Characterization of the melting and wetting of Sn-Ag-X solders

There is tremendous interest at present with Pb-free solder assembly in the surface mount assembly industry in response to recent Japanese and European initiatives and proposed governmental restrictions regarding Pb usage and disposal. Many different solder alloys have been proposed as potential Pb-free solder replacements and the most promising of these fall into the general alloy families of tin-silver (Sn-Ag), tin-silver-copper (Sn-Ag-Cu) and tin-silver-bismuth (Sn-Ag-Bi). Published melting point data on some of these alloys indicates that they should be capable of reduced reflow temperatures relative to the commonly available Sn-3.5Ag alloy, which melts at 221°C. Differential scanning calorimetry (DSC) and reflow visualization was used to characterize the melting and wetting of the Pb-free alloys and generate the practical reflow temperature requirements. This was compared to the DSC data to gain insight on the meaning of the DSC melting data for surface mount applications. The results show that, in general, the wetting performance of the Sn-Ag-Bi alloys are more similar to Sn-Ag and Sn-Ag-Cu than would be predicted by the major onset melting temperature data as measured by the DSC     

Development of conducting adhesive materials for microelectronic applications

Electrically and/or thermally conducting adhesive materials are classified into two categories depending on their conduction modes: isotropic and anisotropic materials. Silver-particle filled epoxy is the most common example of the class of isotropic materials which are conductive in all directions. This material has been long used in the electronic applications as a die-bonding material, where its good thermal conduction rather than its electrical conduction property is utilized. The silver-filled epoxy material has several limitations for high performance electrical interconnections, such as low electrical conductivity, increase in contact resistance during thermal exposure, low joint strength, corrosion issue due to silver migration, difficulty in rework, and so forth. The anisotropic conducting material provides electrical and/or thermal conduction only in one direction. An anisotropic conducting film (ACF) is used for interconnecting TAB mounted chips to a liquid crystal display panel, where fine pitch interconnection and low temperature assembly are required. In this paper, a brief review of the state-of-art conducting adhesive technology is provided. Subsequently, development of new conducting adhesive materials is presented for several different applications, which include high temperature materials for ceramic substrates, and low temperature materials for organic substrates.     

Trends and issues in Pb-free soldering for electronic packaging

A variety of Pb-free solder alloys have been proposed for use as interconnects for electronic packaging including Sn?Ag, Sn?Cu, Sn?Ag?Cu, Sn?Ag?Bi, and Sn?Sb, among others. This paper presents a review of the behavior of promising Pb-free solder alloys as related to their microstructure. Recommendations of optimal alloy composition as a function of performance requirements are given. For surface mount applications, eutectic Sn?Ag?Cu is recommended as the optimal alloy. For flip chip interconnects, the eutectic Sn-Cu alloy has the best performance. The materials and process trends of Pb-free packaging are summarized with optimal conditions identified.     

Study of interfacial reactions between Tin and copper by differential scanning calorimetry

The interfacial reactions at the solder joints of interest in microelectronic structures have been extensively investigated in order to control the soldering process and thereby to provide strong and reliable solder joints. These reactions comprise the dissolution of a base metal into a molten solder, the concomitant growth of intermetallics in the liquid state, and the growth of intermetallics in the solid state. Various experimental techniques employed include conventional cross-sectional metallography, scanning electron microscopy, electron microprobe analysis, x-ray diffraction and others. In this study, the interfacial reaction between tin and copper has been investigated by using differential scanning calorimetry which provides a simple means to study such reactions and complements information obtained from other techniques. The experimental results have been applied to understand the interfacial reactions in two important examples: one for a soldering process with copper metallization, and another for a high conductivity, Pb-free, electrically conducting adhesive which contains tin-coated copper powder as conducting filler particles.     

