Solid-state Aging Research Articles

Development and Evaluation of Direct Deposition of Au/Pd(P) Bilayers over Cu Pads in Soldering Applications

The thermal reliability of Sn-3Ag-0.5Cu/Au/Pd(P)/Cu solder joints was evaluated in this study. After reflow and subsequent solid-state aging (180°C), the reaction product species at the interface included Cu6Sn5 [or (Cu,Pd)6Sn5] and Cu3Sn, and their growth was strongly dependent on the Pd(P) thickness, δPd(P). As δPd(P) increased, the growth of Cu6Sn5 was significantly enhanced, while that of Cu3Sn was suppressed. Computer coupling of phase diagrams and thermochemistry (CALPHAD) analysis showed that minor incorporation of Pd (~2 at.%) into the Cu6Sn5 phase decreased the Gibbs free energy of Cu6Sn5 from −7339 J/mol to −9191 J/mol. This effect might enhance Sn diffusion in Cu6Sn5 but diminish Cu diffusion in Cu3Sn, thereby facilitating the growth of Cu6Sn5 but retarding that of Cu3Sn. High-speed ball shear (HSBS) test results showed that the mechanical properties of the solder joints were slightly enhanced by an increase in δPd(P). These findings suggest that direct deposition of Au/Pd(P) bilayers over the Cu pads can effectively modify the mechanical reliability of solder joints.

Texture evolution and its effects on growth of intermetallic compounds formed at eutectic Sn37Pb/Cu interface during solid-state aging

The growth orientation of Cu6Sn5 intermetallic compounds (IMCs) formed at the eutectic Sn37Pb/polycrystalline Cu interface during solid-state aging was investigated. The results indicate that the interfacial Cu6Sn5 grains exhibit textured growth under solid-state conditions, and their preferred orientations are affected by the initial joint preparation conditions. Cu6Sn5 grains in the [0001] direction normal to the interface are stable in solid and molten Sn37Pb solder at 200°C, but are rapidly consumed at 280°C. This effect leads to the formation of different textures in the Cu6Sn5 layer during the solid-state aging treatment of joints formed at 200 and 280°C. In addition, the influence of texture evolution on the growth of interfacial IMCs was evaluated. The results indicate that Sn diffusion is faster along the [0001] direction of the Cu6Sn5 crystal than along an angle of 25–45° to the [0001] direction; therefore, more IMCs are generated at the interface of the joints formed at 200°C than at those formed at 280°C under the same solid-state reaction conditions.

Development of Metallic Hermetic Sealing for MEMS Packaging for Harsh Environment Applications

Hermetic sealing of microelectromechanical system sensors is indispensable to ensure their reliable operation and also to provide protection during fabrication. This work proposes two prospective candidates for hermetic sealing for rugged environment applications, i.e., Al-Ge and Pt-In. Al-Ge was chosen due to its compatibility with complementary metal–oxide–semiconductor technology. Pt-In possesses the highest remelting temperature among all the solder systems, which is desired for high-temperature applications in both the energy and aerospace industries. The various bonding parameters for Al-Ge eutectic bonding and Pt-In transient liquid-phase (TLP) bonding have been optimized, and their influence on the bond quality is reported. Optimization of bonding parameters has been carried out with the objective of ensuring void-free bonds. A new configuration for stacking Al-Ge thin films has been demonstrated to tackle the issue of loss of Ge prior to bonding, since native Ge oxides are soluble in deionized water. The impact of solid-state aging prior to Al-Ge eutectic bonding has been investigated. The method of tailoring the phases in the Pt-In joint is also discussed. The prospects and constraints of eutectic and TLP bonding from the hermeticity perspective are discussed in detail. Furthermore, changes in the microstructure under aging at 300°C up to 500 h and the resulting influence on the mechanical properties are presented. The overall finding of this work is that Al-Ge can achieve better mechanical and hermetic performance for high-temperature applications.

