Investigating the lead-free soldering of ceramic thick-film circuits

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Dr. Krammer Olivér
Department of Electronics Technology


My project was the comparative study of lead-bearing solder joints and lead-free solder joints on ceramic substrate with palladium-silver pads, and on FR-4 glass epoxy with copper pads. The lead based solder joints were made with one set of soldering, and the lead-free solder joints were formed with several different soldering setups. The lead-free soldering were performed with selective wave soldering system. The parameters I changed were the speed of soldering (10, 12, 15, 20, 25 mm/s) and the temperature of soldering (310 and 330 °C). During the experiment, the quality of the joints were inspected with optical microscope, X-ray microscope, scanning electron microscope. The mechanical properties of the solder joints were characterized by shear test. In the case of ceramic substrate the mechanical strength of the lead-free soldered joints was larger (twice or more) than the lead-bearing soldered joints. The stongest joints were made with 310 °C solder bath temperature and with 12 mm/s soldering speed. With these parameters the average result of the shear test was 25.7 N. The cross-sectional inspections revealed remarkable size (100-250 µm diameter) of voids in the lead-free soldered joints. In these images intermetallic layer was not noticeable. Due to the amount of the voids I decided to analyse the joints with an X-ray microscope too. These inspections confirmed the results of the optical inspections. The maximal size of the voids in the lead-bearing soldered joints was 30 µm. In the lead-free joints on copper pads, more voids that larger than 100 µm can be seen, even in the same joint, but there were joints without voids too. In case of ceramic substrates there were voids in all of the joints. By the results of the inspection with the scanning electron microscope, the intermetallic layer in the lead-bearing solder joints were thicker (9-10 µm) than in the lead-free joints (2.5-8 µm) on ceramic substrate. With 10 mm/s soldering speed on 310 °C soldering temperature, the dissolution of the conductive layer into the solder was significant. The thickness of the intermetallic layer did not correspond with the results of the shear test. The intermetallic layer on the copper pads was too thin to detect. My recommendation for the lead-free soldering parameters in case of ceramic based thick film circuits are the following: 310 °C solder bath temperature and 15 mm/s soldering speed.


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