Michael Clode

Materials and processes for implementing high-temperature liquid interconnects

This paper describes the results of a study investigating liquid solder joints at elevated temperatures (up to 200/spl deg/C). The reactions of eutectic 52In/48Sn solder, which melts at 118/spl deg/C, with various metal barrier layers is presented. The main emphasis of the research was to find a combination of solder and substrate metallization which has good adhesion strength but also remains stable during temperature cycling and high-temperature storage when the solder is molten. Intermetallic growth rates and solder-substrate adhesion strength have been measured for a range of potential barrier layers including Ni, Cr, Pt, Ti, V, Nb, Ta, and W. Of these, only Nb was found to have acceptable properties for a high-temperature barrier layer to In/Sn solder. Other aspects of liquid solder interconnections that have been studied include stability of the molten solder-underfill interface under electrical bias and retention of electrical contact during vibration and phase change. Plastic ball grid array (PBGA) devices have been assembled with Nb barrier layers and liquid solder joints and their reliability during temperature cycling (-20/spl deg/C to +180/spl deg/C) has been compared to PBGA joints with Sn95.5/Ag4/Cu0.5 solder balls.

Dissolution of solids in contact with liquid solder

The dissolution rate of a solid metal such as Cu, in contact with molten solder can be calculated with the use of the Nernst‐Brenner equation. We describe how this equation should be correctly used in cases when the solder is in contact with both the base metal and any intermetallic compounds that have formed. We also show that the concentration of solute in the solder will generally lie between the metastable solubility limit and the equilibrium solubility limit, illustrating these ideas with reference to a system comprising Nb as the base metal and eutectic In‐Sn as the solder, where the concentration levels can be directly correlated to the crystal growth rate.

Cross-Section Preparation for Solder Joints and MEMS Device Using Argon Ion Beam Milling

Mechanical cross-section polishing has traditionally been the method of choice for preparing samples to be examined by scanning electron microscopy (SEM). Although mechanical polishing, allied to selective chemical etching can reveal the most important characteristics of solder joint microstructure, subtle details may be lost. A relatively new cross section polishing method has been developed using an argon ion beam to prepare a flat surface with potentially less sample damage. In this study we compare these two methods of cross section polishing for solder-substrate couples, and for delicate MEMS type structures. Four solder samples were prepared, consisting of SAC (Sn-Ag-Cu) solder, SAC solder on copper substrate, SAC solder on nickel substrate and In-Sn solder on niobium substrate. SEM was used to examine the polished samples and it was found that features such as the internal structure of intermetallic compounds (IMCs) was more readily identified using the new technique. The ion beam milling technique was also found to be more suitable for simultaneous observation of multiple aspects of microstructure (e.g., identification of IMCs in relation to grain boundaries, substrate crystal structure or the eutectic solder structure). The MEMS device cross-sections could only be prepared by the ion beam method as mechanical polishing caused too much damage.

Nanoparticle Enhanced Solders for High Temperature Environments

Nanoparticle enhanced solders have been reported to have superior creep and reliability properties compared to simple alloyed materials. The nanoparticles, typically added at 1-2 wt% concentrations into the solder serve to harden the solder, stabilize the microstructure and improve reliability in high temperature environments. The nanoparticles may be added to the solder before production of solder particles, or added as a separate ingredient of the solder paste. This paper explores the latter approach. For this investigation, nanoparticles composed of a silica dielectric core and Au metallic shell were used, and the efficacy of different synthesis routes compared. In particular, it was found that poly diallyldimethyl ammonium chloride (PDADMAC), served as a better linker molecule than 3-aminopropyltrimethoxysilane (APTMS) for attaching the shell to the core. However, even with solder wettable shells, it was found that the majority of the particles were expelled from the SAC solder during reflow in air, and the causes were examined with the aid of computational fluid dynamics to model the reflow process.

Interfacial Reaction Between Molten Sn-Bi Based Solders and Electroless Ni-P Coatings for Liquid Solder Interconnects

This paper reports on the interfacial reactions and lifetime of electroless Ni-P coatings in contact with molten Sn-Bi based solders. A layer of approximately 4 mum thick electroless Ni-P in contact with the molten Sn-58Bi solder began to fail at 48 h at temperatures between 200degC and 240degC . Elemental additions to modify the solder, included 1-2wt.% of Al, Cr, Si, Zn, Ag, Au, Ru, Ti, Pt, Nb, and Cu. Of these, only Cu modified the interfacial intermetallic compound growth from Ni3Sn4 to (Cu,Ni)6Sn5 , resulting in significantly decreased consumption rates of the Ni-P substrate in contact with the molten solder and increasing the lifetime of the Ni-P layer to between 430 and 716 h. Micro cracks were observed in all but the thinnest Ni-P layers, allowing the solder to penetrate.