NiSn4 formation during the solidification of Sn–Ni alloys

The growth of NiSn4 is studied during the solidification of Sn–Ni alloys containing 0–0.45wt%Ni. Where past research has found metastable NiSn4 after solid state ageing of Ni–Sn couples, it is shown here that NiSn4 also forms during solidification as both a primary and a eutectic phase over a wide range of solidification conditions. The NiSn4 phase has crystal structure isomorphous to PtSn4 (oC20) and no evidence is found for NiSn4 being stable at solidification temperatures. Primary NiSn4 crystal growth was promoted by high cooling rates and a comparison of growth ledges on the facets of primary NiSn4 and Ni3Sn4 crystals suggests easier interface attachment kinetics for NiSn4. The Sn–NiSn4 eutectic grew under all solidification conditions used, with an orientation relationship that produces a low lattice disregistry at the interface plane and the same NiSn4 growth direction as that selected during free growth of primary NiSn4. In hyper-eutectic compositions, both the Sn–NiSn4 and Sn–Ni3Sn4 eutectics grew with the latter growing from primary Ni3Sn4 crystals.

Interfacial reactions between Sn–57Bi–1Ag solder and electroless Ni-P/immersion Au under solid-state aging

Interfacial reactions between Sn–57Bi–1Ag and electroless Ni-P/immersion Au were investigated following isothermal aging at 85 and 130 °C. A long-term aging study confirmed two intermetallics at the solder/substrate interface. With scanning electron microscopy and energy-dispersive X-ray spectroscopy (EDX) analysis, the metastable NiSn4 phase was detected coexisting with the stable Ni3Sn4 phase. The average thicknesses of both intermetallic compounds (IMCs) were graphically plotted versus the square root of aging time. The EDX showed that Ni3Sn4 nucleated first. However, after nucleation, the IMC of the NiSn4 phase grew faster than the one of Ni3Sn4 between 85 and 130 °C. For both temperatures, the growth constants were calculated and the corresponding activation energies were approximated. A high volume of Kirkendall voids appeared along the Ni3Sn4/NiSn4 interface at 130 °C, resulting in dramatic shear strength decline.

Kinetic analysis of Ni 5Zn 21 growth at the interface between Sn–Zn solders and Ni

Ni 5Zn 21 is the primary formed phase in the Sn-9wt%Zn/Ni interfacial reactions. In this study, the growth kinetics of the Ni 5Zn 21 phase was systematically investigated by solid-state aging between 150 °C and 170 °C, and liquid-state aging between 230 °C and 270 °C. The relevant kinetic parameters and the corresponding activation energies were determined. Particularly, the linear growth of the Ni 5Zn 21 layer was clearly found in the early stage of Sn-9wt%Zn/Ni solid-state reactions. When the Ni 5Zn 21 layer reached a critical thickness of about 6–10 μm, depending on the reaction temperature, the growth rate gradually decreased and followed a parabolic relationship. In addition, the reactions of Sn-5wt%Zn and Sn-3wt%Zn solders also demonstrated similar growth behaviors. Nevertheless, the Ni 5Zn 21 growth would significantly slow with decreasing Zn contents. Further, in the reactions with molten Sn–Zn solders, the Ni 5Zn 21 layer growth was also found to be linear prior to becoming about 3 μm thick, and then became parabolic. The appearance of the linear growth in both reflow soldering and isothermal aging suggested that the Sn–Zn/Ni interface was controlled by chemical transformation in the initial stage. The Ni 5Zn 21 growth mechanism is discussed in-depth in this paper.

Coarsening mechanisms, texture evolution and size distribution of Cu 6Sn 5 between Cu and Sn-based solders

During the reflowing procedure, the Cu concentration in the solder affects the coarsening mechanisms of intermetallic compound (IMC) grains. For the Sn3Cu solder, the mean radius of the IMC grains was proportional to the cube root of the reflowing time; while it follows the square root relation with the reflowing time for the SnAgCu and Sn solders. It is proposed that the flux from the substrate was only consumed to coarsen the IMC grains for Sn3Cu solder, while it was separated into two fluxes for the SnAgCu and Sn solders at the reflowing initial procedure. For the Sn3.8Ag0.7Cu/Cu and Sn/Cu couples, the size distribution of the IMC grains well agrees with the modified flux driven ripening (FDR) model when the value of r/〈 r〉 is above 1; while it would closely match with Marqusee and Ross analysis when the value of r/〈 r〉 is below 1. For Sn3Cu/Cu couple, the size distribution of IMC grains shows good agreement with the FDR model. However, for SnPb/poly-Cu, during the solid-state aging procedure, the mean radius of the IMC grains was proportional to the cube root of the aging time. And the size distribution of IMC grains is well consistent with the FDR model.

