Transient Liquid Phase Sintering Research Articles

A Comparison Between Solders & Transient Liquid Phase Sintered Interconnects in High Temperature Multi-Layer Ceramic Capacitors

Abstract There is a long-established market for high temperature multi-layer ceramic capacitors (MLCC) that operate at 150°C and higher in down-hole oil & gas exploration, military and aerospace applications. To maximize the capacitance density and achieve a high degree of mechanical robustness, stacks and leaded form factors have been used with High Melting Point (HMP) Pb-containing solders as the preferred interconnects. However, Pb-containing solders are limited to temperatures below 300°C and are banned from many commercial and automotive applications with further legislation limiting their use planned in the future. Common Pb-free solders such as SAC 305 or SnSb alloys are in widespread use but their performance at prolonged exposures at 200°C is limited. Exposures to high reflow temperatures during assembly, especially successive reflow operations, can also compromise interconnect integrity. Higher temperature gold-containing solders are widely available but these are cost prohibitive and so are not viable for emerging high temperature electronics including higher volume, price sensitive Automotive and Power markets. The development of more energy efficient power converters and inverters based on Wide Band Gap (WBG) semiconductors is driving the adoption of higher temperature electronics in these markets since these operate at higher junction temperatures than traditional silicon. This has led to the development of non-solder interconnects based on sintered silver, nano-metal sintering and transient liquid phase sintering (TLPS) technologies capable of higher temperature performance than common solder based interconnects. The availability of discrete components, such as capacitors, that can operate under these conditions is a key barrier to the development and adoption of high temperature electronics. In this paper the key property differences between solders and TLPS interconnect technologies are compared in detail for MLCC interconnects. The development of a new range of nickel Base Metal Electrode C0G MLCC stacks rated for 200°C is described and performance compared to traditional Precious Metal Electrode (PME) stacks. Thermal cycling performance to 200°C of BME X7R stacks made with 10Sn/88Pb/2Ag solders are compared to similar stacks made with TLPS interconnects of Cu-Sn and In-Ag. The development of leadless stacks, a new bulk capacitance form factor enabled by TLPS technology, is described and their properties compared to traditional stacks.

Microstructure stability of TLPS interconnects in high operating temperature applications

Abstract Interconnect materials for high operating temperature applications are becoming a limiting factor within the chain of materials. While materials such as capacitor dielectrics, semiconductor platforms (e.g. SiC), and baseplate materials (e.g. SiN composites) have paved a pathway to deploying electronics in high operating temperature applications, interconnect materials are a clearly identified weak link. As is often the case in advancements in technology, the materials technologies that were the bottlenecks to advancement give way to new solutions that create new bottlenecks in the material supply chain. Rather than a fluid march towards advancements in new frontiers in electronics, the high operating temperature sphere, like much of advanced electronic, suffers from a ‘slip-fault’ mode of development where advances occur in one segment while others lag behind creating drag on implementation. For high operating temperature applications the available interconnect solutions are becoming the jarring stop to the smooth tectonic shift. Current solutions are diverse: high-lead, gold-based, and nano-sintering and its hybrids, but none are ideal. Even disregarding he toxicity of lead and the ongoing limbo of its regulatory status, the operating temperatures of the high-lead solders are on the low end of the requirements for future harsh environment electronics applications; whereas, the gold and nano-based alternatives have significant cost barriers - either at from the constituent materials perspective or the required investment in new processes. There is also the concern about the assessment of the action of nanomaterials in the waste stream due to their fundamentally different surface reactivity in a variety of situations. Reliance on conventional, solder-type interconnection structures, regardless of composition, introduces the perennial problem of the growth of the interfacial phases due to the essentially unlimited volume of the bulk solder material. The changes in the interfacial structure with additional thermal work - as is provided by high operating temperature applications - creates an environment that is ripe for growth of a variety of failure mechanisms. These failure mechanisms are often related to the uncontrolled laminar growth of intermetallic phases at the interfaces and the mechanical characteristics of these intermetallic phases in comparison with the materials joined and the bulk constituent material of the solder. An alternative class of interconnect materials, transient liquid phase sintering (TLPS) pastes, introduce a joint microstructure that is homogeneous throughout. The interfacial metallurgical reactions with the solderable surfaces are fundamentally similar to those that occur throughout the bulk of the joint. A reactant metal is included in the composition. This reactant metal, most often copper, reacts with and converts the bulk tin in the bulk of the solder interconnect to alloy structures with melting points well above the operating temperatures currently contemplated. At the conclusion of the joining process, which is generally a near drop-in for existing solder reflow processes, there is no large source of unreacted metal (e.g. Sn) that can continue to drive major microstructural changes with the continued thermal work provided by the application environment. For this reason the joints are homogeneous and do not have the free reactants necessary to drive substantial changes in joint morphology during cycling and use conditions. In this paper, we will explore the differences between TLPS joints and solder-type joints with the anticipated thermal work that would be introduced in a high operating temperature environment.

