Product safety and the printed wiring board - impedance control and trace fusing current

Many of today´s printed wiring boards are designed with impedance control as the foremost criterion. The primary goal is to meet radiated/conducted emissions limits and maintain good signal integrity. Depending on the desired trace impedance value, designers size the trace and printed wiring board layers with optimal dimensions. Many high speed circuits depend on strict impedance control to minimize reflections and signal attenuation. Printed wiring boards of today are being designed with high density components. The high density components inherently have many pins and thus, a large amount of interconnecting traces is needed. These traces take up a vast majority of space throughout the printed wiring board. To fit more traces in a limited amount of space, traces are becoming less wide and thinner. Impedance control can still be maintained, however, trace fusing becomes more of a problem to solve. Under normal operating conditions, these signal traces carry no more than a few amps at most. However, under fault conditions, these same traces may have to survive for several minutes at tens of amps before the fault is cleared. As traces get thinner, their current capabilities are decreased, causing a potential product safety problem. The trace can open (trace fusing), causing an interruption of service, or can become hot enough to burn through the printed wiring board and create a fire, before the trace can open. To satisfy both concerns of signal integrity and safety, designers must be aware of the physical characteristics of the signal traces. The traces must be large enough to handle the fault current as not to cause the printed wiring board to burn. Or, size the trace to act like a fuse, and open before the printed wiring board becomes a fire hazard. At the same time, signal integrity must be maintained for circuits to function properly. Traces that open may cause other safety concerns such as interruption of critical communication services, medical monitoring systems or inhibit safety monitoring circuits. The designer must be cognizant of both impedance and fault current requirements, and design a signal trace that meets both requirements.