Influence of Climatic Cycles on Properties of Leadfree Solders

This paper presents complex climatic tests performed in the laboratory of climatechnology of the Department of Electrotechnology. More specifically, it presents the results of climatic shocks administered to specimens of Sn-Pb and leadfree solders, and compares the results. While the experimental results are negative regarding the use of electrical resistance as a method for tracking degradation of the joints, the research has provided a large number of specimens that can be examined for structural changes in the solder after climatic stress.


Accelerated testing
To gain at least rough data on the properties of these new technologies in long-term use, there is generally just one solution: accelerated tests in adverse conditions.
These tests usually make the conditions even worse than expected in real use, in order to gain a margin of security and to obtain results faster.Of course, any such test depends heavily on assumptions that are made and on factors that are simulated, as it is impossible to make complex tests that accurately simulate the real world use.
Accelerated testing of electronic devices and components is specified by a wide field of norms.American military standards are especially useful, as they define a very wide set of operating conditions, test methods, etc.

Specimens
For the purposes of this experiment, several sets of experimental printed circuit boards were prepared.All boards utilized 0R0 SMD resistors (1206), but different mounting technology was used: two types of ECAs (Amepox AX12 and Amepox AX20), two types of leadfree solders (COBAR XM 3S and Kester EM907) and one type of Sn-Pb solder (COBAR S62-GM5) were used.Only the two COBAR solders were examined.

Thermal cycles
The experiments performed by the authors were aimed at comparing leadfree solders and electrically conductive adhesives of various types under cyclic climatic stresses.Thanks to the available equipment, it was possible to perform three different sets of climatic shocks.
First experiment was aimed at rapid changes of outside temperature.The specimens suffered repeated transition shocks between 125 • C dry heat and −45 • C cold.The dwell time at each of these temperatures was 15 minutes -this setup ensured good heating/cooling of the soldered joints.Each specimen suffered 182 such shocks.
The temperature profile of these changes is shown in Figure 2. Thermal cycling produced no significant structural changes, apart from significant surface oxidation of the soldered joints.In contrast to the ECA experiments, no apparent cracks formed.

Moist heat -dry heat cycles
In this subset of experiments, the specimens were subjected to cyclic shocks between moist heat (50 • C, 100% humidity) and dry heat (125 • C).As shown in Figure 5, the increase in electrical resistance is again inconclusive for leadfree solder, and the same applies to Sn-Pb solder (not shown due to space constraints).
Due to nature of the test -it was necessary to dry the specimens before measuring -it was possible to obtain only a much more limited set of values.This type of climatic stress showed a significant increase in the extent and intensity of oxidation.As shown in Figure 7, the surface of the joint is covered with oxides.Unfortunately, as with 2.2, no cracks were observed in the joints.

Moist heat -cold cycles
The last of the settings for climatic shocks utilized moist heat (see above) and cold.As with the other experiments, Figures 8 and 9 show that the electrical resistance of the soldered joints did not react significantly to climatic shocks, even in such adverse conditions.However, the surfaces of the joints again showed significant damage, as shown in Figure 10.The degradation of the surface of the solder, as shown in Figure 10, resembles tin pest.Tin pest is a phase transformation of solid tin caused by low temperatures -it starts to demonstrate itself at roughly 13 • C, and the lower the temperature, the worse the tin pest.Tin pest has again attracted attention with the widespread use of leadfree solders.

Conclusion
The conclusions drawn from this research are, unfortunately, mostly negative.It has been shown that electrical resistance cannot be used to determine damage caused to soldered joints by climatic cycling, at least not in the extent that could realistically be performed in the laboratory belonging to the department.
However the results also show the possibility of a closer study of surface changes to the solder, especially aimed at low-temperature changes.The results from the round of experiments discussed in this article will be extended by obtaining quality cross-sections and examining them using electron microscopy.

Fig. 7 :
Fig. 7: Oxidation of the surface of a soldered joint after moisture-heat cycling

Fig. 9 :
Fig. 9: Resistance change depending on moisture-cold shocks for leadfree solder Pavel ŽÁK was born in 1983.In 2006 he graduated from the bachelor degree program in Electrical Engineering and entered the Information Technology -Power Engineering master degree program at FEE-CTU in Prague.Two years later he graduated with a master's degree in electrotechnology.At present, he is studying a postgraduate doctoral degree program under the supervision of Assoc.Prof. Ivan Kudláček, Ph.D.He is fluent in French and English, and has a basic knowledge of German.His hobbies include electronics, photography, aviation, marine model making and fishkeeping.He works part-time in a private electrophysical laboratory.