Generation of Tin(II) Oxide Crystals on Lead-Free Solder
Joints in Deionized Water |
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Authors: | Hong Chang Hongtao Chen Mingyu Li Ling Wang Yonggao Fu |
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Affiliation: | (1) EMC Corp., Hopkinton, MA 01748, USA;(2) Flextronics, San Jose, CA 95131, USA;(3) Chongqing University, Chongqing, 400044, P.R. China; |
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Abstract: | The effect of the anode and cathode on the electrochemical corrosion behavior of lead-free Sn-Ag-Cu and Sn-Ag-Cu-Bi solder
joints in deionized water was investigated. Corrosion studies indicate that SnO crystals were generated on the surfaces of
all lead-free solder joints. The constituents of the lead-free solder alloys, such as Ag, Cu, and Bi, did not affect the corrosion
reaction significantly. In contrast to lead-free solders, PbO
x
was formed on the surface of the traditional 63Sn-37Pb solder joint in deionized water. A cathode, such as Au or Cu, was
necessary for the electrochemical corrosion reaction of solders to occur. The corrosion reaction rate decreased with reduction
of the cathode area. The formation mechanism of SnO crystals was essentially a galvanic cell reaction. The anodic reaction
of Sn in the lead-free solder joints occurred through solvation by water molecules to form hydrated cations. In the cathodic
reaction, oxygen dissolved in the deionized water captures electrons and is deoxidized to hydroxyl at the Au or Cu cathode.
By diffusion, the anodic reaction product Sn2+ and the cathodic reaction product OH− meet to form Sn(OH)2, some of which can dehydrate to form more stable SnO·xH2O crystals on the surface of the solder joints. In addition, thermodynamic analysis confirms that the Sn corrosion reaction
could occur spontaneously. |
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Keywords: | |
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