Ni3Sn4 Intermetallic Compound Research Articles

Comparative Study of the Dissolution Kinetics of Electrolytic Ni and Electroless NiP Layers by Molten Sn3.5Ag Solder Alloy

Sn-based, Pb-free solders with high a Sn content and high melting temperature often cause excessive interfacial reactions at interfaces. Sn-3.5Ag solder alloy has been used to identify its interfacial reactions with two-metal layer flexile substrates. In this paper the dissolution kinetics of Sn3.5Ag solder on the electrolytic Ni and electroless NiP layer are investigated. It is found that during 1 min reflow the electrolytic Ni layer dissolves much less than the electroless NiP layer due to the formation of Ni3Sn and Ni3Sn2 intermetallic compounds (IMCs) on the electrolytic Ni layer. The faster nucleation of Ni3Sn4 IMC on the NiP layer is proposed as the main reason for the higher initial dissolution rate of the electroless NiP layer. A P-rich Ni layer is formed underneath the Ni3Sn4 IMC due to the solder-assisted reactions. This P-rich Ni layer acts as a good diffusion barrier layer, which decreases the dissolution rate of the NiP layer as compared to that of the Ni layer, but weakens the interface of solder joints and reduces the ball shear load and reliability. Below a certain thickness, the P-rich Ni layer breaks and an increase in the diffusion of Sn atoms through the fractured P-rich Ni layer occurs that increases the growth rate of IMCs again, and thus the dissolution rate of the NiP layer becomes higher again than for the Ni layer. It is found that a 3μm thick NiP layer cannot protect the Cu layer for more than 120 min reflow at 250°C. An electrolytic Ni∕solder system has a relatively higher shear load, a lower dissolution rate of the Ni layer, and is more protective for the Cu layer during extended times of reflow.

The nanoindentation characteristics of Cu6Sn5, Cu3Sn, and Ni3Sn4 intermetallic compounds in the solder bump

In general, formation and growth of intermetallic compounds (IMCs) play a major role in the reliability of the solder joint in electronics packaging and assembly. The formation of Cu-Sn or Ni-Sn IMCs have been observed at the interface of Sn-rich solders reacted with Cu or Ni substrates. In this study, a nanoindentation technique was employed to investigate nanohardness and reduced elastic moduli of Cu6Sn5, Cu3Sn, and Ni3Sn4 IMCs in the solder joints. The Sn-3.5Ag and Sn-37Pb solder pastes were placed on a Cu/Ti/Si substrate and Ni foil then annealed at 240°C to fabricate solder joints. In Sn-3.5Ag joints, the magnitude of the hardness of the IMCs was in the order Ni3Sn4>Cu6Sn5>Cu3Sn, and the elastic moduli of Cu6Sn5, Cu3Sn, and Ni3Sn4 were 125 GPa, 136 GPa, and 142 GPa, respectively. In addition, the elastic modulus of the Cu6Sn5 IMC in the Sn-37Pb joint was similar to that for the bulk Cu6Sn5 specimen but less than that in the Sn-3.5Ag joint. This might be attributed to the strengthening effect of the dissolved Ag atoms in the Cu6Sn5 IMC to enhance the elastic modulus in the Sn-3.5Ag/Cu joint.

Phase analysis and kinetics of solid‐state ageing of Pb‐free Sn3.5Ag solder on electroless NiP substrate

AbstractGrowth kinetics of intermetallic compound (IMC) layers formed between Sn3.5Ag solder and electroless NiP/Cu substrates were investigated at temperatures ranging from 100 to 170 °C for 0 to 60 days. The IMC formed at the interface was mainly Ni3Sn4, and a P‐rich Ni (hereafter Ni3P) layer was also observed as a by‐product of Ni3Sn4 formation, that was localized between the Ni3Sn4 IMC and the electroless NiP deposit layer. This Ni3P layer formed because of the phosphorous accumulation at the interface between the electroless Ni and the IMC layer. It was found that both the Ni3Sn4 IMC and the Ni3P layer were formed due to the solder reaction‐assisted crystallization. Also, another layer with NiSnP was observed between the Ni3P layer and the Ni3Sn4 IMC layer. The layer thickness of Ni3Sn4 and Ni3P were approximately 1.83 and 1.56 µm after 50 days of ageing at 150°C, respectively. Copyright © 2004 John Wiley & Sons, Ltd.

