A connector’s electrical interface depends on a sequence of mechanical events: alignment, contact deflection and sliding engagement.
This contact-design guidance explains how spring geometry, normal force, contact wipe and plating work together—and why PCB spacing and dimensional tolerances belong in connector selection.
Key features and benefits
- Contact deflection generates the force that presses mating surfaces together.
- Contact geometry and spring material govern how the contact responds during mating.
- Sliding engagement provides a cleaning action at the contact interface.
- Plating works with contact geometry and normal force to establish the electrical connection.
- Controlled board spacing and alignment help maintain the intended engagement across manufacturing tolerances.
These characteristics form an interconnected system. Selecting a plating finish independently of contact geometry, or choosing board spacing independently of mating depth, overlooks the mechanical conditions that create the electrical interface.
Technical highlights
Contact deflection creates normal force
During mating, at least one contact normally deflects. The contact’s geometry and material determine its response to this movement.
That deflection generates normal force: the force pressing the mating surfaces together. Insufficient force can produce an unreliable connection, but increasing force without regard to geometry is not a complete solution. The contact must generate the required force while remaining within its intended mechanical limits.
The spring material therefore has two roles. It must provide the mechanical behaviour required for repeatable deflection and the conductivity required for the electrical path. The relationship between these functions is central to connector structure and contact-spring design.
Contact wipe establishes the interface
Contact wipe is the sliding movement between mating contact surfaces as the connector engages. This movement helps remove contamination and surface films from the mating area.
Particles from surrounding processes, dust, oils and oxide films can interfere with electrical contact. Tin-plated surfaces can develop oxide films through exposure to air or humidity, making the cleaning action relevant to interface performance.
Available wipe depends on contact geometry, insertion depth and mechanical tolerances. A nominally mated connector must therefore retain sufficient engagement across the permitted dimensional range, not merely at the nominal assembly position.
Plating and geometry must work together
Connector contacts commonly use copper-alloy spring materials with a plated mating surface. Gold and tin have different interface characteristics and must be considered alongside normal force and wipe.
Gold resists oxidation and requires less normal force than tin. This relationship matters in low-profile, fine-pitch connectors, where contact geometry constrains the available spring movement and force.
The contact finish is consequently part of the mechanical design, not an independent purchasing option. Its interaction with the spring and mating surface is a central consideration in connector materials and processes.
| Design factor | Function | Selection consideration |
|---|---|---|
| Contact geometry | Controls deflection | Mechanical operating limits |
| Normal force | Presses surfaces together | Spring behaviour and finish |
| Contact wipe | Cleans mating interface | Engagement throughout tolerances |
| Plating | Forms contact surface | Force and wipe requirements |
| Board spacing | Sets mating depth | Permitted insertion range |
| Alignment | Positions mating contacts | Combined assembly tolerances |
Application fit
Board-to-board connections
In a board-to-board assembly, stack height—the distance between the connected PCBs—affects contact engagement. Board spacing must remain within the connector’s permitted insertion range so the mating contacts retain the intended wipe.
Connector selection and mechanical stack design should therefore proceed together. Fixing PCB separation before considering mating depth can constrain the contact interface.
Low-profile and high-density assemblies
Reducing connector height creates a trade-off between package size and the mating length available for contact wipe. The contact still needs enough sliding engagement to perform its cleaning action.
Gold-plated interfaces can support contact systems with lower normal force than tin-plated interfaces. Plating choice must nevertheless remain tied to the selected contact geometry and its mating conditions.
Assemblies exposed to vibration and shock
Contact wipe and contact-point redundancy are relevant to connections that must withstand vibration and shock. The contact system must establish the intended interface during mating before it can maintain that interface in service.
Multiple connectors between the same PCBs
Using several connector pairs between two boards introduces interacting dimensional tolerances. Connector dimensions and placement on each PCB contribute to the final alignment.
Contact systems can accommodate some movement, but their deflection and wipe capabilities are finite. Tolerance stack-up analysis should establish whether every connector pair can engage within its intended mechanical range.
Design-in notes for engineers
- Establish PCB stack height together with the mating connector pair, rather than treating board spacing as a separate decision.
- Check minimum and maximum insertion depth against the assembled mechanical stack.
- Evaluate contact wipe across dimensional tolerances, including connector placement on the PCB.
- Assess plating, normal force and contact geometry together.
- For multiple connector pairs, analyse combined alignment tolerances rather than each pair in isolation.
- Include the expected contamination, vibration and shock conditions in contact-system selection.
- Validate the final assembly under its actual operating conditions before production release.
Contact resistance is an electrical measurement, but the interface behind it is created mechanically. Alignment, spring deflection, normal force and wipe must work together to establish the intended connection.
Further reading
- Connector Design/Selection/Assembly
- How Much Gold is Needed for Connector Plating
- Feedback from Connector Failure Analysis
Source
This article is based on the Samtec technical article and official connector-design documentation. Engineers should refer to the current manufacturer datasheet for final design and qualification.
