What are press-fit (eye-of-the-needle) terminals, and what are their primary applications?-ELETE | Professional Connector Manufacturer

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What are press-fit (eye-of-the-needle) terminals, and what are their primary applications?

Sep 02,2026

Press-fit terminals, specifically the eye-of-the-needle (EON) design, are solderless electrical interconnects used to form mechanical and electrical connections between components and plated through-holes (PTH) on a printed circuit board (PCB).

What is an Eye-of-the-Needle（EON）Press-Fit Terminal?

An EON terminal is a stamped, compliant metallic pin characterized by a hollow oval cutout near its center—resembling the eye of a sewing needle.

When the pin is pressed into a PCB's plated hole with a diameter slightly smaller than the pin's outer width, the central beam deforms elastically and plastically. This spring-like compression generates high normal forces against the inner copper walls of the hole, creating a cold-weld gas-tight joint without requiring thermal soldering.

Key Advantages Over Soldering

Eliminates Thermal Stress: Avoids thermal shocking and board warping associated with reflow or wave soldering processes.
High Reliability: Immune to solder joint fatigue, micro-cracking, and thermal expansion mismatches under high vibration.
Eco-Friendly & Clean: Completely solderless, removing lead, flux residues, and the need for post-assembly cleaning.
Easy Repairability: Components can be pressed out and replaced without damaging the PCB hole plating.

Press-Fit Terminal Primary Applications

Industry / Application	Common Uses
Automotive Electronics	Engine control units (ECUs), ABS braking systems, power steering controllers, battery management systems (BMS), and sensor modules where severe vibration and thermal cycling occur.
Industrial Automation	PLC controllers, motor drives, heavy machinery power electronics, and relay switch modules.
Telecommunications & Computing	High-density backplane connectors, blade servers, switches, and power distribution units (PDUs).
Smart Grid & Energy	Solar inverter power stages, smart utility meters, and high-current junction boxes.

How Press-Fit Terminal work?

The working principle of an eye-of-the-needle (EON) press-fit terminal relies on controlled elastic deformation. When inserted into a plated through-hole (PTH) of a printed circuit board, the flexible eye-shaped section compresses within its elastic limit. This high spring force generates continuous normal pressure against the PCB hole wall, forming a reliable, solderless electrical connection through cold welding and friction.

What are the advantages of eye-of-the-needle (EON) terminals compared to traditional soldering?

Press-fit terminals, specifically the Eye-of-the-Needle (EON) design, offer several key advantages over traditional soldering methods like wave or reflow soldering:

No Thermal Stress: Eliminates high-temperature soldering, protecting heat-sensitive electronic components and the PCB substrate from thermal damage or warping.
Superior Mechanical Reliability: The flexible EON zone creates a tight, spring-loaded cold weld within the plated through-hole (PTH), offering exceptional resistance to shock and vibration.
Zero Solder Defects: Eliminates common solder-related failure modes such as cold joints, bridging, solder balls, flux residue contamination, and voids.
Cost-Effective & Eco-Friendly: Removes the need for solder, flux, and cleaning chemicals, reducing process complexity, energy consumption, and environmental footprint.
Easy Repairability: Allows straightforward component replacement and rework without requiring desoldering equipment or damaging the circuit board.

Comparison Criteria	Press-fit Terminals (Eye-of-the-Needle / EON)	Traditional Soldering
Process	Pure mechanical insertion, no heat involved	Requires heating to melt solder (Wave soldering / Hand soldering)
Vibration Resistance	Excellent (Sustained elastic normal force)	Moderate (Solder joints become brittle and prone to fatigue fracture)
Environment & Materials	No solder or flux required	Requires solder paste/wire, requires post-process residue cleaning
Inspection Cost	Only insertion depth and force need monitoring (100% controllable)	Requires X-ray or AOI inspection for internal voids in solder joints

What are the standards for the insertion force and retention force (pull-out force) of press-fit (eye-of-the-needle) terminals?

Insertion force and retention/push-out force are key mechanical parameters used to evaluate the quality of press-fit (eye-of-the-needle) connections.

The primary international industry standard for these connections is IEC 60352-5 (Solderless connections - Part 5: Press-in connections). Additionally, the automotive and industrial sectors often reference IPC-9797 as well as internal OEM specifications such as VW 80000 and USCAR-2.

1. Standard Values and Key Metrics Force requirements are closely linked to nominal terminal dimensions (such as 0.64 mm, 1.5 mm, 2.8 mm, etc.), terminal thickness, and PCB hole diameter. The core principle of standards is: insertion force must not be excessively high (to prevent damage to the PCB and plated through-hole coating), while retention force must not be too low (to ensure vibration resistance and reliable mechanical contact).

