Sn Intermetallic Compound Layer Research Articles

Effects of electromigration on the growth of intermetallic compounds in Cu/SnBi/Cu solder joints

In this study, the effects of electromigration (EM) on the growth of Cu–Sn intermetallic compounds (IMCs) in Cu/SnBi/Cu solder joints under 5 × 103 A/cm2 direct current stressing at 308, 328, and 348 K were investigated. For each Cu/SnBi/Cu solder joint under current stressing, the IMCs at the cathode side grew faster than that at the anode side. The growth of these IMCs at the anode side and the cathode side were enhanced by electric current. The growth of these IMCs at the cathode followed a parabolic growth law. The kinetics parameters of the growth of the IMCs were calculated from the thickness data of the IMCs at the cathode side at different ambient temperatures. The calculated intrinsic diffusivity (D0) of the Cu–Sn IMCs was 9.91 × 10−5 m2/s, and the activation energy of the growth of the total Cu–Sn IMC layer was 89.2 kJ/mol (0.92 eV).

Growth kinetics of Cu–Sn intermetallic compounds at the interface of a Cu substrate and 42Sn–58Bi electrodeposits, and the influence of the intermetallic compounds on the shear resistance of solder joints

This paper explores the growth kinetics of Cu–Sn intermetallic compounds at the interface of a Cu substrate and 42Sn–58Bi electrodeposits obtained from a methane sulphonate bath, and the effect of intermetallic compounds on the shear resistance of solder joints. The growth of the Cu–Sn intermetallic compounds layer [ɛ(Cu 3Sn) + η(Cu 6Sn 5)] during aging exhibits a linear function of the square root of the aging time. The apparent activation energy for the growth of the intermetallic compounds in 42Sn–58Bi electrodeposits on the Cu substrate was 52.48 kJ mol −1. In the early stage of aging, the growth of the intermetallic compound layer contributes to the shear resistance of the solder joints due to the wedge effect of the rough interface between the intermetallic compound layer and the Sn–Bi electrodeposits. However, with further aging, the shear resistance of the solder joint is degraded by the excessive growth of the intermetallic compound layer, which is more brittle than the Sn–Bi electrodeposit layer.

Liquid-state and solid-state interfacial reactions of fluxless-bonded Au–20Sn/ENIG solder joint

Abstract The microstructural evolution and interfacial reactions of fluxless-bonded, Au–20 wt.%Sn/electroless nickel-immersion gold (ENIG) solder joint were investigated during reflow and aging. After the reflow process, only (Au,Ni)Sn intermetallic compound (IMC) was formed at the interface. After prolonged reflow reaction, the (Au,Ni)Sn IMCs that had been formed in the initial reflow stage grew into the solder and a (Ni,Au) 3 Sn 2 IMC was formed at the interface. In the aging treatment, the solid-state interfacial reaction was much faster at 250 °C than at 150 °C. After aging at 250 °C for 100 h, a thick (Au,Ni)Sn IMC layer was formed at the interface. With increasing aging temperature and time, the δ(AuSn)-phase adjacent to the interfacial layer was gradually replaced by the ζ(Au 5 Sn)-phase. The interfacial (Au,Ni)Sn IMC grew with the preferential consumption of the available δ-phase in the solder. The Ni 3 P layer formed from the Ni(P) layer in the initial 500 h of aging was subsequently further transformed into a Ni 5 P 4 layer during further aging treatment between 500 and 1000 h. The original electroless Ni(P) plating layer lost its initial role of a diffusion barrier due to its crystallization. After aging for 1000 h, the Ni 5 P 4 layer was partially destroyed, and the Au–Cu IMCs were formed below the P-rich Ni layer.

Investigation of interfacial reaction between Au–Sn solder and Kovar for hermetic sealing application

The microstructural evolution and interfacial reactions of Au/Sn/Au/Au/Ni/Kovar joint were investigated during aging at 180 and 250 °C for up to 1000 h. The Au/Sn combination formed a rapid diffusion system. Even in non-annealed joint, three phases such as AuSn, AuSn 2 and AuSn 4 were formed. After initial aging at 180 °C, the AuSn, AuSn 2, AuSn 4, Au and Sn phases, which were formed after plating, were fully transformed into ζ-phase and δ-phase, and (Ni, Au) 3Sn 2 intermetallic compound (IMC) layer was observed between the ζ-phase and Kovar. As a whole, the microstructure of the joint was stable during aging at 180 °C. On the other hand, the solid-state interfacial reaction was much faster at 250 °C than at 180 °C. During aging at 250 °C, the Ni layer on the Kovar reacted primarily with the δ-phase in the solder, resulting in the formation and growth of the (Au, Ni)Sn IMC layer at the interface. After aging for 48 h, the Fe–Co–Ni–Au–Sn phase was formed underneath the (Au, Ni)Sn IMC layer. Furthermore, cracks were observed inside the interfacial layers after complete consumption of the Ni layer. The study results clearly demonstrate the need for either a thicker Ni layer or an alternative surface finish on Ni, in order to ensure the high temperature reliability of the Au/Sn/Au/Au/Ni/Kovar joint above 250 °C.