Boron sources as sintering additives for alumina-based refractory castables

Abstract Sintering additives (SA) have great potential to be used in refractories to enhance their thermo-mechanical properties at intermediate temperatures (600–1200 °C). Boron-based compounds are pointed out as suitable options for this purpose as they undergo phase transformations (i.e., decomposition, oxidation, etc.) at relatively low temperatures, which may lead to boron-rich liquid phase generation and transient liquid phase sintering. Considering these aspects, the present study aims to evaluate the role of five different SA [boron oxide (B 2 O 3 ), boric acid (H 3 BO 3 ), sodium borosilicate (BS), magnesium borate (BM) and boron carbide (B 4 C)] when 0.5, 1.0 and 2.0 wt% of these materials were added to alumina-based castable compositions bonded with hydratable alumina. Flowability, apparent porosity, XRD, hot elastic modulus, thermal shock resistance and mechanical tests were carried out in order to identify whether (i) transient liquid phase sintering would be favored in these refractories, and (ii) at which temperature range such compositions might be submitted during service and still present an improved performance when compared to an additive-free castable (reference). Based on the results, incorporating boron oxide into the alumina-based castables was not recommended as this hygroscopic material reduced the flowability of the compositions, preventing a suitable preparation of the samples. The other boron-containing compounds could be directly added to the high-alumina mixtures without resulting in major changes in their processing steps. Nevertheless, the paper highlights the importance of selecting a proper SA source and optimizing its content in the castable composition as these parameters may affect its overall rheology and thermo-mechanical properties. The formulations containing 0.5 wt% of H 3 BO 3 or B 4 C, 1 wt% of sodium borosilicate and 2 wt% of magnesium borate presented enhanced erosion and thermal shock resistances and hot mechanical strength at 1000 °C and 1200 °C, which makes them potential refractory castables to be used in petrochemical and aluminum industries.

A New Method to Fabricate Fe-TiC Composite Using Conventional Sintering and Steam Hammer

The aim of this research paper is to fabricate a Fe-TiC composite by a novel and simple manufacturing method. The latter is based on two cumulative processes; a conventional sintering (transient liquid phase sintering) and a hot forging with steam hammer respectively. The blinder phase of the studied simples is varied from carbon steel to high alloy steel using alloying additive powders. The obtained outcomes showed that after the sintering process, the relative density of the performed simples is improved from 86% to 95.8% without any densification process. Otherwise, in order to ensure maximum densification and enhance in addition the solubility of the alloying additives the hot forging process is then applied. Indeed, the final obtained composite product is a TiC-strengthened steel with a relative density around 99% (about 6.5 g/cm3 of density) wherein 30% (wt.) of spherical and semi-spherical TiC particles are homogeneously distributed in the metal matrix.