Development of conductive adhesive materials for via fillapplications

As mobile computing and telecommunication electronics are spreading fast, a demand for microelectronics packaging schemes for high density, fine pitch, high performance and low cost becomes even more severe. Specifically, the conventional printed circuit board (PCB) with plated-through-hole (PTH) vias is difficult to justify its manufacturing cost as well as the packaging density requirement. To meet this challenge, several new manufacturing schemes of high density and high performance PCB have been recently introduced such as surface laminar circuit (SLC), any layer inner via hole (ALIVH) and others. This new PCB fabrication process is categorized as sequential build-up (SBU) or build-up multilayer (BUM) using laminate-based substrates, where via holes are filled with a conductive paste material to make reliable vertical or Z-interconnects. In this paper, a new electrically conducting paste material to be used for via filling is introduced. The new conducting material consists of a conducting filler powder coated with a low melting point metal or alloy, a mixture of several thermoset resins, and other minor organic additives. By varying the filler content and resin chemistry, several formulations have been produced to fill via holes with a high aspect ratio. Via fill experiments have been performed to demonstrate void-free microstructure with good electrical continuity. Various bulk properties such as thermal, electrical and mechanical have also been characterized to understand the material behavior during via filling as well as the field service     

Effects of novel carboxylic acid-based reductants on the wetting characteristics of anisotropic conductive adhesive with low melting point alloy filler

A low viscosity epoxy resin with carboxylic acid-based novel reductants was introduced to improve the process ability and reliability of an anisotropic conductive adhesive (ACA) resin with a low melting point alloy (LMPA) filler system. The curing degree of the ACA resin and the melting of the LMPA filler were investigated using differential scanning calorimetry (DSC) conducted in the dynamic mode in order to control the curing conditions such as the reaction temperature and a melting temperature of solder. The temperature-dependent viscosity characteristics of the ACA resin were investigated using a rheometer. The compatibility between the viscosity of a polymer matrix and a melting temperature of solder was characterized to optimize the processing cycle. Three different types of carboxyl acid-based reductants were added to remove the oxide layer on the surfaces of the filler particles and the conductive pad. Good wetting properties were achieved between the LMPA filler and the Cu pad in the epoxy resin using these carboxylic acid-based reductants.     

Interfacial Reaction between Zn-Al-Based High-Temperature Solders and Ni Substrate

The Zn-Al(-Cu) eutectic alloys (melting point 381°C) are candidates for use as Pb-free high-temperature solders as a substitute for Pb-based solders, which are suitable for severe working environments such as the engine room of hybrid vehicles equipped with an inverter system as well as a heat engine. In this study, the interfacial reaction between Zn-Al(-Cu) alloys and the Ni substrate during soldering, aging, and thermal cycling was investigated. Semiconductor chips and Ni substrates were soldered with Zn-Al(-Cu) alloys at various temperatures under a nitrogen atmosphere. The soldered assemblies were then heat-treated at 200°C and 300°C to examine the microstructural evolution at the soldered interface. The effect of severe thermal cycles between −40°C and 250°C in air on the microstructure and fracture behavior at the solder joint was investigated. Even after a 1000-cycle test, the thickness of the Al₃Ni₂ layer formed at the interface between the Zn-Al-based solder and the Ni substrate, which is responsible for the damage of the soldered assemblies, was quite small.     

Enhanced solder joint bonding strength of electronic packaging with electrowetting effect

This paper reports a novel method to enhance solder ball or solder ring bonding strength by using electrowetting-on-dielectric (EWOD) effect. With a low melting point, the metal Sn has been widely used in electronic packaging technology. Since Sn will be molten into liquid when the temperature is increased above the melting point, the method for treating liquid can be herein employed. Contact angle of the molten Pb-free balls or ring structure on silicon substrate have been experimentally changed by applying electric field across the thin dielectric film between the molten solder and the conductive silicon substrate. The contact area between the solder and the substrate is enlarged due to the decrease of the contact angle. Our testing results on the EWOD enhanced packaging structures of solder balls, flip-chip and solder ring hermetic package generally show about 50% enhancement in bonding shear strength. The significantly enhanced solder link bonding strength is hopeful for improving packaging reliability and is promising to be used in high performance silicon based electronic or microelectromechanic SiP (system in package) technologies.     