Effect of high temperature storage test on reliability of eutectic Sn–Cu/ENIG solder joint

The interfacial reactions between Sn–0·7 wt-%Cu solder and electroless nickel immersion gold (ENIG) substrate were investigated during multiple reflows and isothermal aging at temperatures between 70 and 150°C for aging times of up to 60 days. After reflowing and aging, the intermetallic compound (IMC) formed at the interface was only (Cu,Ni)6Sn5. In addition, a P rich Ni (Ni3P) layer formed at the interface between the (Cu,Ni)6Sn5 and the Ni–P layer. The thickness of the interfacial (Cu,Ni)6Sn5 IMC layer slightly increased with the multiple reflows and isothermal aging. In contrast, the thickness of the Ni3P layer did not significantly increase. The formation of the (Cu,Ni)6Sn5 IMC layer retarded the consumption of the Ni from the electroless Ni–P substrate. As a result, a very thin Ni3P layer formed at the interface. In the ball shear test performed after aging treatment, the shear strength decreased for all temperatures tested until the samples had been aged for three days, and then remained at a constant value with prolonged aging. In addition, the shear strength could not be significantly related to the aging temperatures used in the present study. All fractures occurred in the bulk solder. The present study shows that the Sn–0·7Cu/ENIG solder joint has superior joint reliability with respect to shear tests.

Effect of surface finish on interfacial reactions of Cu/Sn–Ag–Cu/Cu(ENIG) sandwich solder joints

The interaction between Cu/Sn–Ag–Cu and Sn–Ag–Cu/Cu(ENIG) interfacial reactions has been studied during isothermal aging at 150 °C for up to 1000 h using a Cu/Sn–3.5Ag–0.7Cu/Cu(ENIG) sandwich solder joints. A typical scallop-type Cu–Sn intermetallic compound (IMC) layer was formed at the upper Sn–Ag–Cu/Cu interface after reflow. On the other hand, a (Cu,Ni) 6Sn 5 IMC layer was observed at the Sn–Ag–Cu/ENIG interface. The Cu in the (Cu,Ni) 6Sn 5 IMC layer formed on the Ni side has to be contributed from the dissolution of the opposite Cu metal pad or Cu–Sn IMC layer and the original Cu atom in the Sn–Ag–Cu solder. When the Cu arrived at the interface of the Ni pad, the (Cu,Ni) 6Sn 5 IMC layer was formed on the Ni interface, preventing the Ni pad from reacting with the solder. Although a long isothermal aging treatment at 150 °C was performed, any Ni was not detected in the Cu–Sn IMC layer formed on the upper Cu side. Compared to the single Sn–Ag–Cu/ENIG solder joint, the formation of the (Cu,Ni) 6Sn 5 IMC layer at the lower Sn–Ag–Cu/ENIG interface of the Cu/Sn–Ag–Cu/ENIG sandwich joint retarded effectively the consumption of the Ni from the electroless Ni–P layer, resulting in the formation of the very thin Ni 3P layer. The coupling effect of the Cu/Sn–Ag–Cu/ENIG sandwich joint is helpful to prevent the excessive consumption of Ni substrate and the formation of the brittle Ni 3P and Ni 2SnP layers.

