What Are Factors Affecting Selection of Interconnections?
Selections of the packaging approaches among the various elements is dictated not only by system function, but also by the component types selected and by the operation parameters of the system, such as the clock speeds, power consumption, and heat management methods, and the environment in which the system will operate.
Speed of Operation
The speed at which the electronic system operates is a very important technical factor in the design of interconnections. Many digital systems operate at close to 100MHz and are already reaching beyond that level. The increasing system speed is placing great demands on the ingenuity of packing engineers and on the properties of materials used for PWB substrate.
The speed of signal propagation is inversely proportional to the square root of the dielectric constant of the substrate materials, requiring designers to be aware of the dielectric properties of the substrate material they intend ot use. The signal propagation on the substrate between chips, the so-called time of flight, is directly proportional to the length of the connectors and must be kept short to ensure the optimal electrical performance of a system operating at high speeds.
For systems operating at speed about 25 MHz, the interconnections must have transmission line characteristics to minimize signal losses and distortion. Proper design of such transmission lines requires careful calculation of the conductor and dielectric separation dimensions and their precise manufacture to ensure the expected accuracy of performance. For PCBs, there are two basic transmission line types 1) Stripline, 2) Microstrip.
As the clock rates of the chips increase and as the number of gates per chip grows, there is a corresponding increase in their power consumption. Some chips require up to 30W of power for their operation. With that, more and more terminals are required to bring power in and accommodate the return flow on the ground planes. About 20 to 20 percent of chip terminals are used for power and ground connections. With the need for electrical isolation of signals in high speed systems operation, the count may go to 50 percent.
Design engineers must provide adequate power and ground distribution planes within the multilayer boards (MLBs) to ensure efficient, low resistance flow of currents, which may be substantial in boards interconnecting high speed chips consuming tens of watts and operating at 5V, 3.3V, or lower. Proper power and ground distribution in the system is essential for reducing di/dt switching interference in high speed systems, as well as for reducing undesirable heat concentrations. In some cases, separate busbar structures have been required to meet such high-power demands.
All the energy that has been delivered to power integrated circuits (ICs) must be efficiently removed from the system to ensure its proper operation and long life. The removal of the heat from a system is one of the most difficult tasks of electronic packaging. In large systems, huge heat sink structures, dwarfing the individual ICs, are required to air cool them, and some computer companies have built giant superstructures for liquid cooling of their computer modules. Some computer designs use liquid immersion cooling. Still, the cooling needs of large systems tax the capabilities of existing cooling methods.
The situation is not that severe in smaller, tabletop or portable electronic equipment, but it still requires packaging engineers to ameliorate the hot spots and ensure longevity of operation. Since PWBs are notoriously power heat conductors, designer must carefully evaluate the method of heat condition through the board, using such techniques as heat vias, embedded metal slugs and conductive planes.
As the frequency of operation of electronic equipment increases, many ICs, modules, or assemblies can act as generators of radio frequency (RF) signals. Such electromagnetic interference (EMI) emanations can seriously jeopardize the operation of neighboring electronics or even of other elements of the same equipment, causing failures, mistakes, and errors, and must be prevented. There are specific EMI standards defining the permissible levels of such radiation, and these levels are very low.
Packaging engineers, and especially PWB designers, must be familiar with the methods of reducing or cancelling this EMI radiation to ensure that their equipment will not exceed the permissible limits of this interference.
System Operating Environment
The selection of a particular packaging approach for an electronic product is also dictated by its end use and by the market segment for which that product is designed. The packaging designer has to understand the major driving force behind the product use. Is it cost driven, performance driven, or somewhere in between? Where will it be used: for instance, under the hood of a car, where environmental conditions are severe, or in the office, where the operating conditions are benign? The IPC has established a set of equipment operating conditions classified by the degree of severity.
Product cost has become the most important criterion in any design of electronic systems. Whiel complying with all the aforementioned design and operation conditions, the designing engineer must keep cost as the dominant criterion, and must analyze all potential trade offs in the light of the best cost/performance solution for the product..
The importance of the rigorous cost trade off analysis during the design of electronic product is underscored by the fact that about 60 percent of the manufacturing costs are determined in the first states of the design process, when only 35 percent of the total design effort has ben expended.
Attention to manufacturing and assembly requirements and capabilities during product design can reduce assembly costs by up to 35 percent and PWB costs by up to 25 percent.
Read also: Double-sided PCB Manufacturing.
1, Printed Circuit Handbook by Clyden F. Coombs, Fr., and Happy T. Holden