Diffusion barrier characteristics and shear fracture behaviors of eutectic PbSn solder/electroless Co(W,P) samples

Abstract Diffusion barrier characteristics, activation energy (Ea) of IMC growth and bonding properties of amorphous and polycrystalline electroless Co(W,P) (termed as α-Co(W,P) and poly-Co(W,P)) to eutectic PbSn solder are presented. Intermetallic compound (IMC) spallation and an nano-crystalline P-rich layer were observed in PbSn/α-Co(W,P) samples subjected to liquid-state aging at 250 °C. In contrast, IMCs resided on the P-rich layer in PbSn/α-Co(W,P) samples subjected to solid-state aging at 150 °C. Thick IMCs neighboring to an amorphous W-rich layer was seen in PbSn/poly-Co(W,P) samples regardless of the aging type. α-Co(W,P) was found to be a sacrificial- plus stuffed-type barrier while poly-Co(W,P) is mainly a sacrificial-type barrier. The values of Ea's for PbSn/α-Co(W,P) and PbSn/poly-Co(W,P) systems were 338.6 and 167.5 kJ/mol, respectively. Shear test revealed the ductile mode dominates the failure in both α- and poly-Co(W,P) samples. Analytical results indicated the high P content in electroless layer might enhance the barrier capability but degrade the bonding strength.

Intermetallic compound growth suppression at high temperature in SAC solders with Zn addition on Cu and Ni–P substrates

The effect of adding 0.5–1.5 wt.% Zn to Sn–3.8Ag–0.7Cu (SAC) solder alloy during reflow and solid state ageing has been investigated. In particular, the role of the Zn addition in suppressing interfacial Intermetallic Compound (IMC) growth on Cu and Ni–P substrates has been determined. Solder–substrate couples were aged at 150 °C and 185 °C for 1000 h. In the case of 0.5–1.0 wt.% Zn on Cu substrate, Cu 3Sn IMC was significantly suppressed and the morphology of Cu 6Sn 5 grains was changed, leading to suppressed Cu 6Sn 5 growth. In the SAC–1.5Zn/Cu substrate system a Cu 5Zn 8 IMC layer nucleated at the interface followed by massive spalling of the layer into the solder, forming a barrier layer limiting Cu 6Sn 5 growth. On Ni–P substrates the (Cu,Ni) 6Sn 5 IMC growth rate was suppressed, the lowest growth rate being found in the SAC–1.5Zn/Ni–P system. In all cases the added Zn segregated to the interfacial IMCs so that Cu 6Sn 5 became (Cu,Zn) 6Sn 5 and (Cu,Ni) 6Sn 5 became (Ni,Cu,Zn) 6Sn 5. The effect of Zn concentration on undercooling, wetting angles and IMC composition changes during ageing are also tabulated, and a method of incorporating Zn into the solder during reflow without compromising solder paste reflow described.

Fast Concurrent Growth of Ni3Sn4 and Voids During Solid-State Reaction Between Sn-Rich Solder and Ni Substrates

To simulate the growth of Ni3Sn4 phase layers in Sn-based solder joints with Ni substrates during solid-state aging, Sn/(Cu1−x Ni x )6Sn5/Ni and Sn/Ni diffusion couples were aged isothermally at 180°C and 200°C, and the growth kinetics of the (Ni,Cu)3Sn4 and Ni3Sn4 layers in the respective couples were monitored during the isothermal aging. Once the (Ni,Cu)3Sn4 layer was formed at the (Cu,Ni)6Sn5/Ni interface, it grew unexpectedly fast with concurrent growth of voids formed in the Sn layer during prolonged aging at both temperatures. The results obtained from the various types of diffusion couples revealed that the voids formed in the Sn layer were Kirkendall voids, due to the (Ni,Cu)3Sn4 layer growing predominantly at the (Ni,Cu)3Sn4/Ni interface by fast diffusion of Sn across the (Ni,Cu)3Sn4 layer. It is proposed that the accelerated growth of the (Ni,Cu)3Sn4 and Ni3Sn4 layers after the formation of voids in the Sn layer is due to the relaxation of vacancy oversaturation and the enhanced annihilation rate of incoming vacancies in the presence of the voids in the Sn layer.

