Board-to-Board Connectors: Types, Pitch and Stack Height
A board-to-board connector is an electromechanical component that joins two printed circuit boards (PCBs) without using cables. Board-to-board connectors stack boards vertically, connect boards side-by-side, or attach a daughterboard to a motherboard to save space and shorten electrical connections. Examples include Arduino shields, stacked sensor modules and compact wearable electronics.
What Is a Board-to-Board Connector?
A board-to-board connector system typically consists of a mating pair: a plug (male) and a receptacle (female). The mating parts are soldered to the two PCBs and form a rigid or semi-rigid electrical and mechanical connection. Removing a cable harness can shorten signal paths, reduce cable-related parasitic capacitance and inductance, and save space.
Board-to-board connectors also support modular systems. For example, a main processing board can connect to interchangeable sensor or power boards instead of placing every circuit on one large PCB. The sensor board communicates with the main board through the connector and can be replaced without redesigning the processor board. Separate boards can also simplify manufacturing, testing and field repairs.
If the design requires components separated by long distances or moving parts, wire-to-board and cable connectors may be preferable, as discussed in Flux’s guide to electrical connectors and wiring. A single board with flexible sections can eliminate mating connectors. See Flux’s flex PCB design guide and rigid-flex PCB design overview. For separately manufactured and replaceable boards, board-to-board connectors are often suitable.
Types of Board-to-Board Connectors
Board orientation determines the suitable connector format. Manufacturers also distinguish connector families by pitch, retention and electrical requirements.
Mezzanine / Stacking / Parallel
Mezzanine connectors join parallel boards in a vertical stack, with one board above the other. The format is common in servers, smartphones and industrial controllers. Mezzanine connectors use matching low-profile contact arrays, with contact counts ranging from a few dozen to hundreds for high-density families. Some mezzanine families accommodate board spacing of only a few millimeters.
Coplanar / Edge-to-Edge
Coplanar connectors join boards side-by-side in the same plane. Applications include LED lighting strips and modular backplanes with little available vertical space. Coplanar connections allow engineers to daisy-chain multiple different or identical boards without designing a single oversized PCB.
Right-Angle / Perpendicular
Right-angle connectors join boards at 90 degrees. Examples include backplanes, PCIe cards and plug-in modules. Typically, one board uses a vertical receptacle while the other uses a right-angle plug. The arrangement can support airflow and modular slots in which individual cards can be upgraded without replacing the entire system.
Board-to-FPC
Board-to-FPC connectors connect a rigid PCB to a flexible printed circuit (FPC). Such connectors attach displays, cameras and batteries to main boards in compact electronics. Some feature a zero insertion force (ZIF) or low insertion force (LIF) actuator to secure the flexible circuit. The rigid PCB and flexible circuit provide the rigidity of a standard board for heavier components and routing flexibility in tight spaces.
Pin Header and Socket
A pin header exposes one or more rows of metal pins that mate with a socket on the other board. Pin headers and sockets are often unshielded and used for lower-speed signals, including Arduino shields and prototyping. Standard headers are inexpensive, available in standard pitches, and easy to solder by hand. However, simple headers may lack locking retention and controlled-impedance features.
Floating Connectors
Floating connectors have compliant contacts or mounting structures inside the housing. The compliant interface absorbs misalignment during assembly or operation, including in automotive assemblies. Contact compliance reduces stress transferred to solder joints.
Table 1. Board-to-board connector types, orientations, pitches and applications
| Type | Board Orientation | Typical Pitch | Typical Use |
|---|---|---|---|
| Mezzanine / Stacking | Parallel (stacked) | 0.35 mm to 1.27 mm | High-density vertical integration, smartphones, servers |
| Coplanar / Edge-to-Edge | Horizontal (side-by-side) | 0.5 mm to 2.54 mm | LED lighting strips, extending a flat PCB area |
| Right-Angle / Perpendicular | 90 degrees | 0.8 mm to 2.54 mm | Backplanes, PCIe cards, plug-in modules |
| Board-to-FPC | Parallel or Right-Angle to Flex | 0.2 mm to 1.0 mm | Connecting displays, cameras, or batteries to main boards |
| Pin Header & Socket | Parallel or 90 degrees | 1.27 mm, 2.0 mm, 2.54 mm | Prototyping, low-speed signals, Arduino shields |
| Floating Connector | Parallel or 90 degrees | 0.5 mm to 0.8 mm | High-vibration environments, automotive, absorbing assembly tolerance |
Key Specs to Check When Selecting a Board-to-Board Connector
Compare electrical and mechanical specifications in each manufacturer’s datasheet. Matching pitch alone does not establish connector compatibility.
Pitch
Pitch is the center-to-center distance between adjacent pins. Typical pitch ranges from 0.35 mm for micro-miniature connectors (for example, the Hirose BM28 series) up to 2.54 mm for standard headers. Smaller pitches save board space but require tighter manufacturing tolerances and more precise pick-and-place assembly. A 0.4 mm pitch connector, for example, requires carefully specified solder-paste apertures to prevent bridging. Use the datasheet for exact dimensions and recommended footprints.
