Fractal analysis of intermetallic compounds in Sn–Ag, Sn–Ag–Bi, and Sn–Ag–Cu diffusion couples R. Jayaganthan a,b, ⁎ , K. Mohankumar b,c , V.N. Sekhar b , A.A.O. Tay b , V. Kripesh c a Department of Metallurgical and Materials Engineering, Indian Institute of Technology Roorkee, Roorkee-247667, India b Nano/Microsystem Integration Lab, Department of Mechanical Engineering, National University of Singapore, 117576, Singapore c Institute of Microelectronics, Science Park II, 117685, Singapore Received 4 July 2005; accepted 18 October 2005 Available online 22 November 2005 Abstract The Sn–Ag/Au/Ni–P/Cu, Sn–Ag–Bi/Au/Ni–P/Cu, and Sn–Ag–Cu/Au/Ni–P/Cu diffusion couples were prepared by reflowing the Pb free solders on the top surface metallization of the substrate at 250 °C. They were annealed at 150 and 170 °C for 4, 8, 16, 36, 45 days. The surface morphological features of intermetallic compounds (IMC) formed among the different elements in the solder alloys were characterized by Fractal Dimension using FE-SEM micrographs. The influence of processing parameters on morphological features was studied in the present work. The Box counting technique has been used to measure the fractal dimension of the IMC. It was observed that the morphology of the IMC varies from scallop to planar with increasing annealing time. © 2005 Elsevier B.V. All rights reserved. Keywords: Pb-Free solders; Intermetallic compounds; Fractal dimension 1. Introduction Pb-free solders in electronic packaging industry is gaining importance due to the possible health hazards of Sn–Pb solders. Sn–Ag based solders are identified as potential substitutes for the flip chip packaging applications as the industries are moving towards fine pitch, sub-micron interconnects. The intermetallic compounds (IMC) formed during processing and service temperature are of major concern as it affects joint strength resulting in complete failure of the systems. The formation of IMC occurs at the interface between solder and conducting line during soldering. It is due to the interdiffusion of the parent's phases of M (Cu, Ni, Au) and Sn in the existing IMC layers. The temperature of the solder joints may rise to as high as 100 °C during service conditions and it activates the diffusion process across the interface. Due to that, IMC grows in thickness affecting the reliability of solder joints. Excessive growth of IMC at the joint interface results in brittle layer which detoriates the mechanical integrity of the joints. The fatigue crack growth resistance increases with increasing the roughness of the IMC/ solder interface at the low strain energy release rates as reported in the Yao and Shang's work [11]. Therefore, the strength of the joint depends on the IMC layer thickness as well as the morphology of the layer. Several review papers are published in Pb-free solders [1–4]. Glazer [3] reviewed the mechanical properties, microstructural characteristics, and corrosion behavior of binary Pb free solders. There are two intermetallic compounds (IMC) namely Ni 3 Sn 4 and Ni 3 P formed between Sn–Ag solder and electroless Ni–P substrate [1]. The morphology of Ag 3 Sn appears to be needle like crystals in the solder joint. The interfacial reactions between electroless Ni–P UBM and 95.5Sn–4.0Ag–0.5Cu were inves- tigated by Y. D. Jeon et al. [5]. Two IMC such as (Cu,Ni) 6 Sn 5 and (Ni,Cu) 3 Sn 4 were found in their work. Jang et al. [6] have observed the formation of ternary IMC Ni 4 Cu 7 Sn 6 at the interface between Ni–P UBM and Sn–Ag–Cu alloys. The formation of only (Ni,Cu) 3 Sn 4 IMC at the interface was reported by Kang et al. [7]. However, Zeng et al. [8] have reported the formation of (Cu,Ni) 6 Sn 5 and (Ni,Cu) 3 Sn 4 above and below 0.6 at.% Cu in the solders, respectively. Addition of Bi to Sn–Ag reduces the melting temperature and promotes Materials Letters 60 (2006) 1089 – 1094 www.elsevier.com/locate/matlet ⁎ Corresponding author. Tel.: +91 1332 285869; fax: +91 1332 285243. E-mail address: rjayafmt@iitr.ernet.in (R. Jayaganthan). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.10.090