Mezzanine Connectors are Stacking Up
By Bob Hult, Bishop & Associates Inc.

The issue of system packaging has gained much attention over the past few years, as designers search for better solutions to signal integrity, density, manufacturability, and power and thermal management challenges, as well as cost reductions. Cramming greater functionality in smaller boxes, while producing a modular system that permits user repair and future upgrades, is an ongoing challenge to system packaging engineers.

Traditional card cage designs, which utilize a series of daughtercards plugged into a backplane, provide modular partitioning and communication among the cards. The drive to provide greater PCB surface area for component placement can be satisfied by increasing the size or number of daughtercards, but physical size and high-speed performance criteria create practical limitations to this solution. Efforts to further expand system flexibility and density have driven designers to find ways to utilize every cubic inch within the shrinking envelope, including the space between daughtercard slots. This has given rise to mezzanine card architecture, where the mezzanine card stacks directly above the host board and is electrically connected, and often mechanically supported, by the mezzanine connector. The mezzanine card becomes an optional extension of the daughtercard.

Mezzanine cards are a convenient way to solve a variety of problems, including the ability to provide more PCB space in close proximity to the daughtercard. Some applications have placed the processor, or other high-value component, on a mezzanine card to minimize import taxes. Mezzanine cards can provide the ability to offer a variety of I/O options simply by adding the appropriate mezzanine adapter card.

The I/O adaptor was one of the first mass applications of mezzanine card architecture.

Mezzanine cards can provide a low-cost strategy to address machine obsolescence or the need to expand capacity. Relatively untrained personnel can easily install mezzanine cards in an existing machine to increase memory capacity, upgrade a processor, as well as adapt to new I/O interface requirements, such as an optic link. A daughtercard equipped with mezzanine connectors adds little cost to the basic card, and opens the potential for additional sales of a mezzanine option card in the future.

From a user’s perspective, having the mezzanine option provides an upgrade path that can be populated at any point in the future, extending the useful life of the equipment. Even options that may not have existed at the time the equipment is introduced can be added as they become available. The addition of a card and the installation of new software is generally all that is needed to upgrade a machine.

Designing a system using mezzanine cards allows the product to be partitioned into smaller segments, potentially accelerating the development process, while resulting in smaller, less expensive, field-replaceable units, when repair is necessary. Major OEMs, such as Intel, offer a large variety of pluggable mezzanine cards to complement their products, while other aftermarket suppliers, such as Technobox Inc., produce I/O, feature, and expansion cards that conform to the IEEE-1386-1 standard, and can be installed in a wide range of conforming equipment.

Mezzanine card architecture can also take the form of a system-packaging scheme that eliminates the backplane entirely. For instance, the PC-104 specification defines the electrical and mechanical characteristics of a series of mezzanine cards that stack directly to each other and communicate via a vertical bus of stacking connectors.

The result is a highly modular assembly that facilitates the creation of custom electronic devices using standard off-the-shelf mezzanine cards.

Recently introduced industry standards are playing an increasingly important role in the implementation of mezzanine cards. By tightly defining the mechanical and electrical characteristics of a mezzanine card, including the connectors, users can reduce the design cycle while assuring compatibility among the products of competitive suppliers. The aging IEEE P1386 PCI mezzanine card specification (PMC) is being supplemented by new standards, such as XMC, EPIC Express, and COM Express, which offer higher speed serial performance and greater signal density.

More recently, the introduction of the Advanced Mezzanine Card (AMC) specification, from PICMG, adds mezzanine capability to Advanced TCA backplanes.

The defined interface is a unique one-piece edge connector, which is attached to the daughtercard. The right-angle design allows front access insertion and removal of the mezzanine card, an important feature when the replacement of the mezzanine card must be done without shutting down the equipment.

As system signal speeds continue to increase, and single-ended circuits are replaced by high-speed differential signaling, a new class of mezzanine connectors has evolved, often utilizing internal grounding and shielding structures. Unlike backplane connectors, mezzanine designs have an advantage of typically short circuit lengths from the surface of the daughtercard to the surface of the mating mezzanine card. The fact that the two board surfaces are parallel to each other reduces problems associated with signal skew. Many of these newer interfaces offer a greater selection of stacking heights, higher pin counts, smaller contact centerlines, lower mating and extraction forces, surface mount attachment, and multi-gigabit performance.

The FCI MEG-ARRAY® and GIG-ARRAY™ are examples of high-performance and high-density mezzanine connectors.

The ERNI Electronics MicroSpeed™ mezzanine connector is designed for circuits operating at up to 10 Gb/s, and offers a variety of stacking heights in two-row configurations.

A complementary stacking power module, rated from six to eight amps, fills out the product line.

Samtec offers a huge variety of stacking connectors to support standard, as well as high-speed applications.

Configurations are available with up to 500 surface mount contacts; many featuring integrated shielding systems for high-speed performance.

Major connector manufacturers, such as Amphenol TCS, Tyco Electronics, and Molex, have included mezzanine configurations to complement their lines of high-speed backplane connectors.

The NeXLev® connector, from Amphenol TCS, offers 73 differential pair signals per inch and a bandwidth of up to 12 Gb/s. Stacking configurations of their popular high-speed backplane connectors, such as HDM® and VHDM®, are also available.

Tyco Electronics offers the Mictor and STEP-Z mezzanine connectors in a variety of stacking heights and pin counts.

FCI Electronics has expanded its AirMax VS backplane connector system with the addition of a vertical receptacle.

Given the continuing race for greater system performance in smaller packages, as well as design flexibility, the use of mezzanine card architecture will likely continue to grow. Mezzanine connectors, designed to offer high-speed and high-density with ease of assembly, should continue to experience excellent sales growth.

Bishop & Associates Comments:

• Mezzanine card architecture has existed for many years, but is currently experiencing increased interest due to the potential for greater system density, modularity, and design flexibility. 

• Mezzanine industry standards often specifically define the connector, which can result in multiple sources assuring ease of supply, as well as competitive prices. 

• A wide variety of mezzanine connectors are currently available on the market featuring multiple stacking heights, pin counts, and PCB attachment methods.

• Surface mount attachment offers several PCB processing advantages, and is appearing on many mezzanine connectors. 

• As system speeds have increased, mezzanine connectors have been introduced that provide multi-gigabit performance. Many of these connectors are part of a total system interconnect solution that may include high-speed backplane, midplane, mezzanine, and power connectors.

Robert Hult
Director of Product Technology, Bishop & Associates, Inc.
Robert Hult has been in the connector industry for over 36 years. Hult began his career as a sales engineer for Amphenol. He joined AMP in 1972 and served in several management positions through 1996. In 1997, Hult joined Foxconn as group marketing manager for Intel, Chandler, Arizona, U.S.A. Prior to joining Bishop & Associates, Hult was the regional application engineering manager for Tyco Electronics.

Hult graduated in 1968 from Bradley University with a Bachelor of Science degree in electronics technology and a minor in business.