Suggestions on High-Speed PCB Design of Holes

This Gold Phoenix PCB Knowledge Center article covers high-speed PCB hole design, including blind, buried, and through-hole types, impedance discontinuity, signal reflection, and five layout guidelines for reducing hole parasitics.

In high-speed multilayer circuits, growing data rates make signal integrity a critical design issue which greatly affects the overall performance of a high-speed circuit. As a result, the PCB design of the through-hole, a PCB electrical conductor used to connect the routing in different layers, is very important. The through-hole is comprised of the hole, the solder pad around it, and power layer isolation, and is usually divided into blind hole, buried hole and through hole. In high-speed PCB design, simple holes will cause a great deal of negative effects to the design of the circuit, especially in the high-speed multilayer. Due to the frequency growth and the steepening of the signal rising edge, the impedance discontinuities caused by holes will bring about signal reflection which seriously impacts the system performance and signal integrity.

In order to reduce the adverse effects of hole parasitics, here are some suggestions on PCB design. Firstly, in view of both the cost and signal quality, a reasonable size of the hole is very important. At present, it is difficult to make the hole much smaller. On the other hand, holes with a larger size for power supply or ground wire are also necessary because of the impedance reduction. Secondly, thinner PCB board is conducive to cutting down two of the parasitic parameters of the holes. Thirdly, the holes of the PCB board signal traces should be as few as possible. Fourthly, the holes for power and ground pins should be as near as possible to make sure the lead between the holes and pins as short as possible in order to avoid the increase of inductance. Meanwhile, power and ground leads should be as thick as possible to reduce the impedance. Fifthly, some ground holes could be placed near the holes for the signal transitions between different layers in order to provide the nearest loop for the signal. Even a great amount of extra ground holes can be placed on the PCB. But in the case of great hole density, some solder pads may lead to the formation of breaker slots in copper layers to cut off the circuit; the solution is to move the hole location or cut down the pad size of the holes in this copper layer.

These days, high-speed PCB design is widely used in the field of communications, computers, graphics and image processing and so on. The design of all high-tech value-added electronic products is in the pursuit of low power consumption, low electromagnetic radiation, high reliability, miniaturization, etc. in the high-speed PCB design. In order to achieve the above objectives, the PCB design of holes is an important factor. For high-speed PCB design, continued attention to hole design and signal integrity is an important part of achieving these objectives.