Copper Dissolution in Lead-Free Alloys

Christopher Hunt, Davide Di Maio
National Physical Laboratory

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.

The various factors that influence the dissolution of copper in molten solder were investigated and the important parameters were found to be: temperature, solder composition and flow rate.

A method was developed for comparing the dissolution rate of copper with different solder alloys. The method developed here, presents many advantages. First of all, the detection of the dissolution time was instrumented, hence improving repeatability. This method removes the need for cross sectioning to measure the dissolution rate, and hence the measurement process is very quick, and potentially real time within the machine setup process. Ten copper pads can be tested automatically in rapid succession, insuring constant conditions and repeatability of the results. The procedure is also very flexible and can be adapted and used for different types of soldering machines and providing the same conditions are used, comparables results should be obtainable.

These features make the procedure described here an important tool for future testing in the copper dissolution area. In particular studies that investigate the effect of different copper types could benefit from the flexibility and ability to obtain a large number of results in a short period of time.