Spalling of intermetallic compounds during the reaction between lead-free solders and electroless Ni-P metallization

Intermetallic compound (IMC) spalling from electroless Ni-P film was investigated with lead-free solders in terms of solder-deposition methods (electroplating, solder paste, and thin foil), P content in the Ni-P film (4.6, 9, and 13 wt% P), and solder thickness (120 versus .200 μm). The reaction of Ni-P with Sn3.5Ag paste easily led to IMC spalling after 2-min reflow at 250 °C while IMCs adhered to the Ni-P layer after 10-min reflow with electroplated Sn or Sn3.5Ag. It has been shown that not only the solder composition but also the deposition method is important for IMC spalling from the Ni-P layer. The spalling increased with P content as well as with solder volume. Ni3Sn4 intermetallics formed as a needle-shaped morphology at an early stage and changed into a chunk-shape. Needle-shaped compounds exhibited a higher propensity for spalling than chunk-shaped compounds because many channels among the needle-shaped IMCs facilitated Sn penetration. A reaction between the penetrated Sn and the Ni3P layer formed a Ni3SnP layer and Ni3Sn4 IMCs spalled off the Ni3SnP surface. Dewetting of solder from the Ni3SnP layer, however, did not occur even after spalling of most IMCs.

Growth of an intermetallic compound layer with Sn-3.5Ag-5Bi on Cu and Ni-P/Cu during aging treatment

Growth kinetics of intermetallic compound (IMC) layers formed between the Sn-3.5Ag-5Bi solder and the Cu and electroless Ni-P substrates were investigated at temperatures ranging from 70°C to 200°C for 0–60 days. With the solder joints between the Sn-Ag-Bi solder and Cu substrates, the IMC layer consisted of two phases: the Cu6Sn5 (η phase) adjacent to the solder and the Cu3Sn (e phase) adjacent to the Cu substrate. In the case of the electroless Ni-P substrate, the IMC formed at the interface was mainly Ni3Sn4, and a P-rich Ni (Ni3P) layer was also observed as a by-product of the Ni-Sn reaction, which was between the Ni3Sn4 IMC and the electroless Ni-P deposit layer. With all the intermetallic layers, time exponent (n) was approximately 0.5, suggesting a diffusion-controlled mechanism over the temperature range studied. The interface between electroless Ni-P and Ni3P was planar, and the time exponent for the Ni3P layer growth was also 0.5. The Ni3P layer thickness reached about 2.5 µm after 60 days of aging at 170°C. The activation energies for the growth of the total Cu-Sn compound layer (Cu6Sn5 + Cu3Sn) and the Ni3Sn4 IMC were 88.6 kJ/mol and 52.85 kJ/mol, respectively.

Morphological transition of interfacial Ni3Sn4 grains at the Sn-3.5Ag/Ni joint

The transition in morphology of Ni3Sn4 grains that formed at the interface between liquid Sn-3.5Ag (numbers are in wt.% unless specified otherwise) solder and Ni substrate has been observed at 250–650°C. The morphological transition of Ni3Sn4 is due to the decrease of entropy of formation of the Ni3Sn4 phase and has been explained well by the change of Jackson’s parameter with temperature. According to the variation of solder joint strength with temperature, it decreased rapidly between 350°C and 450°C, where the thickness of the Ni3Sn4 intermetallic compound (IMC) layer was around 6.5 µm. However, the solder joint strength decreased slowly with an increase of soldering time without a significant drop, although the thickness of the IMC was larger than 6.5 µm. The notable drop of solder joint strength and the fracture mode transition with increase of soldering temperature appears to come from excessive lateral growth of IMC grains between 350°C and 450°C.