Low temperature and low pressure bonding of plateless Cu–Cu substrates by Ag-based transient liquid phase sintering

Bonding silicon carbide/gallium nitride (SiC/GaN) based power modules, particularly epoxy-molded modules to heat-substrate and/or heat sink, requires low processing temperature preferably lower than 250 °C, and low pressure as low as 0.1 MPa to prevent damage to the modules. In addition, due to the impracticality of depositing metal-plating to the epoxy-molded module, bonding of plateless Cu-substrates is in great demand. Furthermore, post-processing residual flux cleaning, which is costly and unfavorable to industry need to be avoided as possible by opting out the usage of flux. Up to authors’ knowledge, our study is the first to fulfill all requirements stated above. Transient liquid phase sintering (TLPS) of mixed fluxless Sn–Bi (tin–bismuth) eutectic alloy and Ag (silver) particles was applied to bond plateless Cu–Cu substrates. Sintering temperature of 250 °C, sintering pressure of 0.02 MPa, and reducing environment were applied during processing. The effects of addition amount of Sn–Bi and sintering holding time to the shear strength and microstructure were investigated. The remelting temperature after sintering was also examined. Shear strength of 30 wt% added Sn–Bi was over than 20 MPa, which qualify the requirement of MIL-STD-883K, and larger than conventional Pb-based solder. Formation of intermetallic compounds are thought to strengthen the interface and matrix. Remelting temperature shifted from eutectic temperature of Sn–Bi to approximately 262 °C, allowing the application of modules at higher operating temperature than the processing temperature.

A Transient Liquid Phase Sintering Bonding Process Using Nickel-Tin Mixed Powder for the New Generation of High-Temperature Power Devices

A transient liquid phase sintering (TLPS) bonding process, Ni-Sn TLPS bonding was developed for the new generation of power semiconductor packaging. A model Ni/Ni-Sn/Ni sandwiched structure was assembled by using 30Ni-70Sn mixed powder as the reactive system. The results show that the bonding layer is composed of Ni3Sn4 and residual fine Ni particles with a small amount of Ni3Sn2 at 340°C for 240 min, which has a heat-resistant temperature higher than 790°C. The microstructural evolution and thermal characteristic of the bonding layer for various times at 300°C and 340°C were also studied, respectively. This reveals that, after isothermally holding for 240 min at 300°C and for 180 min at 340°C, Sn has been completely transformed into Ni-Sn intermetallic compounds (IMCs) and the bonding layer is mainly composed of Ni3Sn4 and residual Ni particles. The analysis result for the mechanical properties of the joint shows that the hardness of the bonding layer at 340°C for 240 min is uniform and that the average value reaches 3.66 GPa, which is close to that of the Ni3Sn4 block material. The shear test shows that, as the holding time increases from 60 min to 180 min at 340°C, because of the existence of Sn, the disparity of shear strength between room temperature and 350°C is large. But when the holding time is 180 min or longer, Sn has been completely transformed into Ni-Sn IMCs. Their performances are very similar whether at room temperature or 350°C.

Transient Liquid Phase Sintering Pastes in Heterogeneous Integration

Integrated package technologies continue to be the dominant trend in the electronics packaging industry. In particular, heterogeneous integration of logic and memory or sensing is an enormous growth segment for both mobile electronics and IoT applications. In the mobile microprocessor segment of the field, the most advanced technologies will be implemented in the early adopter class. New package architectures and interconnect schemes will be vetted and implemented without significant cost pressure, performance is the driver. In the IoT segment and downstream mobile, however; lower cost alternatives to cutting edge packaging architectures are needed to drive market growth. Sintering pastes offer an opportunity to cost-effectively enable cutting edge 3D package capability for a wider variety of applications. In this paper we will explore the use of transient liquid phase sintering (TLPS) pastes in package-on-package (POP) schemes for integrated logic with memory or sensing functions in through mold via architectures. Through mold via technology has been well established in the industry and has significantly contributed to the adoption of three dimensional packaging architectures. The advantages of using TLPS pastes in similar structures will be detailed.