Interfacial Reaction of Cu/Sn-Ag/ENIG Sandwich Solder Joint during Aging

The interaction between Cu/Sn-Ag and Sn-Ag/Ni interfacial reactions has been studied during isothermal aging at 150°C for up to 1000h using a Cu/Sn-3.5Ag/ENIG sandwich solder joint. A typical scallop-type Cu-Sn intermetallic compound (IMC) layer formed at the upper Sn-Ag/Cu interface after reflowing. On the other hand, a (Cu,Ni)6Sn5 IMC layer was observed at the Sn-Ag/ENIG interface. The Cu in the (Cu,Ni)6Sn5 IMC layer formed on the Ni side has to be contributed from the dissolution of the opposite Cu metal pad or Cu-Sn IMC layer. When the dissolved Cu arrived at the interface of the Ni pad, the (Cu,Ni)6Sn5 IMC layer formed on the Ni interface, preventing the Ni pad from reacting with the solder. Although a long isothermal aging treatment at 150°C was performed, any Ni was not detected in the Cu-Sn IMC layer formed on the Cu side. Compared to the single Sn-Ag/ENIG solder joint, the formation of the (Cu,Ni)6Sn5 IMC layer of the Cu/Sn-Ag/ENIG sandwich joint retarded effectively the consumption of the Ni from the electroless Ni-P layer.

Effect of isothermal aging on the interfacial reactions between Sn–0.4Cu solder and Cu substrate with or without ENIG plating layer

The interfacial reaction between the Sn–0.4 wt.% Cu solder and two different kinds of substrates (Cu and ENIG (electroless nickel-immersion gold)) during isothermal aging at 150 °C were studied. In case of the Cu substrate, two IMCs (intermetallic compounds) of Cu6Sn5 and Cu3Sn formed at the interface after aging for 250 h. After aging at 150 °C for 1000 h, crack occurred along the interface between the Cu3Sn IMC and Cu substrate. In case of the ENIG substrate, the solder/electroless Ni–P interface exhibited a duplex structure of (Cu,Ni)6Sn5 and (Ni,Cu)3Sn4 after aging at 150 °C for 24 h. Many discontinuous (Cu,Ni)6Sn5 IMCs formed on (Ni,Cu)3Sn4 IMC layer. The presence of P in the electroless Ni–P layer caused complicated interfacial reactions such as formation of a P-rich Ni (Ni3P) and Ni–Sn–P layers. The growth of the Cu–Sn IMC layer formed on the Cu substrate was much faster than that of the Cu–Ni–Sn IMC ((Cu,Ni)6Sn5 and (Ni,Cu)3Sn4) layer formed on the ENIG substrate. In addition, the formation of the Cu–Ni–Sn IMC layer retards the consumption of the Ni from the electroless Ni–P layer. Results of this study showed that the ENIG plating layer acts as a good diffusion barrier for the Sn–0.4Cu solder.

Intermetallic Compound Formation and Growth Kinetics in Flip Chip Joints Using Sn–3.0Ag–0.5Cu Solder and Ni–P under Bump Metallurgy

The interfacial microstructure of Sn–3.0Ag–0.5Cu solder with ENIG (electroless Ni/immersion Au) UBM was studied using scanning electron microscopy and transmission electron microscopy. (Cu,Ni)6Sn5 intermetallic compound layer was formed at the interface between the solder and Ni–P under bump metallization upon reflow. However, after isothermal aging, some AuSn4 but with a certain amount of Ni dissolved in it, i.e., (Au,Ni)Sn4, appeared above the (Cu,Ni)6Sn5 layer. Two distinctive layers, P-rich and Ni–Sn–P, were additionally found from the TEM observation. The analytical studies using EDS equipped in TEM revealed that the averaged composition of the P-rich layer was close to that of a mixture of Ni3P and Ni, while that of the Ni–Sn–P layer was analogous to the P-rich layer containing a small amount of Sn in it. The thickness of the (Cu,Ni)6Sn5 layer increased with increasing aging time and temperature. The layer growth of the intermetallic compound satisfied a parabolic law in the given temperature range. The increase of the intermetallic compound layer thickness was mainly controlled by a diffusion mechanism in the temperature range studied, because the values of time exponent (n) were approximately equal to 0.5. The apparent activation energy for the growth of (Cu,Ni)6Sn5 intermetallic compound layer was 55 kJ/mol.

Microstructure and Mechanical Properties of Sn–0.7Cu Flip Chip Solder Bumps Using Stencil Printing Method

The interfacial microstructure of Sn–0.7Cu solder with ENIG (electroless Ni/immersion Au) was studied using scanning electron microscopy (SEM), electron probe micro analyzer (EPMA) and transmission electron microscopy (TEM). (Cu,Ni)6Sn5 intermetallic compound (IMC) layer was formed at the interface between the solder and Ni–P under bump metallurgy (UBM) upon reflow. The thickness of the (Cu,Ni)6Sn5 IMC layer increased with isothermal aging time. Two distinctive layers, P-rich and Ni–Sn–P, were additionally found from TEM observation. Analytical studies using energy dispersive spectrometer (EDS) equipped in TEM revealed that the composition of the P-rich layer is close to that of a mixture of Ni3P and Ni, while that of the Ni–Sn–P layer is analogous to the P-rich layer but containing a small amount of Sn in it. The shear force of the joints decreased during isothermal aging. The decrease of the shear force should be mainly due to the microstructural degradation within the solder.

