The requirement for smaller electronics to deliver improved performance is nothing new. The demand for connectors that provide higher densities in smaller packages is also not unique. The differences exist in how these requirements are being compensated for during the design and development process.
Variation is a primary challenge with mezzanine connectors, both mechanically and electrically. Connector designers are charged with maximizing connector performance and meeting target design objectives without creating entirely new connector families. If the design requirements are properly defined, the outcome is a connector platform that excels among the competition and achieves broad market acceptance.
Stacking Boards for Flexibility
Designing more features into smaller interconnect packages continues to challenge engineers. A connector’s contact and interface design are key factors for systems requiring higher densities, higher data rates, and/or more power in a smaller package. Much of the progression in the development of mezzanine connectors has been driven by a continuing demand for increased bandwidth in communication systems.
Many advanced server, storage and switching equipment use a high-speed midplane or backplane connector. The systems may need to further accommodate a high-speed connection to a mezzanine card attached to the blades or line cards. Mezzanine connector designs allow engineers to use the space above a daughtercard by stacking boards, increasing the packaging density of a device. The connectors also give designers the flexibility to add functionality into a single card slot.
Industrial products are also beginning to investigate this less known platform to improve and enhance packaging challenges. A robust mezzanine connector can be used at low or high frequencies and can be mechanically designed to accept more demanding install and service routines.
Regardless of where they are employed, variability of a connector platform should also be considered in the beginning of the design phase. Available in stack heights from 5 to 20 mm in 1 mm increments, mezzanine connectors offer a range of mated solutions to accommodate board-to-board spacing requirements. Also, tolerance stack-ups can be minimized if the connector design is less accommodating to such demands.
The right connector platform should already take this into consideration, most often by designing in proper ‘wipe’ or ‘wipe length’. If a connector’s ‘wipe’ is insufficient, mechanical design and tolerances may need to be tightened beyond acceptable cost.
The mezzanine connector will have a minimum of 1 to 1.5 mm of wipe length and subsequently provide granularity of stack heights. Meaning, if a 7.55 mm nominal stack height is needed, then a 7 mm mated solution set should suffice if designed with acceptable wipe length.
In sophisticated devices, the number and density of components on the PCB continues to increase. This increased density usually equates to design complexity in both layout and architecture. Unfortunately, the outcome may mean dealing with more heat, which needs to be removed from the system to maintain operating reliability over the life of the product.
To increase airflow designers look to reduce the height, width, and overall connector profile. Reducing the connector footprint can produce improved airflow and will also provide design engineers with more PCB real estate for additional placement of components.
A connector design and optimal density mean having the ability to be employed strategically on the board, and the nearest devices that would benefit from the proximity of a board-to-board connector solution. Routing hundreds of traces to one large connector can add cost, complexity, and compromise signal integrity.
The ability to place a connector platform designed for multiple mated pairs per board could provide the optimal solution when faced with other design challenges that a large array type connector will introduce. A well designed connector system can meet density requirements without marginalizing electrical or mechanical performance.
Next-generation platforms demand optimal signal integrity performance when routing high-speed signals. However, the size constraint placed on a connector affects signal integrity, especially at higher date rates. Maintaining proper impedance while minimizing the discontinuities can be a challenge, but increased density and overall conductor proximity will likely result in increased crosstalk.
Crosstalk is a critical parameter to consider when selecting a mezzanine connector for high-speed applications. The unwanted noise from the coupling of nearby signal lines typically occurs when two signals are electrically superimposed on each other by inductive and capacitive coupling contributions. This results in distortion of the desired and intended signal.
Connector manufacturers focus on several design methods to address this issue, including the contact design, pin mapping/contact spacing, and shielding techniques.
Manufacturers often aim for the highest bandwidth requirements, without addressing the need for variability, and consequently miss broader marketplace acceptance. Designing for a wider range of applications is essential to providing cost-efficient connector platforms.
The issue of wipe length, for example, provides the mechanism to eliminate unwanted oxidation or contaminants from a conductor surface. Added security measures for system integration can be afforded by generous mis-mating allowances delivered by forgiving contact and housing designs, which provide a longitudinal misalignment tolerance of ± 0.7 mm and angular misalignment tolerance of up to four degrees.
Mezzanine connectors must also provide board retention and stress relief. Increased stack heights and dense daughter cards can add stress on solder joints when mating/un-mating the connector halves. Absorbing added loads can be accomplished in many ways, but taking up more board real estate with the connector is not a likely design objective.
Balancing the terminal design to consume as much area on a pad will improve conductor retention, but a tradeoff exists between signal integrity, power density, and mechanical robustness of the conductors.
Once optimized, enhanced stress relief mechanisms can further the mechanical robustness. Many connector manufacturers have also moved to forming the leads as an integral part of the stamping die, as opposed to terminals that are stamped for SMT termination. This process guarantees a coplanarity of <0.1 mm and virtually eliminates processing defects without compromising robustness.
Hermaphroditic designs are less unique than they use to be, but they still challenge the connector designer. The reason for its acceptance as a interconnect option is primarily due to few application requirements that make this type of interface acceptable.
The right hermaphroditic solution incorporates two points of contact per conductor, for mechanical reliability, especially in applications where shock and vibration are the norm.
Typically available in 3 to 5 mm stack heights, hermaphroditic connectors offer forgiving mis-mating tolerances of up to 0.7 mm side-to-side and ± 4° angular misalignment between the two connector halves and their respective printed circuit boards. This latitude allows multiple connectors to be used on the same board and at various locations without concern of misalignment due to tolerance stack-up.
Open Pin-Field Design
High-speed board-to-board connector systems that feature an open pin field design can accommodate higher differential or single-ended signals, while supporting transmission speeds of more than 25 Gb/s. The unassigned conductors on an open pin field design mezzanine connector allow for maximum grounding and routing flexibility, offering a solution that can be beneficial to various market segments.
Single-ended (or RF) signal characteristics are accommodated by the rows of signal pins that are typically shielded on the perimeter of the connector body, by a designed-for low inductance path to ground. Well designed shields maximize noise suppression and have effectively zero contribution to a system environment that is targeted for stringent noise reduction requirements.
Differential applications benefit from a longitudinal pitch of 1 mm and row spacing of 1.5 mm, as the open pin-field platform permits horizontal or vertical arrangements of differential pair routing schemes. This is important when an engineer is faced with a combination of signal requirements.
Flexible pinout, optimized S:G, shielding contributions, and row-column pitch allow for density and maximum flexibility without forcing the designer into choosing various mezzanine connectors.
As industry requirements continue to evolve, mezzanine connectors will provide an economical way for designers to add functionality and customized options to their systems. Mezzanine connector solutions can combine high-density and high-speed performance designs with mechanical reliability and deliver improved signal throughput.
Filed Under: Rapid prototyping