Stack Height / Mated Height
Mated height is the distance between the facing surfaces of parallel boards when the connectors are fully engaged. Typical ranges span from about 0.6 mm for low-profile mobile devices (for example, Amphenol 103 series) to about 40 mm for tall industrial stacks (for example, Samtec SEARAY). Many connector families offer several plug and receptacle heights to achieve a target board spacing. Calculate clearance for surface-mount components between the boards.
Pin Count
The pin count is the total number of electrical contacts available for signals, power, and ground. High-density arrays can have hundreds of contacts (for example, Samtec SEARAY), while simple power connectors might only have two. Allow contacts for ground returns, shielding and future revisions when space permits.
Current per Contact
Current per contact specifies how much current one contact can carry within the stated temperature-rise limit. High-current power connectors (like the Samtec mPOWER series) are rated up to 18 A per power blade. Conversely, fine-pitch signal contacts are often rated below 0.5 A each. For power connections, verify current ratings and thermal derating under the intended operating conditions.
Voltage
Datasheets identify the continuous operating-voltage rating and the dielectric withstand test voltage. Exceeding the specified voltage limits can cause electrical arcing between closely spaced pins, especially in fine-pitch connectors operating in humid or low-pressure environments. Select a continuous operating-voltage rating above the maximum system voltage, with margin appropriate to the environment.
Data Rate / Signal Integrity
High-speed connectors are engineered with controlled impedance, specialized ground structures, and shielding. Datasheets rate high-speed families by data rate per channel, for example 10, 28 or 56 Gbps. Samtec NovaRay is designed for high-speed links, with supported data rates dependent on the connector assembly and complete channel. High-speed operation depends on connector geometry and board transitions. For background, see Flux’s signal integrity guide.
Mating Cycles
A mating-cycle rating describes how many connection and disconnection cycles the connector can withstand under specified test conditions, including limits on contact resistance and wear. Fine-pitch connectors for phones are often rated for only 10 to 30 cycles (Hirose BM28 is rated for 10). Larger industrial connectors are often rated for hundreds of cycles. Choose a rating consistent with the product’s expected maintenance and replacement cycles.
Misalignment Tolerance / Self-Alignment
Misalignment tolerance is the physical offset a connector can accommodate during mating. Floating connectors typically absorb about ±0.4 mm to ±1.0 mm of X and Y misalignment. For example, JAE’s DW11 floating board-to-board power connector floats ±0.8 mm. Some standard connectors use molded guide posts to align the mating halves before contact engagement, particularly when the connection is hidden during assembly.
Retention
Retention is the force holding the mated pair together, often specified as the withdrawal (unmating) force. For applications subject to vibration, look for a higher withdrawal force, mechanical latches or screws. Depending on the family, retention may come from contact friction, snap-fit features or additional mechanical fasteners.
Operating Temperature
The operating-temperature range specifies rated temperatures for the connector housing and contacts. Typical temperature ranges vary by connector family:
- Commercial examples: −25°C to +85°C.
- Industrial or automotive examples: −40°C to +125°C. Temperatures beyond the specified range can deform the housing or weaken contact spring force.
How to Choose a Board-to-Board Connector
Follow the numbered checklist below to select a connector that meets the mechanical and electrical requirements:
- Determine the board orientation and available space: Decide if the boards will be parallel, perpendicular, or coplanar. Confirm enclosure fit, including both PCB thicknesses, connector mated height and components on the outer surfaces. Check the enclosure’s available space before fixing connector placement.
- Calculate the required pin count: Count data lines, power rails and required ground contacts. Add a 10% to 20% margin for future revisions or extra ground returns. Reserve spare positions when the package size allows.
- Select the stack height: For parallel boards, choose a mated height that clears the tallest components on the facing board surfaces. For a 5 mm tall inductor on the lower board, the mated height must exceed 5 mm plus any necessary clearance. Check the specified clearance and thermal requirements.
- Check the current and voltage requirements: Check contact ratings against expected power demand. For high current, consider a hybrid connector with dedicated power blades alongside signal pins. Check whether a combined power-and-signal part fits the available area.
- Verify the data rate: For high-speed interfaces such as PCIe, USB 3.0 or Gigabit Ethernet, filter connector candidates by the required data rate. For high-speed interfaces, check the manufacturer's S-parameter files to simulate insertion loss and return loss. Check channel loss and reflections at the actual data rate.
- Assess the mechanical environment: For heavy vibration or blind mating, consider a floating connector or a rigid connector with suitable retention and alignment guides. Check solder-joint stress under the specified vibration and mating conditions.
PCB Layout Tips for Board-to-Board Connectors
PCB layout determines the connector pair’s mechanical fit and affects electrical performance. A small layout error can render the assembled boards impossible to mate.
Datum and connector placement on both boards
In electronic design automation (EDA) software, establish a common origin (datum) for both PCB layouts. Account for board orientation and rotation when positioning the mating connectors. Use grid snapping and check connector coordinates in the EDA tool. Check the connector's allowed misalignment in the datasheet. Rigid, fine-pitch connectors can allow only a few tenths of a millimeter.