Liquid Phase Sintered Solders with Indium as Minority Phase for Next Generation Thermal Interface Material Applications

Because of their high thermal conductivity (K), low melting point (T _m), and low shear strength, indium-based materials are excellent candidates for thermal interface material (TIM) applications. However, high In-content solders are expensive and possess low compressive creep strength, which may lead to structural instability following heat-sink attachment. Here, a radically different approach for producing microelectronic solder TIMs based on liquid phase sintering (LPS) is presented, which not only addresses the above problems, but also paves the way for the development of solder TIMs with even higher K than that of In for next generation packages. LPS Sn-In solders, the microstructure of which consists of particles of the high melting phase (HMP) Sn and a smaller amount of intergranular low melting phase (LMP) In, were processed and characterized. Flow stresses close to that of pure In, and electrical/thermal conductivities approximately half that of pure In, were obtained. LPS solder joints between Cu substrates were produced via a single step process combining LPS with joining. The contact thermal resistance of the internal grain boundaries was estimated, and it is inferred that, because of the numerous internal boundaries, the solder/substrate interfaces have a rela- tively small effect on the joint resistance. Based on the estimated boundary resistance, a previously developed model was utilized to predict the thermal conductivity of the LPS solder as a function of HMP type, volume fraction, and particle size. Preliminary results for LPS solders with Cu as the HMP phase are also presented.     

AgCu28共晶钎料的铺展性研究

针对陶瓷DIP外壳钎焊时,常出现AgCu28钎料流淌的问题,而钎料铺展性对钎料的流淌起着重要作用,为此研究了在不同的生产工艺条件下,AgCu28共晶钎料在镀有不同厚度镍的金属化陶瓷基板和4J42可伐合金片上的铺展性。结果显示钎料在化学镀镍层上比在4J42合金带上的铺展面积大,并存在一个临界镀镍层厚度(0.5~1.0 靘),超过这一临界值,铺展面积显著下降。钎料铺展面积随着焊接温度、时间而改变,控制在钎料熔点以上的停留时间是减少钎料过分铺展的关键。     

A review: On the development of low melting temperature Pb-free solders

Pb-based solders have been the cornerstone technology of electronic interconnections for many decades. However, with legislation in the European Union and elsewhere having moved to restrict the use of Pb, it is imperative that new Pb-free solders are developed which can meet the long established benchmarks set by leaded solders and improve on the current generation of Pb free solders such as SAC105 and SAC305. Although this poses a great challenge to researchers around the world, significant progress is being made in developing new solder alloys with promising properties. In this review, we discuss fundamental research activity and its focus on the solidification and interfacial reactions of Sn-based solder systems. We first explain the reactions between common base materials, coatings, and metallisations, and then proceed to more complex systems with additional alloying elements. We also discuss the continued improvement of substrate resistance to attack from molten Sn which will help maintain the interface stability of interconnections. Finally, we discuss the various studies which have looked at employing nanoparticles as solder additives, and the future prospects of this field.     

Inkjettable conductive adhesive for use in microelectronics and microsystems technology

Ink jet is an accepted technology for dispensing small volumes of material (50–500 picolitres). Currently traditional metal-filled conductive adhesives cannot be processed by ink jetting (owing to their relatively high viscosity and the size of filler material particles). Smallest droplet size achievable by traditional dispensing techniques is in the range of 150 μm, yielding proportionally larger adhesive dots on the substrate. Electrically conductive inks are available on the market with metal particles (gold or silver) <20 nm suspended in a solvent at 30–50 wt%. After deposition, the solvent is eliminated and electrical conductivity is enabled by a high metal ratio in the residue. Some applications include a sintering step. These nano-filled inks do not offer an adhesive function. Work reported here presents materials with both functions, adhesive and conductive. This newly developed silver filled adhesive has been applied successfully by piezo-ink jet and opens a new dimension in electrically conductive adhesives technology.The present work demonstrates feasibility of an inkjettable, isotropically conductive adhesive in the form of a silver loaded resin with a two-step curing mechanism: In the first-step, the adhesive is dispensed (jetted) and precured leaving a ‘dry’ surface. The second step consists of assembly (wetting of the 2nd part) and final curing.     