Maximum Insertion Force:

Typically, maximum insertion force for a single pin is capped under 100 N to 150 N (for common 0.64 mm / 0.8 mm automotive signal terminals, the typical limit is usually 30 N to 80 N per pin).
Excessive force may cause PCB copper cracking, terminal bending, or excessive metal debris (shaving).

Minimum Retention Force / Push-out Force:

IEC 60352-5 requires that single-pin retention force must not fall below 20 N or 30 N (depending on pin specifications).
In automotive applications, residual retention force is often required to remain ≥ 20 N after thermal shock, mechanical vibration, and aging tests to ensure no looseness under extreme vibration environments.

2. Insertion Force-Distance Profile In automated production and quality monitoring, focus extends beyond peak force to the curve profile throughout the process:

Initial Contact Phase: The terminal tip enters the PCB guidance hole, causing force to rise rapidly.
Elastic Deformation Phase: The eye-of-the-needle structure collapses and deforms, with force reaching its peak (Peak Force).
Final Seating Phase: Upon reaching the set depth, press force stabilizes, completing mechanical locking.

3. Key Factors Affecting Insertion and Retention Forces

Key Factor	Impact on Force
Final PCB Hole Size (PTH Size)	Smaller hole sizes lead to a sharp increase in insertion force or even terminal breakage; larger hole sizes result in insufficient retention force and increased contact resistance.
Plating Type & Thickness in PTH	Variations in friction coefficient exist among HASL, ENIG, and Immersion Tin. Hot air solder leveling (HASL) and Immersion Tin can act as lubricants, typically resulting in lower insertion forces.
Press-in Speed	Excessively high speeds may lead to localized stress concentration; automated press-fitting speed is usually set between 10 to 50 mm/s.
Terminal Plating	The thickness of tin or silver plating on the terminal surface impacts friction and the micro cold-welding effect.

Specific Requirements for PCB Plated Through-Hole (PTH) Dimensions in the IEC 60352-5 Standard

IEC 60352-5 (Solderless connections - Part 5: Press-in connections) enforces strict dimensional control on PCB Plated Through-Holes (PTH). Because eye-of-the-needle (EON) terminals rely on the elastic contact pressure from the hole wall to establish a gas-tight connection, the drill size, copper plating thickness, and finished hole size must be maintained within tight tolerance limits.

1. Core Dimensional and Tolerance Specifications

IEC 60352-5 specifies corresponding PCB hole dimensions based on nominal terminal sizes (such as common 0.64 mm, 0.8 mm, and 1.2 mm grid pins). Key control requirements include:

Finished Hole Diameter (df):
- Tolerance Requirement: Typically controlled within ±0.05mm to ±0.08 mm (e.g., φ 1.00±0.05 mm or φ1.45 ±0.05 mm).
- An undersized hole leads to excessive insertion force that damages the hole wall copper plating, whereas an oversized hole fails to provide sufficient retention force.
Hole Wall Copper Thickness (c):
- Average Thickness: Must not be less than 25 um.
- Minimum Local Thickness: Must not be less than 20um at any point.
- Adequate copper thickness prevents the copper wall from stretching or breaking through during terminal insertion.
Surface Finish Thickness:
- Immersion Tin (Sn): Typically specified between 0.8 um and 1.5 um.
- Hot Air Solder Leveling (HASL): 1um to 15um (due to poor thickness uniformity, high-end automotive electronics lean toward Immersion Tin or ENIG).
- ENIG (Electroless Nickel / Immersion Gold): Nickel layer 3 um~6 um, Gold layer 0.03 um~0.1 um.

2. Plated Through-Hole (PTH) Cross-Section Structure and Key Parameters A press-fit plated through-hole consists of the drill hole, electroplated copper layer, and surface finish layer. The dimensional relationships are outlined in the table below:

Dimension / Feature	Symbol	Standard Typical Requirements	Function / Considerations
Drill Hole Diameter	dk	0.10 ~ 0.15 mm larger than finished hole diameter	Reserves clearance/thickness for subsequent copper plating and immersion tin
Plated Copper Thickness	c	Average ≥25 um, local ≥20 um	Provides mechanical strength and electrical conductivity; withstands terminal contact pressure
Finished Hole Diameter	df	Depends on terminal specification (tolerance ± 0.05 mm）	The actual contact interface after terminal press-fitting
Pad Diameter	a	≥ df + 0.30 mm	Prevents pad peeling/delamination during press-fitting

3. Examples of Common Standard Terminal Sizes and Corresponding Hole Diameters

According to the IEC 60352-5 specification, typical matching requirements for mainstream terminals and PCB finished holes on the market are as follows:

0.64 mm Signal Terminals (Common in automotive and industrial applications): Finished hole diameter is typically specified as φ 0.60± 0.05 mm or φ 1.00± 0.05mm.
1.2 mm / 1.5 mm Power Terminals (High-current applications): Finished hole diameter is typically specified as φ1.45 ±0.05 mm or φ1.60± 0.05mm.