Development of gold based solder candidates for flip chip assembly

Abstract

Flip chip technology is now rapidly replacing the traditional wire bonding interconnection technology in the first level packaging applications due to the miniaturization drive in the microelectronics industry. Flip chip assembly currently involves the use of high lead containing solders for interconnecting the chip to a carrier in certain applications due to the unique properties of lead. Despite of all the beneficial attributes of lead, its potential environmental impact when the products are discarded to land fills has resulted in various legislatives to eliminate lead from the electronic products based on its notorious legacy as a major health hazard across the spectrum of human generations and cultures. Flip chip assembly is also now increasingly being used for the high-performance (H-P) systems. These H-P systems perform mission-critical operations and are expected to experience virtually no downtime due to system failures. Thus, reliability of the solder joint is a major critical issue. This reliability is directly influenced by both the phases in the bulk solder and also the intermetallic compounds formed between the solder and the solder wettable layer of the under-bump metallization during both the wetting reaction or/and the solid state ageing. In the present work, an attempt has been made to develop new solder alloys for flip chip assembly using the CALPHAD approach based on gold, the safest element among all the elements being considered for this application. Specifically, efforts have been made to predict the phases in the bulk solder of the promising solder candidates and also the intermetallic compounds formation, using the CALPHAD approach.

Introduction

First level packaging involves interconnecting of the chip to a carrier. The two prominent techniques for doing this are wire bonding and flip chip assembly. Flip chip technology has been widely accepted as a technology for maximum miniaturization and is rapidly replacing wire bonding. Typical applications today are mobile products such as cellular phones or GPS devices [1]. Advanced packaging technologies such as flip chip technology are required because electronic devices are operating faster and becoming smaller, lighter and more functional [2].

Flip chip technology offers several advantages especially for high-density currents because the whole chip surface may be used for a large number of I/O pads in an area array configuration. Short interconnection lengths result in excellent electrical performance of interconnects as well. Flip chip assembly is also being used for automotive diesel control since it is mechanically the most rugged interconnection method. Above all, flip chip technique is probably the lowest cost interconnection for high volume automated production [3], [4]. Due to all these advantages, flip chip assembly are now increasingly being used for high-performance (H-P) systems, i.e. servers, storage, network infrastructure/telecommunication systems [5].

Flip chip assembly requires the usage of high melting point solders for certain applications like those of H-P systems. In these cases, the solder should have a solidus temperature above 270 °C in order to prevent the solder joint within the component from remelting when the component is subsequently soldered during second level packaging. The liquidus temperature should also be below 350 °C, which is determined by the polymer materials used in the substrate. Increasing the lead content and reducing the tin content result in solders with substantially high melting points. Common versions of high lead containing solders currently being used for these applications are Pb–10Sn/Pb–5Sn, having a melting range of 275–302 °C and 308–312 °C, respectively, [6]. Lead in particular is being preferred for this application requiring high reliability because of its unique properties like low melting point, malleability, low surface tension, excellent conductivity, high resistance to corrosion and its tendency of not forming intermetallics with the commonly used solder wettable layer of the under-bump metallization (UBM) [7].

Given the legacy of lead as a pervasive environmental pollutant and the potency of its health impacts, it is not surprising that jurisdictions across the world have tried to limit its use. The current drive to eliminate lead from solder materials began with the concern over water distribution systems where direct population exposure to lead from potable water was demonstrated. Subsequently, the European Union (EU) legislation, “Restriction of the Use of Hazardous Substances (ROHS) in Electrical and Electronic Equipment” (Directive 2002/95/EC) effectively bans the use of lead (Pb) and several other substances in electrical products [8]. However, the European commission (EC) specifically granted a special use of lead in solder exemptions, independent of concentration, for certain applications involving the use of flip chip technology [9]. The intent was to preserve the reliability of flip chip solder joints since it is also being used for H-P systems which perform mission-critical operations, so it is imperative that they maintain continuous and flawless operation over their life time. H-P systems are expected to experience virtually no down time due to system failures [5]. It is still unspecified exactly how long the exceptions, covering the Pb-rich solders, will be in force [10].

Au–20Sn (weight-percent), a eutectic composition with a melting point of 280 °C could be considered for flip chip assembly yet this eutectic alloy is brittle. Many Au based ternary alloys adhering to the solidification criterion generally undergo eutectic or close to eutectic reaction since gold undergoes eutectic reaction with Ge, Sb, Sn and Si. Moreover, gold has been classified as the safest element by both EPA – US (Environmental protective agency – United States) and OSHA (Occupational and safety health administration) among all the elements considered for this application [11], [12]. Gold also possesses some unique properties required for high melting point solders like: good corrosion and oxidation resistance, excellent bio-compatibility and workability, good thermal and electrical conductivity, low natural radius of curvature and the tendency of not spontaneously forming intermetallics with the commonly used solder wettable barrier layer. Thus, gold based ternary systems could probably be a potential substitute for high lead containing solders used in flip chip assembly for certain applications.

The microelectronics industry is extremely cost conscious. The history of the industry has been to continuously produce higher performance at lower costs. Cost competitiveness in the electronics industry is maintained by reducing the cost of individual components to a minimum, in order to maximize the overall cost reduction [13]. Considering gold, as a potential substitute for lead would substantially increase the cost of high melting point solders. Though the initial costs would be higher, but there would potentially be a value to the recycled product and no disposal costs associated with the solders, resulting in lower life time costs for the product. In addition, refining costs becomes important when considering initial material selection. The refining of gold recovered from electronic waste scrap is easier and cheaper than refining of other noble elements [14]. It has been reported that 47% of the total consumption of gold was recovered and successfully recycled in 2007 [15].

Experimental determination of a ternary system is extremely time consuming. If the thermodynamic description of all the binary systems is available, it is possible to estimate a phase diagram of higher component systems from the extrapolation of the binary systems using the CALPHAD method. This method attempts to provide a true equilibrium calculation by considering the Gibbs energy of all phases and minimizing the total Gibbs free energy of the system. One of the most significant advantages of the CALPHAD methodology is that since the total Gibbs energy is calculated, it is possible to derive all of the associated functions and characteristics of phase equilibria. The quality of the extrapolation to a ternary system from the descriptions of the three binary systems depends not only on the accuracy of the calculation of each binary system but also on the magnitude of possible ternary interactions and the occurrence of ternary intermetallic compounds. The probable magnitude of these ternary interactions can be estimated from the properties of the binary systems. From the calculated phase diagrams, the melting temperature range, i.e. the solidification path, as well as the susceptibility to intermetallic formation with various solder wettable layer of the UBM can be estimated [16], [17]. Thus, in the present work, focus has been put on determining all the gold based ternary systems adhering to the solidification criterion of the flip chip assembly, prediction of phases in the bulk solder and the susceptibility of IMCs formation between the solder alloys and the commonly used solder wettable layers like Au, Cu, Ni and Pd.