Pin Row Female Insertion Force Design: Material Selection and Structure Optimization

Introduction

The pin bus connector is the most commonly used product in board-to-board connections, and its plugging force is the core indicator for measuring mechanical performance and user experience.The insertion and removal force is too large, the assembly is difficult and easily damaged; if the insertion and removal force is too small, the contact is unreliable and the vibration resistance is poor.How to find the best balance between contact reliability and ease of operation is a key issue in the design of the pin header.This paper deeply analyzes the design points of plugging force from the three dimensions of material selection, structural design, and process control.

1. Basic concept of plugging force

1.1 Definition of Plugging Force

The insertion and extraction force of the female needle row includes two indicators:

• Insertion Force: Maximum thrust required for matched males and females
• Extraction/Withdrawal Force: Maximum pull force required to pull out after pairing

The pull-out force is usually greater than the insertion force, because there is static friction between the contact surfaces when pulling out, and the elastic deformation returns with a hysteresis effect.

1.2 Composition of the plugging force

The plugging force is mainly composed of the following parts:

II. Influence of material selection on plugging force

2.1 Contact material

The material of the row of pin female contacts (row of pin pins, row of mother shrapnel) directly affects the insertion and removal force and the retention force.

Selection principles:

• Require low plugging force: Choose materials with low elastic modulus (such as brass)
• Require high retention: Choose materials with high yield strength (such as beryllium copper)
• Require long life: Choose materials that are resistant to fatigue (such as beryllium copper, phosphor bronze)

2.2 Insulator material

Insulator (plastic body) material affects two aspects of plugging force:

1. Rigid: Affects pin/shrapnel fixation stiffness

2. Coefficient of friction: Plastic friction resistance of the guiding part

Material	Modulus of Elasticity (GPa)	Coefficient of friction (counter-plastic)	High Temperature Resistance	Description
PA66	3~3.5	0.3~0.4	120℃	Versatile, low-cost
PA6T/PA9T	3.5~4.5	0.35~0.45	150~170℃	High temperature resistance, high strength
PBT	2.5~3.0	0.3~0.4	120℃	Low cost and stable size
LCP	10~15	0.2~0.3	200℃+	Extremely high rigidity, low friction

2.3 Coating Materials

The effect of the coating on the plugging force is mainly reflected by the friction coefficient:

Plating	Coefficient of friction (for same coating)	Abrasion resistance	Influence of plugging force
Jin vs Jin	0.3~0.5	Poor (Soft)	Medium
Tin to Tin	0.5~0.8	Poor (sticky)	Larger
Nickel to Gold	0.4~0.6	General	Medium
Palladium vs Gold	0.3~0.4	Good	Smaller

Note:The tin coating may produce a “cold welding” phenomenon at high temperatures and pressures, resulting in an abnormal increase in plugging force or even adhesion.

III. Influence of structural design on plugging force

3.1 Needle Row Structure Parameters

pin Diameter/Width

• the thicker the pin, the greater the amount of interference with it, and the greater the plugging force.
• Common specifications: 0.5mm (2.54 mm pitch), 0.4mm (2.0mm pitch), 0.3mm (1.27 mm pitch)

pin Head Shape

Head Shape	Insertion Force	Pull-out force	Features
Cone	Small	Large	Good orientation, commonly used
Spherical	Small	Medium	Uniform contact
Tangent	Medium	Medium	Simple processing
Right Angle	Large	Large	Difficult to insert, good anti-vibration

Recommended design:Cone + cylinder combination type, the angle of the guide section is 15° ~ 30°, and the length of the matching section cylinder is 0.5~1.0mm.

3.2 Row Bus Structure Parameters

Shrapnel form

The contact shrapnel design of the row mother is the most critical factor in determining the insertion force:

Shrapnel form	Plug Force Level	Number of Contact Points	Applicable scenarios
Single Shrapnel	Small	1 point	Low-cost, low-plug force requirements
Double Shrapnel (Opposite Clip)	Medium ~ Large	2 o’clock	General-purpose type, good reliability
Triple Shrapnel	Large	3 points	Highly reliable, high vibration environment
Cantilever beam type	Small ~ Medium	1 ~ 2 o’clock	Fine pitch, low plugging force
Torsion spring type	Medium	2 o’clock	High life, mid-plug force

Excess volume design

Interference refers to the amount of spacing in the free state of the mother shrapnel that is less than the diameter of the pin.

