Effects of the Al Pr Additions on the Wetting Oxidation and Mechanical Properties of Zn-25Sn High Temperature Pb-free Solder Alloys

Abstract

Zn-25Sn based alloys possess great potential as candidates for high temperature lead- free solders But it exhibits poor oxidation resistance due to the high zinc content This study investigated various solder related properties of the high temperature Zn-25Sn- XAl (X=0~0 25 wt%) -YPr ( Y=0~ 0 15wt%) Pb-free solders The aspects investigated include the wetting performance feasibility of Al and rare earth Pr addition for enhancing the wettability wetting and interfacial reaction between solder and Cu substrate oxidation resistance of the solders microstructure and mechanical properties of the solders The experimental investigations conducted were wetting balance test scanning electron microscopy (SEM) electron probe microanalysis (EPMA) energy dispersive spectroscopy (EDS) equipped in transmission electron microscopy (TEM) thermal gravimetric analysis (TGA) and tensile tests For clearifying the relationship between wetting time and the initial performance in dipping process the initial stages of non-reactive and reactive wetting behaviors were investigated The initial stage of wetting curve was divided into three stages The liquid impeding force of the first stage was measured by wetting balance and was theoretically analyzed by Navier-Stokes equation for comparison An equation in terms of wetting force wetting time viscous force and buoyance force was proposed to describe the wetting curve of this stage The viscous force was found to decrease quickly in the second stage The third stage was described by the traditional wetting expressions The dynamic contact angles of each stage were estimated theoretically The contact angles of the first stage were found to increase rapidly upon dipping and then reach a steady value The angles decreased rapidly in the second stage Finally the angles decreased gradually to reach the thermodynamic equilibrium contact angle described by the Young’s equation Based on the equations aforementioned the initial AB stage of Zn-25Sn-(0 0 01 0 05 wt% ) Pr was investigated The contact angle decreases with time due to the good wetting between solder and Cu substrate The larger values of slope of the initial stage reflected the better wettability The addition of 0 01wt% Pr in Zn-25Sn increased the wetting force However dewetting was observed under air atmosphere for the Zn-25Sn Pr-containing solders and solders containing 0 01wt% Al It was also found that the dewetting of Pr-containing solders could be avoided under argon atmosphere as well as with higher Al additions The wettability and wetting force were improved upon the increasing addition of Al in the range of 0-0 09 wt% in Zn-25Sn alloy The thickness of intermetallic compounds layers increased with Al additions The additions of 0 01 0 05 wt% Pr in Zn-25Sn-0 05Al enhanced the wettability of solders and depressed the growth of intermetallic compounds However the higher additions of 0 08 0 15 wt% Pr in Zn-25Sn-0 05Al degraded the wettability while enhanced the growth of the intermetallic compound The phases PrZn3 Pr3Sn5 and (Cu Al)4Zn were formed at the solder/substrate interface The results of thermal gravimetric analysis (TGA) indicate that the corrosion resistance of the various solders decreases in the sequence Zn-25Sn-0 05Al-0 05Pr > Zn-25Sn-0 05Al> Zn-25Sn-0 01Al> Zn-25Sn> Zn-25Sn-0 01Pr> Zn-25Sn-0 05Pr It was found that the addition of 0 05 wt% Al and 0 05 wt% Pr largely increases the oxidation activation energy A comparison of the results of electron spectroscopy for chemical analysis on the oxidized specimen Zn-25Sn Zn-25Sn-0 05Al and Zn-25Sn-0 05Al-0 05Pr indicated that the Al and Pr prominently accumulated at the surface of solder The investigation by high resolution transmission electron microscopy revealed the existence of Al2O3 and Pr2O3 in the surface oxide films of Zn-25Sn-0 05Al and Zn-25Sn-0 05Al-0 05Pr The oxide film also consists amorphous (Al Zn Sn)Ox and (Al Pr)Oy It is believed that the formation of the compact oxide film is responsible for the oxidation resistance of the solders The addition of Al in Zn-25Sn-XAl ( X= 0 0 01 0 03 0 05 0 09 wt% ) tends to refine the grain size and increase the undercooling behavior of the solder The increasing addition of Al up to 0 09wt% enhanced the ultimate tensile strength (UTS) of the alloy from 67 28 to 78 61MPa (16 84% improved) and the yield strength from 42 52 to 52 81MPa (24 2% improved) The strain of the solder degraded from 39 02% to 32 83% when the addtion of Al increased from 0 to 0 09wt%

Date of Award	2017 Feb 9
Original language	English
Supervisor	Kwang-Lung Lin (Supervisor)