Background

Since the 1930’s and 1940’s many methods have been used for making electrically conductive interconnects on insulating substrates. The often conflicting requirements of cost, electrical insulation, thermal management, track definition, track adhesion, signal transfer speed, current-carrying capability, resistance to adverse working environments, physical strength, performance at high frequencies and so on, have meant that varying degrees of success have been achieved. However, during the last few years the process of copper-plating ceramic has been greatly improved. It can now be considered as a serious alternative to well-established interconnect methods such as etched Cu/epoxide-glass laminates, ‘screen-and-fire’ thick-film circuits, evaporated-metal-on-glass thin-film circuits and etched thick-film.
Copper-Plated Ceramics

Copper-plated ceramic addresses the interconnection requirements of many of the latest electronics systems. The key features stem from the combination of the insulator, the conductor and the manufacturing methods.
Materials and Sizes

The normal insulator used is 96% alumina ceramic. When the highest performance at frequencies above 5GHz is required, 99.5% alumina is used, but since the prices differ by a factor of ten, it is used only when applications require lower losses at higher frequencies. Individual circuits can be produced in any size up to the dimensions of the ceramic panel used, usually 100mm x 100mm or 150mm x 100mm. If more than one circuit fits on to the panel, they are usually made in multiples, with each circuit printed onto prescribed section.
Why Use Alumina for the Insulator?

The electrical properties of alumina are excellent. The insulation resistance is very high and, over the range that affects circuit boards, does not change significantly with either temperature or humidity. The thermal conductivity is reasonable, being a little under one tenth that of copper but around one hundred times that of most organic materials.
Why Use Copper for the Conductor?

The conductor is pure electroplated copper. For all practical purposes only silver has higher electrical conductivity, but copper is cheaper, much more metallurgically stable and its solderability, regarding tin-lead solders, is very well understood. Conductor thickness can range between 5μm and 100μm with only a minimal additional cost for higher thickness. At the highest thickness some loss of resolution capability occurs - the ultimate resolution is mainly limited by the surface quality of the substrate. The conductors have near rectangular cross-sections, very low surface area to volume ratio and the top surface of the conductor is as smooth as that of the substrate. The electrical resistance of the tracking is about 1 milliohm per square for a thickness of 25μm
Other Materials

Despite the fact that pure copper is eminently suitable for solder assembly, it is often necessary to add other materials for various reasons, such as:

· Preventing oxidation during storage, aesthetics, wirebonding and protection against solder-bridging and physical damage

· Organic anti-tarnish coatings prevent oxidation and discolouration by sulphur-containing gases. Plated gold (sometimes over plated nickel) is used both for aesthetics and where wire-bonding is contemplated

· Tin-lead can give retention of solderability during longer storage periods

· Photo-imaged solder mask minimises solder‑bridges and physical/handling damage

· Electroless silver is also available.

These finishes may cover the entire circuit or can be applied only in selected areas.