Recent advances of conductive adhesives as a lead-free alternative in electronic packaging: Materials, processing, reliability and applications

Abstract

Tin–lead solder alloys are widely used in the electronic industry. They serve as interconnects that provide the conductive path required to achieve connection from one circuit element to another. There are increasing concerns with the use of tin–lead alloy solders in recognition of hazards of using lead. Lead-free solders and electrically conductive adhesives (ECAs) have been considered as the most promising alternatives of tin-lead solder. ECAs consist of a polymeric resin (such as, an epoxy, a silicone, or a polyimide) that provides physical and mechanical properties such as adhesion, mechanical strength, impact strength, and a metal filler (such as, silver, gold, nickel or copper) that conducts electricity. ECAs offer numerous advantages over conventional solder technology, such as environmental friendliness, mild processing conditions (enabling the use of heat-sensitive and low-cost components and substrates), fewer processing steps (reducing processing cost), low stress on the substrates, and fine pitch interconnect capability (enabling the miniaturization of electronic devices). Therefore, conductive adhesives have been used in liquid crystal display (LCD) and smart card applications as an interconnect material and in flip–chip assembly, chip scale package (CSP) and ball grid array (BGA) applications in replacement of solder. However, no currently commercialized ECAs can replace tin–lead metal solders in all applications due to some challenging issues such as lower electrical conductivity, conductivity fatigue (decreased conductivity at elevated temperature and humidity aging or normal use condition) in reliability testing, limited current-carrying capability, and poor impact strength. Considerable research has been conducted recently to study and optimize the performance of ECAs, such as electrical, mechanical and thermal behaviors improvement as well as reliability enhancement under various conditions. This review article will discuss the materials, applications and recent advances of electrically conductive adhesives as an environmental friendly solder replacement in the electronic packaging industry.

Introduction

Semiconductor electronics industry has made considerable advances over the past couple decades, while the essential requirements of interconnects among all types of components in all electronic systems remain unchanged. The components need to be electrically connected for power, ground and signal transmissions, where lead-containing solder especially eutectic tin/lead (Sn/Pb) solder alloy has been the de facto interconnect material in most areas of electronic packaging. Such interconnection technologies include pin through hole (PTH), surface mount technology (SMT), ball grid array (BGA) package, chip scale package (CSP), and flip–chip technology [1], [2], [3]. Fig. 1 shows the schematic illustration of bonding between the component and the substrate via interconnect material.

There are increasing concerns with the use of tin–lead alloy solders. First, tin–lead solders contain lead, a material hazardous to human and environment. Each year, thousands of tons of lead are manufactured into various products, especially consumer electronic products (e.g. cell phones, pagers, electronic toys, PDA (personal digital assistant), etc.) which tend to have a short life cycle (2–3 years) and millions of such lead-containing products simply end up in landfills. According to 2001 U.S. Geological Survey, the total lead consumption by the U.S. industries in 2000 was 52,400 metric tonnes. More than 10% of the lead (5430 metric tonnes) was used to produce alloy solders. Worst of all, most of the electronic products have a very short service life. Recycling of lead-containing consumer electronic products has proven to be very difficult. Japan has banned the use of lead in all their new electronic products in January 2005, and European Union plans to ban all imports of lead-containing electronics in July 2006 [4], [5]. In the US, legislations in limiting the use of lead have been introduced in both the Senate and the House of Representatives [6]. In response to the new legislations, most major electronic manufacturers have stepped up their search for alternatives to lead-containing solders.

To date, these efforts have been focused on two alternatives: lead-free solders and polymer-based electrically conductive adhesives (ECAs) [7], [8], [9], [10]. The most promising lead-free alloys contain tin as the primary element, because it melts at a relatively low temperature (232 °C), inexpensive and easily wets other metals. Depending on the applications, a number of lead-free solder alloys have found their way in commercial products [8], [11], [12], [13]. However, most currently commercial lead-free solders, such as tin/silver (Sn/Ag), tin/silver/copper (Sn/Ag/Cu), have higher melting temperatures (T_m of Sn/Ag and Sn/Ag/Cu are 217 and 221 °C, respectively) than of the conventional tin–lead eutectic solder (183 °C). Therefore, the reflow temperature during electronic assembly must therefore be raised by 30–40 °C. This increased temperature reduces the integrity, reliability and functionality of printed wiring boards, components and other attachment, therefore it also severely limits the applicability of these metal alloys to organic/polymer packaged components and low-cost organic printed circuit boards. Although some low melting point alloys are available such as tin/indium (Sn/In, T_m 120 °C), tin/bismuth (Sn/Bi, T_m 138 °C), tin/zinc/silver/aluminum/gallium (Sn/Zn/Ag/Al/Ga, T_m 189 °C) [14], their material properties and processibility in assembly are still of concern. Several review papers have been published on the development and current status of lead-free solders [6], [15], [16], [17], [18], [19], [20].

On the other hand, electrically conductive adhesives (ECAs) mainly consist of an organic/polymeric binder matrices and metal filler. The conductive fillers provide the electrical properties and the polymeric matrices provide the physical and mechanical properties. Therefore, electrical and mechanical properties of ECAs are provided by different components, which is different from the case for metallic solders that provide both electrical and mechanical properties. Compared to the solder technology, ECAs offer numerous advantages, such as environmental friendliness (elimination of lead usage and flux cleaning), mild processing conditions, fewer processing steps (reducing processing cost), and especially, the fine pitch capability due to the availability of small-sized conductive fillers [7], [21], [22], [23], [24], [25], [26]. However, like all lead-free materials, currently commercialized ECAs still have some limitations and challenging properties, such as a lower electrical and thermal conductivity compared to solder interconnects, conductivity fatigue in reliability tests, limited current carrying capability, metal migration fatigue in reliability and high voltage tests, and poor impact strength [7], [8], [21]. Table 1 gives a general comparison between tin–lead solder and generic commercialized ECAs [27].

In recognition of the importance and issues of conductive adhesives, there have been worldwide efforts on the research and development of high performance ECAs in recent years. The efforts so far have been mainly addressed on various material properties and assembly aspects of conductive adhesives. The scope of this review is to summarize some recent activities and advances on electrically conductive adhesives. It should be noted that the research and study on ECAs are still quite active and currently, no commercially available conductive adhesive can replace tin–lead metal solders in all applications, especially for high-power devices such as microprocessors. Our purpose of this review is to have a better understanding of the nature of conductive adhesives as well as the significant progress made on the materials research and development.