Electroless plating on silver is a low cost alternative to printing of mixed metals or pure gold paste systems on LTCC. It overcomes the necessity to have material transitions from inner to outer layers or from conductor lines to wire bonding- or solder-pads.
Since no commercial process and material set for silver thick film conductors has been available on the market a proprietary Ni/Pd/Au coating technology was developed for the use on silver inks for LTCC and Al2O3-ceramic as a base for both soldering and wire bonding.
The work included the screening of different chemicals as well as several silver paste systems from two commercial vendors. Conductor adhesion, plating layer thicknesses, plating accuracy, (lead free) solderability and gold wire-bondability were assessed to optimize the process.
Layers of about 5 microns Ni, (0.1 to 0.3) microns Pd and (0.05 to 0.15) microns Au were electrolessly deposited. The developed Ni-Pd-Au finish is an economical alternative with only about a quarter of the cost compared to the conventional use of silver, silver / palladium and gold compounds for ceramic substrates. This technology allows coating of the structures down to a fine pad size of 200×200 microns and a minimum line width of 100 microns, without reducing the adhesion mechanism between thick-film metallization and ceramic substrate.
By covering of pure conductors with high temperature glass or dielectrics, further material saving is possible. Besides, the process offers also very good coating of structures in cavities.