Development and Evaluation of Equipment Enhancements for Transient Liquid Phase Bonding (TLPB) and Sintering

Operating devices at higher temperatures in automotive applications and the use of wide bandgap materials have driven the development of new bonding technologies as silver sintering and transient liquid phase bonding (TLPB). Considering their specific demands a dedicated tool for processing in batch was developed and characterized. Press unit containing a flexible elastomer membrane was implemented in a vacuum reflow soldering chamber. It allows bonding of various devices with different thickness without using height compensating tooling. Pressures up to 0.5 MPa, temperatures of maximum 300 °C and reducing atmosphere were utilized to demonstrate the process capability for void free low pressure silver sintering and Cu-Sn TLPB.

Transient liquid phase sintering materials (TLPS) vs. conventional solders for high temperature MLCC interconnects.

Abstract There is a growing need within the electronics industry for interconnect materials that can meet higher processing and operating temperatures. The solidus to liquidus range of current tin (Sn) based solders and other lead (Pb) solders are narrower than traditional SnPb solders as the industry is moving to higher temperatures. Transient Liquid Phase Sintering (TLPS) is a technology that offers low processing temperatures while having high temperature capabilities but has had limited exposure outside of the Semiconductor industry. Results of mechanical testing as well as product environmental data is presented herein as TLPS is used as a mechanical and electrical interconnect for Multi-Layer Ceramic Capacitors (MLCC).

Alternative Metallurgies for MEMS Lid Seal

Abstract MEMS devices are a segment of the electronics industry enjoying explosive growth. With the advent of the internet of things (IoT), the trajectory for growth will continue to be steep and price pressures on both the devices and packaging will be intense. Current MEMS packaging schemes rely heavily on an architecture that employs a metal-can or laminate “lid” attached to the device package with a solder ring. This ring serves to connect the package elements together mechanically and to protect the device from noise (electronic or acoustic) and environmental damage. Like most modern electronic devices, MEMS devices are subject to the challenges imposed by the requirement for lead-free packaging. As with many devices, solder connections formed within the package have a similar composition, and therefore a similar reflow profile, to the solder used to attach the packaged components to a substrate. Devices that yield well at the manufacturer are thus susceptible to field failure at the assembly site induced by remelting of the solder within the package during assembly reflow. Unlike in SIP packages where the solder joints in the module are often encased in mold compound, the ring seal in MEMS devices is exposed and unreinforced resulting in failures such as lid shift, solder flow or flux contamination in the device cavity, and package deconstruction - particularly during rework. These type of post-fabrication yield losses can be very costly. Transient liquid phase (TLPS) sintering paste compositions are a new class of solder replacement materials that can be processed at typical reflow temperatures, but which do not remelt when subjected to subsequent thermal excursions. TLPS pastes combine solder alloy particles and reactive metal particles in proportions such that the solder ‘thermosets’ during a typical solder reflow cycle. This ‘thermosetting’ behavior results in a joint that does not remelt at the original reflow temperature, and thus enables the highly reliable electrical interconnect essential for assemblies that will undergo subsequent reflow excursions. TLPS pastes are similar to solders in many respects. The electrical, thermal and mechanical performance is similar to conventionally used tin-based solders. The TLPS pastes are stored and applied like solder pastes. The reflow cycles used to form the TLPS paste joints are also similar to those for forming solder joints. However, unlike solder, TLPS materials do not change shape during reflow or wet substrates beyond the deposition footprint. Also unlike solder, the metals within the bulk of the TLPS interconnect react with one another resulting in a stable interconnect with a thermally robust bonding structure to joining surfaces, even after subsequent thermal cycles. These attributes present both unique opportunities and unique challenges for addressing MEMS lid seal applications. This paper will explore the key material and engineering challenges for implementation of TLPS paste in MEMS lid seal applications.

High Aspect Ratio Bumping Process with Solder Bumps Included Sintered Pillars

Abstract This study provides new method for fine pitch bumping described as sintered pillar (SP) bumping method, which enables the formation of high aspect ratio bumps for stable assembly process. The SP bumps include sintered pillars formed by the transient liquid phase sintering process, and both SP bumps and sintered pillars are used with conventional paste printing process and equipment. By optimizing the sintered pillar paste, we achieved approximately 25% increase in bump height compared to those formed by the conventional paste printing process. We propose that the SP bumping method is one of the solutions for fine pitch bumping.

