via a re-flow soldering process. The samples were stored at 150 °C for 200, 400, 600, 800, and 1100 h. The microstructures of the samples after aged at 150 °C for various times were characterized using optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and the analyzed of solder joint shear strengths. The joint strength between Sn–8Zn–3Bi and Cu pad was about 4.0 ± 0.3 kg, while the strength between Sn–9Zn–lAl and Cu pad had values of 2.6 ± 0.1 kg. Both kinds of solder joints exhibited reduced strengths with increasing aging times. After aging at 150 °C for 1100 h, the joints strengths of Sn-8Zn-3Bi and Sn–9Zn–lAl were 1.8 ± 0.3 and 1.7 ± 0.3 kg, respectively. Both the Sn–8Zn–3Bi and Sn–9Zn–lAl joints showed brittle fracture behaviors. A flat layer of Cu 5Zn 8 intermetallic compound (IMC) was formed between Sn–8Zn–3Bi solder and Cu pad after reflow. When the aging time was increased to 400 h, Zn-depletion and formation of Cu 6Sn 5 IMC were observed in the solders due to the interaction between the tin and zinc compounds. The interaction between Sn–9Zn–lAl solder and Cu pad had similar behavior, however, Cu 6Sn 5 IMC formed in Sn–9Zn–lAl solder when after aging at 150 °C for 600 h. As the aging time increased, both types of solders generated clear IMC spalling layers with large and continuous voids. Those voids substantially decreased the joint strength.

Effect of strain waveform on creep‐fatigue life for Sn–8Zn–3Bi solder

ABSTRACTThis paper describes the creep‐fatigue life of Sn–8Zn–3Bi under push–pull loading. Creep‐fatigue tests were carried out using Sn–8Zn–3Bi specimens in fast–fast, fast–slow, slow–fast, slow–slow and hold–time strain waveforms. Creep‐fatigue lives in the slow–slow and hold‐time waveforms showed a small reduction from the fast–fast lives but those in the slow–fast and fast–slow waveforms showed a significant reduction from the fast–fast lives. Conventional creep‐fatigue life prediction methods were applied to the experimental data and the applicability of the methods was discussed. Creep‐fatigue characteristics of Sn–8Zn–3Bi were compared with those of Sn–3.5Ag and Sn–37Pb.

Correlation between interfacial microstructure and shear behavior of Sn–Ag–Cu solder ball joined with Sn–Zn–Bi paste

Sn–8Zn–3Bi solder paste was applied as a medium to joint Sn–3.2Ag–0.5Cu solder balls and Cu/Ni/Au metallized ball grid array substrates at 210 °C. Sn–Ag–Cu joints without Sn–Zn–Bi addition were also conducted for comparison. The shear behavior of the specimens was investigated after multiple reflow and thermal aging. For each strength test, more than 40 solder balls were sheared. The shear strength of Sn–Ag–Cu specimens kept constant ranging from 15.5 ± 1.3 N (single reflow) to 16.2 ± 1.0 N (ten reflows) and the fractures occurred in the solder. Shear strength of Sn–Ag–Cu/Sn–Zn–Bi specimens fell from 15.9 ± 1.7 N (single reflow) to 13.4 ± 1.6 N (ten reflows). After single reflow, Sn–Ag–Cu/Sn–Zn–Bi specimens fractured in the solder along Ag–Au–Cu–Zn intermetallic compounds and at Ni metallization. After ten reflows, fractures occurred in the solder and at solder/Ni–Sn–Cu–Zn intermetallic compound interface. The shear strengths of the Sn–Ag–Cu and Sn–Ag–Cu/Sn–Zn–Bi packages changed little after aging at 150 °C. Sn–Ag–Cu/Sn–Zn–Bi joints kept higher strength than Sn–Ag–Cu joints. Sn–Ag–Cu joints fractured in the solder after aging. But the fractures of Sn–Ag–Cu/Sn–Zn–Bi specimens shifted to the solder with aging time.

Effect of multiple reflows on mechanical strength of the interface formed between Sn–Zn–Bi solder and Au/Ni/Cu bond pad

In this work, the shear strengths and interfacial reactions of Sn–8Zn–3Bi and Sn–8Zn–1Bi (wt%) solders with Au/Ni/Cu ball grid array (BGA) pad metallization were systematically investigated after multiple reflows. The peak reflow temperature was fixed at 230 °C. After the shear test, fracture surfaces were investigated using a scanning electron microscope equipped with an energy dispersive x-ray spectrometer. Cross-sectional studies of the interfaces were also conducted to correlate with the fracture surfaces. Two failure modes, ball cut and pad lift, were assessed for the different solders and reflow cycles. It was found that the shearing forces of both the Sn–Zn–Bi solder joints tended to increase slightly with an increase in the number of reflow cycles due to augmentation of the shearing area. A layer-type spalling of the interfacial intermetallic compounds (IMCs) was observed very early in the liquid-state reaction for the solder alloys. The active nature of the Zn confirmed an instant reaction zone at the interface to maintain the bonding between the solder and the substrate.