Characterizations and catalytic properties of fine particles of Ni–Sn intermetallic compounds supported on SiO 2

Fine particles of Ni–Sn intermetallic compounds (IMCs) of Ni 3Sn, Ni 3Sn 2, and Ni 3Sn 4 with specific crystal structures were selectively prepared on SiO 2 by chemical vapor deposition (CVD) of Sn(CH 3) 4 onto Ni/SiO 2. X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR) measurements indicated that the near surface of these fine particles with atomic Ni/Sn ratios of 3/1, 3/2, and 3/4 are similar to those of corresponding unsupported Ni–Sn IMCs. X-ray diffraction (XRD) patterns of the fine particles showed peaks too broad to determine these crystal structures. Each unsupported Ni–Sn IMC was clarified to exhibit a specific Ni K-edge and Sn K-edge X-ray absorption near-edge structure (XANES) spectrum. The crystal structures of fine particles were by the identified by the fingerprinting method by Ni K-edge and Sn K-edge XANES spectra. Both these XANES spectra of Ni–Sn IMC/SiO 2 catalysts with Ni/Sn atomic ratios of 3/1, 3/2, and 3/4 were similar to those of unsupported Ni 3Sn, Ni 3Sn 2, and Ni 3Sn 4, respectively. The particles would have an average diameter of 3–4 nm obtained by TEM. Each Ni–Sn IMC particle showed intrinsic properties for the adsorption of carbon monoxide and hydrogen. The fine-particle catalysts exhibited high benzene selectivity (>99% C) at high conversion for cyclohexane dehydrogenation. The selectivity was almost the same as that of the Ni–Sn IMC/SiO 2 catalysts with large particle sizes (about 15 nm), whose crystal structures were clearly determined by XRD, but was completely different from that of the Ni/SiO 2 catalyst. The activities of fine-particle catalysts were about 15 times higher than those of the large-particle catalysts.

Effects of substrate metallizations on solder/underbump metallization interfacial reactions in flip-chip packages during thermal aging

Effects of Ni and Au from Ni/Au substrate metallizations on the interfacial reactions of solder joints in flip-chip packages during long-term thermal aging were systematically investigated. It was found that both Au and Ni influenced the solid-state interfacial reactions, underbump metallization (UBM), and intermetallic compound (IMC) evolution. Because large amounts of Ni could incorporate into IMC to form a multicomponent (Cu, Ni)6Sn5 phase during assembly reflow, while Au could only affect the reaction during thermal aging through the reconfiguration of AuSn4 phase, Ni had stronger effects on solid-solution type Ni–V UBM consumption than Au. It was found that the UBM consumption process was faster in the eutectic SnPb solder system than that in the SnAgCu solder system during aging. A porous structure was formed in the UBM layer after Ni in UBM was consumed. Electrical resistance of flip-chip packages increased significantly after the porous structure reached certain extents. The results showed that the diffusion process of Ni from UBM and Sn from solder in the presence of (Cu, Ni)6Sn5 or (Ni, Cu)3Sn4 phase at solder joint interfaces could be much faster than that in the case of binary Cu6Sn5 or Ni3Sn4 IMC.

Interface microstructure between Fe-42Ni alloy and pure Sn

The interfacial reaction and microstructure of Sn with an Fe-42Ni substrate soldered at 250 °C were studied primarily using transmission electron microscopy. Apparent double reaction layers formed between Fe-42Ni and Sn of the same phase of FeSn2 containing Ni from 2 to 6 at.% as a substitution impurity for Fe. The first layer, which faced an Fe-42Ni substrate, was thin and grew with a flat interface. The second layer, which adhered to Sn solder, showed needle/square pillar-shaped small crystalline structures. Between these two layers, voids were frequently observed. The morphological difference between these two layers and voids could be attributed to the slow-fast diffusion mechanism during soldering. The fast diffusion occurring along the surfaces or the grain boundary of FeSn2 in the second layer led to the growth of FeSn2 products into Sn melt. Simultaneously, the slow diffusion penetrating the Fe-42Ni and FeSn2 grain contributed to the formation of the first layer and voids. Ni from Fe-42Ni can dissolve into Sn melt at the reaction temperature. The dissolved Ni formed platelets of Ni3Sn4 intermetallic compound inside a β-Sn matrix on solidification or on cooling. In contrast, Fe formed FeSn2 crystallines directly along the interface, because its activity was higher than that of Ni in a Sn liquid at reaction temperature. The joint of Fe-42Ni/Sn/Fe-42Ni, reacted for 2 min, had a joint strength of about 65 MPa, and maintained this high strength regardless of the reaction time.

