A Test Methodology for Copper Dissolution in Lead-Free Alloys

Lead-free selective soldering can result in extended times at high temperatures, which in turn can result in excessive dissolution of exposed copper, such as plated through holes. This phenomenon is more severe with lead-free, since the alloys have higher melting points, hence requiring longer times for the PTH to reach the higher temperatures, and the alloys typically have a greater capacity to dissolve copper. This paper discusses a test method that characterises the dissolution rate of copper from PWBs. A PWB design was created that allowed the time for the dissolution of a copper pad to be measured. With this quantitative method a soldering process or an alloy can be characterised in terms of dissolution rates at specific conditions of temperature and flow rate. This methodology provided repeatable measurements that allowed the various experimental parameters to be isolated. Particular attention was paid at the flow rate of the molten solder. In fact, different alloys at the same temperature can have considerably different flow rates, due to the different viscosity at that temperature. The performances of seven lead-free alloys and a typical 60/40 Sn-Pb alloy were compared at three temperatures. NPL worked with a number of partners using different alloys and copper types to measure the relative rate of copper dissolution. This work shows that some of the current alloy developments now offer superior performance to SnPb at the same temperature. Interestingly intermetallic formation between the alloy systems varies considerably. The copper type on the PWB is also influential, with significant differences between electroplated, electrodeposited and reverse treated. Introduction The advent of lead-free soldering has resulted in the introduction of a number of high tin solder alloys, which for selective soldering applications typically means solders with melting temperatures in excess of 220°C. The higher melting points of the lead-free alloys also dictate higher processing temperatures, and usually higher contact times, as the components take longer to heat up to the higher processing temperature. The copper dissolution process is temperature and solder alloy dependent. The solubility of copper in the new lead free alloys with tin compositions of at least 95% tin is potentially higher than that of tin-lead solder. Hence, the higher temperature and solubility effects can significantly increase the risk of damaging copper dissolution. Exposed surface copper can be removed, disconnecting the land from the track. The formation of a solder joint during soldering requires a reaction between the solder and the metallisation of the substrate. This reaction involves a dissolution process, which occurs through an intermediate phase, an intermetallic that forms at the interface. The intermetallic itself is soluble in molten solder, and hence the intermetallic substrate interface proceeds into the substrate with time. The nature of this intermetallic and its thickness will be significant in controlling the overall copper dissolution process. A number of alloys are now available where additions significantly below 1% affect a number of material properties, of which the solubility of copper is an important parameter. Factors Affecting Dissolution of Copper When a copper substrate is in contact with molten solder, atoms of copper will tend to diffuse into the molten metal and a diffusion zone is created on top of the copper surface [1]. The Nernst Brunner equation, often referred as Dybkov's analysis was found to be valid to explain this part of the dissolution process: ) ( = c c V S k dt dc s  (1) where cs is the solubility of copper in the molten solder at the specific temperature, c is the concentration of copper in the solder, S is the surface area of the copper, V is the volume of the molten solder and k is a constant [2, 3, 4]. As originally published in the IPC APEX EXPO Proceedings.