Single-board solutions save space by placing all of a system’s electronics on a small, low-cost printed circuit board. For single board computers (SBCs), designers must strive to configure as much processing power, functionality and I/O as possible on a single board, but the reality is that in industrial, consumer and medical applications, many times a single board is not the best solution and multiple printed circuit boards are required. In this case, board-to-board (BTB) connectors become very important.
Even with all the design effort put into designing multiple boards for a system, failure to select the right BTB connector can completely destroy the design. This can result in form factor or signal integrity issues up front, or possibly later in the field with usage (or abuse) failures.
This article examines the design issues that drive the demand for BTB connectors and the factors designers must consider when selecting a BTB connector from the many available options. These factors include circuit performance, production requirements, usage models, ease of maintenance, signal type, connector size and number of contact positions, radio frequency interference (RFI) and electromagnetic interference (EMI), and more. Phoenix Contact’s BTB connector solutions are used as an example of how they can solve a designer’s board connectivity problems.
Why use BTB connectors?
From a design, manufacturing, and sales perspective, there are at least ten situations where it makes more sense to use two or more blocks of interconnected printed circuit boards than a single board.
Form factor constraints that limit the overall size of using a single larger board approach require a three-dimensional layout to take advantage of the available package depth.
Low-level, high-sensitivity analog I/O or RF circuits cannot be placed near high-speed, noisy digital circuits.
High voltages are present, and engineering practices as well as regulatory standards require mandatory isolation measures.
Thermal considerations require that hotter components be placed in separate locations to improve heat dissipation and thermal management.
Multiple product versions may have to use or reuse an established circuit segment, such as a core processing board that can be paired with a basic multi-line user display and buttons, or a more complex graphical touch screen for a different model of alarm or sensor system.
Production requires the use of special components, such as power supply units and heat sinks that require special manufacturing/assembly processes or manual insertion, while the remaining components can be inserted and soldered automatically.
Vendors want to upgrade a feature in the system (e.g., processor and memory), but want to keep the analog functionality intact to increase technical confidence and amortize costs.
Field experience has shown that a component of a system (such as externally facing I/O) is more likely to require field replacement, while internal core functions (such as processors and memory) will have a longer mean time between failures (MTTF).
Some components require thicker PCB materials and heavier copper cladding, such as for power supply devices.
EMI/RFI needs to be considered and attended to, and functions must be isolated from each other, and perhaps even RF shielded for parts of the circuit.
Clearly, there are many valid design, production and support reasons for choosing or sticking with multiple printed circuit boards. This is the case for many applications, including industrial control systems, motor controls, programmable logic controllers (PLCs), alarm and security devices, medical systems such as portable X-ray machines or ultrasound instruments, and devices with different human-machine interfaces (HMIs)
How to Choose a BTB Connector
Once a decision has been made to use two or more connected printed circuit boards, the designer must then select the appropriate BTB connector. In almost all cases, it is not just a matter of finding a connector pair that meets the basic specifications, but it is also wise to first find a range of fully compatible connectors with different BTB options so that design choices are not limited in advance.
Connector offerings are so diverse that even a glance at the connectors available from one supplier can make it difficult to decide, but it’s not. When designers focus on their priorities, constraints, and must-haves, the range of specific connector options available often becomes quite small. In addition, so many connector style options mean that designers can find an optimal match that balances the inevitable technical trade-offs with minimal compromise.
Designers can use advanced computer-aided design (CAD) tools to model possible physical configurations and possible BTB orientations, including mezzanine, daughter card and coplanar, as well as unconstrained over-hole ribbon cable (Figure 2). However, there is no need to “jump straight to CAD,” as simpler techniques can also be very effective for initial evaluation and have been successfully employed, including the use of cardboard models to evaluate various board sizes and arrangements.
Figure 2: Board-to-board connections can be made in a variety of orientations and arrangements, including mezzanine, daughter card, coplanar, and unconstrained ribbon cable
Explore the freedom
In addition to basic orientation, so many connector versions provide designers with layout and placement options. For example, instead of using one connector with more pins, designers can choose to use two smaller BTB connectors, each with fewer pin positions. Doing so simplifies board layout and avoids the need for some signals to span the entire length of the printed circuit board.
