Bend Test

Stress Test Background

Bend Testing is a stress test which subjects the unit under test (typically printed wiring boards and handheld electronic products) to monotonic flexural loading. Monotonic bend testing is conventionally used to characterize fracture strength of interconnects between board and components or the interconnection structure within the board. A typical test set up is shown in Figure 1.

Test Set Up and Stress Profile Description

For a typical 4-point bend test set up as shown in Figure 1,the unit under tests is typically placed on a pair of fixed anvils, and a second pair of movable anvils is used to apply load on the unit. For monotonic bend testing, load is applied on the unit with movable anvils from the top at a constant speed, and loading can be removed when failures are detected. Functional tests are recommended to be performed continuously by monitoring the electrical resistance and strain of the interconnects of interest.

Parameters Determination

Target stress level, in this case crosshead speed, crosshead displacement

Target stress level is the stress level to be reached for the purpose of the stress test to be achieved. Therefore, it should be determined based on the purpose of defect precipitation.

Crosshead speed

Crossheadcontrols the movement of the movable anvils and it is approximately proportional to the strain rate of the test unit, which is usually set to be constant. A minimum crosshead speed that can cause a nominal global PWB strain rate of 5000µStrain/s is recommended to avoid an overstate fracture strength of the interconnects of interest. Different crosshead speed can be selected for different purposes.

For design improvement purposes (HALT), crosshead speed can be selected according to the desired stress level to be reached after HALT. For example, if impact resistance of interconnects is to be improved, a higher crosshead speed can be selected to simulate a high impact situation.

For screening purposes (HASS, ESS), a lower crosshead speed can be chosen to avoid impact on the good interconnects after the screen. However, the minimum crosshead speed recommended by IPC should be taken as a reference.

Maximum crosshead travel displacement

Maximum crosshead displacement is approximately proportional to the maximum strain of the unit under test. Different crosshead speed can be selected for different purposes.

For design improvement purposes (HALT), maximum crosshead displacement should be selected according to the desired stress level to be reached after HALT. For example, a higher maximum crosshead displacement should be selected for a unit that can be subjected to high level bending in its operating condition.

For screening purposes (HASS, ESS), a smaller crosshead displacement should be chosen to avoid overstressing the good interconnects.

Standards with Stress Test Description

For Design Improvement Purpose

For Testing/ Acceptance Purpose-System Level:

IPC/ JEDEC 9702: Monotonic Bend Characterization of Board-Level Interconnects

IEC 61189-2: Test Methods for Electrical Materials, Printed Boards and Other Interconnection Structures and Assemblies- Part 2: Test Methods for Materials for Interconnection Structures, Test 2M08: Flexural Strength

IPC TM 650 Section TM 2.4.4B(1994): Test Method Manual: Flexural Strength of Laminates (at Ambient Temperature)

IPC-TM-650 Section TM 2.4.5.1 (2000): Test Method Manual: Flexibility- Conformal Coatingt

For Testing/ Acceptance Purpose-Component Level

Possible Defects Precipitated by this Stress