Electrical resistance of Sn–Ag–Cu ball grid array packages with Sn–Zn–Bi addition jointed at 240 °C

Sn–8Zn–3Bi solder paste and Sn–3.2Ag–0.5Cu solder balls were reflowed simultaneously at 240 °C on Cu/Ni/Au metallized ball grid array substrates. The joints without Sn–Zn–Bi addition (only Sn–Ag–Cu) were studied as a control system. Electrical resistance was measured after multiple reflows and aging. The electrical resistance of the joint (R1) consisted of three parts: the solder bulk (Rsolder bulk, upper solder highly beyond the mask), interfacial solder/intermetallic compound (Rsolder/IMC), and the substrate (Rsubstrate). R1 increased with reflows and aging time. Rsolder/IMC, rather than Rsolder bulk and Rsubstrate, seemed to increase with reflows and aging time. The increase of R1 was ascribed to the Rsolder/IMC rises. Rsubstrate was the major contribution to R1. However Rsolder/IMC dominated the increase of R1 with reflows and aging. R1 of Sn–Zn–Bi/Sn–Ag–Cu samples were higher than that of Sn–Ag–Cu samples in various tests.

Effect of substrate metallization on interfacial reactions and reliability of Sn–Zn–Bi solder joints

The scope of this paper covers a comprehensive study of the lead-free Sn–Zn–Bi solder system, on Cu, electrolytic Ni/Au and electroless Ni(P)/Au surface finishes. This includes a study of the shear properties, intermetallic compounds at the substrate–ball interface and dissolution of the under bump metallization. The Sn–8Zn–3Bi (wt.%) solder/Cu system exhibited a low shear load with thick IMCs formation at the interface. The dissolution of the Cu layer in the Sn–Zn–3Bi solder is higher than that of the other two Ni metallizations. It was found that the formation of a thick Ni–Zn intermetallic compound (IMC) layer at the solder interface of the electrolytic Ni bond pad reduced the mechanical strength of the joints during high temperature long time liquid state annealing. The solder ball shear-load for the Ni(P) system during extended reflow increased with an increase of reflow time. No spalling was noticed at the interface of the Sn–Zn–3Bi solder/Ni(P) system. Sn–8Zn–3Bi solder with electroless Ni(P) metallization appeared as a good combination in soldering technology.

Correlation between localized strain and damage in shear-loaded Pb-free solders

A digital image correlation (DIC) algorithm was employed to measure microscopic strain-field evolution in shear-loaded model solder interconnections made out of a number of Sn-based alloys. Four different solder alloys studied were Sn–36Pb–2Ag, Sn–3.8Ag–0.7Cu (SAC), Sn–3.3Ag–3.82Bi, and Sn–8Zn–3Bi. The measured strain fields were correlated with damage observed at the scale of the sample, and at microscopic length scales. Local strain differs significantly from applied global strain and has been shown to depend on the geometry of the samples as well as the microstructure (on a grain level) of the solder. Strain fields in all solder interconnections were found to localize near but not at the solder–substrate interface and along grain boundaries in the solders. The eventual failure path as observed on the scale of the sample (parallel to the two solder–substrate interfaces with a cross-over from one interface to the other somewhere in the connection) showed a good correlation with measured strain fields in all interconnections. In contrast to the similarity on a macroscopic scale, on a microscopic scale the failure mechanisms were observed to be material specific.

Spallation of interfacial Ag–Au–Cu–Zn compounds in Sn–Ag–Cu/Sn–Zn–Bi joints during 210 °C reflow

Sn–8Zn–3Bi solder paste was applied as a medium to joint Sn–3.2Ag–0.5Cu solder balls and Au/Ni/Cu metallized ball grid array substrates at 210 °C. The spallation behavior of Ag–Au–Cu–Zn compound was studied as the Sn–Ag–Cu/Sn–Zn–Bi joints were reflowed respectively for 5, 20 and 30 s. After reflow for 5 s, the cracks were formed between Ag–Au–Cu–Zn compounds and Ni metallization. With further reflow time of 20 s, the cracks were propagated. The crack formation and propagation between Ag–Au–Cu–Zn compounds and Ni metallization, and the instability of Ag–Au–Cu–Zn IMCs at Ni layer might lead to Ag–Au–Cu–Zn compound spallation as the molten solder might flow to the gap zones, exerting a lifting force to the Ag–Au–Cu–Zn compounds during soldering. As a result, the reasons for the crack formation were discussed.

