Feature Article - Hybrid Microcircuit Packaging, Part 1 - By Christopher Henderson

In this section, we’ll provide an overview of hybrid microcircuit packaging. Although this technology may seem old to many viewers, hybrid microcircuits are still used extensively in military and space applications.

Popular in the 1970s and 1980s, hybrid microcircuits (Figure 1) are still common in many military applications. Hybrid microcircuits are composed of multiple technologies like resistors, capacitors, and integrated circuits. They have several distinct advantages for high reliability and extreme environments. They can be hermetically sealed to keep out moisture, and can be made using materials and processes that permit operation over a wider temperature range than plastic encapsulated microcircuits. Hybrid microcircuits can be manufactured with thick film or thin film technology, and can be formed to create leads for surface mount or pins for through-hole mount.

Let’s begin by looking at the materials used in thick film and thin film hybrid microcircuits (Figure 2). The elements of interest would be electrodes, a metal that makes contact to a non-metallic part of the hybrid, terminals that make contact to other metals, interconnect layers, resistors, and dielectric layers. There are thick film materials to do these functions as well as thin film materials. We’ll begin with thick film materials.

Thick film materials typically create layers greater than 25 microns thick. Silver and gold-based materials are common for conductors. Silver, silver-palladium, or silver-palladium-platinum pastes are used in a screening technology to create conductive traces on the substrate (Figure 3). These materials require an 850°C firing temperature to bond the material to the substrate. Gold-based materials are also common; however, the cost of gold is making this approach more expensive. Gold-platinum and gold-palladium-platinum are also quite expensive. The firing temperature is a few degrees higher than for that of silver. There are also some thick film dielectric materials. Many of these are proprietary, though.

Thin film technology is manufactured differently than thick film technology hybrids (Figure 4). A thin film technology hybrid uses layers that are between 0.1 and 5 microns; therefore, the substrate must be extremely flat and uniform. This requires a fine-grained material like alumina, aluminum nitride or quartz. Sometimes they are polished to create a flatter, more uniform surface. The SEM image on the right shows an example of a small-grained, flat substrate. Thin film layers can be vacuum deposited through evaporation, or they can be sputtered. Numerous metals and metal alloys can be sputtered on a substrate. Some of the common ones are listed here. Metals can also be plated as well. Gold, nickel, and copper can be vacuum deposited or deposited through electroless deposition.

There are three common substrate types: aluminum oxide or alumina, beryllium oxide, and aluminum nitride. Alumina is the most common substrate. It has a coefficient of thermal expansion that is relatively low, and it is inexpensive to manufacture. Figure 5 shows some of the common properties of alumina. Beryllium oxide has some nice properties—including a higher thermal conductivity—but beryllium is toxic. Therefore, special handling procedures are required. Many electronics manufacturers do not want to deal with the liability of beryllium, so it is not commonly used anymore. A substitute for beryllium oxide is aluminum nitride. It has a higher thermal conductivity, and better-matched coefficient of thermal expansion to silicon. As a result, it is receiving more attention.

Let’s move on and discuss resistors (Figure 6). Resistors are a common element within a hybrid microcircuit. They can be made using thick or thin film technologies. Lower resistance thick film resistors (a) are made of conductive metals, while higher resistance elements can be made from ruthenium oxide and other materials. These materials are typically deposited on alumina substrates. Thin film resistors (b) can be made from nickel alloys, nichrome, nichrome and tantalum pentoxide stacks, tantalum nitride, and other materials. The substrates are typically silicon dioxide or quartz. Thin film resistors can be made with top contacts for wirebonding, or with back contacts for better high frequency performance.

Capacitors are another common element in a hybrid microcircuit. Capacitors can also be made with thick and thin film technologies (Figure 7). Thick film capacitors (a) are typically made from ceramic or tantalum with electrodes at either end for connection, while thin film capacitors (b) are made using an aluminum-silicon dioxide-silicon, or MOS structure. They can also be made as metal-insulator-metal structures. Thin film capacitors are typically wirebonded.