Development of Simple Electrolytes for the Electrodeposition of Pb-Free, Sn-Based Alloy Solder Films
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0993-E04-07
Development of Simple Electrolytes for the Electrodeposition of Pb-Free, Sn-Based Alloy Solder Films Chunfen Han, Qi Liu, and Douglas G. Ivey Chemical and Materials Engineering, University of Alberta, Edmonton, T6G 2G6, Canada
ABSTRACT A simple, cyanide-free, slightly acidic, chloride-based solution has been developed for the electrodeposition of Sn-0.7wt%Cu solder films. The solution contains only one additive (triammonium citrate) other than the metal salts and is stable for more than 36 days. The optimum citrate concentration is 0.30 mol/L for 0.22 mol/L of SnCl2ï2H2O. The addition of 0.003 mol/L of CuCl2ï2H2O to this solution can produce near eutectic (Sn-0.7 wt% Cu) deposits, on Au, Ni or Cu substrates, at current densities of 10-12.5 mA/cm2. Plating rates are in the 20-27.5 µm/hr range at room temperature. Copper is evenly distributed (~0.7wt%Cu) through most of the thickness, with slightly higher Cu levels at the solder/Ni interface. After solder reflow at 260∞C, two phases are present, Sn plus either Cu6Sn5 for Cu substrates or (Cu,Ni)6Sn5 for Ni substrates. INTRODUCTION Most microelectronic assemblies in use today utilize Sn-Pb solders for interconnection [1,2]. Increasing health and environmental concerns worldwide regarding the toxicity of Pb have prompted countries, such as Japan and those from the European Union, to pass legislation prohibiting or restricting the use of Sn-Pb solders, which has stimulated the development of a large number of Pb-free alternatives [1]. The most promising of these are Sn-based, particularly the Sn-0.7wt% Cu eutectic alloy and eutectic or near-eutectic SAC (Sn-Ag-Cu) alloys. Conventional Sn-Cu alloy plating solutions are either strongly alkaline with alkali cyanides or alkali pyrophosphates as complexing agents [3] or are acidic and based on sulfuric acid [4], pyrophosphate-iodide [5] or methanesulfonic acid (MSA) [6]. In the acidic solutions, many other chemicals, such as polyoxyethylene laurylether, triethanolamine, sorbitol, sodium gluconate, 1,4hydroxybenzene, Trion X100 or polypropylene glycol, may be added as complexing agents. In fact, some Sn-based alloy electroplating baths contain as many as 13 additives. The additives are necessary to improve solution stability and deposit morphology. However, they also make the electrodeposition process difficult to control. The composition of alkaline baths is often simpler; however, they are usually not compatible with photoresists when plating on patterned substrates. In this paper, a simple, cyanide-free, slightly acidic, chloride-based solution has been developed for the electrodeposition of Sn-0.7 wt% Cu solder films on various substrates, including Au, Cu and Ni. Copper is a very common substrate material used in electronic packages and Ni is used as a surface finish because of its lower reaction rates, relative to Cu, which helps prevent rapid reaction between the solder and the Cu layer below [7,8]. EXPERIMENTAL PROCEDURE The plating solutions used for electrodeposition contained only SnCl2ï2H2O, CuCl2ï2H2
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