Comparative study of wetting behavior and mechanical properties (microhardness) of Sn–Zn and Sn–Pb solders

In this paper the solder balling, wetting, spreading, slumping and microhardness testing of the Sn–Zn based solders have been compared with the Sn–Pb solder. Two types of solders (Sn–9Zn and Sn–8Zn–3Bi) have been investigated along with Sn–37Pb solder for reference. The variation of these tests has been done as a function of reflow temperature from 220–250°C. Solder balls of these three solder pastes after 15min heating at 230°C show no ball formation surrounding the central ball. Spread test shows that above 240°C Sn–9Zn is very good and can be comparable to Sn–37Pb. The wetting angle of Sn–9Zn (39°) at 250°C is even lower than the Sn–37Pb solder (41°). In case of Sn–8Zn–3Bi, the wetting angle is very high (77°) at 220°C, which is unacceptable but it drops down to 48° at 250°C. Line profiles of slump test show that after preheating at 160°C, Sn–9Zn behaves similar to Sn–37Pb with better distinction in the finer pitch (120μm). Microhardness shows two different characteristics for eutectic and non-eutectic solder pastes. Hardness of Sn–37Pb and Sn–9Zn (eutectic) decreases with increasing reflow temperature while the microhardness of Sn–8Zn–3Bi (non-eutectic) increases with increasing reflow temperature. Microstructural characterization at 220 and 250°C shows grain coarsening in Sn–37Pb and Sn–9Zn solders, which cause the hardness to drop a little. For Sn–8Zn–3Bi, with increasing temperature the amount of hard Bi segregation increases which is the main cause of the rise in hardness. SEM images show the formation of Pb rich islands in Sn–37Pb, formation of Zn rod from spheroids in Sn–9Zn and precipitation of Bi-rich phase in Sn–8Zn–3Bi are the important features that contribute to different hardness nature.

Effect of sample perimeter and temperature on Sn–Zn based lead-free solders

This paper presents studies conducted on Sn–40Pb, Sn–9Zn and Sn–8Zn–3Bi lead-free solders for electronic applications. Differential Scanning Calorimeter (DSC) profile and the microstructure of the solders were discussed. The wettability between different copper perimeter and molten solders with the presence of flux at 250 °C was investigated. The withdrawal force divided by the sample perimeter was used to calculate the surface tension of the molten solders. The wetting properties of the solders were tested at different temperatures using two types of fluxes. The surface tension of Sn–40Pb, Sn–9Zn and Sn–8Zn–3Bi solders was 0.357, 0.481 and 0.4523 N/m, respectively, and these values showed good agreement with reported data. The Cu 6Sn 5 IMC phase was formed between Sn–40Pb solder and Cu substrate. The Sn–9Zn solder exhibits Cu 6Sn 5 and γ-Cu 5Zn 8 phases with Cu substrate while Sn–8Zn–3Bi solder forms a mixture of Cu–Sn + Cu–Zn phase as a first layer from the solder and γ-Cu 5Zn 8 phase as a second layer next to the Cu substrate. For Sn–8Zn–3Bi solder the MHS flux improves the wetting force and contact angle at higher temperatures. Increase in sample perimeter has no effect on contact angle but the addition of Bi to the Sn–Zn system reduces the surface tension and contact angle.

Interfacial microstructure and shear behavior of Sn–Ag–Cu solder balls joined with Sn–Zn–Bi paste

The Sn–3.2Ag–0.5Cu solder balls were joined with Sn–8Zn–3Bi paste on Cu/Ni/Au metallized BGA substrates at 240 °C. The shear behavior of thus produced BGA package was investigated after multiple reflow and thermal aging. The two solders have mixed together and have no visible interfacial boundary. Moreover the void and the indent can be clarified occasionally close to the interface after reflow. The shear strength falls from 18.0 ± 1.6 N (one cycle reflow) to 14.8 ± 1.5 N (five cycle reflow) and remains roughly unchanged after 5 cycles up to 10 cycles (14.9 ± 2.1 N). Four types of fractures are observed in single cycle cases. The solder in which the voids exist may be sheared to descend the joint strength. The thickness of Ni–Sn–Cu–Zn compounds appears to be irrelevant to the shear behavior from 5 to 10 cycles. The shear strength drops from single to five cycles may be due to the spallation of excessive brittle Ni–Sn–Cu–Zn compounds into the solder which degrades the joint strength. The shear strength of the Sn–Ag–Cu/Sn–Zn–Bi package changes little after aging at 150 °C. The strengths are 17.5 ± 2.9, 16.9 ± 2.7, 17.6 ± 2.6 and 16.8 ± 2.4 N after aging for 100, 200, 500 and 1000 h, respectively. As aging time increases, fractures shift to occur in the solder.

