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.

3.14.1 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.

3.14.2 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.

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.

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.

3.14.3 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.

3.14.4 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.

Final Checklist: Press-Fit Implementation Mandates