锡和锡基合金镀层的可焊性研究

Studies on Solderability of Tin and Tin Alloy Coatings

讨论了锡和锡基合金镀层的表面元素组成和表面氧化物的生成及其与可焊性的关系。锡和锡基合金包括Sn，Sn－Bi，Sn－Pb以及它们组合镀层的表面元素组成和表面氧化物的生成情况采用X-光电子能谱（XPS）和俄歇电子能谱（AES）进行研究，这些镀层在155℃，16h热老化前后的可焊性采用润湿称量法测定。研究结果表明在铜基体上电镀光亮锡和锡基合金镀层的可焊性均优于相应的非光亮镀层，这是由于光亮镀层的表面结构致密、孔隙率小。故抗氧化能力增强。相同厚度（约10μm）的光亮Sn－Bi和Sn－Pb镀层的可焊性较光亮Sn镀层更好，这是由于Bi和Pb作为相应镀层的组份与纯Sn比较能提高润湿力和减少表面氧化物的生成。Pb－Sn／Sn和Pb－Sn／Sn－Bi组合镀层的研究结果表明，组合镀层中Cu－Sn金属间化合物层更薄，这是由于以高Pb－Sn（Pb含量＞60wt％）为中间层的组合镀层能阻挡Cu－Sn金属间化合物的生成与扩散，而且，中间Pb－Sn层的Pb无论在室温或经热老化处理后均已扩散到Sn或Sn－Bi镀层表面。组合镀层表面氧化物厚度也较相应的Sn或Sn－Bi镀层要少。Pb－Sn／Sn镀层在面氧化物是锡氧化物，而扩散到表面的Pb仍以金属态存在。Pb－Sn／Sn－Bi镀层中的Bi和扩散到表面的Pb也仍以金属态存在。

In this paper, surface elemental composition and the oxide film formation on the surface of electroplated tin and tin alloys and its relation to soderability have been discussed.X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) were used to study surface elemental compositions and the oxide film on electroplated tin and tin alloys including Sn, Sn-Bi, Sn-Pb and their combination coatings. The corresponding solderability of these coatings before and after 155℃,16 h heat ageing were determined by wetting balance method.The results show that the solderability of bright tin and tin alloy coatings is superior to that of corresponding non-bright coatings. This is due to the surface structure of bright coatings being more dense and less pores, so more resistant to be oxidized in atmosphere. The solderability of bright Sn-Bi and Sn-Pb coatings is better than bright Sn coating with same thickness (~10μm).This is due to the Bi or Pb, as constitutions of coatings, increasing the wetting ability of coatings and decreasing the formation of surface oxides on Sn-Pb or Sn-Bi coatings more than pure Sn coa ting.The results of combination coatings including Pb-Sn/Sn and Pb-Sn/Sn-Bi on show that the Cu-Sn intermetallic layer is thinner for duplex coatings. This is due to high Pb-Sn(Pb>6O wt. % )as inner coating inhibiting the formation and diffusion of Cu-Sn intermetallic compounds. Moreover, the Pb from the inner Pb-Sn layer diffuse to the top Sn or Sn-Bi surface at both room temperature or after heat ageing. The surface oxides are also thinner on combination duplex coatings than that on corresponding Sn or Sn-Bi coatings. The surface oxides present are mainly tin oxides with the diffused Pb being present as metallic lead. The diffused Pb and Bi in Pb-Sn/Sn-Bi duplex coating are both present as metallic forms. This is due to Sn being more easily oxidized than Bi and Pb in atmosphere. The combined effect of Bi and Pb makes the solderability of Pb Sn/Sn-Bi duplex coating much improved. The influences of inorganic and organic impurities on solderability of coa tings are also discussed.Based on above results, the main factors influencing solderability of tin and tin alloy coatings are surface elemental composition and thickness of surface tin oxide film formed on coatings.