Electrochemical corrosion study of Pb-free solders

This paper presents an investigation on the corrosion behavior of five solders by means of polarization and electrochemical impedance spectroscopy (EIS) measurements. The Sn–9Zn and Sn–8Zn–3Bi solder, in comparison with the Sn–3.5Ag–0.5Cu and Sn–3.5Ag–0.5Cu–9In solder, were tested in 3.5 wt% NaCl solution and 0.1 wt% adipic acid solution, respectively. The Sn–37Pb solder was for reference in this work. The polarization curves indicated that the Sn–9Zn and Sn–8Zn–3Bi solder showed the worst corrosion resistance both in the salt and acid solutions, in terms of corrosion-current density, corrosion potential, linear polarization resistance, and passivation-current density. Meanwhile, the Sn–3.5Ag–0.5Cu solder remained the best corrosion characteristics in both solutions. It was found that due to microstructure alteration, Bi additive to the Sn–9Zn solder improved the corrosion behavior in the salt solution, whereas decreased that in the acid solution. However, the additive of In degraded the Sn–3.5Ag–0.5Cu solder in both solutions. The EIS results agreed well with the noble sequence of the five solders subjected to the two solutions with polarization. The equivalent circuits were also determined. Nevertheless, the four Pb-free solders exhibited acceptable corrosion properties since there was not much difference of key corrosion parameters between them and the Sn–37Pb solder.

Effect of microstructural evolution on electrical property of the Sn–Ag–Cu solder balls joined with Sn–Zn–Bi paste

Sn–8Zn–3Bi solder paste and Sn–3.2Ag–0.5Cu solder balls were reflowed simultaneously on Cu/Ni/Au metallized ball grid array (BGA) substrates. The correlation between microstructural evolution and the electrical resistance of the joints under various testing conditions of reflow cycles and heat treatment was investigated. The electrical resistance of the Sn–Ag–Cu joints without Sn–Zn–Bi was also conducted for comparison. The average resistance values of Sn–Ag–Cu and Sn–Ag–Cu/Sn–Zn–Bi samples changed, respectively, from 7.1 (single reflow) to 7.3 (10 cycles) mΩ and from 7.2 (single reflow) to 7.6 (10 cycles) mΩ. Furthermore, the average resistance values of Sn–Ag–Cu and Sn–Ag–Cu/Sn–Zn–Bi samples changed, respectively, from 7.1 (aging 0 h) to 7.8 (aging 1000 h) mΩ and from 7.2 (aging 0 h) to 7.9 (aging 1000 h) mΩ. It was also noticeable that the average resistance values of Sn–Ag–Cu/Sn–Zn–Bi samples were higher than those of Sn–Ag–Cu samples in each specified testing condition. The possible reasons for the greater resistance exhibited by the Sn–Zn–Bi incorporated joints were discussed.

Reliability of Sn–8 mass%Zn–3 mass%Bi Lead-Free Solder and Zn Behavior

In order to evaluate the reliability of Sn–8 mass%Zn–3 mass%Bi (hereafter, Sn–8Zn–3Bi) solder, thermal shock tests (temperature range: from 233 to 353 K) and high temperature humidity tests (test condition: 353 K, 85% relative humidity) were conducted. The PCB (Printed Circuit Board) pads were finished by OSP (Organic Solderability Preservative), Sn and Ni/Au. The electric part for test was QFP (Quad Flat Package), and the lead was plated by Sn–3 mass%Bi. The joint strength was estimated by pull testing, and Zn-phase behavior in solder joint was examined. The pull strengths of Sn–8Zn–3Bi joints in the as-soldered state were measured to be about 16N, irrespective of the PCB pad coatings. After thermal shock testing (hereafter, TS test) up to 1000 cycles, the pull strengths decreased to around 14N. Meanwhile, after high temperature humidity testing (hereafter, HTH test) up to 1000 h, the pull strengths deceased to around 6N, irrespective of the PCB pad coatings. The reason for the drastic decrease of pull strength after the HTH test was proven to be a crack that was observed along the zinc oxide-phase. The Zn-phase in the Sn–8Zn–3Bi solder changed from a separate acicular shape to a lined-up branched arrangement after HTH tests of 1000 h.

