As-bonded State Research Articles

Application of organosilanes in the preparation of metal surfaces for adhesive bonding

This paper deals with research into the influence of surface preparation before adhesive bonding of galvanised and non-galvanised steel substrates. The steel substrates were experimentally treated with chromate-free zirconate passivation, iron phosphate, zinc phosphate and an experimental organosilane-based preparation. The roughness of the treated substrates was evaluated by a set of selected vertical and horizontal parameters. The corrosion rate of the treated substrates was tested by electrochemical impedance spectroscopy and linear polarisation. Single-lap joints were created using one-component epoxy/PVC-polymerblend adhesive with glass beads, to ensure an evenly glued gap. The joints were tested for shear strength by tension, in accordance with STN EN 1465 in the as-bonded state, as well as after the climatic cyclic test PV1200. The results show that the adhesive used had a higher adhesion to galvanised substrates with a specific morphology. Adhesion to non-galvanised substrates has been significantly improved by using organosilane for surface preparation. The organosilane layer, containing epoxy organic functional groups, was incorporated into the organic binder of the adhesive and showed increased bond strength.

Interfacial reactions and structural properties of explosively welded titanium/copper plates

In this study, the effect of post-processing heat treatment on the interface morphology and kinetics of intermetallic phase growth in titanium/copper bimetallic sheets were analyzed using a scanning electron microscopy and mechanical testing. Ti/Cu composite was fabricated by explosive welding. Then, clads were subjected to the post-welded annealing at 973 K for times ranging between 15 min and 100h. In the as-bonded state, the interface shows a wavy character with a limited number of melted zones, which are mostly composed of phases that are not observed in the Ti-Cu equilibrium phase diagram. Short-term annealing is shown to transform non-equilibrium phases into equilibrium ones, such as TiCu4 and Ti3Cu4. For longer annealing times, the growth of four intermetallic sublayers (Ti2Cu, TiCu, Ti3Cu4, and TiCu4) distributed along the entire interface is detected. However, the growth kinetics of particular sublayers was different. Volume fraction estimation of intermetallic phases at annealing times under study revealed the largest thickness for TiCu and TiCu4 phases. The nano-hardness of intermetallic sublayers was found to drop with the Ti content.

Evolution of Transient Liquid-Phase Sintered Cu–Sn Skeleton Microstructure During Thermal Aging

The evolution of the transient liquid-phase sintered (TLPS) Cu–Sn skeleton microstructure during thermal aging was evaluated to clarify the thermal reliability for die-attach applications. The Cu–Sn skeleton microstructure, which consists of Cu particles connected with Cu–Sn intermetallic compounds partially filled with polyimide resin, was obtained by the pressure-less TLP sintering process at 250 °C for 1 min using a novel Cu-solder-resin composite as a bonding material in a nitrogen atmosphere. Experimental results indicate that the TLPS joints were mainly composed of Cu, Cu6Sn5, and Cu3Sn in the as-bonded state, where submicron voids were observed at the interface between Cu3Sn and Cu particles. After thermal aging at 150, 175, and 200 °C for 1000 h, the Cu6Sn5 phase fully transformed into Cu3Sn except at the chip-side interface, where the number of the submicron voids appeared to increase. The averaged shear strengths were found to be 22.1 (reference), 22.8 (+3%), 24.0 (+9%), and 19.0 MPa (−14%) for the as-bonded state and specimens aged at 150, 175, and 200 °C for 1000 h, respectively. The TLPS joints maintained a shear strength over 19 MPa after thermal aging at 200 °C for 1000 h because of both the positive and negative impacts of the thermal aging, which include the transformation of Cu6Sn5 into Cu3Sn and the formation of submicron voids at the interface, respectively. These results indicate an excellent thermal reliability of the TLPS Cu–Sn skeleton microstructure.

Effect of Au Addition on the Microstructure and Properties of Ag-4Pd Bonding Wires

Silver-based bonding wires such as Ag-4Pd and Ag-8Au-3Pd have drawn remarkable attention in the packaging industry because they are cheaper and more conductive than Au- and Cu-based wires, respectively. This study aimed to investigate the intermetallic compound (IMC) formation and growth at the bonding interface between Ag-4Pd wire and Al-pads and between Ag-8Au-3Pd wire and Al-pads. The as-bonded and reliability-tested Ag-4Pd/Al and Ag-8Au-3Pd/Al specimens were then investigated by transmission electron microscopy (TEM) and scanning transmission electron microscopy equipped with energy-dispersive X-ray spectroscopy (STEM-EDS). The bonding properties were examined by ball shear and wire pull tests. In the as-bonded state, hexagonal close-packed (HCP) (Ag, Pd)2Al and HCP Ag2Al were formed at the Ag-4Pd/Al interfaces, whereas dual phase consisting of face-centered cubic Ag alloy with HCP precipitates (Ag, Au, Pd)2Al crystals and HCP Ag2Al layers were observed at the Ag-8Au-3Pd/Al interfaces. The IMCs showed significant growth and oxidation during reliability tests at 130 °C and 85 pct relative humidity for 192 hours. Alloying Au in Ag-4Pd wires promoted the growth of the IMC layer and it also enhanced the mechanical properties in the as-bonded material. By contrast, overgrowth of the IMCs in the Ag-8Au-3Pd/Al system induced microcrack formation in bonding and thus degraded the reliability of the material.