Effect of reflow time on interfacial reaction and shear strength of Sn–0.7Cu solder/Cu and electroless Ni–P BGA joints

The interfacial reaction and shear strength between the eutectic Sn–0.7wt.% Cu solder and two different kinds of ball grid array (BGA) substrates (Cu and Au/Ni–P/Cu) during reflow at 255°C for up to 30min were studied. With the solder joints between the Sn–0.7Cu solder and Cu substrates, a Cu6Sn5 intermetallic compound (IMC) was formed at the interface. In the case of the electroless Ni–P substrate, the IMC formed at the interface was mainly (Cu, Ni)6Sn5. Also, a P-rich Ni (Ni3P) layer was observed as a by-product of the Cu–Ni–Sn reaction. The Ni3P layer was between the (Cu, Ni)6Sn5 IMC and the electroless Ni–P layer. The thickness of the Cu6Sn5, (Cu, Ni)6Sn5 and Ni3P layer was found to increase with the reaction time. The growth of the Cu6Sn5 IMC layer for Cu substrate was faster than that of the (Cu, Ni)6Sn5 IMC layer for electroless Ni–P substrate. The Ni content in the (Cu, Ni)6Sn5 IMC layer formed at the interface increased with increasing reaction time. In the ball shear tests, the shear strength value did not change much as a function of reflow time. In all samples, the fracture mainly occurred in the bulk solder. However, for the Sn–0.7Cu/electroless Ni–P joint reflowed for 10min, the fracture occurred in ductile and slightly brittle mode.

Development of Cu–Sn intermetallic compound at Pb-free solder/Cu joint interface

The development of Cu–Sn intermetallic compound (IMC) at the solder/Cu joint interface had been studied using two Pb-free solders, Sn–3.8Ag–0.7Cu and Sn–2Ag–0.8Cu–0.6Sb alloys. Meanwhile, 100Sn/Cu joint was applied for comparison. Both Pb-free solder joints were found with thinner Cu–Sn IMC layers at as-soldered state due to the slower dissolution rate of intermetallic compound into the liquid Pb-free solders during reflow and, consequently, slower growth rates of Cu–Sn IMC during the solid-state thermal aging at 125 °C, where Sn–2Ag–0.8Cu–0.6Sb solder joint gave the minimum value. Thermodynamic analysis showed that such phenomena could be attributed to the reduction of the driving force for Cu–Sn IMC formation due to the existence of Ag and Sb atoms.

Growth Kinetics of Cu–Sn Intermetallic Compounds at Interface of 80Sn–20Pb Electrodeposits and Cu Based Leadframe Alloy, and Its Influence on the Fracture Resistance to 90°-Bending

Electrodeposits of 80Sn–20Pb were formed on a Cu-based leadframe alloy from organic sulfonate solutions under different plating conditions, and were then aged at 100 to 180°C to examine the effects of the microstructure of electrodeposits on the growth kinetics of the Cu–Sn intermetallic layer formed at the electrodeposit/substrate interface, and the resultant influence on the fracture resistance of the electrodeposits under bending. The Cu–Sn intermetallic compound layers [ε (Cu3Sn) + η (Cu6Sn5)] increase linearly in thickness with the square root of aging time. However, the growth rate of the intermetallic compounds layer exhibits significant differences depending on the deposition condition; it is fastest in an extremely fine elecrodeposit formed using pulse current in a bath without any grain refiner, but is slowest in that formed using dc current in a bath with grain refiner, in spite of an equivalent grain size in both electrodeposits. This is due primarily to the inhibiting effects of the grain refiner remaining at grain boundaries of the electrodeposit on the diffusion of Sn across the intermetallic compounds/electrodeposit interface. The occurrence frequency and the degree of opening of surface cracks, formed when the aged samples are subjected to the 90°-bending test, are found to be associated with the growth rate of the intermetallic compound layer and the ductility of the electrodeposit. For samples aged under the same conditions, the sample electrodeposited from the solution containing grain refiner shows the highest resistance to surface cracking during the 90°-bending test due to the lower growth rate of its intermetallic layer and the higher ductility of the electrodeposit compared with those of other samples.