Reliability and interfacial reaction of lead-free solder alloys doped with rare earth elements

Abstract

Solder is widely used as interconnection materials such as between the silicon die (or chip) and packaging substrate, and between electronic components and printed circuits board (PCB). As a joining material, solder provides electrical connection, thermal dissipation and mechanical support in electronic assemblies. The performance and quality of the solder joint are crucial to the overall functions of all electronic devices and circuits. Eutectic Sn-37wt%Pb (Sn-Pb) solder has been used extensively in the assembly of modern electronic circuits. Due to the inherent toxicity of lead (Pb), environmental regulations around the world have been targeted to eliminate the usage of Pb-bearing solders in electronic assemblies. This has triggered the development of “Pb-free” solders used in electronic industries. In order to become a successful solder material, Pb-free solder alloys need to be reliable over long term use. Even though many Pb-free solder alloys possess higher strength than the traditional Sn-Pb ones, there still exist reliability problems such as solderability and interfacial reaction on metallization pad of packaging substrate. It is the aim of this study to develop new Pb-free solders with better properties. The new Pb-free solders were developed by doping trace amounts of rare earth (RE) elements, cerium (Ce) and lanthanum (La) in the ratio 1:1, into Sn-based solders. These solder alloys were in eutectic composition, mainly Sn-9wt%Zn (Sn-9Zn), Sn-3.5wt%Ag (Sn-3.5Ag) and Sn-0.7wt%Cu (Sn-0.7Cu). The amounts of RE additions were from 0.05wt% to 0.5wt%. In general, the resulting RE-doped Pb-free solders were found to have better performances than their original ones. The microstructures of all the Pb-free solders were refined and become more uniform after addition of RE elements. For example, with the addition of 0.25wt% RE elements in Sn-Ag, the coarse β-Sn grain size was reduced and the intermetallic compound (IMC) particles become finer in eutectic colonies. A change of microstructure was in turn to affect the mechanical properties. Since only trace amounts RE elements were added into the Pb-free solders, the melting temperatures were changed only slightly comparing with their original ones. When some Pb-free solders were doped with a small percentage of RE elements, their tensile and creep properties were improved. The ultimate tensile strength and creep resistance were increased with RE amounts in the Pb-free solders. However, the elongation to failure of the solder alloys became lower. Wetting behavior of Pb-free solders was very important to the interconnection of electronic packages, especially in soldering. A solderability test was used to evaluate the wetting properties of Pb-free solders, which were doped with RE elements, on copper coupons. The wetting force and angle were usually measured to express the wetting behavior. In general, a given solder using a particular flux, the solderability was improved as the soldering temperature was increased. When the flux used was more activated, the wetting behavior was also improved. The addition of RE elements helped to enhance the wetting force and reduce the wetting angle. It also showed that amount of RE additions needed to be optimized for the best wetting performance. Apart from this, intermetallic layer (IML) growth at the interface would degrade the solder joint reliability, especially during thermal aging. Excessive IML growth would deteriorate the joint strength due to its brittle characteristics. The formation of IML in Pbfree solders had become more essential than that in Sn-Pb solders. It was because the soldering temperature for Pb-free solders was always higher than that for Sn-Pb solders. It meant that a faster IML growth occurred in Pb-free system. It was found that Pb-free solders doped with RE elements had lower IML thickness at after soldering and during thermal aging. In order to ensure the feasibility of newly Pb-free-RE solders application, certain Pb-free- RE solders had been formed as solder balls to bond on the Ball Grid Array (BGA) package. Interfacial reaction and ball shear strength were investigated upon 150oC thermal aging. It was found that both Sn-Ag-RE and Sn-Cu-RE solders had higher shear strength than solders without RE additions for all aging times. On the other hand, the Sn- Zn solder had higher shear strength than the Sn-Zn-RE solder for all aging times. However, the shear performance of Pb-free RE solder systems was still better than Pbbearing solders for all aging times. Hence, Sn-9Zn-RE, Sn-3.5Ag-RE, and Sn-0.7Cu-RE solders were reliable to be used.

Date of Award	4 Oct 2004
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Lawrence WU (Supervisor)