Solder joint reliability of TFBGA assemblies with fresh and reworked solder balls

In this article, the solder joint reliability of thin and fine-pitch BGA (TFBGA) with fresh and reworked solder balls is investigated. Both package and board level reliability tests are conducted to compare the solder joint performance of test vehicle with fresh and reworked solder balls. For package level reliability test, ball shear test is performed to evaluate the joint strength of fresh and reworked solder balls. The results show that solder balls with rework process exhibit higher shear strength than the ones without any rework process. The results also exhibit that the different intermetallic compound (IMC) formation at solder joints of fresh and reworked solder balls is the key to degradation of shear strength. For board level reliability tests, temperature cycling and bending cyclic tests are both applied to investigate the fatigue life of solder joint with fresh and reworked solder balls. It is observed that package with reworked solder ball has better fatigue life than the one with fresh solder ball after temperature cyclic test. As for bending cyclic test, in addition to test on as-assembled packages, reworked and fresh samples are subjected to heat treatment at 150 °C for 100 h prior to the bending cyclic test. The purpose is to let Au–Ni–Sn IMC resettle at solder joints of fresh solder ball and examine the influence of Au–Ni–Sn IMC on the fatigue life of solder joints (Au embrittlement effect). The final results confirm that reworked solder balls have better reliability performance than fresh one since Au embrittlement dose exist at fresh solder ball.

Isothermal aging characteristics of Sn–Pb micro solder bumps

The shear strength of the under bump metallurgy (UBM) structure in both the high-melting solder bump and low-melting solder bump after aging were evaluated. Scanning electron microscopy and transmission electron microscopy were examined in the intermetallic compounds (IMCs) and bump joint profiles at the interface between solder and UBM. In 900 h aging experiments, the maximum shear strength of Sn–97wt.%Pb and Sn–37wt.%Pb decreased by 25% and 20%, respectively. The growth of Cu 6Sn 5 and Cu 3Sn was ascertained by the aging treatment. The crack path changes from the interior of a solder to the IMC interface. Compare with the Cu–Sn IMC, the amount of Ni–Sn IMC was small. The Ni layer is considered as the diffusion barrier.

Investigation of Interfacial Reactions between Sn–Ag–Bi–In Solder and (Cu, Electroless Ni–P/Cu) Substrate

Abstract The growth kinetics of intermetallic compound layers formed between Sn–3Ag–8Bi–5In solder and (Cu, electroless Ni–P/Cu) substrate were investigated at temperatures between 70 and 170°C for 1 to 60 days. The layer growth of the intermetallic compounds in the Sn–Ag–Bi–In/Cu and Sn–Ag–Bi–In/electroless Ni–P systems satisfied the parabolic time law. As a whole, because the values of time exponent have approximately 0.5, the layer growth is mainly controlled by diffusion. The apparent activation energies for the growth of the Cu6Sn5 and Ni3Sn4 intermetallic compounds are 81.4 and 67.1 kJ/mol, respectively.

Intermetallic compound layer formation between Sn–3.5 mass %Ag BGA solder ball and (Cu, immersion Au/electroless Ni–P/Cu) substrate

The growth kinetics of intermetallic compound layers formed between eutectic Sn–3.5Ag BGA (ball grid array) solder and (Cu, immersion Au/electroless Ni–P/Cu) substrate by solid-state isothermal aging were examined at temperatures between 343 and 443 K for 0–100 days. In the solder joints between the Sn–Ag eutectic solder ball and Cu pads, the intermetallic compound layer was composed of two phases: Cu6Sn5 (η-phase) adjacent to the solder and Cu3Sn (e-phase) adjacent to the copper. The layer of intermetallic on the immersion Au/electroless Ni–P/Cu substrate was composed of Ni3Sn4. As a whole, because the values of the time exponent (n) are approximately 0.5, the layer growth of the intermetallic compound was mainly controlled by a diffusion-controlled mechanism over the temperature range studied. The growth rate of Ni3Sn4 intermetallic compound was slower than that of the total Cu–Sn(Cu6Sn5+Cn3Sn). The apparent activation energy for growth of total Cu–Sn(Cu6Sn5+Cu3Sn) and Ni3Sn4 intermetallic compound were 64.82 and 72.54 kJ mol−1, respectively.