Development of new no-flow underfill materials for both eutectic Sn-Pb solder and a high temperature melting lead-free solder

In recent years, no-flow underfill technology has drawn more attention due to its potential cost-savings advantages over conventional underfill technology, and as a result several no-flow underfill materials have been developed and reported. However, most of these materials are not suitable for lead-free solder, such as Sn/Ag (m.p. 225/spl deg/C), Sn/Ag/Cu (m.p. 217/spl deg/C), applications that usually have higher melting temperatures than the eutectic Sn-Pb solder (m.p. 183/spl deg/C). Due to the increasing environmental concern, the demand for friendly lead-free solders has become an apparent trend. This paper demonstrates a study on two new formulas of no-flow underfill developed for lead-free solders with a melting point around 220/spl deg/C. As compared to the G25, a no-flow underfill material developed in our research group, which uses a solid metal chelate curing catalyst to match the reflow profile of eutectic Sn-Pb solder, these novel formulas employ a liquid curing catalyst thus provides ease in preparation of the no-flow underfill materials. In this study, curing kinetics, glass transition temperature (Tg), thermal expansion coefficient (TCE), storage modulus (E') and loss modulus (E') of these materials were studied with a differential scanning calorimetry (DSC), a thermo-mechanical analysis (TMA), and a dynamic-mechanical analysis (DMA), respectively. The pot-life in terms of viscosity of these materials was characterized with a stress rheometer. The adhesive strength of the materials on the surface of silicon chips were studied with a die-shear instrument. The influences of fluxing agents on the materials curing kinetics were studied with a DSC. The materials compatibility to the solder penetration and wetting on copper clad during solder reflow was investigated with both eutectic Sn-Pb and 95.9Sn/3.4Ag/0.7Cu solders on copper laminated FR-4 organic boards.     

Wetting and Soldering Behavior of Eutectic Au-Ge Alloy on Cu and Ni Substrates

Au-Ge-based alloys are interesting as novel high-temperature lead-free solders because of their low melting point, good thermal and electrical conductivity, and high corrosion resistance. In the present work, the wetting and soldering behavior of the eutectic Au-28Ge (at.%) alloy on Cu and Ni substrates have been investigated. Good wetting on both substrates with final contact angles of 13° to 14° was observed. In addition, solder joints with bond shear strength of 30 MPa to 35 MPa could be produced under controlled conditions. Cu substrates exhibit pronounced dissolution into the Au-Ge filler metal. On Ni substrates, the NiGe intermetallic compound was formed at the filler/substrate interface, which prevents dissolution of Ni into the solder. Using thin filler metal foils (25 μm), complete consumption of Ge in the reaction at the Ni interface was observed, leading to the formation of an almost pure Au layer in the soldering zone.     

Influence of Surface Segregation on Wetting of Sn-Ag-Cu (SAC) Series and Pb-Containing Solder Alloys

Wetting of Sn-Ag-Cu (SAC) series solder alloys to solid substrates is strongly influenced by surface segregation of low-level bulk impurities in the alloys. We report in situ and real-time Auger electron spectroscopy measurements of SAC alloy surface compositions as a function of temperature as the alloys are taken through the melting point. A dramatic increase in the amount of surface C (and frequently O) is observed with temperature, and in some cases the alloy surface is nearly 80 at.% C at the melting point. The C originates from low-level impurities incorporated during alloy synthesis and inhibits wetting because C acts as a blocking layer to reaction between the alloy and substrate. A similar phenomenon has been observed over a wide range of (SAC and non-SAC) alloys synthesized by a variety of techniques. That solder alloy surfaces at melting have a radically different composition from the bulk uncovers a key variable that helps to explain the wide variability in contact angles reported in previous studies of wetting and adhesion.     

Towards model-based engineering of optoelectronic packaging materials: dielectric constant modeling

Increases in data transmission speeds of optoelectronic devices have consequently increased high-frequency requirements for optoelectronic packaging materials including substrate, EMC/EMI shielding, adhesive and encapsulant (molding and underfill) materials. Most of those materials are polymer/filler composites, and critical materials properties for the device design and packaging include the effective dielectric constant, dielectric loss and their frequency and filler concentration dependence. This work presents a systematic theoretical investigation of the effective dielectric constant of polymer/filler composite materials, and its dependence on the filler concentration, the filler/polymer interaction, and the size of fillers. Our results demonstrate that, in contrary to the prevailing views, the filler concentration dependence of the effective dielectric constant is non-monotonic. Depending on the dielectric constant ratio between filler and polymer matrix, and the degree of interaction between filler and matrix, the effective dielectric constant exhibits an extreme as a function of filler concentration. In addition, our model is demonstrated to contain the Maxwell–Wagner formulation as an asymptotic limit. The present results have significant implications to the targeted formulation of optoelectronic packaging materials.     