Characteristics of copper-doped SrFe0.75Mo0.25O3 − δ ceramic as a cathode material for solid oxide fuel cells

In this study, the densification, microstructure and electrical properties of SrFe0.75Mo0.25−xCuxO3−δ ceramics were investigated. The densification of SrFe0.75Mo0.25O3−δ ceramics were significantly enhanced by Cu substitution through transient liquid phase sintering at high temperatures. Rietveld refinement of the diffraction data confirmed the structure as an orthorhombic (Pnma) perovskite and no secondary phase was observed. 7.6% and 45.0% of Fe and Mo in the SrFe0.75Mo0.25O3−δ ceramic were in the valence states of 3+ and 5+, respectively, while these percentages in SrFe0.75Mo0.05Cu0.20O3−δ ceramic increased to 13.8% and 50.2%, respectively. On the other hand, 92.4% and 55.0% of Fe and Mo for the former were in 2+ and 6+ valence states respectively. However for the later these percentages dropped to 86.2% and 49.3% respectively. The calculated δ value increased from 0.46 for the SrFe0.75Mo0.25O3−δ ceramic to 0.87 for the SrFe0.75Mo0.05Cu0.2O3−δ ceramic, indicating that the concentration of oxygen vacancies increased with Cu substitution. The average thermal expansion coefficient was approximately 21.6×10−6K−1 for the SrFe0.75Mo0.05Cu0.2O3−δ ceramic in the temperature range from 25 to 600°C, and larger than that of the SrFe0.75Mo0.25O3−δ ceramic (14.4×10−6K−1). The electrical conductivities of SrFe0.75Mo0.25−xCuxO3−δ ceramics were consistent with the small polaron hopping conduction mechanism at low temperatures and with a metallic behavior above the transition temperature of 600°C attributed to loss of lattice oxygen. The electrical conductivity increased by more than three times from 36.5S·cm−1 for x=0 to 135.6S·cm−1 for x=0.2 at 600°C, due to an increase in oxygen vacancies, the change in the proportions of Fe2+/Fe3+ and Mo6+/Mo5+ redox couples, and the increase in the grain size in the microstructure.

Transient liquid phase spark plasma sintering of B4C-based ceramics using Ti-Al intermetallics as sintering aid

Fully-dense B4C-based ceramics were fabricated using Ti-Al intermetallics as sintering aid by Spark Plasma Sintering at a low temperature of 1700°C whilst applying 32MPa uniaxial pressure. The influence of the intermetallic additions on the phase composition, morphology and mechanical properties of the ceramics were investigated. The intermetallics flowed and react with B4C to form a three-phase composite containing B4C, TiB2 and Al4C3 during the transient liquid phase sintering process. The TiB2 and Al4C3 particles were located on the boundaries between B4C grains and effectively inhibited the growth of B4C, which improved the mechanical properties. Good bonding between B4C and the phases formed was confirmed by TEM analysis. The specimens with 5wt.% Ti-Al provide an attractive combination of homogeneous morphology and excellent mechanical properties, including a Vickers hardness of 33.5±0.4GPa, a flexural strength of 506±16MPa and a fracture toughness of 5.5±0.1MPam0.5.

Enhanced pressureless bonding by Tin Doped Silver Paste at low sintering temperature

The nanosilver sintering die-attach technique has been a promising alternative for wide band gap semiconductors. However, it is less preferable in industry because of its high sintering temperature. Recently research has been initiated to develop transient liquid phase sintering (TLPS) solder paste for use in electronics packaging. In this article, in order to lower the sintering temperature of nanosilver paste, we develop a novel tin (up to 10wt%) doped silver paste (TDSP) and a sintering profile with the highest processing temperature of 235°C based on TLPS. Sintered TDSP is Ag/Ag3Sn/Ag–Sn solid solution composites. The composites have a microstructure of Ag matrix grains reinforced by Ag3Sn and Ag–Sn solid solution within the matrix grains. And this microstructure endows the sintered Ag+4%Sn with a pressureless bonding strength of 23MPa. The improved mechanical properties of sintered TDSP are attributed to second-phase strengthening and solid solution strengthening mechanisms. However, the overmuch formation of brittle Ag3Sn phase is the main reason resulting in sharp decrease of bonding strength when the Sn content over 5wt%. The new TDSP technology is expected to be applicable to a wide range of power semiconductors devices, such as organic devices and printed circuit boards. Furthermore, it provides new strategies for low-temperature sintering.

