Encapsulation of lead-free Sn/Zn/Bi solder alloy particles by coating with wax powder for improving oxidation resistance

Abstract

An encapsulation treatment of lead-free Sn/Zn/Bi solder powder was investigated for improving the oxidation resistance. Sn-8mass%Zn-3mass%Bi alloy particles were coated with a wax (12-hydroxystearic acid) powder by means of a dry mechanical treatment method using a ball mill. In order to determine the optimum operating conditions of the ball mill in the wax-coating treatments, the compressive energy required for deforming a single Sn/Zn/Bi alloy particle was measured with an unconfined compression tester and the mechanical energy applied to the alloy particles in the ball mill was estimated using the results of the compression test. The optimum operating conditions were determined based on both the applied energy and the flowability of solder pastes, and the wax-coated alloy particles maintaining the spherical shape were obtained under the conditions. The wettability test and the solder balling test for the solder pastes containing the wax-coated alloy particles stored at room temperature in air were carried out to evaluate the oxidation resistance performance. The wax-coated alloy particles had an excellent wettability compared with the original alloy particles, and the oxidation resistance of the Sn/Zn/Bi solder powder was improved by the encapsulation treatment.

Characterization of media mills based on mechanical energy applied to particles

“…When the tin/zinc/bismuth alloy powder was used as a stresssensitive material, the empirical formulas corresponding to Eqs. (1)-(5) were different from those in the system of cop- per powder; however, we confirmed that the applied energy estimated from the deformation of alloy powder was almost agreed with that from the copper powder under a given milling condition in a ball mill (Iwasaki et al, 2005). Accordingly, the applied energy thus estimated is independent of the physical properties of stress-sensitive materials used and expresses the characteristics of the mill.…”

“…The authors have proposed two different methods for estimating experimentally the net applied energy using stress-sensitive materials, in which the applied energy is estimated based on the disintegration of the agglomerates of fine pigment (iron oxide) particles in the powder bed of calcium carbonate and the plastic deformation of spherical soft metal particles (Iwasaki et al, 2001b(Iwasaki et al, , 2005. The former is applied to the characterization of vessel-rotating and stirring type of powder mixers in which relatively low stress fields are formed in the powder bed; the latter for grinding and compounding machines in which higher compressive and shear stresses apply to the particles.…”

“…It has been proven that the applied energy estimated from the disintegration of fine pigment agglomerates is almost agreed with that from the deformation of metal particles . Using the evaluation results of the applied energy for a bead mill, we successfully synthesized the composite particles with the bead mill, in which soft metal particles were coated with fine ceramic particles (Iwasaki et al, , 2004 and wax powder (Iwasaki et al, 2005) maintaining the original shape of metal particles.…”

Thermal and mechanical properties of Sn–Zn–Bi lead-free solder alloys

“…Sn-Zn solders posses several fascinating features such as low cost as well as low reflow temperature of 220 • C. In addition, Sn-9Zn eutectic alloy has a melting temperature (198 • C) close to that of Sn-Pb eutectic alloy (183 • C), and offers better mechanical properties than the conventional Sn-Pb solders [4,5]. In spite of the Zn in Sn-Zn system is easily oxidized to form certain corrosion products (e.g., Zn oxide/hydroxide and ZnCl 2 ) which leads to poor wettability and reduces the reliability of solder joints, certain Sn-Zn composition region can act as a sacrificed anode with a good anticorrosive property for the ferrous base metals [6][7][8][9][10]. However, the application of the Sn-Zn alloy needs to think of these specific phenomena, caused by the high activity of Zn.…”

Effects of Ag and Cu addition on microstructural properties and oxidation resistance of Sn–Zn eutectic alloy

“…Unfortunately, despite their excellent mechanical properties, the Sn-Zn eutectic alloy is susceptible to both oxidation and corrosion. Many researchers have devoted considerable time to overcome this drawback, or at least to improve the soldering properties of the Sn-Zn eutectic alloy by adding a third or fourth element such as Ag [1][2][3][4][5], Al [6][7][8], Bi [9][10][11][12][13], In [14,15], or Ga [16]. Among them, the addition of Bi to the Sn-Zn eutectic alloy imparts superior soldering properties, such as high joining strength, good wettability, and low melting temperature, in electronic packaging [9,10].…”