Solid–Solid Reaction Between Sn-3Ag-0.5Cu Alloy and Au/Pd(P)/Ni(P) Metallization Pad with Various Pd(P) Thicknesses

The effect of Pd(P) thickness on the solid–solid reaction between Sn-3Ag-0.5Cu and Au/Pd(P)/Ni(P) at 180°C was investigated in this study. The reaction was conducted after reflow, thereby removing the Au/Pd finish before the solid-state reaction. The reaction products included (Cu,Ni)6Sn5, Ni2SnP, and Ni3P, and their growth strongly depended on the Pd(P) thickness, especially for the former phases [i.e., (Cu,Ni)6Sn5 and Ni2SnP]. As the Pd(P) thickness increased from 0 μm, to 0.1 μm, to 0.22 μm, the (Cu,Ni)6Sn5 exhibited a needle-like dense layer, chunk-like morphology, and discontinuous morphology, respectively. The alternative phase (Ni2SnP) behaved in a manner opposite to that of (Cu,Ni)6Sn5, growing with a discontinuous morphology to a dense layer with increasing Pd(P) thickness. However, this strong dependence disappeared when the solder joints were subsequently subjected to solid-state aging. The (Cu,Ni)6Sn5 and Ni2SnP both became layered structures for all cases examined. A high-speed ball shear (HSBS) test was conducted to quantify the mechanical response of the interfacial microstructures. The HSBS test results showed that any initial difference in shear strength caused by the various Pd(P) thicknesses could be reduced after the solid-state aging, which is consistent with the microstructural evolution observed. The mechanical strength of the solder joints was decreased due to the presence of a bi-intermetallic structure of (Cu,Ni)6Sn5/Ni2SnP at the interface. Detailed analysis of the growth of (Cu,Ni)6Sn5 and Ni2SnP is also provided.

Suppressing effect of 0.5 wt.% nano-TiO 2 addition into Sn–3.5Ag–0.5Cu solder alloy on the intermetallic growth with Cu substrate during isothermal aging

This study investigated the effects of adding 0.5 wt.% nano-TiO 2 particles into Sn3.5Ag0.5Cu (SAC) lead-free solder alloys on the growth of intermetallic compounds (IMC) with Cu substrates during solid-state isothermal aging at temperatures of 100, 125, 150, and 175 °C for up to 7 days. The results indicate that the morphology of the Cu 6Sn 5 phase transformed from scallop-type to layer-type in both SAC solder/Cu joints and Sn3.5Ag0.5Cu–0.5 wt.% TiO 2 (SAC) composite solder/Cu joints. In the SAC solder/Cu joints, a few coarse Ag 3Sn particles were embedded in the Cu 6Sn 5 surface and grew with prolonged aging time. However, in the SAC composite solder/Cu aging, a great number of nano-Ag 3Sn particles were absorbed in the Cu 6Sn 5 surface. The morphology of adsorption of nano-Ag 3Sn particles changed dramatically from adsorption-type to moss-type, and the size of the particles increased. The apparent activation energies for the growth of overall IMC layers were calculated as 42.48 kJ/mol for SAC solder and 60.31 kJ/mol for SAC composite solder. The reduced diffusion coefficient was confirmed for the SAC composite solder/Cu joints.

Intermetallic compound identification and Kirkendall void formation in eutectic SnIn/Cu solder joint during solid-state aging

Microstructural investigations were performed on the interfacial reactions between eutectic SnIn solder and Cu substrate during reflowing at 433 K and solid-state aging at 373 K. Cu2(In,Sn) was identified as the only intermetallic compound (IMC) at the interface, which consists of two sublayers with different morphology, a fine-grained sublayer at the Cu side and a coarse-grained sublayer at the solder side. During solid-state aging, voids were found between these two Cu2(In,Sn) sublayers but not at the substrate interface, which is also attributed to the Kirkendall effect considering the different diffusion fluxes of Sn or In and Cu atoms in different sublayers.