For example, Phoenix Contact’s FINEPITCH 1.27 series (1.27 mm (mm) pitch) is available in 12, 16, 20, 26, 32, 40, 50, 68, and 80-pin versions. Note: 1.27 mm is exactly 0.05 inches, or 50 mils, which is a common pitch. Consider two vertical female connectors in this family: a 26-contact 1714894 with a width of 21.6 mm and a 12-contact 1714891 with the same other specifications but a width of 12.71 mm, which is slightly more than half the width of the 26-contact version (Figure 3).
The loss of board space associated with using two smaller connectors in different locations on a printed circuit board is negligible, and is often offset by the reduced alignment space required for the printed circuit board and improved signal integrity. Similarly, Phoenix Contact’s FINEPITCH 0.8 series (0.8 mm pitch) includes a range of 0.8 mm pitch connectors ranging from 12-pin 9.58 mm long 1043682 connector sockets all the way up to 80-pin versions (Figure 4).
Another issue is the height of the connector, allowing designers to ensure that two neatly aligned parallel boards are mated to each other within the enclosure and that each board is optimally positioned. The processor board can be mounted to the back of the product enclosure, while the second board with the user display and buttons can be placed flush with the front panel.
As a result, the connectors have the same number of pins, length and width, but there is one major difference: their height. By mixing different heights, various inter-board spacings can be supported, called stacking heights. For example, the vertical female connector in Phoenix Contact’s FINEPITCH 1.27 series is available in 6.25 and 9.05 mm heights, while the mating vertical male connector is available in 1.75 and 3.25 mm heights.
In addition, it is very important that the mating pairs have a “swabbing length” of 1.5 mm while maintaining a reliable surface contact swabbing length of 0.9 mm. As a result, there is a non-stepping range of continuous available spacing from board to board: 8.0 mm to 13.8 mm (Figure 5). Phoenix Contact’s FINEPITCH 0.8 series connectors use a similar scheme with different heights and swab lengths compared to the FINEPITCH 1.27 series and support a continuous range of 6 to 12 mm. The inherent flexibility of the BTB mating distance also provides the added benefit of relaxed assembly tolerances in production.
Figure 5: Since the male and female connectors in the FINEPITCH 1.27 series offer several independent height options and long swaging lengths, the actual BTB stack height can be any value between 8.0 and 13.8 mm
Supports EMC and RF requirements
High-density, multi-contact BTB connectors are expected to support bandwidths well beyond power and low-frequency signal needs, thus minimizing the need for multiple discrete cable assemblies (a single cable supporting a single signal). Key parameters are the connector’s performance in the gigahertz range and its ability to maintain signal integrity at these frequencies. Electromagnetic compatibility (EMC) is also considered to ensure that high-speed signals are not affected in the connector, nor are they affected by nearby signals.
Some connector families feature unique designs that meet bandwidth and EMC considerations. For example, Phoenix Contact’s FINEPITCH 0.8 series supports data rates up to 16 gigabits per second (Gb/s) and includes multiple post-mating connector-to-connector shield paths (Figure 6) for excellent EMC performance (Figure 7).
Figure 7: Picture of the electric field around a FINEPITCH 0.8 series connector showing shielding performance; dark blue indicates electric field strength from 0 to 0.1 volts per meter (V/m) and dark red indicates 1.0 V/m
These connectors provide S-parameters to support high-fidelity RF signal path modeling, as well as data on insertion loss, far-end crosstalk (FEXT) measured on the receiver side, and near-end crosstalk (NEXT) measured on the transmitter side (Figure 8)
Figure 8: Graph of 0 to 10 GHz insertion loss (left) and near-end crosstalk (right) for high data rate connectors such as the FINEPITCH 0.8 series
In-Depth Analysis
While connector functionality may seem simple, there are other factors to consider when choosing the right connector family. These include
Compatibility with standard high-volume production processes (loading and soldering), which also requires a high degree of coplanarity throughout the connector body, typically better than 0.1 mm.
Reliable performance is guaranteed even when the contact surface plating wears off after repeated plugging and unplugging; 500 cycles is considered the highest performance level. After 500 insertions and removals, Phoenix Contact’s FINEPITCH 0.8 series maintains a contact resistance of less than 20 milliohms (mΩ), while the FINEPITCH 1.27 series remains below 25 mΩ (in accordance with IEC 60512-2-1:2002-02).
