After 3 years development, including 2 years of practical testing in the industry up to commercial scale, we can report [1] about an interesting new application: the preparation of the end surface of printed circuit boards. The places, where the diodes, resistors etc. have to be mounted and to be connected with the copper based printed circuits, have to be solderable for a period of at least 1 year in all climates. This is specified in the industry by > 4 hrs at 155șC, followed by a complex solderability testing procedure.
Copper alone does not meet this requirement at all, also not when coated with organic coatings (like benzo-triazoles) or chemically deposited tin. Only gold (with a Ni interlayer), Palladium or a thick layer (10-17 ”) of solder tin (aplied by a melt and then hot air levelling process) do the job.
We have found, that a very thin (80 nm in average) coating of PAni - deposited from a water dispersion [2] - followed by a special chemical tin deposition of only 0,5 - 1 ” thickness, allows to reach far more than 4 hrs 155șC-ageing plus full industrial soldering process.
Here, PAni's function is four-fold: (1) it passivates the copper [3], (2) it catalyzes the tin deposition so that also with increased Cu concentration used tin deposition baths, only pure tin is being deposited, see fig. 2, (3) the interdiffusion of Sn and Cu is inhibited, (4) the oxidation of the final tin surface is decreased.
This application of Organic Metals is based on one of the most advanced dispersions which have been developed to date. It meets long term and high-end specifications.
figure caption:
fig 2: Reaction scheme for the catalytic function of polyaniline during the formation of the passivative oxide layer between iron and a polyaniline coating
[1] for the first time in a scientific journal
[2] this is the first and commercial water dispersion of PAni ef. Ormecon Chemie data sheet “ORMECON CSN, PCB 7000”
[3] cf fig. 1 in [....] Adv. Mater. 6, No. 3 (1994) 226-28
