The Dependence of Composition, Cooling Rate and Size on the Solidification Behaviour of SnAgCu Solders

Abstract

The scope of this study is to investigate the influences on the solidification of the microstructure of SnAgCu solders. It will be shown that the solidification process depends on solder composition, specimen size and manufacturing conditions. The influence of solder composition has been investigated on bulk solder ingots by varying the Ag content from 3.0 wt% to 3.8 wt% and the Cu content from 0.4 wt% to 1.5 wt%. The influence of an additional Au content was investigated on a SnAg3.OCuO.5AuO.14 solder. Solidification experiments with different cooling rates from 0.006 K/s to 0.6 K/s have been carried on bulk solder ingots (length 23 mm; 0 7 mm) of the alloys SnAg3. OCuO.5, SnAg3.8CuO.7 and SnAg3.5CuO.4. The results will point out the influence of cooling gradients on the microstructure.In order to compare the microstructure of these large specimen with real solder joints, solidification experiments with cooling rates from 0.33 K/s to 10.9 K/s have been carried out on SnAg3.OCuO.5 solder balls with four different sizes (0 130 ptm, 0 270 ptm, 0 590 ptm and 0 1100ptm). Phase sizes and shapes as well as grain orientations have been investigated and compared. Experiments on directed solidification have been carried out in order to investigate the dendritic growth of f-Sn dendrites. For that purpose a temperature gradient was generated inside the solder during solidification. IntroductionDuring the last years many studies and investigations on the solidification behaviour of lead free solders were published in order to improve the reliability of lead free interconnects [1][2][3][4]. A mayor influence on material behaviour is the microstructure that is formed during the solidification process. This microstructure is responsible for the mechanical properties of the interconnection and depends on various influences. Therefore it is necessary to understand the solidification process and its influences in order to interpret differences in material behaviour.The microstructure of SnAgCu consists of three phases: f-Sn and the intermetallic compounds Ag3Sn and Cu6Sn5. According to the phase diagram the eutectic composition with the melting point at 217°C is around Sn95.5Ag3.8CuO.7 [5]. It is known that changes in composition can change size, shape and number of those phases [1,5,6]. The determining process for the formation of the microstructure is the reflow process. This dynamic process is not comparable with the state of equilibrium in the phase diagram [2]. That means that the parameter cooling rate influences the phase growth of intermetallics and changes the phase diagram [3,4].Apart from the choice of the Ag and Cu content in the solder paste, it has to be considered that the composition also changes during the reflow process. This is caused by the dissolution of substrate metallisation [7] such as Ni/Au, Ag and Cu. This depends on peak temperature and time above liquidus of the reflow profile. That means that soldering on a Cu metallisation will cause an increase of the Cu content and a decrease of the thickness of t...

The effect of downscaling the dimensions of solder interconnects on their creep properties

“…Experiments from [25] give a reasonable explanation, why the microstructures between bulk specimens, medium sized solder balls and tiny flip chip joints are different. As can be seen from the polarized light images in Fig.…”

The scaling effect on microstructure and creep properties of Sn-based solders

“…The grain structure is very comparable to the structures of the small SnAg3.5 balls shown in figure 2. Therefore it can be concluded that the solidification experiments on small solder balls presented in [9][10][11][12][13][14] are significant to explain the microstructure in small solder joints. [15].…”

“…In order to achieve a better understanding of solidification in small volumes Mueller and Coworkers carried out a number of solidification experiments on small solder balls [8][9][10][11][12]. The details of these experiments are described in [8,9]. Briefly, solder spheres of diameters between 130 11m and 1100 11m consisting of various SnAg and SnAgCu alloys were solidified at different cooling rates (0.15 Kls to 10.5 Kls).…”

“…la) [7]. After slow solidification (undercooling �T = 8 K) the microstructure contained needle shaped intermetallic Detailed description of experimental procedure can be found in [8,9]. In order to achieve a better understanding of solidification in small volumes Mueller and Coworkers carried out a number of solidification experiments on small solder balls [8][9][10][11][12].…”

“…In order to determine the undercooling of the Detailed description of experimental procedure can be found in [8,9]. solder, the temperature was measured in the solder by a thermocouple (Tl).…”