Kinetics of intermetallic phase formation at the interface of Sn–Ag–Cu–X (X = Bi, In) solders with Cu substrate

The effects of Bi and In additions on intermetallic phase formation in lead-free solder joints of Sn–3.7Ag–0.7Cu; Sn–1.0Ag–0.5Cu–1.0Bi and Sn–1.5Ag–0.7Cu–9.5In (composition given in weight %) with copper substrate are studied. Soldering of copper plate was conducted at 250 °C for 5 s. The joints were subsequently aged at temperatures of 130–170 °C for 2–16 days in a convection oven. The aged interfaces were analyzed by optical microscopy and energy dispersive X-ray spectroscopy (EDX) microanalysis. Two intermetallic layers are observed at the interface – Cu 3Sn and Cu 6Sn 5. Cu 6Sn 5 is formed during soldering. Cu 3Sn is formed during solid state ageing. Bi and In decrease the growth rate of Cu 3Sn since they appear to inhibit tin diffusion through the grain boundaries. Furthermore, indium was found to produce a new phase – Cu 6(Sn,In) 5 instead of Cu 6Sn 5, with a higher rate constant. The mechanism of the Cu 6(Sn,In) 5 layer growth is discussed and the conclusions for the optimal solder chemical composition are presented.

Integrated Batteryless Electron Timer

From the viewpoint of information security, the semiconductor timing devices are reviewed, and a promising cell with floating gate (FG) is proposed as an integrated batteryless electron timer. The first issue is the difficulty in the timing precision, which is related to the trap-detrapping phenomena in the tunnel oxide between the FG and the silicon surface. The basic idea to resolve this issue is to monitor the trap-free cells among a plurality of prepared cells. The integrated batteryless electron timer is composed of a plurality of single-polysilicon-type solid-state aging devices that are connected in parallel. The first sample is fabricated in a standard complementary metal-oxide-semiconductor process, and the measurements clearly exhibit the first evidence that we succeeded to remove the trap-detrapping-related fluctuation in the ticking operation. The resultant secondary issues on the precision, i.e., the manufacturing fluctuation (subjecting to the central-limit theorem) and the temperature dependence, are also briefly discussed.

Computational investigation of intermetallic compounds (Cu6Sn5 and Cu3Sn) growth during solid-state aging process

Computational investigations of the morphological evolution and growth kinetics for intermetallic compounds (IMCs, Cu6Sn5 and Cu3Sn) formed during reaction and aging between Sn-based solder and a copper substrate are presented. Cu-substrate, Cu3Sn (ɛ phase) and Cu6 Sn5 (η phase) layers (or grains), as well as the Sn-liquid phase (or Sn-solid phase) are considered during the soldering (solid-state aging) process. In the simulation, interface regions are defined by the coexistence of two or more phases (at triple points) at a computational grid point. The simulation is performed through the multiphase-field approach. In the phase-field simulation, the grain boundary (GB) diffusion of the η phase as well as the interfacial energy between this phase and the solder alloy are treated as model parameters. Variation of these parameters allows the investigation of the effects of short-circuit diffusion paths and GB wetting on the morphological evolution of the IMC layers. The simulations addresses the growth kinetics of the two IMC layers (Cu6Sn5 and Cu3Sn) during the two processes up to 14h, illustrating the variation of η and ɛ IMC thickness and the number of η and ɛ grains as the microstructure coarsens.

An Investigation of Diffusion Barrier Characteristics of an Electroless Co(W,P) Layer to Lead-Free SnBi Solder

Diffusion barrier characteristics for eutectic SnBi solder/electroless Co(W,P) couples were investigated via liquid-state aging at 250°C and solid-state aging at 120°C. At the couple interface, CoSn3 intermetallic compound (IMC) spallation was observed for the SnBi/amorphous Co(W,P) couple subjected to liquid-state aging. In contrast, no spallation of IMCs was observed for the SnBi/amorphous Co(W,P) couples subjected to solid-state aging. For the SnBi/polycrystalline Co(W,P) couple, a thick IMC layer was observed adjacent to a tungsten-enriched amorphous interfacial layer regardless of aging conditions. IMC formation in all samples indicated that Co(W,P) is essentially a sacrificial barrier to SnBi solder. However, amorphous Co(W,P) might also exhibit stuffed-type barrier behavior due to its relatively high phosphorus (P) content. Analytical results indicated that the P content in Co(W,P) is a crucial factor affecting the structural evolution at the SnBi/electroless Co(W,P) interface.

