Choosing connectors that meet increased speed and density requirements for low-profile 1U and 2U rack-mountable systems is a complex task. Increases in signal-pin counts and power consumption have turned system packaging design for optimal airflow into an increasingly difficult challenge.
Common to network interfaces, storage, and telecom equipment, these lowprofile systems require long-lasting connector configurations that can survive next-gen upgrades.
Several connector configurations are available to meet requirements for 1U and 2U systems. Mezzanine and coplanar boardto- board configurations increase overall system performance and flexibility, especially in compact designs with a defined envelope and sparse board real estate.
Due to the complexities involved, it is important for engineers to include connector and contract manufacturers as well as silicon vendors and printed-circuit board (PCB) fabricators early in the design process. Most manufacturers will walk through the part selection process with the engineer to help determine which interconnect solution meets the necessary speed, density, cost, quality, and long-term reliability requirements. This will ensure the engineer chooses the optimum configuration.
Actually, some connector manufacturers will create models that show users how high-speed channels will perform in their specific design. Also, they can create an electrical model for full channel analysis, which the customer can use to predict the electrical performance of a system before tooling it.
HIGH-SPEED DESIGN CHALLENGES
To uphold a system’s longevity, engineers must account for next-generation advances in their current designs. This is a tricky task because racks used in expensive 1U/2U systems must last through two or more generational upgrades.
For example, a 20-circuit connector today may need to support higher data rates for Gen 2 or Gen 3 protocols in the future, which typically increases the pin count. As a result, many engineers are designing their systems with headroom for increasing speed capabilities. This helps ensure a product can incorporate new technologies. It also ultimately reduces the cost of system upgrades.
Operability issues, such as airflow and board density, go hand in hand in the design of small-envelope systems. Dense boards generate a significant amount of heat and require more airflow. Early in the design process, it is important to consider adequate cooling for both current and future needs. Cooling utilities, i.e., heatsinks and fans, improve thermal management by spreading heat and blowing air through the box.
However, if hot air simply recirculates through the system, the equipment may still overheat. Open space in a design helps introduce fresh air, minimizing the amount of hot air present. Ultimately, the necessary amount of airflow determines the stack height between boards, so all components must be strategically spaced within the overall system.
Compact high- speed configurations can lead to compromised signal integrity, so designers must find a balance between optimum transmission speeds and envelope size. It is also critical to create a system satisfying both bandwidth and space requirements.
As data speeds increase, more connector shielding is required to protect against crosstalk during data transmission. Grounding connector pins helps facilitate the shielding process, but it can increase the connector size. Plus, taller stack heights increase signal path distance and may impair signal integrity.
PCB adhesion is another consideration since components are typically surfacemount (SMT) or through-hole. SMT devices may have J-lead tails, ball-grid array (BGA), or solder-charge technology and sit on the top of one side of a board. SMT adhesion provides benefits in its ability to place components on both sides of the PCB and is typically preferred from a signal-integrity standpoint.
One drawback of SMT is the lower adhesion force to the board and longer exposure to higher oven temperatures than through-hole, wave-solder, and press-fitcompliant components require. Fortunately, there are interconnect configurations that address these low-profile 1U/2U design challenges.
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