Interfacial evolution and bond reliability in thermosonic Pd coated Cu wire bonding on aluminum metallization: Effect of palladium distribution

In this paper, the growth kinetics of Cu–Al intermetallic compounds formed during isothermal annealing of Pd–Cu wire bonds with different palladium distribution at 175°C are investigated by electron microscopy and compared to bare Cu wire bonds. Transmission electron microscopy (TEM) was used to provide high resolution imaging of the Cu–Al IMCs in the as-bonded state and TEM-EDX used to analyze the concentrations of Pd at the bond interface in the as-bonded state. Cu–Al IMCs were found to grow thicker with increasing annealing duration. The growth kinetics of the Cu–Al IMCs were correlated with the diffusion process during thermal annealing. The IMC thickness for Pd–Cu wire bonds with Pd at the bond interface was found to be thinner as compared to that for Pd–Cu wire bonds with no Pd at the bond interface. Thus, the presence of palladium at the bond interface has slowed down the IMC growth. Nano-voids were found in the Pd–Cu wire bonds with Pd at the bond interface, but not in the Pd–Cu wire bonds with no Pd at the bond interface. The IMC growth rate for the Pd–Cu bonds with no Pd was found to be close to that for bare Cu for the initial annealing durations. Corresponding bond pull testing showed that Pd–Cu wire bonds containing Pd have best preserved the bond strength after 168h aging at 175°C due to the beneficial presence of Pd.

Role of intermetallics on the mechanical fatigue behavior of Cu–Al ball bond interfaces

The mechanical fatigue behavior of Cu–Al interfaces occurring in thermosonic ball bonds –typically used in microelectronic packages for automotive applications – is investigated by means of a specially designed fatigue test technique. Fully reversed cyclic shear stresses are induced at the bond interface, leading to subsequent fatigue lift off failure and revealing the weakest site of the bond. A special focus is set on the role of interfacial intermetallic compounds (IMC) on the fatigue performance of such interfaces. Therefore fatigue life curves were obtained for three representative microstructural states: The as-bonded state is compared to two annealed states at 200 °C for 200 h and at 200 °C for 2000 h respectively. In the moderately annealed state two IMC layers (Al2Cu, Al4Cu9) could be identified, whereas in the highly aged state the original pad metallization was almost entirely consumed and AlCu is formed as a third IMC. Finally, the crack path is traced back as a function of interfacial microstructure by means of electron microscopy techniques.Whereas conventional static shear tests reveal no significant decrease of the bond shear force with increased IMC formation the fatigue tests prove a clear degradation in the cyclic mechanical performance. It can be concluded that during cycling the crack deflects easily into the formed intermetallics, leading to early failure of the ball bonds due to their brittle nature.

Growth of Intermetallic Compounds in Thermosonic Copper Wire Bonding on Aluminum Metallization

Interface evolution caused by thermal aging under different temperatures and durations was investigated by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that approximately 30-nm-thick and discontinuous Cu-Al intermetallic compounds (IMCs) were present in the initial bonds before aging. Cu-Al IMCs grew under thermal aging with increasing aging time. The growth kinetics of the Cu-Al IMCs was correlated to the diffusion process during aging; their combined activation energy was estimated to be 1.01 eV. Initially, Al-rich Cu-Al IMCs formed in the as-bonded state and early stage of aging treatment. Cu9Al4 was identified by selected-area electron diffraction (SAD) as the only type of Cu-Al IMC present after thermal aging at 250°C for 100 h; this is attributed to the relatively short supply of aluminum to the interfacial reaction.

Detailed investigation of ultrasonic Al–Cu wire-bonds: I. Intermetallic formation in the as-bonded state

Scanning and transmission electron microscopy were used to study intermetallic formation and the interface morphology in copper wire-bonds. The ends of copper wires were melted in air and in a protective environment to form wire-balls. The protective environment enabled formation of symmetrical and relatively defect-free copper balls, together with a smaller heat affected zone (in comparison with wires melted in air). Detailed morphological and compositional characterization of the Al–Cu as-bonded interface was conducted using scanning and transmission electron microscopy, on specimens prepared by focused ion beam milling. Discontinuous and non-uniform intermetallics were found in regions where high localized stress was introduced during the wire-bonding process. The main intermetallic phase was found to be Al2Cu.

