Use High-Speed Board-to-Board Connectors to Increase Circuit Density While Improving Performance

Electronic devices are shrinking while data rates are increasing. For designers, these trends require incorporating more circuitry into smaller spaces while maintaining data rates, reliability, and signal integrity. Designers must also address airflow for cooling and physical separation to minimize electromagnetic interference (EMI).

A common solution to increasing circuit density is printed circuit board (pc board) stacking. The use of daughter and mezzanine sub-boards yields more circuit board real estate while providing paths for cooling and signal isolation.

This article briefly reviews the challenges designers of high-speed circuits face. It then introduces board-to-board connectors from Würth Elektronik and shows how they can be used to provide reliable signal connections while maintaining signal integrity.

Mezzanine boards

A mezzanine board layout consists of two parallel pc boards stacked one on top of the other and connected via board-to-board connectors (Figure 1, left).

Figure 1: Shown are examples of a range of mezzanine-mounted pc boards (left); secondary boards can be mounted on connectors or with surface-mount or threaded spacers (right). (Image source: Würth Elektronik)

This board-to-board arrangement of two pc boards provides additional physical space for circuits. It can be used to increase volumetric efficiency, allow interchangeability, or provide physical separation to improve airflow and reduce EMI. Board-to-board connectors interconnect boards; no cables are used. Mezzanine board connectors offer a range of stack heights that determine board spacing. The upper boards can be supported and retained by the connector or secured with surface-mount or threaded spacers for greater vibration and shock resistance (Figure 1, right).

Signal integrity considerations

Signal integrity describes how a signal is distorted or attenuated when transmitted from one board to another through a connector. Some of these effects, such as contact resistance, are not frequency-dependent and can be easily accounted for and corrected.

However, two key frequency-dependent signal integrity parameters are reflection coefficient (ρ) and transmission coefficient (t) (Figure 2). The transmission coefficient is typically expressed as insertion loss in decibels (dB). Reflection coefficient (return loss) is due to data signals being reflected to the source when a step in impedance value is encountered. Insertion loss quantifies attenuation through the transmission path. Both are dependent on the impedance of the connector (Z_CAB) relative to the pc board line impedance (Z_s).

Figure 2: The return loss and insertion loss are dependent on the impedance of the connector relative to the pc board line impedance. (Image source: Würth Elektronik)

Transmission loss reduces the signal amplitude as it passes through the connector and is proportional to the path length and the connector's geometry. Some energy may also be lost due to near-end crosstalk (NEXT) or far-end crosstalk (FEXT). The return loss and transmission coefficient are frequency-dependent parameters that depend on the difference between the connector impedance (modeled as a cable) and the pc board transmission line impedance, assumed to be 50 Ω in this example. The reflection and transmission coefficients are defined by the equations shown.

The graph in Figure 2 shows the variation of those parameters as a function of the connector (cable) impedance. If the connector impedance is 50 Ω, the theoretical return loss is zero, and the transmission coefficient is 100%, indicating no loss. If the connector impedance differs from 50 Ω, the parameters vary proportionally to the difference from 50 Ω and with frequency. In a connector, impedance depends on the insulating material used and the geometry of the contact pins, including their width, length, and spacing (pitch). It is also affected by the wiring of the adjacent pins.

There are two common wiring configurations for transmitting high-speed data (Figure 3): single-ended, with the data signal referenced to ground, and differential, with two complementary signal lines; the data signal amplitude is the difference in their voltages. Differential signaling is used to reduce noise and interference on its dual signal lines. In general, differential signaling is used at the highest data rates. Data signals are typically paired with one or more ground signals to reduce noise pickup.

Figure 3: Shown are three common signal wiring configurations that illustrate the use of intermediate ground conductors to reduce noise and interference pickup. (Image source: Würth Elektronik, modified by author)

Single-ended wiring is typically designed for a characteristic impedance of 50 Ω, while differential wiring is designed for 100 Ω. The pin selections from the connector to the board can affect the performance of the ground conductors.

From a signal integrity perspective, board-to-board connectors must be designed to support specified impedances and data rates.