Interfacial Bonding Behavior with Introduction of Sn–Zn–Bi Paste to Sn–Ag–Cu Ball Grid Array Package During Multiple Reflows

Sn–8Zn–3Bi solder paste and Sn–3.2Ag–0.5Cu solder balls were reflowed simultaneously on Cu/Ni/Au metallized ball grid array (BGA) substrates to investigate the interfacial bonding behaviors for multiple reflow cycles at two different soldering temperature of 210 and 240 °C. The different intermetallic compounds that formed at the interface after one reflow cycle were respectively the island-shaped Ag–Au-Cu-Zn (near the solder) compounds and the Ni–Sn–Cu-Zn (near the metallized pad) compounds in 210 or 240 °C soldering systems. Layered Ag–Au–Cu–Zn, Ag5Zn8, and Ag–Zn–Sn compounds were also observed within the solder near the interface after single reflow cycle. After ten reflow cycles, the Ag–Au–Cu–Zn compounds significantly decomposed, while the Ag3Sn and Ni–Sn–Cu–Zn compounds coarsened obviously.

Investigations on microhardness of Sn–Zn based lead-free solder alloys as replacement of Sn–Pb solder

The microhardness of the Sn–Zn based solder pastes have been compared with the Sn–Pb solder paste. Two types of solder such as Sn–9Zn and Sn–8Zn–3Bi have been investigated along with Sn–37Pb solder for reference. The variation of microhardness with reflow temperature from 220 to 250 °C shows two different characteristics for eutectic and non-eutectic solder pastes. Hardness of Sn–37Pb and Sn–9Zn (eutectic) decreases with increasing reflow temperature while the microhardness of Sn–8Zn–3Bi (non-eutectic) increases with the increasing reflow temperature. Microstructural characterization at 220 and 250 °C shows grain coarsening in Sn–37Pb and Sn–9Zn solders, which cause the hardness to drop a little. For Sn–8Zn–3Bi, with increasing temperature the amount of hard Bi segregation increases which is the main cause of the rise in hardness. SEM images show the formation of Pb rich islands in Sn–37Pb, formation of Zn rod from spheroids in Sn–9Zn and precipitation of Bi rich phase in Sn–8Zn–3Bi are the important features that contribute to the different hardness nature. Again the effect of cooling rate on hardness was also studied and compared among these three solder pastes. The result shows that the Sn–9Zn is the most sensitive to cooling rate. With the change in cooling rate, the hardness increase in Sn–9Zn is greater (58% after water cooling) than in Sn–37Pb (30%) and Sn–8Zn–3Bi (33%). The hardness profile along the distance from the centre shows that the hardness gradient is also found highest on Sn–9Zn solder paste whereas similar trendlines were found in Sn–37Pb and Sn–8Zn–3Bi solder. SEM images of air- and water-cooled microstructures show preferential Zn formed at the edge in Sn–9Zn solder while Pb rich islands at the centre (for Sn–37Pb) and precipitation of Bi at the edge (for Sn–8Zn–3Bi) was found along with grain refinement.

Interfacial reactions and impact reliability of Sn–Zn solder joints on Cu or electroless Au/Ni(P) bond-pads

Sn–9Zn and Sn–8Zn–3Bi solder balls were bonded to Cu or electroless Au/Ni(P)pads, and the effect of aging on joint reliability, including impact reliability, was investigated. For the purpose of quantitatively evaluating the impact toughness ofthe solder joints, a test similar to the classic Charpy impact test was performed.The interfacial compounds formed in the solder/Cu joint during soldering wereCu–Zn intermetallic compounds (IMCs), not Cu–Sn IMCs. One of the Cu–Zn IMCs, γ–Cu5Zn8, thickened remarkably with aging, and eventually its morphology changed from layer-type into discontinuous. The rapid growth of the γ–Cu5Zn8 and void formation at the bond interface led to the significant degradation of the joint reliability due to a ductile-to-brittle transition of the joint. Meanwhile, the compound formed in the solder/Au/Ni(P) joint during soldering was a Au–Zn IMC, above which Zn redeposited during aging. Both the dissolution and diffusion of Ni into the solders were extremely slow, which contributes to negligible void formation at the bond interface. As a result, the solder bumps on the Au/Ni(P) pads were able to maintain the high joint strength and impact toughness even after prolonged aging.