TEM microstructural analysis of As-Bonded Al–Au wire-bonds

In this study the interface morphology of a model 99.999% (5N) Au wire bonded to Al pads in the as-bonded state was examined by scanning/transmission electron microscopy with energy dispersive spectroscopy. Specimens for transmission electron microscopy were prepared using the lift-out method in a dual-beam focused ion beam system. Analysis of the bond microstructure was conducted as a function of the Al pad content and as a function of the bonding temperature. Additions of Si and Cu to the Al pad affect the morphology and the uniformity of the interface. A characteristic-void line is formed between two intermetallic regions with different morphologies in the as-bonded samples. According to the morphological analysis it was concluded that a liquid phase forms during the bonding stage, and the void-line formed in the intermetallic region is the result of shrinkage upon solidification and not the Kirkendall effect.

ジルコニア／アルミニウム合金の接合強度に対するアルミニウム合金熱処理による寸法変化の影響

High strength aluminum alloys, 2024 and 7075, were joined to zirconia ceramic with buffer layers, respectively, and their strengths were compared. The as-bonded state strength of 2024 aluminum alloy joint was 198MPa and higher than that of 7075 aluminum alloy joint, 112MPa. The reason of the low strength of 7075 aluminum alloy joint was that the difference in thermal co-efficiency and dimensional change of 7075 aluminum alloy was larger than these of 2024 aluminum alloy. The bonding strength of 7075 aluminum alloy joint decreased after aging, because dimensional contraction during aging increased the residual stress.

窒化けい素セラミックとアルミニウム合金接合体の強度に及ぼす低温後熱処理とＭｇ添加の影響

An aluminum alloy was joined to a silicon nitride ceramic with an Al-27.1Cu-4.7Si brazing filler metal at a brazing temperature of 813 K in a nitrogen gas atmosphere. A heat treatment at 723 K (post heat treatment at low-temperature) was performed after the joining. The joint strength was estimated from the shearing strength, and the effects of post brazing heat treatment at low-temperature and Mg content in the aluminum alloy on the joint strength were examined. Though the joint strength in the as-bonded state was very low, it increased to about 65 MPa after post brazing heat treatment at low-temperature for 7.2 ks. The post brazing heat treatment at low-temperature caused the decomposition of Al2Cu layer at the joining interface and the Cu diffusion into the aluminum alloy. Mg addition to the aluminum alloy increased the joint strength. The effects of Mg were the dissolution of an oxide scale on the aluminum surface and the enhancement of Cu diffusion into the aluminum alloy.

Direct Solder Bump Formation Technique on Al Pad and Its High Reliability

A new technique of direct solder bump formation on an Al pad for flip-chip interconnection was developed by using a rapidly solidified Sn-1Ag-1Zn-5Sb alloy wire. The solder balls made from the wire by arc discharge in Ar containing H2 were bonded to an Al-1Si layer by thermosonic bonding at 423 K in air. The addition of Zn and Sb into the Sn-base alloy was effective in obtaining a good contact state between the ball and the Al-1Si layer. The shear strength of the Sn-base alloy bump is 61 MPa in the as-bonded state and remains almost unchanged even after aging for 400 h at 473 K. The fracture morphology also remains unchanged upon aging. Furthermore, there is no distinct increase in the interdiffusion distance between Sn and Al. These results allow us to expect practical use of the direct solder bump formation technique on Al pad with high reliability.

アルミナセラミックスと炭素鋼との拡散接合

The effect of an insert metal on the diffusion bonding of alumina ceramics to carbon steel has been invsetigated with particular reference to the relaxation of thermal stress which is set up in the cooling process from bonding temperature owing to the difference in thermal expansion between the alumina and steel. The insert metal used was a combination of a titanium foil 20μm thick and a copper foil 0.05-4mm thick. All joints obtained were fractured in the alumina on tensile test. However, the fracture strength increased with the increase in the thickness of the copper foil, approaching a saturated value (30MPa) for foil thickness of 2-4mm. In the alumina ceramics cracks were observed in the as-bonded state. The crack decreased in length with the increase of the thickness of the copper foil and could not be observed when the foil thickness was more than 2mm. According to a stress analysis using finite element method (FEM), the crack was initiated at the position where the thermal stress took a maximam value, and the direction of the crack was nearly normal to that of the maximam stress. The analysis using FEM indicated also that the thermal stress decreased with increasing the thickness of the copper foil because the plastic flow of the copper foil relaxed the stress. Consequently, the formation of the crack in the alumina ceramics is attributable to the thermal stress caused by the difference in the thermal expansion between the alumina and steel. The crack and thermal stress are considered to cause the change in the fracture strength of the joint with the thickness of copper foil. A combination of the titanium foil and a nickel foil 2mm thick was also applied as an insert metal. However, the fracture strength of the joint was considerably low.