Effect of crystallinity and thickness on the diffusion barrier behavior of electroless nickel deposit between Cu and solder

The crystallinity of electroless nickel deposit was manipulated by applying bath stabilizer including lead acetate and thiourea. A crystalline deposit and a higher deposition rate of electroless nickel were achieved with thiourea than with lead acetate. The effect of crystallinity on the diffusion barrier performance of the electroless nickel deposit was studied between solder and Cu deposit. The thickness of the electroless nickel deposit investigated includes 1, 3, and 5 /spl mu/m. It was found that both crystalline and amorphous deposits perform similarly in barrier performance except when the thickness of the deposit is as thin as 1 /spl mu/m. Cross sectional elemental analysis results indicate that a 3 /spl mu/m thickness deposit can withstand ten times of reflow without counter diffusion between Sn and Cu, although Ni-Sn intermetallic compounds were formed. The 3 /spl mu/m thickness is also adequate for barrier function after 1000 hours of aging at 150/spl deg/C.

Phenomena of electroless Ni-P and intermetallic-compound stripping and dissolving in Sn-Bi and Sn-Pb solder joints with Au/EN/Cu metallization

Electroless Ni-P (EN) has been popularly adopted and used as a diffusion barrier in the under-bump metallurgy (UBM) for flip-chip application. The EN with different P contents was first deposited on activated Cu/Al2O3 substrates. To prevent the EN from oxidation, a thin Au coating was further plated on the EN/Cu/Al2O3 substrates. Two types of solder alloys (63Sn-37Pb and 42Sn-58Bi) and two compositions of EN (Ni-5.5wt.%P and Ni-12.1wt.%P) were employed to investigate the interfacial reaction in the joint of solder/Au/EN/Cu. Occurrence of EN and intermetallic compound (IMC) stripping and dissolving was revealed. After annealing, Ni3Sn4 and Ni3P formed between the solder and the EN in all joints. However, some of the Ni3Sn4 IMCs stripped into the solder for a longer annealing time. The stripped EN was first observed in the Sn-Bi/Au/Ni-5.5wt.%P/Cu/Al2O3 joints annealed at 185°C for 180 min. The stripped IMCs and the EN then dissolved in the solder and formed the Ni-P-Cu-Sn-Pb solid solution in the Sn-Pb/Au/Ni-5.5wt.%P/Cu/Al2O3 joints annealed at 200°C or 240°C. The phenomenon of IMC stripping was found in all joints. However, both the stripping and dissolving of EN was only observed in joints with Ni-5.5wt.%P. The tendency of IMC stripping was related to the amount of IMCs, while the EN stripping corresponded to the surface condition of the EN.

Interfacial reaction of Pb-Sn solder and Sn-Ag solder with electroless Ni deposit during reflow

The interfaces between electroless Ni-P deposit and Pb-Sn solder and Sn-Ag solder were formed by reflowing for different time periods to examine their microstructures and microchemistry. It was found that the Pb-Sn solder interface is more stable than the Sn-Ag solder interface. Sn-Ag solder reacts quickly with the electroless Ni-P deposit and forms nonadherent Ni-Sn intermetallic compounds (IMCs). Pb-Sn solder reacts slowly and forms adherent Ni-Sn IMC. A P-rich Ni layer, revealed as a dark layer under scanning electron microscopy (SEM), is formed on the electroless Ni-P deposit due to the solder reaction. For short reflow times, this P-rich Ni layer consists of only Ni3P compound, but during prolonged reflow, new crystals of Ni2P, Ni5P4, and NiP2 are also found to be formed from the amorphous electroless Ni-P layer.