Ratio of interference amount to pin diameter	Plug Force Level	Exposure Reliability
5%~10%	Low	General
10%~15%	Medium	Good
15%~20%	Higher	Excellent
More than 20%	High	Excellent

Key Design Points:

• Too little interference: unreliable contact, susceptible to vibration
• Too much interference: high plugging force, easy to wear, short life
• General recommendation: 10% ~ 15% excess volume

3.3 Guided Structural Design

Good guided design significantly reduces insertion force and improves assembly efficiency:

Guiding Angle Design:

• Needle head chamfer: 30° ~ 45°, length 0.2~0.5mm
• Female inlet chamfer: 20° ~ 30°, depth 0.3~0.8mm
• The flatter the guide angle, the smaller the insertion force, but the longer the guide stroke

Staying and Positioning:

• Dislocated retaining columns: prevent reverse insertion
• Positioning columns: precisely aligned to reduce lateral resistance upon insertion

3.4 Calculation of multi-pin plugging force

When the number of pins is large, the total plugging force is roughly equal to the single pin plugging force multiplied by the number of pins, but the following corrections need to be considered:

1. Manufacturing error: The amount of interference of each pin varies, and the actual total plugging force is slightly lower than the theoretical value

2. Assembly skew: When matching, it is impossible to fully align, creating additional resistance

3. Positioning Columns/Freezing: Increases additional insertion resistance

Empirical formula:

“`

Total insertion force = single pin insertion force × number of pins × 0.85 + positioning resistance

Total pull-out force = single pin pull-out force × number of pins × 0.9

“`

(0.85 and 0.9 are non-uniform correction coefficients between pins)

IV. Effect of process control on plugging force

4.1 Stamping process

Process parameters	Effect on plugging force	Control points
Stamping burr	Burrs increase friction and wear	Burr ≤ 5μm
Dimension accuracy	Plugging force fluctuates due to interference deviation	Within ± 0.02 mm
Surface finish	Low finish increases the coefficient of friction	Ra≤0.8μm
Material deformation hardening	Cold hardening improves strength and hardness	Control the number of stampings

4.2 Electroplating process

Process parameters	Effect on plugging force	Control points
Plating thickness	Too thick to increase the effective size, increase the interference	Control tolerances according to specifications
Plating uniformity	Unevenness leads to discrete plugging forces	Reasonable choice of roll plating/hanging plating
Coating hardness	Hard gold is more wear-resistant than soft gold	Select according to life requirements
Finish before plating	Affects final surface roughness	Pre-plating polishing

4.3 Injection molding process

Process parameters	Effect on plugging force	Control points
Dimension accuracy	Shrapnel slot size affects positive pressure	Within ± 0.03 mm
Burr/Burr	Increased friction and stagnation	Mold Repair Control Burr
Material shrinkage	Affects the actual fit gap	Optimizing Injection Molding Parameters

V. Standards and tests related to plugging force

5.1 Test Standards

Standard	Scope of application	Main Content
EIA-364-13	Connector Plug Force Test	Insertion force, pull-out force test method
EIA-364-09	Mechanical life test	Change in plugging force after multiple plugging
IEC 60512-4	Mechanical test of the connector	Includes plugging force and mechanical life
GB/T 5095.4	Electronic device connector test	Equivalent to IEC 60512

5.2 Test Methods

Test conditions:

• Test speed: 25mm/min (standard) or according to specifications
• Alignment accuracy: ± 0.05 mm
• Test environment: 23 ± 5 ℃, humidity 45% ~ 75%

Test items:

1. First plug force: Force value for 1st insertion and extraction

2. Rated Plug-in Force: The average plugging force of the 2nd ~ 5th times (excluding the first effect)

3. Post-life plugging force: Change in force value after N plugs

5.3 Reference to typical industry specifications

Spacing	Single pin insertion force	Single-pin pull-out force	Remarks
1.27 mm pin header	0.3~0.8N	0.5~1.0N	Single point of contact
2.0mm pin header	0.5~1.2N	0.8~1.5N	Single point of contact
2.54 mm pin header	0.8~1.8N	1.0~2.0N	Single point of contact

>Note: The above is a typical range, and the specifications are subject to the product specifications.The insertion force of the double contact point is about 1.5 to 2 times that of the single contact point.

