Electronic devices are constantly shrinking, while data transmission rates are constantly increasing. For designers, to comply with this trend, they must be able to integrate more circuits in a smaller space while maintaining data transmission rate, reliability, and signal integrity. Designers must also address the issues of air cooling and physical isolation to minimize electromagnetic interference (EMI) as much as possible.
Stacking printed circuit boards (PC boards) is a common method to increase circuit density. By using daughter boards and sandwich daughter boards, more circuit board space can be obtained, while constructing cooling and signal isolation paths.
This article briefly outlines the various challenges faced by high-speed circuit designers. Then introduce W ü rth Elektronik's board to board connectors and explain how to use these connectors to achieve reliable signal connections while maintaining signal integrity.
Sandwich panel
The layout of the sandwich panel consists of two parallel circuit boards stacked vertically, which are electrically connected through board to board connectors (Figure 1, left).
Multiple column sandwich mounted circuit boards
Figure 1: The left image shows examples of multiple sandwich mounted printed circuit boards (PCBs); The figure on the right shows the installation method of the sub board, which can be installed through connectors, surface mount technology, or threaded isolation columns. (Image source: W ü rth Elektronik)
This board to board arrangement consisting of two circuit boards brings more physical space to the circuit. This structure can be used to improve volumetric efficiency, achieve interchangeability, or form physical isolation to improve airflow and reduce EMI. Board to board connectors are directly connected to the circuit board without the use of cables. The sandwich panel connector can achieve multiple stacking heights with determined board spacing. The upper circuit board can be supported and fixed by connectors, or fixed with surface mount or threaded isolation columns to enhance vibration and impact resistance (Figure 1, right).
Consideration factors for signal integrity
Signal integrity describes how signals are distorted or attenuated when transmitted from one circuit board to another through connectors. Some of these effects, such as contact resistance, are frequency independent and can be easily incorporated into calculations and corrected.
However, the two key signal integrity parameters related to frequency are the reflection coefficient (ρ) and the transmission coefficient (t) (Figure 2). The transmission coefficient is usually expressed in decibels (dB) using insertion loss. Reflection coefficient (return loss) is caused by the reflection of data signals back to the signal source when encountering impedance value steps. Insertion loss is used to quantify the attenuation of the transmission path. Both depend on the connector impedance (ZCAB) relative to the PC board line impedance (Zs).
Both return loss and insertion loss depend on the impedance of the connector
Figure 2: Return loss and insertion loss depend on the connector impedance relative to the PC board line impedance. (Image source: W ü rth Elektronik)
The transmission loss will reduce the amplitude of the signal passing through the connector and is proportional to the path length and the geometric structure of the connector. Near end crosstalk (NEXT) or far end crosstalk (FEXT) can also cause some energy loss. Return loss and transmission coefficient are frequency dependent parameters that depend on the difference between the connector impedance (simulated as a cable) and the circuit board transmission line impedance (assumed to be 50 Ω in this example). The reflection coefficient and transmission coefficient are defined by the formulas shown.
Figure 2 shows the variation of these parameters with the impedance of the connector (cable). If the impedance of the connector is 50 Ω, the theoretical return loss is zero and the transmission coefficient is 100%, indicating no loss. If the impedance of the connector deviates from 50 Ω, the changes in relevant parameters will be proportional to the deviation value and frequency of the connector impedance from 50 Ω. In connectors, impedance depends on the insulation material used and the geometric structure of the pins, including width, length, and distance (spacing). In addition, the wiring of adjacent pins can also have an impact on it.
There are two common wiring configurations for transmitting high-speed data (Figure 3): one is a single ended structure, where the data signal is referenced to ground; Another type is the differential structure, which uses two complementary signal lines, and the amplitude of the data signal is the voltage difference between the two signal lines. Differential signals are used to reduce noise and interference on dual signal lines. Generally speaking, differential signals are used for applications with the highest data rates. Data signals are typically paired with one or more ground signals to reduce noise pickup.