Lead Terminal Strength Testing for Electronic Components: Standards, Methods, and Applications

Lead terminal strength testing, also known as termination robustness testing, evaluates the ability of electronic component leads, pins, and terminals to withstand tensile, bending, and torsional stresses encountered during assembly, handling, and field service. The test is specified in IEC 60068-2-21 (Environmental Testing, Test U), GB/T 2423.29, and equivalent JIS standards, and is a mandatory reliability qualification item for capacitors, resistors, semiconductor devices, transformers, and connectors. This article explains the test principles, applicable sample types, test items, standard test methods, acceptance criteria, and typical industrial applications.

Technical Overview

Lead terminal strength testing is a mechanical robustness test that verifies the structural integrity of the terminations (leads, pins, tabs, and terminals) attached to an electronic component. During printed circuit board assembly and product use, component leads are subjected to multiple mechanical stresses: axial tension during automatic insertion and lead forming, repeated bending during lead trimming and rework, and torsion during nut tightening or lead straightening. A weak or defective termination can result in lead breakage, seal failure, internal disconnection, or intermittent electrical faults that appear only after the product has been fielded.

The internationally recognized test method is IEC 60068-2-21, Environmental Testing, Part 2-21: Test U, Robustness of Terminations and Integral Mounting Devices. The Chinese national standard GB/T 2423.29 is technically equivalent, and the Japanese standard JIS C 60068-2-21 adopts the same framework. The test family is subdivided into tension (Ua1, Ua2), bending (Ub), and torque (Uc, Ud) procedures, and is routinely referenced by component-level specifications such as the IEC 60115 series for fixed resistors and the IEC 60384 series for fixed capacitors.

Because the test applies loads up to and beyond the mechanical limits of the terminations, it is classified as a destructive test. Samples subjected to terminal strength testing must not be reused for other qualification tests or shipped as production units.

Testing Samples

Axial-Leaded Components

Axial through-hole parts such as film resistors, axial film capacitors, suppression chokes, and axial diodes have one lead exiting each end of the body along the component axis. These leads are tested in tension along the lead axis and in bending at the point where the lead exits the component body.

Radial-Leaded Components

Radial parts, including aluminum electrolytic capacitors, radial ceramic and film capacitors, radial inductors, and LED lamp packages, have two or more leads exiting the same face of the body. Each lead is tested individually for pull and bend strength, with particular attention paid to leads attached to internal electrode structures that may delaminate under load.

Semiconductor Packages and Integrated Circuits

Discrete packages such as TO-92, TO-220, and TO-247, as well as dual in-line (DIP) packages, are tested for lead robustness to confirm the strength of the lead-frame-to-plastic-body interface. Test U procedures are also applied to the gull-wing and J-bend leads of leaded surface-mount IC packages where the relevant detail specification requires it.

Surface-Mount Devices

For surface-mount components with castellations, L-shaped terminals, or metal end caps (such as chip resistors and multilayer ceramic capacitors), termination strength is more commonly assessed through pull-off and shear tests on soldered joints; however, the lead-robustness principles of IEC 60068-2-21 are applied to the terminations themselves where the detail specification calls for it.

Electromechanical and Passive Power Components

Relays, transformers, power inductors, circuit breakers, filter modules, terminal blocks, and screw-mounted components with threaded studs or tab terminals are tested for terminal strength, including torque strength of screw terminations and pull-out strength of quick-connect tabs.

Testing Items

Depending on the component type and the governing detail specification, the following test items are performed:

• Tensile (pull) strength: resistance of the termination to a steady axial pulling force.
• Thrust/push strength: axial compression applied to leads or terminals where specified.
• Bending strength: resistance to repeated 90-degree bending of the lead at the body exit point.
• Torque strength (rigid terminations): resistance of screw, stud, and tab terminations to a specified tightening torque.
• Torque stress (flexible leads): resistance of a flexible lead to twisting about its own axis through a defined angle.
• Lead fatigue: repeated low-level bending cycles where the detail specification defines a fatigue requirement.
• Post-test electrical verification: measurement of electrical parameters and continuity after mechanical loading.

Testing Methods

Sample Preparation and Test Conditions

Leads are straightened, where necessary, by hand or with pliers applying only the force required to achieve the required straightened length, typically not less than 10 mm for small components unless otherwise specified. Tests are conducted under standard atmospheric conditions for testing, normally 15 °C to 35 °C with relative humidity of 45 % to 75 % and atmospheric pressure of 86 kPa to 106 kPa. The number of terminations tested per sample lot and the sampling plan are defined by the relevant detail specification.

