While the use of electronic products spans many applications, their internal physical construction or packaging usually follows a similar hierar­ chical structure. In most current systems, four structural levels can be iden­ tified (Fig. 8.1). At the smallest level, the electronic circuitry is contained within the semiconductor chip (or die). The chip must be protected from the environment, electrical signals must be routed to and from it, and

dissipation of the heat generated due to the electrical losses, with acceptable resulting temperature rises, must be ensured. This may be accomplished through the use of a chip package. If multiple chips are housed in a single package for enhanced electrical performance and compactness, the resulting configuration is called a multichip module. The next common structural level is a circuit card or board, which houses many individual packages and provides the appropriate electrical interconnections between them through a network of conducting traces. Such traces can be on single surface layer or within multiple layers of conducting planes to reduce size. Many boards are next housed within a chassis or box. The heat generated within the overall electronic system is rejected to the environment at this level. The box may be hermetically sealed, may have vents, or be equipped with fans for air circulation. One or more of the walls may contain heat exchanges for heat removal.