Nanomechanical properties of Ag solder bumps doped with Pd and Au

Highlights

• We evaluated the Pd and Au doped on Ag solder alloy using nanoindentation.

• H enhancement as the RT and TTT and particular for Pd 2% and Au 3%

• Changeable H and E are due to responses of bonding strength of doped Ag solder.

• Stress exponent S2 (Pd:3%) > S3 (Pd:5%) > S4 (Pd:2%-Au:3%)and bond strength (S4 > S3 > S2) are inversely related from creep test.

Abstract

As an approach to increasing their reliability, we doped Ag solder bumps with Pd and Au alloys to enhance their creep resistance, which we characterized using a nanoindentation technique. The hardnesses of the pure Ag and the Ag solders doped with Pd:3%, Pd:5%, and Pd:2%-Au:3% were 0.6 ± 0.1, 1.8 ± 0.2, 2.8 ± 0.3, and 3.1 ± 0.5 GPa, respectively. Although the hardness of the Ag solder was enhanced significantly when it contained Pd:2%-Au:3%, it exhibited a lower creep exponent (n > 2). The mechanism of the thermal recovery of the sample appears to be associated with activation of dislocation sources—also the reason for the decrease in hardness. The compounds doped with Pd:2%-Au:3% displayed significantly greater bond strengths and creep exponents, whereas the Pd-only compounds exhibited poorer reliability.

Introduction

The fabrication of intermetallic compound (IMC) solders at an interface is crucial for good adhesion under bump metallurgy (UBM) [1], [2], [3]. For electronics assemblies, lead (Pb)-free solders are preferred over tin (Sn)-Pb solders for use in bump fabrication [4]. The Pb-free solders are acceptable as Sn-rich alloys when they incorporate elements such as silver (Ag), copper (Cu), indium (In), and antimony (Sb). Typically, high-Sn-content solders react rapidly with Cu UBM, creating a thick layer of CuSn IMC as a result of interdiffusion, such that the solder joints of IMCs decay and, thus, form microcracks [5]. Silver metal is generally used as a solder in microelectronics packaging because it has a unique combination of a low melting point and good wettability, strength, and creep [6], [7], [8]. In particular, doped Ag alloys are believed to be promising materials for creating bumps with greater connection reliability [9]. It has been suggested that a rapid interaction tends to form a substantial amount of IMCs, which can be brittle—an important and challenging problem affecting solder reliability [10], [11], [12]. Shock impact, bending mechanical damage, and plastic strain deformation are common in integrated circuit packaging. The periodicity of plastic deformation stresses can lead to fatigue and failure of solder bump interconnects [13]. Thermal cycling, thermal and mechanical shock, vibration, and bending processes are generally used as accelerated reliability tests for solder bumps [14]. To address these problems, it will be necessary to increase the solder reliability that existing Pb-free solders can endure, or develop new solder composites. Nanoindentation technology can be used to evaluate the mechanical properties of IMCs [15], [16], [17] because it enables more precise measurements of mechanical performance on the nano- and microscale. The alloy content improves the resistance of Ag solder bumps to thermal shock. Accordingly, to meet the requirements for the conductive paste bonding process, an Ag system IMC must have good resistance to thermal fatigue, high ductility, and better solderability as a conductive solder bump [8]. In terms of reliability, doping with gold (Au) and/or palladium (Pd) can enhance the hardness (H) and Young's moduli (E) of Ag solder bumps. Variations in metal–metal bond strength have effects on stability. In bulk AuAg alloys, the metal–metal bond strength follows the order AuAu > AuAg > AgAg. Thus, increasing the number of Au atoms would generate more AuAg bonds, which are stronger than AgAg bonds, resulting in greater stability of AuAg alloys [18].

Although nanomechanical behavior generally distributes thermal shock on a solder array, little is known from nanoindentation studies about the significant differences between Ag solder bumps and Ag solder bumps doped with Au and/or Pd in printed circuit boards (PCBs). The data suggests that the Pd and/or Au atoms in the Ag solder play significant roles in preserving the integrity of the solder joint. Preexisting grain boundaries can be used with the nanoindentation technique to determine the stress concentration [19], [20], [21]. In addition, the nanomechanical aspects of Au and/or Pd doping as a means of inducing changes in Ag solder bumps and the effects of thermal temperature treatment (TTT) have not yet to be reported in detail. Furthermore, although a mechanical test may well be useful for examining the well-grained microstructures developed in Ag solder bumps, it may not be ideal for determining creep resistance and even less so for measuring grain boundary sliding. An attractive method of enhancing the creep resistance of solder materials is to adopt a solder bump approach. In this study, we employed the nanoindentation technique to analyze the values of H and E of Ag solder bumps at RT and TTT. In addition, we adopted the creep method to track the change in the duration of grain boundary formation.