Radial and angular misalignment also occurs when the two boards are mated with the corresponding connectors.
The latter misalignment problem is a realistic factor for designers to consider. Ideally, the centerlines of the male and female connectors will be perfectly centered and free of slope to each other. Given the small size of these fine-pitch connectors, this misalignment does not seem to be allowed, but good connector design can allow for a certain degree of mismatch between the two parameters.
The ScaleX technology of the FINEPITCH 0.8 and FINEPITCH 1.27 series solves this real problem very well. These series offer a special housing geometry that not only protects the contacts from damage in case of misalignment, but also provides corresponding tolerance compensation with a center offset of ±0.7 mm and a tolerance of ±2°/±4° along the diagonal and longitudinal axes respectively (Figure 9).
Figure 9: Realistic alignment is never perfect, so FINEPITCH 0.8 mm and FINEPITCH 1.27 connectors allow for tilt and longitudinal angular misalignment of up to ±2°/±4° and eccentric radial misalignment of up to 0.7 mm, respectively.
What you can’t see is also important
While connectors do not have the nanometer dimensions of integrated circuits, the mechanics of connector contacts contain tiny components with tight tolerances and ultra-thin precious and non-precious metal plating, and their bodies are precision molded parts. Given the size of the metal contact area and the way these contacts are “buried” in the housing, you cannot see how a highly reliable contact area is formed.
At these sizes, not only does the design need to be precise, but it also needs to be able to be implemented at the micro component level in high volume production. This is why the FINEPITCH 0.8 series with ScaleX technology has a unique dual contact approach. When mated, the contacts (AD and female element) are vibration-proof in a very narrow space. In addition, these contacts feature gull-wing solder pins, which are optimal for automated soldering processes.
Circuits where direct connection of the board is not possible
Although direct placement and connection of BTBs is an attractive option, there are some situations where it is not possible to directly mate and connect two or more printed circuit boards via BTB connectors. This may be due to the form factor of the overall product package, the shape of the board, electrical and electronic considerations when placing the board, or thermal issues.
To address these situations, Phoenix Contact’s FINEPITCH 1.27 series also offers female insulated piercing connectors (IDC) for use with flat cables. Using these flexible, flat ribbon cable connections between two printed circuit boards allows them to be physically isolated rather than electrically isolated, and the boards do not have to be placed parallel or perpendicular to each other. As with the BTB connectors, these products are also available in a full range of 12 to 80 pin positions; Phoenix Contact’s 1714902 is a free-hanging 12-pin version (Figure 10). A panel mount version is also available.
The flat cable for IDC BTB layouts is also a carefully designed product with AWG 30 (0.06 mm²) Leeds conductors and a choice of three insulation types: basic PVC (-10°C to +105°C), high temperature (-40°C to +125°C) and halogen-free versions. Specifications for some facilities require halogen-free products to prevent fires and to create a “charcoal” coating to reduce emissions of toxic carbon-containing gases and reduce the visibility of soot and carbon particles.
With five different cable orientations and connector arrangements (Figure 11), nine connector sizes supporting 12 to 80 pin positions, very short 5 cm (about 2 inches) to much longer 95 cm (about 37.5 inches) flexible cable lengths, and three available insulation types, these options yield more than 10,000 possible permutations. Since it is impractical to stock all of these products, these IDC cable assemblies are produced on demand, depending on the connector/cable pair and configuration required.
Figure 11: Three arrangements and orientations (out of five) of IDC cable connectors are shown, providing designers with maximum cable placement flexibility and minimal restrictions by simplifying cable routing and placement.
Wrap-up
Connectors and interconnects are key elements in completing a design and need to be given proper consideration in advance. When using multiple printed circuit boards, BTB connectors provide a convenient, reliable, high-performance technology to connect two or more boards in a variety of layouts.
The nuances and complexity of these connectors are often underestimated, but as mentioned above, precision engineered BTB connectors such as Phoenix Contact’s FINEPITCH 0.8 and FINEPITCH 1.27 series enable high interconnect density, superior mechanical performance, compatibility with production processes and procedures, and electrical performance that meets today’s complex product design data rate and EMC requirements in today’s complex product designs.
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