Examples of board-to-board connectors

A good option for signal connectors in high-speed data applications is the WR-BTB series from Würth Elektronik. These are surface-mount board-to-board connectors available with 40, 80, or 100 pins and a pitch of 0.80 mm, as well as 64 pins with a pitch of 1.00 mm. The 1.00 mm pitch, 64-pin connectors are compatible with IEEE 1386 mezzanine connector requirements. The 0.80 mm pitch connectors are polarized to prevent reverse mating. Multiple stacking heights are available for each pin count.

All WR-BTB connectors feature copper-alloy contacts, selectively gold-plated, with a contact resistance of 50 mΩ or less, depending on the pin count. The connectors' bodies are made from guaranteed halogen-free plastic, making them more environmentally friendly without sacrificing strength, electrical resistance, soldering temperature resistance during pc board assembly, or fire protection rating. They operate over a temperature range from -55 to 85°C. Additionally, they are RoHS 3 compliant.

Unlike RF connectors, WR-BTB connectors do not have a fixed characteristic impedance; it depends on the contact dimensions, the dielectric constant of the underlying board, and the pc board wiring layout, among other things. The WR-BTB connector designs minimize signal reflections in high-speed pc board systems for 50 Ω single-ended or 100 Ω differential-pair transmission lines. For example, the 0.8 and 1 mm pitch connectors are compatible with PCIe 2.0 signaling or USB 2.0 differential signaling at 480 megabits per second (Mbits/s).

An example of a specific WR-BTB plug/receptacle connector pair is the 658158303064 64-pin plug (Figure 4, left) and its 658101003064 mating receptacle (Figure 4, right). These are shrouded 64-pin connectors with a 1.00 mm pitch and a contact width of 0.30 mm. The connectors are rated for a working voltage of 100 V_AC and a current of 1000 milliamperes (mA). The maximum contact resistance of these connectors is 30 mΩ. Both connectors feature integrated surface-mount board guides and include pick-and-place (PnP) clips. These provide a flat surface for PnP machine vacuum nozzles to pick up the connectors without damaging the contacts.

Figure 4: Shown is a 64-pin, 1.0 mm pitch, plug/receptacle pair with PnP clips. (Image source: Würth Elektronik)

The highest available pin count in this product family is 100 pins, such as the 658855603100 0.80 mm pitch 100-pin plug that mates with the 658807713100 receptacle. These connectors have a 50 VAC voltage rating and can handle currents up to 500 mA. The maximum contact resistance is 50 mΩ.

Stacking heights are selected by choosing specific combinations of plug and receptacle pairs. The available stacking heights depend on the pin count and pitch (Figure 5).

Figure 5: Stacking heights are selectable based on connector pitch and lead count. (Image source: Würth Elektronik, modified by author)

To see how this works, the stacking height of the 658158303064 plug and 658101003064 receptacle (highlighted in blue) is 14.75 mm when mated. If the receptacle is changed to a 658105303064 (highlighted in green), the stacking height is 9.75 mm. With two plug components and three receptacles, six stacking heights, ranging from 7.75 to 14.75 mm, are available for 64-pin 1.0 mm connectors. The 0.80 mm pitch connector offers a wider range of stacking heights.

In contrast, the 658855603100 0.80 mm pitch 100-pin plug mated with the 658807713100 receptacle offers only a single stacking height of 10 mm.

Applications

Board-to-board connectors are used across a broad range of applications, including consumer electronics, vehicular systems, industrial automation, medical devices, and many more.

Mezzanine boards, using board-to-board connectors, may be used in the following circumstances:

• For subassemblies that require improved airflow and physical space for cooling
• To enable the use of a common subassembly across multiple product models to reduce costs
• To simplify assembly by allowing the two boards to be assembled separately before they are connected
• To allow pc boards to be disconnected and reconnected, enabling design flexibility
• For specialized circuits, such as radio-frequency (RF) or high-voltage (HV) power supplies, which can be isolated as mezzanine subassemblies
• To easily upgrade boards

These are only a few of the functions enabled by a mezzanine board with board-to-board connectors.

Environmental and safety certifications

WR-BTB connectors are certified to or compliant with the common environmental and safety standards applicable to connectors (Figure 6).

Certification:

Figure 6: Shown are the WR-BTB environmental and safety certifications. (Image source: Würth Elektronik)

Conclusion

Würth Elektronik board-to-board connectors used in mezzanine configurations enhance the volumetric efficiency, signal integrity, and reliability of electronic devices. They also provide more efficient airflow for cooling, improve electromagnetic isolation, and simplify assembly.