Development of Novel Pre-alloyed PM Steels for Optimization of Machinability and Fatigue Resistance of PM Components

It is well known that a large proportion of ferrous PM components require secondary machining operations for dimensional conformance or for producing geometrical features that cannot be generated during die compaction. Nevertheless, the machining behavior of PM parts is generally characterized as being “difficult” due to the presence of residual porosity that lowers thermal conductivity and induces interrupted cutting. Several admixed additives such as MnS and BN-h can be used to improve the machining behavior of PM steels. Nevertheless, their negative effect on mechanical properties, especially fatigue resistance, makes their utilization uninteresting for the fabrication of high-performance PM steel components. This article summarizes the work carried out to develop a novel PM steel that was especially engineered to form machinability enhancing precipitates. This new material is pre-alloyed with tin (Sn) in order to form Cu-Sn (Cu(α)) precipitates during transient liquid phase sintering. The newly developed material presents machinability improvement of 165% compared to reference material used in the PM industry as well as increases in toughness and fatigue resistance of 100% and 13%, respectively.

On the sintering mechanisms and microstructure of aluminium–ceramic cenospheres syntactic foams produced by powder metallurgy route

Metal matrix syntactic foams of ceramic (mullite) cenospheres (10–40% v/v) embedded in pure aluminium matrix, were fabricated by powder metallurgy technique at different sintering temperatures (610–710°C). Density, vis-a-vis porosity and microstructural characteristics were examined by using scanning electron microcopy and energy dispersive X-ray spectroscopy. A first systematic description of the sintering mechanism of the composites is presented. The in situ reaction between silica (phase of mullite) and aluminium particles, apart from any degradation of the mullite cell wall and the precipitation of silicon in the matrix, changes the sintering mechanism of syntactic foams due to the formation of eutectic aluminium–silica liquid quantities. Transient liquid phase sintering, with surface material diffusion, and liquid phase sintering, with bulk diffusion, selected to be the main sintering mechanisms, are described in detail for sintering temperatures below and above the melting point of aluminium matrix, respectively.

FEEDSTOCK FLOW CHARACTERIZATION AND PROCESSING OF POROUS NITI BY METAL INJECTION MOULDING (MIM)

Porous NiTi alloy with a nearly three-dimensionally interconnected pore structure has been successfully fabricated by a transient liquid phase sintering following the metal injection moulding (MIM) process, using a mixture of Ni and Ti elemental powders. The elemental Ni and Ti powders mixture was mixed with a binder system, comprised mainly polyethylene-glycol (PEG) in an alternative technique using a speed mixer, principally incorporating a dual asymmetric centrifuge (DAC). The powder-binder mixture was then characterized using a capillary rheometer at various temperatures and shear rates. It was found that the feedstock exhibited pseudo-plastic behaviour, which is favourable for the MIM process. A temperature range of 120oC - 130oC was considered as the optimum operating condition for the injection moulding processing. The parts were moulded into cylindrical shapes, leached in warm water (60oC for 10 hours), thermally debound in argon and subsequently sintered in a vacuum furnace at four different temperatures ranging from 950oC to 1250oC. All samples underwent expansion in both diameter and height after water leaching and sintering. The XRD results showed that increasing the sintering temperature resulted in a major fraction of the B2 NiTi phase due to phase homogenization and subsequently decreased the amount of secondary phases such as NiTi2 and Ni3Ti. Besides that, the formation of the transient liquid phase during sintering enabled major fraction of pores to be developed with porosity and average size of 39 - 45% and 100 - 120 μm, respectively. The porous parts produced have a great potential to be used as an implant in biomedical applications.

