Design Articles

Mini-Circuits has developed products having a lead-less LTCC (Low Temperature Co-fired Ceramic) substrate that serves as the base of the unit. Examples of such products are the SBTC series of power splitters and the DBTC series of directional couplers.

Metallized pads on the bottom surface, typically deposited with palladium-silver, form the electrical connections and mechanical mounting points. The absence of leads makes possible the ultimate in packing density for a given component size. As with other kinds of surface mount components, lead-less LTCC is mounted on a user’s motherboard via stencil printing of solder paste followed by reflow. An optimum choice of solder formulation and a well controlled reflow process are required to ensure proper wetting of both the LTCC component pads and the motherboard metallization. Adequate wetting is essential for reliable connections.

Solders suitable for lead-less LTCC

Both tin-lead eutectic (Sn/Pb 63/37%) and silver-bearing solder (Sn/Pb/Ag 62/36/2%) have been used successfully with lead-less LTCC. The solder paste must be formulated especially for stencil printing. A well chosen solder paste will not require raising the peak temperature, relative to what is needed for other kinds of components, in order to obtain good wetting.

Examples of solder pastes that have been used successfully without providing an inert atmosphere in the conveyor oven are Kester Part No. 57-3201-4813 (Easy Profile 256, Sn/Pb) and EFD Inc. type 6-SN62-575 (Sn/Pb/Ag). These materials contain “no-clean” flux. Solder pastes containing water-soluble flux are generally more active in removing oxidation from surfaces to be soldered.