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3.14 Press-Fit Technology

Press-fit is a mechanical interconnection technology that uses a compliant pin design to create a gas-tight, reliable electrical connection with the plated through-hole (PTH) barrel without requiring soldering. This technology is mandatory for applications demanding high mechanical integrity, current capacity, or ease of connector replacement/rework, such as backplanes, power supplies, and high-speed data connectors.

3.4.14.1 The Mechanism: Cold Welding and Reliability

Press-fit relies on the elastic and plastic deformation of both the pin and the PTH barrel to achieve a secure contact.

  • Pin Design: The pin is designed with a specific compliant zone (often an eye-of-the-needle or tuning fork shape). As the pin is inserted into the hole, this zone compresses, partially deforming the PTH barrel.
  • Gas-Tight Seal: The continuous outward pressure exerted by the compressed pin against the copper barrel creates a gas-tight seal (a cold-weld zone). This seal prevents oxygen and contaminants from reaching the contact surfaces, ensuring long-term low contact resistance and high reliability, often exceeding that of a standard soldered joint.
  • Rework: Press-fit allows for the non-destructive removal and replacement of connectors, which is impossible with standard PTH soldering without risking pad damage.

3.4.14.2 Design Mandates: Hole Tolerance and Pad Stack

The success of press-fit is highly dependent on tight control of the PTH dimensions. Unlike soldering, where the solder fills the gap, press-fit relies on a specific interference fit.

A) Finished Hole Diameter (FHD)

The PTH's Finished Hole Diameter (FHD) tolerance is the most critical parameter.

  • Tolerance: The tolerance stack-up must be exceptionally tight, often ± 0.05 mm (± 2 mil) or less, compared to ± 0.10 mm for standard PTHs.
  • Design-in: The PCB fabricator must be informed that the holes are designated for press-fit, as they require higher precision drilling, plating uniformity, and strict quality control on the final hole size measurement.

B) Pad Stack Requirements

The pad geometry must accommodate the stress induced during insertion.

  • Annular Ring: The pad's copper area ≥ 0.15 mm radial annular ring) must be robust enough to prevent pad lift or barrel damage during pin insertion and removal.
  • Ground/Power Planes: Pins connecting to internal ground or power planes require thermal relief only if they are later to be selectively soldered. If the connection is exclusively press-fit, the pin should connect directly to maximize the mechanical bond and minimize contact resistance.

3.4.14.3 Assembly and Process Control

Press-fit assembly is a mechanical process requiring specialized, high-force equipment and mandatory post-assembly inspection.

  • Equipment: Insertion requires a dedicated servo-driven press capable of applying hundreds of kilograms of force with high precision. Never use a hammer or improvised tools, as this introduces uncontrolled mechanical stress, leading to immediate pin failure or latent component damage.
  • Force Monitoring: The press equipment must monitor and log the insertion force curve for every connector.
    • Maximum Force: Ensures the press stops before damaging the PCB or connector body.
    • Minimum Force: Ensures the compliant pin achieves the required minimum compressive force needed for a reliable gas-tight seal.
  • Inspection: Post-assembly verification must be performed. Methods include:
    • Visual Inspection (VI): Checking the final stand-off height (the distance between the connector body and the PCB surface).
    • Electrical Test: Measuring the contact resistance (≤ 5 mΩ typical) to verify the electrical integrity of the cold weld.

3.4.14.4 Press-Fit vs. Solder: Risk and Cost

The decision to use press-fit is a strategic trade-off involving upfront NRE/CapEx versus long-term flexibility and reliability.

Factor

Press-Fit (Mandate)

Soldering (Standard)

Reliability

Superior mechanical robustness and high current capacity; guaranteed gas-tight seal.

Joint reliability depends heavily on flux chemistry and thermal profiling.

Rework/Repair

Non-destructive removal/replacement is feasible with specialized tools.

Component removal requires desoldering, risking pad damage and high rework cost.

Hole Cost

High PCB Cost: Requires tight hole tolerance (low yield for the fabricator).

Low PCB Cost: Standard plating tolerance is acceptable.

Assembly Cost

High CapEx: Requires dedicated press equipment and tooling.

Low CapEx: Uses existing wave/selective solder machines.

Final Checklist: Press-Fit Implementation Mandates

Parameter

Mandate

Rationale

Hole Tolerance

PCB fab spec must call out a tight FHD tolerance (≈ 0.05 mm).

Ensures the interference fit required for a gas-tight electrical seal.

Insertion Equipment

Dedicated servo-driven press is mandatory; no manual tools allowed.

Provides controlled, high-force insertion while monitoring stress.

Force Logging

Insertion force curve must be monitored and logged per connector (minimum/maximum force).

Verifies the mechanical quality of the cold weld and prevents component damage.

Post-Assembly Check

Stand-off height is inspected; electrical test verifies low contact resistance.

Final quality gate for mechanical seating and electrical integrity.

Pad Integrity

Annular ring integrity checked (must be ≥ 0.15 mm) after insertion.

Ensures the copper pad stack is not damaged or lifted by the insertion stress.


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