Tao-Chih Chang

Electrochemical Behaviors of the Sn-9Zn-xAg Lead-Free Solders in a 3.5 wt % NaCl Solution

The electrochemical behaviors, microstructures, and corroded products of Sn-9Zn-xAg lead-free solders were investigated in this study. The 63Sn-37Pb and Sn-3.5Ag solder alloys were also tested for comparison. The Sn-3.5Ag solder alloy had a higher equilibrium potential (-0.44 V SCE ) than the ones of 63Sn-37Pb (-1.10 V SCE ) and Sn-9Zn (-1.43 V SCE ). The Ag addition enhanced the corrosion resistance of the Sn-9Zn solder alloy. Passivation behavior occurred in the solder alloys used in this study except the 63Sn-37Pb one. X-ray diffraction patterns showed that the Zn segregated in the Sn-9Zn solder alloy as solidified, but it dissolved when 0.5 wt % Ag was added to the solder alloy. The AgZn 3 and Ag 5 Zn 8 were found in the Sn-9Zn-1.5Ag solder alloy but they were substituted by the Ag 3 Sn when the Ag content in the solder alloy was above 2.5 wt %. However, they were the initial sites for pits formation. The corroded product of SnCl 2 was observed in all solder alloys tested. In addition, the ZnCl 2 , ZnO, and SnO were observed in the Sn-9Zn-xAg solder alloys.

Effect of thermal cycling on the adhesion strength of Sn-9Zn-xAg-Cu interface

The effect of thermal cycling on the adhesion strength of the Sn-9Zn-xAg-Cu interface has been investigated by using pull-off tester, X-ray diffractometer, scanning electron microscope and energy dispersive spectrometer. The Sn-9Zn-xAg lead-free solders offer a better thermal cyclic resistance than the 63Sn-37Pb and Sn-9Zn solder alloys. The adhesion strength of the Sn-9Zn-Cu interface increases from 4.4 /spl plusmn/ 0.4 MPa to 13.8 /spl plusmn/ 0.9 MPa with increasing the thermal cycles from zero to three times but it decreases to 8.5 /spl plusmn/ 0.8 MPa for five cycles. The Sn-9Zn-xAg solder alloys (x=0.5, 2.5, and 3.5 wt%) have a similar tendency and the maximum adhesion strength of 21.41 /spl plusmn/ 1.5 MPa for the Sn-9Zn-2.5Ag solder alloy has been obtained after three thermal cycles. The adhesion strength of the Sn-9Zn-1.5Ag-Cu interface increases from 7.8 /spl plusmn/ 0.6 to 16.6 /spl plusmn/ 0.9 MPa with increasing the thermal cycles from 0 to 5 times.

Effect of heat treatment on the interfacial adhesion strength between Sn-Zn-xAg lead-free solders and Cu substrate

Sn-9Zn eutectic solder alloy is a potential candidate to replace 63Sn-37Pb alloy as interconnecting material in the electronic packaging industry, but the inferior wettability and poor oxidation resistance make this alloy unsuitable for wide application. In this study, different Ag contents were added to the Sn-9Zn alloy to improve the wettability, oxidation resistance, and adhesion strength at the interface. A pull-off test was used to estimate the adhesion strength of the interface between Sn-Zn-xAg solders and Cu substrates, scanning electron microscopy (SEM) was utilized to observe the fracture morphology, and energy dispersive spectrum (EDS) was employed to analyze the distribution of elements. Finally, X-ray diffraction (XRD) was exploited to identify the intermetallic compounds (IMCs) formed at the interface.