There are many factors to consider during the design stages of circuit board manufacturing, but doubtless speed and size will be among them. More to the point, manufacturers want to minimise size and maximise speed without incurring additional costs.

Multichip modules (MCMs) represent a technology that meets such needs. The modules consist of two or more chips, mounted by wire-bonding, flip-chip, or tape automated bonding (TAB) onto a second-level package that is then placed onto the board.

Within the module are various types of connection – between the elements of the module, or between the elements and the substrate. The second-level package may also contain plated through-holes and vias, and is attached to the board by pin, pad or solder bumps.
The substrate is an important part of the package, providing mechanical support, electrical conductivity or insulation and/or thermal conduction. Substrates can be routable or non-routable, but most MCMs are routable. As such they give signal, power and ground routing, whereas non-routable substrates act as heat sinks.

Routable substrates can be categorised further according to the technology used to make them. MCM-L is based on laminated PCB technology, and is the cheapest method for substrate production. The laminates are made from an organic material, such as epoxy glass. This technique offers the advantage that the lead frame can be integrated with the substrate to simplify assembly and aid heat distribution. Organic laminates, however, have a low routing density, poor thermal conduction, and a high coefficient of thermal expansion. Newer materials, for example polyimides, can overcome these problems and can be adapted to suit individual needs.

The use of ceramics in the manufacture of substrates presents a number of advantages. They are electrically non-conductive and can be used for the module package body as well as the substrate. Known as MCM-C, ceramic substrates can be thick-film multilayer, high or low-temperature co-fired ceramic.

A third type of process, MCM-D, is used when high densities and fine-line geometries are required. Ceramic or silicon is used as the substrate base, onto which conductor and dielectric layers are deposited. MCM-D technology allows the addition of electrical enhancements such as decoupling capacitors and photo-etchable resistor films. A major disadvantage of this technology, though, is the cost. Due to a generally low demand and low production volume, this is the most expensive option.

Cost is not the only influence on the choice of substrate, of course. Routing and performance, number of signal layers (and therefore via requirement), ease of production and versatility are all necessary considerations.

Other options relate to mounting and package selection. Possible mounting technologies include surface mount, insertion mount and chip-on-board. Package types available are single or dual in-line, pin-grid array, small outline package, and flatpack , among others. Selection criteria includes the total number of inputs and outputs, packaging efficiency and package size.

MCMs allow a reduction in the overall size of a board by bringing several components together in a single package. Doing so also results in higher signal speeds and a maximisation of thermal performance – without an increase in weight or cost.

A big advantage over standard PCBs is that MCMs can provide better signal integrity in high bandwidth and high clock-rate designs. The packaging of key components responsible for specific functions increases the upgradeability of the motherboard.

Another very important feature of MCMs is that they are highly customisable. By selecting from a range of substrate, package and mounting technologies, with an unlimited choice of component combinations, the manufacturer can design an MCM to meet any need. Such needs usually involve speed and size reductions.

A relatively recent development is the digital MCM (dMCM). An example of the use of a dMCM is the development by Fujitsu and Sun Microsystems of a dMCM incorporating a digital microprocessor. The design allows the proximity of chip connections that gives the processor close access to the main memory. The result is a network module with a 75% reduction in size from the system board solution.

The range of improvements brought to PCBs when MCMs are added, with little extra cost, means they are a vital technology for the continued development of faster, smaller, lighter electronic products.