Bandwidth demand drives connector design
Connectors form an important link in a signal’s transmission chain. Therefore, when designing high-speed systems, don’t overlook them. With the right parts, proper PCB materials, and proper design, you can meet the demand for high data rates.
Mechanical vs. electrical interconnects
Prior to today’s high speed design requirements, the issues of concern for interconnects were the physical aspects of their designs and factors that influenced those designs. These aspects and factors include:
Physical aspects;
– Stack height or distance between boards
– Pitch
– Power
– Termination types required (through-hole, press-fit, SMT)
– Latches or locks
– Supports/Constraints such as standoffs, brackets, chassis slots or frames
Influential elements;
– Processing and operating temperature
– Humidity ranges
– Shock and vibration requirements
– Safety issues and specifications
– Adherence to environmental standards such as RoHS compliance, lead-free standards
– Packaging and auto placement requirements
As system speeds have increased, interconnect design is no longer based solely on mechanical requirements. Designers now have to take into account electrical performance issues such as insertion loss, return loss, crosstalk, skew, and propagation delay, to name a few. As with every aspect of high-speed product development, the successful design of any interconnect involves achieving the right balance between maximising the physical and mechanical strength of the interconnect while optimising SI (signal integrity). Interconnect design and manufacture has to be approached as part of the overall system level design process.
It’s all about the footprint
The essence of any interconnect design is the footprint, which is sometimes defined by mechanical considerations and other times by SI considerations. Specifically, the factors that affect SI performance include footprint configuration, size, spacing of the pads, PTH (plated through hole) vias, and BOR (breakout region). A graphic of footprint/routing design is shown in Figure 1. Additional factors to consider include maintaining signal integrity through the space/trace relationship and determining via diameters, tolerance stack-ups, number of layers needed, signal to ground ratios and taking into account overall board density.
Figure 1. Connector footprints are defined by signal integrity and mechanical considerations.
BOR refers to the traces and ground planes in the vicinity of the connector. This is the area where the traces are “broken out” from an optimised, consistent transmission-line environment and routed as required to attach to the connector terminals. In terms of the connectors chosen for high speed performance PCBs, connectors must be evaluated in the context of their use in a real world environment. “Connector-only” data can be useful for comparing the relative performance between two similar connectors, but it may be very far removed from the performance you obtain in an actual application.
Laminate choice is another increasingly important factor in cable and connector design. While FR-4 isn’t the best laminate choice for high speed/high bandwidth designs, it’s still the preference of many product developers because of its low cost and broad availability.
Accounting for electrical performance is also necessary for the design and manufacture of stripline connectors. In one of Samtec’s designs, an integral ground blade is incorporated between two rows of contacts. That’s because PCB can become the limiting factor if the connector is not designed correctly. The connector has to have the required number of vias and a decent number of layers.
All connectors need to be both routable and palatable [acceptable]. For example, if 16 layers are required for the routing of a connector, there has to be something really special about the connector or it will design itself out of a lot of systems. This is why it’s important to have a thorough understanding of the PCB to which the connector is linked while also taking into account factors such as design optimisation, manufacturability, and final product implementation.
Pin geometry
The design and fabrication of the contact system is a critical component for the design and manufacturability of any high speed, high bandwidth interconnect. As with all other elements of hardware, the pin geometry needs to be designed such that the focus is upon maintaining signal integrity by reducing crosstalk, coupling and reflections. That’s accomplished by controlling impedance and reducing loss.
Specific pin geometry factors to take into consideration include:
– Eliminating as many signal path disruptions as possible by maintaining a consistent cross section. Examples include limiting the use of barbs and reducing stubs. To a certain extent, cross section changes can be remedied by proper placement of the plastic contact housing to eliminate air gaps.
– Keeping the mating of the contact system as short and straight as possible.
– Having contacts that mate on the smooth milled surface of the contact thereby enabling higher cycle life and providing superior electrical properties.
– Determining optimal placement of signal contacts and grounds so the connector can withstand the physical/mechanical requirements of the overall PCB. Figure 2 depicts an optimised contact design.
Figure 2. Contact design is essential to the signal integrity of any interconnect system,
Board-to-board interconnects
Dielectric considerations are primarily a mechanical moulding question that arises after the footprint is baked-out [finalised] and the routing and construction of the connector comes into play. Furthermore, in board-to-board configurations, the connector is tuned section-by-section. In this instance, the connectors are divided into determinate areas, retention areas, and beam areas. Each of these areas is then analysed in greater detail to ensure that the transition between the zones within the connector and within the substrate all match. This ensures optimal signal integrity, impedance control, and loss mitigation.
Summary
Today’s rapidly increasing bandwidth needs challenge the capability of traditional design approaches that utilise existing technology components. While connectors are often considered to be the “back end” of many product implementations they too need to be held to the same design criteria as all other elements within any high speed/high bandwidth product.
Brian Vicich is VP of Engineering at Samtec US.
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