Thermodynamic database on microsolders and copper-based alloy systems

Recent progress on the thermodynamic databases of calculated phase diagrams in microsolders and Cu-based alloys is presented. A thermodynamic tool, Alloy Database for Microsolders (ADAMIS), is based on comprehensive experimental and thermodynamic data accumulated with the calculation of phase diagrams (CALPHAD) method and contains eight elements, namely, Ag, Bi, Cu, In, Sb, Sn, Zn, and Pb. It can handle all combinations of these elements and all composition ranges. The elements of Al and Au have also been added to ADAMIS within a limited range of compositions. Furthermore, a database of Cu-based alloys, including binary (Cu-X), ternary (Cu-Fe-X, Cu-Ni-X, and Cu-Cr-X), and multicomponent (Cu-Ni-Cr-Sn-Zn-Fe-Si) systems, has also been developed. Typical examples of the calculation and application of these data-bases are presented. These databases are expected to be a powerful tool for the development of Pb-free solders and Cu substrate materials as well as for promoting the understanding of the interfacial phenomena between them in electronic packaging technology.     

Development of Bi-base high-temperature Pb-free solders with second-phase dispersion: Thermodynamic calculation, microstructure, and interfacial reaction

Bismuth and its alloys are candidates for Pb-free high-temperature solders that can be substituted for conventional Pb-rich Pb−Sn solders (melting point (mp) = 573 – 583 K). However, inferior properties such as brittleness and weak bonding strength should be improved for practical use. To that end, BiCu−X (X=Sb, Sn, and Zn) Pb-free high-temperature solders are proposed. Miscibility gaps in liquid BiCu−X alloys were surveyed using the thermodynamic database ADAMIS (alloy database for micro-solders), and compositions of the BiCu−X solders were designed on the basis of calculation. In-situ composite solders that consist of a Bi-base matrix with fine intermetallic compound (IMC) particles were produced by gas-atomizing and melt-spinning methods. The interfacial reaction between in-situ composite solders and Cu or Ni substrates was investigated. The IMCs at the interface formed a thin, uniform layer, which is an appropriate morphology for a reliable solder joint.     

低熔点合金在电连接头中的应用性能研究

针对输配电线路电连接头发热导致能源损耗甚至引发事故的问题,文中研究了低熔点合金在电连接头中的应用性能。选用在铜界面上润湿性较好的低熔点合金Sn-3.5Ag、Sn-1.6Cu、Sn-Zn-0.7Cu,设计其在电连接中的连接工艺并制作试样。利用ERESCO MF4便携式X光机判断连接后界面的可靠性,再搭建SLQ-2000A升流实验系统并模拟实际工况。最后,测试了低熔点合金和电力复合脂试样在额定通流值为180 A时,持续通流时间4 h过程中试件的发热情况。研究结果表明,低熔点合金作为电连接接触面材料后试样的发热量均低于电力复合脂连接试样的发热量。     

Interfacial reaction studies on lead (Pb)-free solder alloys

Recently, the research and development activities for replacing Pb-containing solders with Pb-free solders have been intensified due to both competitive market pressures and environmental issues. As a result of these activities, a few promising candidate solder alloys have been identified, mainly, Sn-based alloys. A key issue affecting the integrity and reliability of solder joints is the interfacial reactions between a molten solder and surface finishes in the solder joint structures. In this paper, a fundamental study of the interfacial reactions between several Pb-free candidate solders and surface finishes commonly used in printed-circuit cards is reported. The Pb-free solders investigated include Sn-3.5 Ag, Sn-3.8 Ag-0.7 Cu, and Sn-3.5 Ag-3.0 Bi. The surface finishes investigated include Cu, Au/Ni(P), Au/Pd/Ni(P), and Au/Ni (electroplated). The reaction kinetics of the dissolution of surface finishes and intermetallic compound growth have been measured as a function of reflow temperature and time. The intermetallic compounds formed during reflow reactions have been identified by SEM with energy dispersive x-ray spectroscopy.