Mezzanine connectors face dramatic challenges in high-data-rate telecom and datacom systems. Needed: a high-speed, high-density parallel board connector solution.
By Magnus Henzler
Currently, digital signal speeds on PCBs are moving up to 5 Gbit/s, and signal speeds of up to 10 Gbit/s are expected to get into the designs the next few years. These trends are driving the need for enhanced backplane and mezzanine connector technologies to meet the signal-integrity requirements of these systems.
The dramatic increase in bandwidth has impacted connector performance in terms of capacitance, crosstalk, impedance, inductance, and reflection. Several industry standards have bus structures based on high-speed lanes, or channels, using 2.5 to 3.187 Gbits/s per lane. These include 10G Ethernet (IEEE 802.3ak - CX4), InfiniBand, 10G Fibre Channel, Serial Attached SCSI (SAS), and Serial ATA2 (SATA-2). Consequently, systems need high-performance connectors that not only meet higher electrical specifications, but also meet high-density requirements.
Several high-speed backplane connectors have emerged. The telecom and datacom markets are the drivers for higher speed connectors, particularly for backplane, board-to-board, and I/O products. These connectors are designed to solve signal bandwidth, signal loss, and cable management challenges. To accommodate the high speeds, designers have many special requirements to consider, particularly in the area of grounding, contact grids, and shielding.
A great challenge is in the grounding system. Ground planes in the connector and special placement of the signal pins relative to the ground planes can control higher frequencies. Parallel to the high-speed, high-density trend is miniaturization, which is playing an important role in some consumer and telecommunications applications.
In new digital systems, data has to be switched and transferred at multi-gigabit speeds. Therefore, designers must consider connectors as part of the transmission line, and take care of impedance, propagation delay, skew, and crosstalk issues. The backplane has become the core of interconnection between each of the system elements. Mezzanine connectors and cable interconnects to the backplane are the infrastructure that enables systems to operate at multi-gigabit speeds (see Fig. 1). So, designers are now including the backplane and mezzanine connectors into their initial design process.
 FIGURE 1. Mezzanine connectors permit high-speed signals to be shared between boards in a board-to-board manner. |
In high-speed systems, differential signaling is replacing single-ended signaling. This means the signal is represented as a voltage difference between a pair of dedicated lines. Thus, differential signaling requires two separate lines for each signal, but it offers greater isolation from noise (crosstalk and electromagnetic interference), and it can operate at lower signal voltages.>
Multi-gigabit backplane and mezzanine connectors must carefully control impedance throughout the connector signal path. Since characteristic impedance is a function of geometry and materials, each of these variations can alter the impedance and generate reflections. The spacing of conductors and integrated ground shields greatly determine the target impedance, which is typically 50-Ω signal to ground in single-ended, and 100-Ω signal-to-signal in differential applications.
Two paths
To control impedance, reduce crosstalk, and improve coupling between differential pairs, high-speed connectors have evolved into fully shielded structures that perform like a twin-axial cable. Differential pair signals must arrive at their destination at the same time, so the length of the conductive path through the connector is carefully designed to minimize differences in pair length, and the resulting signal skew. The capacitance of the plated through-hole is a significant source of noise generation and reflections. Therefore, connectors with surface-mount technology (SMT) or through-hole-reflow terminals offer the best signal-integrity benefits.
It is possible to implement backplane and high-speed mezzanine connectors, as well as high-performance cable assemblies for applications up to 10 Gbit/s. One example was designed specifically for high-speed differential signaling, and has been widely adopted in a variety of applications, including the AdvancedTCA specification. “L”-shaped integrated shields provide mechanical rigidity and isolation between the differential pairs for better signal integrity through the connector. The connector has been proof-tested in circuits operating at up to 10 Gbit/s. There are two commercial sources for these connectors, and one offers a cable connector based on the design. This makes it possible to transmit data securely and reliably via thin cables, and with high signal integrity, in applications with high-speed differential signaling in the range from 2.5 to 10 Gbit/s.
Mezzanine requirements
Just as backplane connectors and related cable assemblies ensure the high-speed connections from the boards to the backplane and to peripheral devices, mezzanine connectors are high-density, space-saving stacking connectors for parallel connection of PCBs. They can be used to place high-pin-count devices onto mezzanine cards to simplify board routing without compromising system performance.