Mounting holes and standoffs sized to the mated height
For boards secured with standoffs, select lengths compatible with the connector’s mated height and the board mounting geometry. A mismatch puts continuous stress on solder joints and may eventually cause fatigue and failure. Short standoffs can compress the connector housing. Oversized standoffs may prevent full mating.
Ground pins between high-speed pairs
Assign ground pins adjacent to high-speed differential pairs to minimize crosstalk and provide a clear return path. A common pattern is signal-signal-ground-signal-signal. Adjacent grounds help separate the pairs and support a controlled return path through the connector interface. Follow standard high-speed PCB design layout rules to maintain impedance through the transition.
Power pin derating
When passing power through multiple adjacent pins, derate the current capacity. The heat generated by clustered power pins reduces the safe operating current per pin compared to a single isolated contact. Check the datasheet's derating curve based on the number of energized pins. For example, a pin rated for 1 A alone may carry less when ten adjacent pins are all energized.
Keep-outs
Respect the manufacturer's recommended keep-out zones around the connector footprint. The specified clearances prevent interference with the mating housing and leave access for rework tools. Placing tall components too close to the connector can block the mating half from fully seating. Review our PCB footprint design guide for more details on managing component clearances.
Assembly and mating force
Support the PCB directly beneath the connector during mating. High pin-count connectors require significant insertion force, which can flex or crack an unsupported board during assembly.
Avoid placing delicate components like ceramic capacitors near the connector footprint, as board flexure during mating can easily fracture them.
Tolerance stack-up of the two boards
Account for PCB fabrication tolerances, pick-and-place accuracy and enclosure mounting-point alignment. If the tolerances stack unfavorably, rigid connectors will bind. Calculate worst-case mechanical offsets to confirm that production boards can mate without binding.
Common Mistakes When Selecting and Laying Out Board-to-Board Connectors
Board-to-board connector errors often involve mechanical alignment, retention or return-path planning. Check the corresponding corrective steps before layout sign-off.
Ignoring tolerance stack-up
Assuming perfect alignment between multiple rigid connectors on the same board is a common cause of binding. Two rigid connectors between the same PCBs can bind if manufacturing variations change connector spacing. PCB fabrication, connector housing and component placement each contribute to the combined offset.
- Correction: Use a single high-pin-count connector, or use a floating connector for the second interface to absorb the offset.
Insufficient retention force for vibration
Relying on the friction of a standard pin header in a high-vibration environment is a common oversight. Repeated vibration can gradually loosen the connection, especially in automotive or industrial equipment.
- Correction: Specify a connector with a mechanical latch that locks the mating halves, or use appropriately torqued standoffs to secure the board stack.
Forgetting to specify the correct stack height
A matching family name does not confirm the mated height of a particular plug-and-receptacle pair. An incorrect mated height can cause component interference or prevent the boards from reaching enclosure mounting points.
- Correction: Check the exact plug and receptacle part numbers against the manufacturer’s stack-height matrix, which may offer many height combinations.
Poor ground return paths
Routing high-speed signals through the connector without providing adjacent ground pins creates an impedance mismatch and ruins signal integrity. A longer return-current path increases loop inductance and may increase electromagnetic emissions or cause EMI test failures. For more detail, see Flux’s PCB impedance control guide.
Designing a Multi-Board Product in Flux
First, Flux can design a product with several boards. The main board stays in the parent project. Each daughterboard gets a separate Flux project.
Next, before creating the board projects, the Flux agent writes a floorplan document with one dimensioned brief per daughterboard: the outline in millimeters, finished thickness, layer count, mounting holes, each connector with its part, side, position, rotation and mating direction, height limits and keep-outs, the board's position in the parent assembly, and the nets that cross to the main board. The user approves the floorplan first.
After floorplan approval, each board is designed in its own project. Flux brings each populated 3D board into the parent assembly at the planned position and measures clearances, overlaps, connector-to-opening offsets and board thickness in millimeters.
Finally, Flux holds routing until the fit checks of every board pass. If a check finds a conflict, Flux stops and waits for a user decision. For example, a drone builder can put the flight controller, ESC and radio boards in one stack and check that the stack fits before routing a trace.
Plan a multi-board design in Flux before routing.
FAQs
The terms overlap. Both describe connectors joining parallel boards in a stack. "Mezzanine" is common for high-density and high-speed parts. "Stacking connector" often refers to header-and-socket parts with long pins.
Use the mated height from the datasheet for the exact plug and receptacle part numbers. The mated height is the distance between the facing board surfaces. Select enough clearance for the tallest intervening component and size standoffs for the approved mated geometry.
Yes, with a suitably rated power or hybrid connector. Some power blades are rated up to 18 A, for example Samtec mPOWER. Fine-pitch signal contacts generally carry less current. Check the selected part’s datasheet and derating curve.
Yes. High-speed families have controlled impedance and ground structures, and some support 56 Gbps or 112 Gbps PAM4 per channel. Use the manufacturer’s S-parameter models and assign grounds between differential pairs as specified for the connector.
Use flex or rigid-flex when boards must move, fold or sit at odd angles, or to eliminate connectors. Use a board-to-board connector when modular boards need separate assembly, testing and replacement.
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