Patent TW-200830967-A (TW)
Title: A novel via structure for improving signal integrity
Inventor: HSU HSIUAN-JU
IPC: H05K3/42
Filed: 2007-01-11
Abstract: The embodiment of the invention is about a novel via structure which can be incorporated into printed circuit boards, integrated circuit packages, and integrated circuits in order to reduce crosstalk, to improve signal integrity and to achieve EM emission compliance. A 4-layer (2 signal layers and 2 power layers or 2 signal layers and 2 ground layers) circuit board assembly was used for demonstrating the effect of the novel via structure. The same concept can be applied to any multi-layer circuit board. Layers that have an electrical property can be added above, under, or within the basic 4-layer circuit board to achieve a multi-layer circuit board. For 2-layer and 3-layer circuit boards, a deformed version of the proposed via structure based upon the same concept will be needed for a coplanar waveguide configuration. The 4-layer circuit board can be separated into 3 types of circuit structures. Type A (see figure 2): one signal layer is on the top layer; 2 ground layers or 2 power layers represent the two middle layer; and the other signal layer is on the bottom layer. The signal via connects the transmission lines that lie on the top and bottom layers. The ground via or power via is plated surrounding the signal via, i.e., the ground via or power via is concentric to the signal via. The plated ground via or the power via needs to be kept a non-trivial distance (such as the diameter of the anti-pad) from the signal via to avoid shorting. The impedance of the signal via can be properly designed by adjusting this distance between the signal via and the ground via (or power via). The thickness of the plated ground via (or power via) must be larger than the skin depth at the frequency of the signal. It is assumed that copper will be used for the ground via or power via in this implementation. The plated via connects the third layer of the ground plane (or power plane) to the fifth layer of the ground plane (or power plane) to be ground via (or power via). Note that it is necessary that the third layer and the fifth layer have the same electrical properties. With this configuration the signal via can be isolated to prevent electromagnetic coupling with other vias (not shown in the figure) on the circuit board. Also, the plated ground via (or power via) provides a voltage reference or a complete current return path for the signal propagating through the signal via. Eventually, the impedance discontinuity between the transmission line and the signal via will be reduced so that the subsequent reflection effect and, hence, the return loss will be minimized. Due to the separation of the signal vias by using the plated ground via (or power via), the crosstalk between signal vias will also be reduced to improve the quality of the signals. Consequently, the radiation on the edge of the circuit board (EMC/EMI problem) will be minimized as well. Type B (see figure 4): the top layer and bottom layer are ground layers or power layers. The third layer and the fifth layer are signal layers. The signal via connects the transmission line on the third layer with the transmission line on the fifth layer. A ground via or power via is created to connect the top layer with the bottom layer. The diameter of the ground via or power via is smaller than that of the signal via to avoid shorting. The impedance of the signal via can be properly designed by controlling the distance between signal via and the ground via (or power via) as well. The ground via or power via is concentric to the signal via. Note that it is necessary that the top layer and the bottom layer have the same electrical properties. The thickness of the plated ground via or power via is larger than the skin depth associated with the frequency of the signal. The ground via or power via is assumed to be a highly conductive metal, such as copper, in this implementation. Also, the ground via (or power via) provides a voltage reference plane or a complete current return path for the signal propagating through the signal via. Finally, the impedance discontinuity between the transmission line and the signal via will be reduced so that the subsequent reflection effect and, hence, the return loss will be minimized. Consequently, the radiation on the edge of the circuit board (EMC/EMI problem) will be minimized as well. Type C (see figure 2 or figure 4): Refer to Type A or Type B. The signal layers are replaced with power layers or ground layers. The resulting configurations, i.e., the stack from the top to the bottom layers, turn out to have the generic structure: power-ground-ground-power or ground-power-power-ground. When the signal via is replaced with a power via or a ground via, the via structure will become a ''capacitor'', an element that can store electrical energy. The value of the capacitor can be properly designed by filling the space between the power via and ground via with a material having a selected permittivity value, or by adjusting the distance between the power via and ground via. The addition of a capacitor will benefit the power integrity because the voltage fluctuations between the power plane and the ground plane can be minimized. Finally, the overall signal integrity of the system will be improved due to the reduction of the voltage fluctuations in these layers.