Tensile Test (Test Ua1 / Ua2)

The component body is clamped and a steady axial pulling force is applied to the termination through an appropriate clamp or hook. The force is increased gradually and then held constant for at least 10 seconds (commonly 10 s to 30 s). The test force is selected from the cross-sectional area of the lead in accordance with IEC 60068-2-21, with typical values:

• Cross-sectional area up to 0.05 mm²: 1 N
• Over 0.05 mm² to 0.07 mm²: 2.5 N
• Over 0.07 mm² to 0.14 mm²: 5 N
• Over 0.14 mm² to 0.20 mm²: 10 N
• Over 0.20 mm² to 0.50 mm²: 20 N
• Over 0.50 mm² to 1.20 mm²: 30 N
• Over 1.20 mm²: 40 N

For leads with insulated sleeving or special configurations, the test Ua2 procedure and the forces specified in the detail specification apply. Test Ua2 also addresses the tension behavior of leads intended for wrapping connections.

Bending Test (Test Ub)

The lead is clamped at a defined distance from the component body and bent through 90 degrees in one direction and returned to its original position, at a controlled rate of approximately one to two seconds per bend, unless the detail specification states otherwise. The standard severity is two bends (one full out-and-return cycle), with three or more bends applied only where specified. For components with multiple leads, the bending direction is chosen to be the most onerous for the termination design. Bending rates, cycle counts, and bend orientation should be strictly controlled because they strongly influence results.

Torque Tests (Test Uc / Ud)

For rigid terminations such as screw terminals, threaded studs, and integral mounting devices, test Uc applies a defined tightening torque, held for a specified duration such as 10 seconds, to verify that the termination and the component body withstand assembly loads without damage or turning. For flexible leads, test Ud twists the lead about its own axis through a specified angle, commonly 90 degrees or 180 degrees, over a defined active length, to verify conductor and lead-frame integrity under torsional stress.

Acceptance Criteria

A sample is considered to have failed if any of the following is observed after testing:

• The lead breaks, loosens, or is pulled out of the component body.
• The termination or the body shows visible cracking, delamination, or deformation that affects function.
• Sealed components exhibit damage to the seal or encapsulation.
• Post-test electrical measurements fall outside the limits defined in the detail specification.

Force gauges and test fixtures used for these procedures must be calibrated and traceable to national or international measurement standards, and testing should be performed by accredited third-party laboratories or qualified in-house reliability facilities.

Applications

Terminal strength testing is applied throughout the electronics industry for several purposes:

• Design qualification of new component types before release to production.
• Periodic reliability audits and lot acceptance testing of passive components such as resistors, capacitors, and inductors.
• Incoming inspection screening by electronics manufacturers to validate supplier process quality.
• Supplier comparison and sourcing decisions based on quantified mechanical robustness data.
• Failure analysis, where the test is used to reproduce lead breakage or termination pull-out field failures.
• Compliance demonstration for product safety and reliability requirements in consumer electronics, industrial controls, telecommunications, lighting, medical equipment, and automotive electronics.
• Process validation for lead forming, lead trimming, and automated insertion operations that impose mechanical loads on terminations.

Components intended for vibration- and shock-exposed environments benefit especially from verified termination strength, because lead integrity directly governs survival under dynamic mechanical loading.

FAQ

What is lead terminal strength testing?

It is a mechanical robustness test that applies controlled tensile, bending, and torsional loads to the leads, pins, or terminals of an electronic component to confirm that the terminations will survive assembly handling and service stress without breaking, loosening, or losing electrical continuity.

Which standards specify this test?

The primary standard is IEC 60068-2-21 (Test U: Robustness of Terminations and Integral Mounting Devices). GB/T 2423.29 and JIS C 60068-2-21 are equivalent national adoptions, and component family standards such as the IEC 60115 and IEC 60384 series reference Test U in their general and detail specifications.

What force is applied during the pull test?

The pulling force depends on the cross-sectional area of the lead, ranging from 1 N for very fine leads up to 40 N for leads larger than 1.2 mm², with the force held steady for at least 10 seconds according to IEC 60068-2-21.

How is the bending test performed?

The lead is bent 90 degrees away from the body and returned to its original position at a controlled rate. The standard severity is two bends, and the bend direction is selected to place the most severe stress on the termination.

Is the test destructive?

Yes. Because loads approaching the mechanical limits of the terminations are applied, tested samples must be scrapped and must not be used for other qualification tests or shipped as production parts.

How many samples are required?

The sample size and the number of terminations tested per component are defined by the relevant detail specification; common practice is to test all terminations on a small sample of components, or a statistically defined subset for components with many leads.

What equipment is used?

Motorized or manual lead integrity testers with calibrated force gauges, bending fixtures, and torque fixtures are used. Equipment calibration must be traceable, and testing is typically performed by accredited third-party laboratories or certified internal reliability laboratories.