Several popular mezzanine standards like CMC, PMC, and VIM have emerged to support the configuration versatility and performance required by designers of board-based systems. The ANSI-VITA 32 Processor PMC (PrPMC) standard extends the functionality of CMC/PMCs (PCI Mezzanine Cards). Further extensions are now being developed by VITA, in the VITA 42.x XMC (switched fabric mezzanine) draft standards. PICMG is also developing a new PICMG AMC.x AdvancedMC series of mezzanine specifications aimed at AdvancedTCA applications. These new mezzanine standards target the high-speed, high-bandwith I/O signaling that is needed for modern switch fabrics.
The requirements for new mezzanine connectors are, first and foremost, high-speed and high-density. The connectors should be able to support signals up to 10 Gbit/s. They also should handle both differential signals and standard, single-ended signals. These needs require a special contact design and configuration. To save space, a low-profile design and compact layout is appealing. In response, a new family of high-density parallel board connectors now meets the increasing bandwidth demands in mezzanine applications. It handles single-ended and differential signals with data rates up to 10 Gbit/s, and supports automatic SMT assembly in a low-profile (5-mm) package.
Speedy solution
The modularly designed, shielded, 1.0-mm connector system consists of two terminal banks and two outwardly arranged shield plates. The connector provides a closed-entry design based on reliable dual-beam leaf contacts. These MicroSpeed connectors are intended for space-saving board-to-board connections with short board distances (see Fig. 2). It is equally suitable for differential and single-ended signals.
 FIGURE 2. Microspeed connectors take advantage of the dual-spring contact design perfected by ERNI and scaled to accommodate 1.0 mm connectors with 0.7 mm of tolerance. |
Areas of application include modern telecommunication and data transmission systems with speeds up to 10 Gbit/s, as well as technologies in the fields of measurement, medicine, and government applications. Further, the SMT version of this technology can be processed automatically using the conventional SMT assembly and reflow-soldering process.
 FIGURE 3. Optimal crosstalk reduction is achieved with a vertical (transverse to longitudinal direction) configuration of the signal pairs. |
The longitudinal pitch of the MicroSpeed connectors is 1.0 mm, and the transverse pitch is 1.5 mm (for impedance purposes). Differential signal pairs can be arranged horizontally or vertically (see Fig. 3). Optimized crosstalk behavior is achieved with a vertical (transverse to longitudinal direction) configuration of the signal pairs and the pairing arrangement of signal and shield contact pairs. Measurements and eye diagrams at 10 Gbit/s have demonstrated that crosstalk is less than 2% at 100-ps rise time, and the reflection factor is less than 5% with a board thickness of 1.6 mm (FR4, 100-mm trace lengths). Furthermore, the MicroSpeed connectors exhibited 100% co-planarity with a tolerance of ±0.05 mm. Because of this tolerance compensation, several mezzanine connector arrangements are possible on one board without mating or contact problems.
 FIGURE 4. Soldering test boards were used to verify the expected soldering reliability of MicroSpeed connectors (mid-left of the board). |
In this connector solution, the target impedance is reached: 100 Ω for differential signals and 50 Ω for single-ended signals. For signal terminals processed using SMT, the user can select between SMT or THR terminal options for the ground contacts. The ground contacts also assure robust connector strain relief. The length of the connector modules, 27-mm long with 50 signal contacts and two shield plates, enables them to be easily lined up with virtually no loss of space. The concept permits additional pins to be added or removed, in six steps, up and down.
Solder and performance tests
Several testboards assembled with MicroSpeed connectors were used to examine the soldering quality of the SMT and THR/PIP contacts. For example, a 2.0-mm-thick PCB with MicroSpeed connectors was reflow-soldered using a Sn62Pb36Ag2 solder paste. A laser-cut, 150-µm stencil was used in a closed squeegee system. Both versions with SMT and THR terminals showed similar soldering results. What’s more, these connectors are also RoHS-compliant.
 FIGURE 5. The eye diagram at 10 Gbit/s demonstrates that signal integrity was preserved even at that high rate. |
Signal integrity was measured with special testboards (200-mm FR4, microstripline included) using a network analyzer. The measured eye opening for the longitudinal contact configuration at 10 Gbit/s (rise time, tr = 35 ps) for a mated signal pair ±0.5 V (1.0 V) was 640 mV (64%). The related result for the transverse layout was 680 mV (68%).
MAGNUS HENZLER is product marketing manager at ERNI Electronics, 3005 E. Boundary Terrace, Midlothian, VA 23112. Tel: (804) 228-4100; email: Magnus.Henzler@erni.de.
