A significant increase in the price of copper is expected in the coming years, particularly due to the global trend towards electromobility. Against this background and in conjunction with weight aspects, the use of aluminium alloys with sufficient strength is an alternative for certain applications in automotive electronics. In the field of electronic connecting elements, the associated surfaces are very often tin-based.
Coating aluminium substrates with tin or tin alloys is state of the art, but requires a large number of pre-treatment and process steps and is therefore very time-consuming. With regard to implementation in continuous processes such as coil coating, a significant reduction in process times would be desirable. At the same time, the pre-treatment and coating process would need to be adapted to higher-alloy aluminium alloys, which have the corresponding strength.
The main objective of the research project was the production of adhesive tin alloy coatings on various aluminium substrates with process times that are justifiable in coil coating lines. In consultation with the PA members, a maximum treatment time of 30 s per treatment step was considered economically acceptable; however, a treatment time of 10 s was aimed for. Within the project, two commercially available zincate processes (DIAPREP from IPT GmbH and Chemofit from Chemopur GmbH) were examined for their suitability to act as adhesion promoters for different aluminium strip qualities (Al99.5, AlMg3 and AlZn5.5MgCu) prior to electroplating with tin or tin alloys. Commercial products were also used as tin electrolytes: both a strongly acidic tin-silver electrolyte (Slotoloy SNA30, Schlötter), which is suitable for strip plating lines, and a weakly acidic tin-zinc electrolyte (Dipsol SZ 240, Dipsol).
Adhesion-resistant coatings were also achieved on higher-alloy aluminium substrates with process times of 30 s, although a (nickel) intermediate layer was sometimes required for the tin-silver alloy coating. The degreasing of the aluminium alloys could not be guaranteed in the short process time, as a boiling degreasing process with a usual treatment time of up to 30 minutes must be used for this. Electrolytic degreasing for aluminium alloys is not available. The surface quality of the aluminium alloys proved to be of particular importance for achieving adhesive and corrosion-resistant coatings. It has been shown that cracks in the surface or precipitation of alloy constituents can lead to the formation of blisters or layer detachment. In addition, the test results indicate that apparently identical compositions of aluminium alloys do not necessarily have identical properties or requirements for pre-treatment prior to coating. Therefore, if necessary, adjustments must be made in order to achieve consistently adhesive coatings. In principle, it can be said that even higher-alloyed aluminium substrates, which are necessary for use in plug connections, can be coated with tin alloy coatings to ensure good adhesion. The aim of the project was achieved.
Acknowledgement
The IGF project 21009 N of the research association Edelmetalle + Metallchemie was funded by the Federal Ministry for Economic Affairs and Energy via the AiF as part of the programme for the promotion of joint industrial research (IGF) on the basis of a resolution of the German Bundestag.
