New SMP-MAX Connector Eases Board-to-Board Mating

The evolution of board-to-board RF connectors came from two natural trends of the wireless market: make it smaller, make it cheaper. Miniaturization saves both space and weight, while accommodating more capabilities. While wireless handsets are an obvious example of smaller, lighter, cheaper, the same trend is true throughout the entire wireless system.



Connectors need to be smaller because the components and pc boards have also become smaller. For example, RRH (remote radio head) are significantly more compact, allowing easy installation even on top of a tower or in dense urban environments. This reduction in size required, first, a simpler interconnect system, replacing bulky RF cable assemblies by simple board-to-board RF connectors able to handle power levels exceeding 100 W.



As RF engineers started to use these board-to-board RF connectors widely, they required a lower cost solution manufactured at very high volumes.



Shrinking electronic components and packages present two mechanical concerns for interconnections. Small size makes visual alignment more difficult than with larger components. Small size also makes components less robust and therefore more prone to damage if abused or mishandled. While a large connector can allow a certain degree of “brute force” alignment, microminature connectors demand more highly engineered solutions.



There are three main types of misalignment.



Radial misalignment occurs when the pin contact and receptacle are not aligned on the same center axis.



Angular misalignment occurs when the pin contact is at an angle with respect to the receptacle.

Axial misalignment occurs when the pin contact does not seat fully within the receptacle. Axial misalignment is of concern with RF connectors because it can result in impedance mismatches, causing reflections and increasing VSWR.





Another important aspect is the pull-in range angle, which is the “gathering” ability to bring misaligned parts into alignment. The pull-in range angle is at least equal to the working angular misalignment, but in most cases, this pull-in angle is much larger.



The obvious issue with alignment is to avoid stubbing of the two mating parts. Tolerating some misalignment during the mating process avoids possible damage from stubbing. It can also allow blindmating, whereby visual alignment is not necessary. Many blindmating approaches use a funnel concept to gather in and center a component. Blindmating capabilities exemplify the move from pure performance to a user-friendly design. Not only does the connector work, it is easier for technicians to plug and unplug.



Misalignment and Board-to-Board RF Connectors

Traditional microminiature RF board-to-board connectors, such as the MCX, do not tolerate misalignment and are particularly intolerant of varying spacing between boards. Traditional SMP connectors, along with MMBX and IMP connectors, allow limited misalignment but still require relatively precise mechanical alignment.



Board-to-board SMP connectors—including SMP, SMP-Spring, and SMP-MAX— consist of three parts: two receptacles that mount on each board and an adapter that plugs into the receptacles. Adapters typically snap into one receptacle to provide a fixed, semipermanent connection and slide into the second receptacle for the separable connection. Adapters are available in different lengths from less than ½ inch to more than 3 inches to accommodate different board-to-board spacings.



In practice, angular and axial misalignments are of greater concern to designers and assemblers. Radial misalignment is essentially a mechanical issue compensated for by careful design of both boards and connectors. Management of angular and axial misalignment is critical to maintaining high levels of signal integrity. Gaps resulting from alignment issues can cause impedance variations, cause reflections, and increase the VSWR. While this is true from both types of misalignment, angular misalignment can also add mechanical stress to the parts and lower reliability.



To accommodate greater misalignment than allowed by traditional SMP configurations, a second generation—SMP-Spring— used a spring-loaded adapter. While the SMP-Spring tolerates large radial and angular misalignment, it is particularly effective in dealing with variations in board-to-board spacings. Because the spring loading ensures that the adapter is fully seated in the receptacle, SMP-Spring connectors offer low, consistent VSWR. The drawback is that the complexity of the spring system makes the solution more expensive.



A third generation—SMP-MAX—has been recently introduced to provide even larger misalignment tolerances. As shown in Figure 2, the SMP-MAX combines a “catcher’s mitt” funnel on the receptacle to help funneling the connectors, which is critical in blindmate conditions. To handle axial misalignment, it uses an impedance-matching insulator that’s optimized for operating gaps when the adapter is not fully engaged in the receptacle. Gaps up to 0.094 inch (2.4 mm) are allowed with no significant changes to VSWR. The center contact of the board-mount receptacle uses a conical shape to prevent additional stress and increase reliability.



Third-generation SMP-MAX connectors tolerate large misalignments

Because of the significant reduction in component complexity, SMP-MAX connectors offer lower costs than their spring-loaded counterparts while offering wider misalignment tolerances. Figure 3 shows typical design goals for alignment for board designers and the alignment tolerances for SMP-MAX connectors. The center oval is the typical misalignment target most designer factor into their design. The outer edge of the outer oval is the tolerance of the SMP-MAX board-to-board connector. The distance from the outer edge to the inner oval represents the additional misalignment headroom offered by the SMP-MAX design. Such margins mean a more robust system design and represent peace of mind in knowing board-to-board mating is easier, faster, and less prone to damage. The advantage is a comfortable margin of misalignment headroom. Electrical performance is also well within the requirements of today’s and emerging wireless standards. VSWR is under 1.2 up to 3 GHz.





SMP-MAX provides generous alignment headroom over typical board-design goals.

SMP-MAX Performance

Figure 4 compares performance of SMP, SMP-Spring, and SMP-MAX connectors. SMP-MAX connectors offer flexibility in application: The obvious tradeoff in the SMP-MAX design is a reduced operating frequency in exchange for high misalignment tolerances and higher power-handling capabilities. Since current and emerging mobile applications have typical frequency requirements between 800 MHz and 2.5 GHz, the SMP-MAX connectors operating frequency range amply meets application requirements.

It also offers cost savings, both in procurement and in application. The generous misalignment tolerances mean less chance of damage from mismating and less exacting PCB design requirements.

• Compatible with SMP receptacles, so that legacy boards don’t have to be refitted. The “fixed” side of the interconnection can use older receptacles

• Available with or without the catcher’s mitt alignment funnel

• Higher power-handling , up to 300 W, depending on temperature and frequency

• Available in symmetrical and asymmetrical configurations. In symmetrical configurations, the adapter has the same interface at both ends, but requires different receptacles to snap-in mating on one end and slide-on mating at the other. Symmetrical configurations reduce the possibility of error during assembly, but require stocking of three different parts rather than two.

• Available for through-hole or surface-mount assembly



Conclusion

The newest generation, SMP-MAX, give designers high tolerances at a much friendlier price point.