3.4 Press-Fit Technology

Press-fit connectors turn plated through-holes into precision sockets, creating solderless, gas-tight joints that can carry high current and endure harsh environments. Their success depends less on the press machine itself than on the quality of the hole—its size, plating, finish, and cleanliness. When holes are right and the board is firmly supported, compliant pins seat smoothly, monitored by force–displacement curves that reveal every detail of engagement. The result is a robust electrical connection that avoids thermal stress, simplifies rework, and can outlast soldered joints in demanding applications.

3.4.1 What press-fit is (and why you’d choose it)

A compliant pin (eye-of-the-needle, dual-beam, etc.) elastically compresses as it’s pressed into a plated through-hole (PTH). The spring force makes a gas-tight metal-to-metal contact—no solder, less heat, great for high-current/backplane connectors, reworkable assemblies, and mixed processes.

Key idea: the hole is half the connector. If the hole stack isn’t right, no press or program can save it.

3.4.2 Hole & finish prep (the #1 success lever)

Design and fabricator notes that make compliant pins happy:

Plated hole construction

Finished hole size: follow the connector datasheet; target an interference (pin diagonal − hole Ø) that the supplier specifies. As a starting feel: many eye-of-the-needle pins like ~0.03–0.09 mm interference.
- Example: pin diagonal 0.92 mm → finished hole 0.86–0.89 mm (verify with supplier table).
Copper in hole wall: robust builds like ≥25 µm (Class 3) hole wall copper; more on high-current boards.
Annular ring: ≥ 0.25–0.30 mm radial; bigger near connector tangs to resist peel/lift.
Barrel straightness/roughness: tight fab control; avoid nodules or voids (X-section first article).

Surface finish (inside the barrel)

Preferred: matte Sn (or SnPb where allowed), HASL (level, not lumpy), or ImmSn.
Think twice: ENIG puts Ni in the stack—some pin designs tolerate it, others don’t (higher insertion force / fretting risk). Use only if the connector maker approves.
Avoid contamination: no solder, flux, or mask in the hole. Specify no tenting on PTHs for press-fit zones; mask clearance +0.10–0.15 mm beyond pad.

Panel & layout tips

Keep stiffeners/backing copper near big connectors so the panel doesn’t “drum” during press-in.
Put tooling holes close to the pin field for accurate fixturing.
Don’t route microvias into the barrel; keep planes/teardrops symmetrical to avoid tilt.

3.4.3 Pins, plating, and lube (match the pair)

Pin style: eye-of-the-needle (controlled spring), dual-beam, or solid press-in posts. The data sheet defines pin diagonal, recommended hole Ø, and insertion force per pin.
Pin finish: Sn over Cu is common; some use Sn over Ni.
Lubricants: many press-fit pins carry a dry polymer lube. It lowers force scatter and fretting. Don’t solvent-strip it in cleaning; verify chemistry compatibility during NPI.

3.4.4 Press equipment & setup (parallel, supported, calm)

Press type: pneumatic or servo press with a calibrated load cell and Z encoder. Servo gives the best force–displacement trace.
Nest/backup: rigid support plate directly under the pin field (as close as mask allows). No air gaps.
Planarity & skew: use lead-ins/pilots on the tool; check connector coplanarity; keep press platens parallel.
Speed: steady, not a slam—3–15 mm/s works for most; let force rise smoothly.
Temperature & moisture: room temp, dry PCBs. Moisture in barrels lifts copper.

Never hammer pins or use a hand arbor without a load readout on production hardware.

3.4.5 Force–displacement (F–Z): your truth meter

A good press-in makes a recognizable curve:

Free travel: near-zero force until pins touch holes.
Engagement ramp: force rises as beams compress and scrape oxides.
Controlled plateau: pins sliding at roughly steady force.
Seating knee: sharp rise as shoulders/face seat on PCB.
Hold & release: brief dwell, then unload.