Application of transient liquid phase sintering (TLPS) interconnect material for high temperature Pb-free RoHS compliant MLCC lead attachment

Transient Liquid Phase Sintering is a process that provides high temperature Pb-free RoHS compliant interconnect solutions that exceed the high temperature capabilities of Pb-Sn solders. KEMET, working in collaboration with Ormet Circuits Inc. has successfully applied Ormet's TLPS technology to a line of Leaded Multi-layer Ceramic Capacitors (MLCC) components for high temperature applications. The material is Pb-free, RoHS compliant and able to withstand process and operating temperatures > 400°C while having initial processing temperatures of less than 300°C. Potential applications for TLPS are in the automotive, aerospace, oil, gas, and geothermal exploration industries where electronics are being exposed to higher operating temperatures and require robust interconnects capable of withstanding harsh environments.

Transient Liquid Phase Sintering Pastes as Solder Alternatives in High Temperature Applications

High operating temperature electronics are a growing market in the electronics industry. Most of the components and substrates necessary to support the production of these harsh environment devices are now available, but the interconnect materials for component, module and board level assembly are lagging in development. Currently the high operating temperature markets are being served by lead-bearing solders and expensive alloys such as gold-tin or gold-germanium. Lead has been banned from the majority of electronics applications in many areas of the world, but so far has been exempted in the high temperature solder applications due to the lack of an adequate replacement. Even without the impending regulatory restrictions, the melting temperature of the lead containing solders is marginal for the next generation of high operating temperature electronics. Transient liquid phase (TLPS) sintering paste compositions are a new class of solder replacement materials that can be processed at typical reflow temperatures, but which do not remelt when subjected to subsequent thermal excursions. TLPS pastes combine solder alloy particles and reactive metal particles in proportions such that the solder ‘thermosets’ during a typical solder reflow cycle. This ‘thermosetting’ behavior results in a joint that does not remelt at the original reflow temperature, and thus enables the highly reliable electrical interconnect essential for high operating temperature applications. TLPS pastes are similar to solders in many respects. The electrical, thermal and mechanical performance is similar to conventionally used tin-based solders. The TLPS pastes are stored and applied like solder pastes. The reflow cycles used to form the TLPS paste joints are also similar to those for forming solder joints. However, unlike solder, TLPS materials do not change shape during reflow or wet substrates beyond the deposition footprint. Also unlike solder, the metals within the bulk of the TLPS interconnect react with one another resulting in a stable interconnect with a thermally robust bonding structure to joining surfaces, even after subsequent thermal cycles. Though not a practical replacement for solder in common assembly operations, TLPS technology is an attractive solder alternative for specialty interconnect applications. In this paper, the nature and characteristics of TLPS pastes will be will be explored in comparison to solder paste materials commonly used in high operating temperature applications.

Microstructure of Transient Liquid Phase Sintering Joint by Sn-Coated Cu Particles for High Temperature Packaging

We develop a transient liquid phase sinter (TLPS) bonding using Sn-coated Cu micro-sized particles. With this bonding process, a thermally stable joint comprising Cu3Sn phase and a dispersion of ductile Cu particles can be obtained. The particle paste, which contained Cu particles with a thin Sn coating and terpineol, was used to join Cu substrates. The setup was bonded at 300 °C for 30s under an applied pressure of 10 MPa using a thermo-compression bonding system under a formic acid gas atmosphere for reducing the oxide layer on the Sn coating and the Cu substrate. After bonding, the TLPS joint showed a thermally stable microstructure with a good shear strength, which was fully consisted of Cu3Sn intermetallic compounds matrix and embedded ductile Cu particles. The kinetics of the microstructure transformation and high temperature reliability of the TLPS joint were investigated. After 300 °C isothermal aging for 200h, the shear strength and microstructure of the TLPS joints showed almost unchanged. The results demonstrate that joint with high-melting-point obtained by the TLPS bonding using Sn-coated Cu particle paste has the potential to fulfill the requirement of high temperature electronic packaging.