VI. Plugging Force Design Optimization Strategy

6.1 Ways to Reduce Plugging Force

1. Decrease the amount of infringement: Reduce interference under the premise of ensuring reliable contact

2. Optimized Orientation Angle: Increases guide angle and reduces insertion resistance

3. Choose low friction coating: e.g. gold-plated, palladium-plated

4. Selection of low modulus of elasticity materials: e.g. brass instead of phosphor bronze

5. Optimize shrapnel structure: Increase shrapnel length, decrease stiffness

6. Reduce the number of contact surfaces: Single shrapnel instead of double shrapnel (reduced reliability)

6.2 Ways to Improve Retention

1. Increase Excess Volume: Increases positive contact pressure

2. Increase Contact Points: Double shrapnel, triple shrapnel design

3. Selection of highly elastic materials: Beryllium copper, tin bronze

4. Increase the number of shrapnel: Multiple shrapnel in parallel

5. Optimized surface treatment: Appropriate increase in surface roughness (moderate)

6. Add clasp/lock: Mechanical locking structure

6.3 Balanced Design Principles

The core of the plug force design is to find a balance between the following three:

1. Exposure ReliabilitySufficient positive pressure is → required

2. Operational convenienceIt → shouldn’t be too hard to plug.

3. Product life→ Wear-resistant, anti-fatigue

Design process suggestions:

1. Determine first the application’s requirements for contact reliability and plug-and-plug life

2. Choose the right material and shrapnel structure

3. Estimate the plugging force through formulas or simulations

4. Production of hand plate samples for actual testing and verification

5. Iterative optimization based on test results

VII. Frequently Asked Questions and Countermeasures

7.1 Excessive plugging force

Reason	Countermeasures
Excessive volume design	Reduce the amount of interference and optimize the shrapnel structure
Rough surface, high friction	Improve surface finish, choose low friction coating
Poor orientation design	Increase the guide angle, lengthen the guide stroke
The row pin is not aligned with the row mother	Add positioning columns, optimize guide structure
The material is too hard	Switch to more resilient materials

7.2 Pull-out force is too small/retention force is insufficient

Reason	Countermeasures
Insufficient Excess Volume	Increase Excess Volume
Shrapnel deformation	Changing materials, optimizing heat treatment
Excessive wear	Increased coating hardness and wear resistance
Stress relaxation	Use stress relaxation resistant materials
Temperature rise causes force to drop	Use materials with good high temperature performance

7.3 Large dispersion of plugging force

Reason	Countermeasures
Stamping size unstable	Improve mold accuracy and stamping process
Uneven coating thickness	Optimized plating process
Injection Molding Size Fluctuations	Optimizing Injection Molding Parameters and Molds
Inconsistent assembly	Strengthen assembly process control

Conclusion

The insertion and removal force design of the needle row mother is a comprehensive embodiment of multi-disciplinary materials, structures, and processes.Reasonable plugging and unplugging forces must not only have sufficient contact positive pressure to ensure reliability, but also cannot greatly affect assembly operation and service life.Through careful material selection, structural design and process control, the best balance between reliability and convenience can be found.

Shenzhen Rui Xin Sheng Ye Electronic Technology Co., Ltd. has a complete range of needle row mother products, covering a full pitch from 1.27 mm to 2.54 mm, supporting single/double rows, straight/bent feet, patches/plug-ins and other specifications.We have extensive experience in plug-in force design and testing, and can provide customers with customized plug-in force optimization solutions to meet the needs of different application scenarios.