Failure Mechanisms of Dummy IGBT Assembles Constructed using Liquid In-Sn/Nb System

Liquid solder joints have previously been proposed in order to improve the reliability of solder joints in general, and especially for those that are operated at elevated temperatures. The solder is designed to melt during high temperature operation, releasing the stresses on the joint. The component will remain mechanically attached to the substrate by use of a polymer underfill or glob-top. Assemblies of dummy insulated gate bipolar transistor (IGBT) devices were constructed using the In-Sn as a low melting point solder and Nb as a barrier layer, on both device bond pads and connecting wires. Silicone and epoxy based adhesives were used as glob top materials, and alumina was used as the substrate. Thermal cycling carried out between -20 and +125 degC lead to rapid joint failure, and analysis of the joints showed that the wires had moved under mechanical tests. Further testing is underway.

Edge effects in intermetallic compound crystal growth between Nb and molten 52In–48Sn solder

A solder/substrate couple consisting of a Nb substrate and eutectic 52In–48Sn solder shows promise for high-temperature liquid solder interconnects. In this letter, the intermetallic compound (IMC) crystal growth between vacuum-sputtered Nb film and molten 52In–48Sn solder at four temperatures; 192, 220, 240, and 260°C, was investigated employing samples with a solder/substrate interface area of 7mm by 7mm. It was found that at all four temperatures, the IMC crystals nucleated and grew first at the corners of the substrate, followed by the edges, and then gradually covered the entire interface. This result, can be explained if the initiation of IMC crystal growth is diffusion controlled, and the value of the Nb diffusion coefficient is estimated with the aid of a three-dimensional numerical simulation.

Interactions between liquid Sn-Bi based solders and contact metals for high temperature applications

Liquid solder interconnects are promising as an alternative approach to conventional high melting point solder interconnects for applications where the operating temperature is likely to exceed 125/spl deg/C. In order to ensure that a liquid solder interconnect remains in contact with the terminations on the component and the substrate, and that electrical contact between them remains unbroken, there must be some growth of an intermetallic compound (IMC) at the interfaces between the solder and the contact metallizations. However, given that IMC growth is generally much faster when the solder is liquid, the growing IMC must act as a strong diffusion barrier to suppress further IMC growth. This paper presents preliminary studies of liquid-phase Sn-Bi based solders that result in stable interfaces between the solders and three common contact metallizations, consisting of electroless Ni(P)/Au, of Cu and of Ti-W. Small quantities (1 or 2 %) of an additional element, including Cr, Si, Zn, Ag, Au, Al and Cu, have been alloyed with the eutectic Sn-Bi composition to find an effective inhibitor additive that can achieve a strong IMC diffusion barrier. IMCs and their growth rates, as well as the consumption rates of the three contact metallizations in contact with the molten solders, were investigated. Storage temperatures of 200/spl deg/ and 240/spl deg/ were used, with storage times ranging between two hours and one month. Results to date show that suitable additives can significantly reduce IMC growth rates for both the Ni(P)-Au and Cu contact metallizations, while no appreciable IMC growth is observed for Ti-W in contact with both the original and the various alloyed Sn-Bi based solders. Based on the current results, criteria to further assist the design of feasible molten liquid solder - contact metallization systems have been deduced.

Benefits of wheel-tool gap sensing in Conform/spl trade/ extrusion machinery

This paper presents the results from the successful production trials of a gap sensing system for Conform/spl trade/ extrusion machinery and associated benefits. It is critical to maintain a precise pre-defined wheel-tool gap for the efficient running of the Conform extrusion process and to maintain product quality. A high-temperature capacitive gap sensing system is designed and implemented on a copper Conform extrusion machine in a production plant. First, it is shown that machine setup times prior to production can be reduced from 35 to 5 min with active gap sensing. The sensors can be used for on-line direct gap measurement and control, and, for the first time, provide a detailed view of extrusion zone gap behavior during a full production cycle. From the experimental results, three different stages of a Conform extrusion cycle, start, slugs feed, and steady-state extrusion, can be clearly identified. The gap sensor is used to evaluate the relationship between gap size, and waste levels. It is shown that there is a linear reduction of waste levels from 20% to 2%, when the gap size is reduced from 1 to 0.15 mm.

Modelling Zener Pinning: A Comparison of Different Computer Simulation Methods

We compare the ability of three different types of microstructural model to simulate particle pinning. The microstructural models are the Phase Field model, the Front Tracking model and the Monte Carlo Potts model. The same 3D test geometry is simulated using each method. This is an hexagonal network with spherical particles located at the centre of each hexagonal grain. The hexagonal grain network provides a constant driving force for a moving boundary and includes triple line and quadruple point motion. This geometry allows detailed investigation of the boundary/particle interaction. The pinning force acting on the migrating curved grain boundary is calculated and compared with theoretical predictions for each model.