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

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

Under bump metallurgy study for Pb-free bumping

The demand for Pb-free and high-density interconnection technology is rapidly growing. The electroplating-bumping method is a good approach to meet fine-pitch requirements, especially for high-volume production, because the volume change of patterned-solder bumps during reflow is not so large compared with the stencil-printing method. This paper proposes a Sn/3.5Ag Pb-free electroplating-bumping process for high-density Pb-free interconnects. It was found that a plated Sn/Ag bump becomes Sn/Ag/Cu by reflowing when Cu containing under bump metallurgy (UBM) is used. Another important issue for future flip-chip interconnects is to optimize the UBM system for high-density and Pb-free solder bumps. In this work, four UBM systems, sputtered TiW 0.2 µm/Cu 0.3 µm/electroplated Cu 5 µm, sputtered Cr 0.15 µm/Cr-Cu 0.3 µ/Cu 0.8 µm, sputtered NiV 0.2 µm/Cu 0.8 µm, and sputtered TiW 0.2 µ/NiV 0.8 µm, were investigated for interfacial reaction with electroplated Pb/63Sn and Sn/3.5Ag solder bumps. Both Cu-Sn and Ni-Sn intermetallic compound (IMC) growth were observed to spall-off from the UBM/solder interface when the solder-wettable layer is consumed during a liquid-state "reflow" process. This IMC-spalling mechanism differed depending on the barrier layer material.

Reliability study of the electroless Ni–P layer against solder alloy

Copper (Cu) has been widely used in the under bump metallurgy of chip and substrate metallization for chip packaging. However, due to the rapid formation of Cu–Sn intermetallic compound (IMC) at the tin-based solder/Cu interface during solder reaction, the reliability of this type of solder joint is a serious concern. In this work, electroless nickel–phosphorous (Ni–P) layer was deposited on the Cu pad of the flexible substrate as a diffusion barrier between Cu and the solder materials. The deposition was carried out in a commercial acidic sodium hypophosphite bath at 85 °C for different pH values. It was found that for the same deposition time period, higher pH bath composition (mild acidic) yields thicker Ni–P layer with lower phosphorous content. Solder balls having composition 62%Sn–36%Pb–2%Ag were reflowed at 240 °C for 1 to 180 min on three types of electroless Ni–P layers deposited at the pH value of 4, 4.8 and 6, respectively. Thermal stability of the electroless Ni–P barrier layer against the Sn–36%Pb–2%Ag solder reflowed for different time periods was examined by scanning electron microscopy equipped with energy dispersed X-ray. Solder ball shear test was performed in order to find out the relationship between the mechanical strength of solder joints and the characteristics of the electroless Ni–P layer deposited. The layer deposited in the pH 4 acidic bath showed the weak barrier against reflow soldering whereas layer deposited in pH 6 acidic bath showed better barrier against reflow soldering. Mechanical strength of the joints were deteriorated quickly in the layer deposited at pH 4 acidic bath, which was found to be thin and has a high phosphorous content. From the cross-sectional studies and fracture surface analyses, it was found that the appearance of the dark crystalline phosphorous-rich Ni layer weakened the interface and hence lower solder ball shear strength. Ni–Sn IMC formed at the interfaces was found to be more stable at the low phosphorous content (∼14 at.%) layer. Electroless Ni–P deposited at mild acidic bath resulting phosphorous content of around 14 at.% is suggested as the best barrier layer for Sn–36%Pb–2%Ag solder.

Interfacial Reactions Between Sn-58 mass%Bi Eutectic Solder and (Cu, Electroless Ni-P/Cu) Substrate

The growth kinetics of intermetallic compound layers formed between eutectic Sn–58Bi solder and (Cu, electroless Ni–P/Cu) substrate were investigated at temperature between 70 and 120◦C for 1 to 60 days. The layer growth of intermetallic compound in the couple of the Sn–58Bi/Cu and Sn–58Bi/electroless Ni–P system satisfied the parabolic law at given temperature range. As a whole, because the values of time exponent (n) have approximately 0.5, the layer growth of the intermetallic compound was mainly controlled by diffusion mechanism over the temperature range studied. The apparent activation energies of Cu6Sn5 and Ni3Sn4 intermetallic compound in the couple of the Sn–58Bi/Cu and Sn–58Bi/electroless Ni–P were 127.9 and 81.6 kJ/mol, respectively.