Diffusion Barrier Characteristics of Electroless Co(W,P) Thin Films to Lead-Free SnAgCu Solder

Diffusion barrier and bonding characteristics of amorphous and polycrystalline electroless Co(W,P) layers (α-Co(W,P) and poly-Co(W,P)) to SnAgCu (SAC) solder were investigated. In the SAC/α-Co(W,P) sample subjected to liquid-state aging at 250°C, the spallation of intermetallic compound (IMC) into solder, a nano-crystalline P-rich layer at SAC/Co(W,P) interface, and the recrystallized Co(W,P) containing Co2P precipitates were observed. As to the SAC/α-Co(W,P) sample subjected to solid-state aging at 150°C, a thick IMC layer neighboring to the P-rich layer formed at the solder/Co(W,P) interface. Liquid-state aging resulted in an IMC mixture without spallation whereas solid-state aging induced a layer-like IMC in SAC/poly-Co(W,P) samples. Amorphous W-rich layers were also observed in SAC/poly-Co(W,P) samples and it could not inhibit subsequent alloy reactions in the samples. Analytical results indicated the α-Co(W,P) is a sacrificial- plus stuffed-type barrier whereas the poly-Co(W,P) is mainly a sacrificial barrier. The activation energies of IMC growth were 110.7 and 81.8 kJ/mol for SAC/α-Co(W,P) and SAC/poly-Co(W,P) samples, respectively. High P content in α-Co(W,P) was found to degrade the bonding strength to the solder as revealed by the shear test and the control of P content would be a key issue for electroless plating layer applied to under bump metallurgy as the diffusion barrier.

Interdependent Intermetallic Compound Growth in an Electroless Ni-P/Sn-3.5Ag Reaction Couple

The interfacial microstructure of electroless Ni-P/Sn-3.5Ag solder joints was investigated after reflow and high-temperature solid-state aging to understand its interdependent growth mechanism and related kinetics of intermetallic compounds (IMCs) at the interface. The reflow and aging results showed that mainly three IMC layers, Ni3Sn4 ,N i 2SnP, and Ni3P, formed during the soldering reaction. It was found that the Ni3Sn4 and Ni3P layers grow predominantly as long as the electroless Ni-P layer is present; however, once the Ni-P layer is fully consumed, the Ni2SnP layer grows rapidly at the expense of the Ni3P layer. A transition in the Ni3Sn4 morphology from needle and chunky shape to scallop shape was observed after the solid-state aging of reflowed samples. The kinetics data obtained from the growth of compound layers in the aged samples revealed that initially the growth of the Ni2SnP layer is controlled by diffusion, and subsequently by the rate of reaction after the Ni-P metallization is fully consumed. It was found that complete transformation of the electroless Ni-P layer into a Ni3P layer results in the rapid growth of the Ni2SnP layer due to the dominating reaction of Sn with Ni3P. The apparent activation energies for the growth of Ni3Sn4 ,N i 2SnP, and Ni3P compound layers were found to be 98.9 kJ/mol, 42.2 kJ/mol, and 94.3 kJ/mol, respectively.

Effect of adding Ce on interfacial reactions between Sn–Ag solder and Cu

We investigated the effect of adding cerium (Ce) to low Ag content Sn–1.0wt.%Ag solder on the interfacial reactions between the Sn–1.0Ag solder and Cu substrate. The formation and growth of interfacial intermetallic compounds (IMCs) between the Sn–1.0Ag–0.3Ce solder and Cu substrate were studied and the results were compared to those obtained for the Ce-free Sn–1.0Ag/Cu and most promising Sn–3.0Ag–0.5Cu/Cu systems. The addition of Ce to the Sn–Ag solder significantly reduced the growth of the interfacial Cu–Sn IMCs, retarded the interfacial reactions between the solder and the substrate, and prevented the IMC from spalling from the interface. The Sn–1.0Ag–0.3Ce solder alloy had a good interfacial stability with the Cu substrate during solid-state isothermal aging in the viewpoint of IMC growth.