Four things you need to know about PCB differences and pass

In high-speed printed circuit boards (PCBS), through-holes have been criticized for reducing signal integrity. pcb stackup calculator However, the use of holes is unavoidable. On a standard board, the components are located on the top layer, and the wiring of the differential pairs is located on the inner layer. Crosstalk between the electromagnetic radiation and the line pairs in the inner layer is very low. Components on the board plane must be connected to the inner layer through holes.

Fortunately, an information-transparent through-hole can be designed to minimize the impact on performance.

1. Pass the basics of architecture

Let's first look at the element that connects the top transmission line to the inner layer in a simple through-hole. 1 oz vs 2 oz Figure 1 is a 3D diagram showing the through-hole structure. There are four basic components: signal through-hole, through-hole stub, through-hole pad and isolation pad.

The through hole is a metal cylinder plated on the outside of the through hole between the top and bottom layers of the circuit board. The signal hole is connected with a transmission line on a different layer. Through hole stump is the unused part of through hole. The through hole gasket is a circular gasket that connects the through hole to the top or internal transmission line. Isolation discs are annular gaps in each power supply or ground layer to prevent short circuits to the power supply and ground layer.

2. Electrical characteristics of through-hole components

As shown in Table 1, let's take a closer look at the electrical system properties of each of our perforated components.

A simple path is a pi-type network consisting of capacitor-inductor-capacitor (C-L-C) elements in two adjacent layers. Table 2 shows the effect of the through hole size.

By balancing the size of inductance and parasitic capacitance, it is possible to design a through-hole with the same characteristic impedance as the transmission line, which will not have a special impact on the work of the circuit board. There is no simple formula for converting through hole dimensions to C and L components. The 3D electromagnetic (EM) field solver can predict the structural impedance based on the dimensions used in the PCB layout. By repeatedly sizing the structure and running 3D simulations, the through-hole size can be optimized to meet the required impedance and bandwidth requirements.

3. Design transparent differential holes

As we discussed in A previous post, in order to achieve differential pairing, lines A and B must be highly symmetrical. These pairs are routed on the same level. If you need to pass holes, you must drill holes in the adjacent positions of the two wires. Because the two holes of the differential pair are very close together, an elliptical disconnector shared by the two disconnectors can reduce parasitic capacitance instead of using two separate disconnectors. Ground holes are also placed next to each hole to provide ground loops for holes A and B.

Do not forget that when the data transmission rate has exceeded 10Gbps, the hole stump will seriously affect the integrity of the high-speed economic development signal. Fortunately, there is a backhole PCB manufacturing technology process that can be analyzed to drill holes in unused through the cylinder. Depending on the design tolerances of the manufacturing process in China, the backside drilling removes unused through-hole metal and minimizes the through-hole stump to less than 10mil at maximum capacity.

A three-dimensional electromagnetic simulator is used to design a differential channel according to the required impedance and bandwidth. It's an iterative process. This process is repeatedly adjusted by sizing and running electromagnetic simulations until the desired impedance and bandwidth are reached.

4. How to verify performance

The differential through-hole design shown in Figure 2 has been built and tested. The test sample consists of a pair of differential wires on the top layer, followed by differential holes on the internal differential wires, and then a second pair of differential wires connected again to the BGA ground pad on the top layer. The total length of the signal path is about 1330 mil. I used a differential time domain reflectometer (TDR) to measure differential impedance, a network analyzer to measure bandwidth, and a high-speed oscilloscope to measure the eye map of the data to understand its effect on the signal. Figures 3, 4, and 5 show the impedance, bandwidth, and eye map, respectively. The left picture shows the test results when backdrilling is used, and the right picture shows the test results when backdrilling is not used. In the bandwidth baud diagram in Figure 5, we can clearly see that when the data rate is greater than 10Gbps, reverse drilling is critical to achieving high performance.