Board-to-Board Interconnects for Modular Design and Automated Manufacture

By Ryan Smart | October 01, 2024

Design is a multi-faceted challenge. Not only are engineers expected to deliver products with higher performance and more features in ever smaller form factors, they must also complete designs in short timeframes to secure competitive advantage. In addition, there is a growing need for designs to be manufactured at high speeds without compromising quality or operational reliability during product lifetime.

Meeting these requirements has a bearing on both the design process and the materials and components selected. Modular design, for example, has become more popular, allowing key sub-sections (such as I/O, processor, and analog signal processing) to be built onto separate boards that can be re-used across several products. This reduces the design risk and development time and, through the combination of several smaller PCBs, offers more flexibility in addressing challenging form factors.

There are many benefits to taking this approach, especially getting to market quickly with a reduced level of design risk. However, careful consideration needs to be given to how to connect the various modular boards.

Single or multiple connectors

Clearly, the boards in a modular design need to reliably connect with each other, allowing the passage of signals and power from one board to another. While using a single connector between boards is relatively simple, in many cases this is not ideal. Firstly, it means that multiple types of signal (power, digital clocks, digital data, and analog) have to pass through the connector, which can introduce crosstalk and other noise. Secondly, all the PCB traces have to be routed to the connector. Apart from the space required, long traces run the risk of introducing noise and propagation delays that can negatively impact circuit operation.

For this reason, and especially on larger boards, a preferred approach is to use fine-pitch interconnects comprising many pins across multiple connectors. This improves flexibility, reduces trace lengths, and allows different types of signal to have their own dedicated contacts. However, with multiple board-to-board connectors deployed between a board pair, mechanical alignment and positional tolerance become a more critical aspect of the design. Precise placement on the PCB is critical, while the need for automated manufacturing demands that this placement be performed at high speed. Unfortunately, with two or more connectors mating the same two PCBs, the level of placement accuracy required is often outside the parameters of even the best modern manufacturing processes.

This is a problem because misalignment by even sub-millimeter distances — whether due to PCB manufacture, paste location, pick-and-place accuracy, or a combination of these factors — will put undue stress (such as excessive side-loading) on the board-to-board connectors. In turn, this can cause damage, leading to quality and yield issues during manufacture and the potential for reduced system reliability during operation in the field.

Mitigating misalignment with floating connectors

To address these challenges, an innovative type of board-to-board connector has been developed. So-called floating connectors derive a degree of elasticity from a combination of a housing that can move laterally and longitudinally and a novel contact design featuring a spring-like mechanism. In this type of connector, the housing is suspended by the contacts, which are usually housed in the male half of the connector.

Floating connectors can accommodate misalignment during automated manufacture

The degree of two-directional movement afforded by this arrangement generally exceeds the potential misalignment that can occur as a result of the manufacturing process. This allows pick-and-place processes to be deployed for boards with multiple connectors in cases where the sum of misalignments might otherwise present challenges for high-speed, high-precision mating. As a result, designers can focus on design rather than manufacturing requirements, leaving them free to deploy multiple floating board-to-board connectors per board pair. Decisions about type and quantity of connectors can be based purely upon the needs of the circuit rather than any limits associated with interconnects.

The spring-based suspension mechanism inherent in floating connectors will also contribute to connection integrity during operation, not least in applications that encounter some levels of vibration or shock, whether it be the daily use of a portable device or the operation of a moving vehicle or a factory automation system. For example, floating connectors can mitigate the possibility of performance degradation or failure from fretting, when long-term vibration causes the plating to wear from rigid mating pins, exposing the underlying alloy to potential oxidation.

Innovative contact design provides benefits in floating connectors

While floating connectors can be used universally, in many applications normal board-to-board connectors are more than adequate. In assessing which technology is more appropriate for a particular application, designers should consider the following:

Would the design benefit from multiple connectors per board pair?
What is the mechanical accuracy of the automated manufacturing process?
How much will the final product be exposed to shock and vibration during normal use?
How often will the boards be separated and replaced during the product lifetime

The answers to these questions should guide the designer as to which path to take.

The connector selection process should also consider environmental aspects, especially the operating temperatures that the connector may be exposed to. This is particularly relevant for designs that have been enclosed to protect sensitive electronics and where temperature changes can contribute to stress on connectors, especially if the materials differ or there are differences in board thicknesses.

Finally, it should be noted that almost all applications will require the connector to be made from materials that are fully RoHS-compatible and free of REACH SVHCs such as lead, brominated flame retardants, red phosphor (PFOS/PFOA), or antimony.

Floating connector developments

Recent years have seen significant growth in the types, capabilities, and availability of floating connectors.

Today’s designers can choose from product families that offer a variety of connector pitches, usually in the sub-millimeter (0.5 mm > 0.8 mm) range, and contact counts up to and beyond 160 per connector. Options for vertical and horizontal female connectors allow boards to be connected parallel to each other or at right angles, while a broad spectrum of mounting heights down to as little as 6 mm allows engineers to choose solutions that meet the inter-board space requirements of increasingly compact form factors.

Among the latest advances have been the launch of connectors that support data rates up to 12 Gb/s, allowing them to deliver the performance expected by standards such as SAS (serial attached SCSI), which is widely used in enterprise server and storage applications. At the same time, technologies that mix signals and power in a single connector will support applications where traces need to carry heavier currents.

As well as their inherent spring-like structure design, many of the latest floating connectors offer further support for accurate alignment and robust interconnects through features such as location pegs and retention tabs.

Retention tabs and through-board pins enhance the mechanical rigidity of floating connectors

The former help to eliminate movement during solder reflow, while the latter provide additional mechanical strength for surface mount connectors. Furthermore, among the connectors that mix signals and power, there are products that feature through-board solderable posts to augment both location accuracy and mechanical strength, ensuring resistance to shocks.

The trend towards modular design means that fine-pitch board-to-board connectors play an increasingly important role in providing the flexibility needed to meet application requirements. At the same time, the complexity and component density of modern systems means there is a need to deploy multiple board-to-board connectors for signals and power per board pair. However, this introduces the potential for misalignment during automated pick-and-place and solder reflow processes, leading to potential manufacturing quality issues and problems with in-service reliability.

These issues make early consideration of methods for connecting pairs of PCBs fundamental. The choice of interconnects will not only ensure an optimized product that is compatible with modern production processes, it can also enhance reliability during lifetime operation. In particular, the use of floating connectors that accommodate and absorb misalignments and small movements in multiple axes is becoming much more prevalent across a variety of applications, ranging from industrial automation to electric vehicles and security systems to IoT devices.

Visit Harwin to learn more about floating connectors and other innovative solutions.