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 robustmechanical interconnection technology that uses a compliant pin design to create a gas-tight, reliable electrical connection thatwith avoidsthe 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)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.

3.4.1 The springMechanism: forceCold makesWelding 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 metal-to-metalseal (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:—no solder,Press-fit less heat, greatallows for high-current/backplanethe non-destructive removal and replacement of connectors, reworkable assemblies, and mixed processes.

  Key idea: the holewhich is halfimpossible thewith connector.standard IfPTH thesoldering holewithout stackrisking isn’tpad right, no press or program can save it.damage.

  
  

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3.4.2 Design Mandates: Hole &Tolerance finishand prepPad (the #1 success lever)Stack

DesignThe andsuccess fabricatorof notespress-fit thatis makehighly compliantdependent pinson happy: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 PlatedFinished holeHole constructionDiameter (FHD) tolerance is the most critical parameter.

• Finished hole size:Tolerance: followThe thetolerance connectorstack-up datasheet;must targetbe anexceptionally tight, often 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 0.86–0.8905 mm (verify± 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.

  3.4.3 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 supplierhigh table).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.
  • Copper in hole wall:Inspection: robustPost-assembly buildsverification likemust be performed. Methods include:
    • Visual Inspection (VI): Checking the final ≥25stand-off µmheight (Classthe 3)distance holebetween wall copper; more on high-current boards.
    • Annular ring: ≥ 0.25–0.30 mm radial; bigger nearthe connector tangsbody to resist peel/lift.
    • Barrel straightness/roughness: tight fab control; avoid nodules or voids (X-section first article).

    Surface finish (insideand the barrel)

    PCB
    • Preferred: matte Sn (or SnPb where allowed), HASL (level, not lumpy), or ImmSnsurface).
    • ThinkElectrical twice:Test: Measuring the ENIGcontact resistance puts(≤ 5 mΩNi intypical) to verify the stack—someelectrical pinintegrity designs tolerate it, others don’t (higher insertion force / fretting risk). Use only ifof the connectorcold maker approves.weld.
    • Avoid
  contamination:

  3.4.4 noPress-Fit solder,vs. flux,Solder: orRisk maskand inCost

  the

  The hole.decision Specifyto no tenting on PTHs foruse press-fit zones;is maska clearancestrategic +0.10–0.15trade-off mminvolving beyondupfront pad.

NRE/CapEx

Panelversus &long-term 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.

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3.4.3 Pins, plating,flexibility 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.

reliability.

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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.

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

A good press-in makes a recognizable curve:

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

Signal on the F–Z traceFactor	WhatPress-Fit it likely means(Mandate)	ActionSoldering (Standard)
Too low, too flatReliability	OversizedSuperior holesmechanical /robustness under-platedand pinshigh /current missingcapacity; lubeguaranteed gas-tight seal.	HoldJoint lot;reliability gaugedepends holes;heavily checkon pinflux batchchemistry and thermal profiling.
Jagged spikesRework/Repair	Burrs,Non-destructive roughremoval/replacement barrels,is tiltedfeasible entrywith specialized tools.	InspectComponent holeremoval roughness;requires slowdesoldering, speed;risking improvepad nestdamage and high rework cost.
NoHole seating kneeCost	High PCB Cost:Not fullyRequires seatedtight /hole wrongtolerance Z(low stopyield for the fabricator).	Low PCB Cost:Verify Z;Standard inspectplating standofftolerance heightis acceptable.
ForceAssembly far above specCost	High CapEx:Holes undersizeRequires /dedicated ENIGpress +equipment tightand pin / no lubetooling.	Low CapEx:Stop beforeUses damage;existing measure;wave/selective consultsolder suppliermachines.

Log

Final &Checklist: compare:Press-Fit storeImplementation the curve (or its key features) per insertion. Run SPC on peak and plateau forces.

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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.

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3.4.7 Process flow (first article → volume)

1. FA/Tool prove-out
2. • 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.
2. Production
3. • 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.
3. Change control
4. • Any PCB finish or connector lot change → run the FA mini-suite again.
     

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3.4.8 Troubleshooting (symptom → smallest fix)Mandates

SymptomParameter	Likely causeMandate	First fixRationale
HighHole insertion forceTolerance	HolesPCB undersize;fab ENIGspec friction;must missingcall lubeout a tight FHD tolerance (≈ 0.05 mm).	MeasureEnsures holes;the switchinterference tofit Snrequired barrelfor finisha nextgas-tight rev;electrical confirm pin lube; reduce speed slightlyseal.
PinsInsertion don’t seat (no knee)Equipment	ZDedicated stopservo-driven short;press nestis flex;mandatory; tallno soldermanual beadstools on padsallowed.	VerifyProvides Z;controlled, stiffenhigh-force backup;insertion keepwhile holesmonitoring free of solder
Cracked barrel / lifted pad	Excess force; thin copper; poor support	Halt; improve support; check copper ≥25 µm; revisit hole sizestress.
HighForce contact resistanceLogging	OxideInsertion film;force poorcurve engagementmust be monitored and logged per connector (minimum/maximum force).	EnsureVerifies compliantthe beamsmechanical landed;quality useof approvedthe lubecold pins;weld considerand Snprevents barrelcomponent finishdamage.
Push-outPost-Assembly lowCheck	Stand-off heightOversize holes;is worninspected; pinselectrical test verifies low contact resistance.	GaugeFinal &quality sort;gate newfor connectormechanical lot;seating holdand WIPelectrical integrity.
Shavings/debrisPad Integrity	RoughAnnular holes;ring dryintegrity entrychecked (must be ≥ 0.15 mm) after insertion.	ImproveEnsures platingthe smoothness;copper ensurepad pinstack lube;is vacuumnot atdamaged pressor lifted by the insertion stress.

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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.

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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
  

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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.