Study of copper barrel plating dissolution of printed circuit board with multiple exposure thermal stress testing
The copper barrel plating dissolution of printed wiring boards (PWB) occurring with multiple thermal stress testing had been studied in this paper. The thickness reduction of copper barrel plating and related copper dissolution rate were measured and compared for test specimens under the conditions of multiple thermal stress cycles (1X-6X) with two test temperatures (288°C / 260°C) in accordance with IPC-TM-650, Test Method 2.6.8, modrfred. Before all thermal stress tests, the solder alloy element concentration (wt%) of the lead-free solder pot in-use had been tested by a solder paste vendor and the result was acceptable and normal as compared with IPC new released guidelines for diagnosing contamination of SAC305 solder pots / baths. Moreover, the practical affecting factors of copper dissolution during thermal stress and lead-free soldering process had been discussed in this paper, including the direct relevance of copper dissolution to printed circuit board performance and reliability.
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.
Experimental study of thermal modeling for stratospheric platform airships
R&D work on the Japanese stratospheric platform airship is discussed from the viewpoint of the thermal modeling. The work includes two experiments on the thermal characteristics of the 25m-long airship and the 35m-long PV-equipped airship model. The validity of the current thermal model was confirmed by comparing the calculated results with the measured data. The other important work is the Ground-to-Stratosphere (GTS) flight test. A 47mlong airship-shaped balloon was flown to the stratosphere. The flight simulation program into which the thermal model was integrated, was developed and applied successfully to predict the ascent behavior and the flight trajectory.