Signal on the F–Z trace	What it likely means	Action
Too low, too flat	Oversized holes / under-plated pins / missing lube	Hold lot; gauge holes; check pin batch
Jagged spikes	Burrs, rough barrels, tilted entry	Inspect hole roughness; slow speed; improve nest
No seating knee	Not fully seated / wrong Z stop	Verify Z; inspect standoff height
Force far above spec	Holes undersize / ENIG + tight pin / no lube	Stop before damage; measure; consult supplier

Log & compare: store the curve (or its key features) per insertion. Run SPC on peak and plateau forces.

3.4.6 Acceptance criteria (mechanical, electrical, visual)

Define these before the build; copy from the connector spec and your class requirements.

Mechanical

Insertion force per pin / per connector within supplier limits.
Retention (push-out) force ≥ spec (often >30–80 N/pin depending on type—use the supplier number).
Board bow during press < 1.5–2.0 mm across the field (or strain <500–700 με near risk parts).

Electrical

Contact resistance initial within spec (typ. ≤10 mΩ/pin, check datasheet).
Current/temperature rise meets connector rating (sample test at rated current).
Insulation resistance to neighbors per product class.

Visual

Pins seated to standoffs, uniform protrusion if through-board; no cracked mask, no barrel splits, no lifted annular rings.
No shavings around holes (vacuum and inspect).
Orientation/polarizing features correct.

For safety/harsh-duty builds, add aging: thermal cycles, humidity bias, vibration, then repeat resistance/retention.

3.4.7 Process flow (first article → volume)

FA/Tool prove-out
- Gauge finished hole Ø across panel; check copper & finish with microsection.
- Press a coupon row; record F–Z curves; measure push-out/contact resistance.
- Freeze recipe ID: speed, Z stop, envelope limits.
Production
- 100% F–Z monitoring on the press (envelope match).
- Patrol checks (per lot): hole gauge on 5 boards, retention on 1 connector, electrical spot-check.
Change control
- Any PCB finish or connector lot change → run the FA mini-suite again.

3.4.8 Troubleshooting (symptom → smallest fix)

Symptom	Likely cause	First fix
High insertion force	Holes undersize; ENIG friction; missing lube	Measure holes; switch to Sn barrel finish next rev; confirm pin lube; reduce speed slightly
Pins don’t seat (no knee)	Z stop short; nest flex; tall solder beads on pads	Verify Z; stiffen backup; keep holes free of solder
Cracked barrel / lifted pad	Excess force; thin copper; poor support	Halt; improve support; check copper ≥25 µm; revisit hole size
High contact resistance	Oxide film; poor engagement	Ensure compliant beams landed; use approved lube pins; consider Sn barrel finish
Push-out low	Oversize holes; worn pins	Gauge & sort; new connector lot; hold WIP
Shavings/debris	Rough holes; dry entry	Improve plating smoothness; ensure pin lube; vacuum at press

3.4.9 Data to keep (makes audits easy)

Per press event: SN, connector PN/lot, peak/plateau force, Z at seat, pass/fail envelope.
Per lot: hole gauge study (min/mean/max), microsection on first lot, push-out sample, resistance sample.
Changes: PCB finish, connector plating/lube, press tool maintenance.

3.4.10 Pocket checklists

Before NPI

Connector datasheet hole table copied to fab notes
PCB finish for press-fit confirmed (Sn/ImmSn/HASL preferred)
Hole wall copper ≥25 µm; annular ring ≥0.25–0.30 mm
First-article microsection planned

Press setup

Rigid backup nest under pin field; planarity checked
Servo press with load cell + Z encoder; envelope loaded
Speed 3–15 mm/s; Z stop verified on scrap board
Vacuum for debris; ESD grounding

First article

F–Z curves match expected shape/limits
Retention and resistance meet spec
Visual: seated, no barrel/pad damage

Production patrol

Hole gauges on sample; F–Z SPC in control
One connector push-out per lot (sacrificial)
Log connector & PCB lot changes

By locking hole specifications, finishes, and press controls into the quality plan, press-fit becomes a stable, low-risk interconnect method. With consistent monitoring and clear acceptance criteria, assemblies gain durable reliability without the hidden costs of cracked barrels or weak contacts.