A pull-off test, also called stud pull test, is a type of test in which an adhesive connection is made between a stud and a carrier (or object to be tested) by using a glue, possibly an epoxy or polyester resin, that is stronger than the bond that needs to be tested. The force required to pull the stud from the surface, together with the carrier, is measured. Simple mechanical hand-operated loading equipment has been developed for this purpose. When higher accuracy is required, tests can be performed with more advanced equipment called a bond tester. A bond tester provides more control and possibly automation. Applying the glue automatically and curing with UV light is the next step in automation. This methodology can also be used to measure direct tensile strength or/and the bond strength between two different layers.
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MIL-STD-883 methods .9 destructive bond pull test[1] and .1 flip chip pull off test[2] apply, as well as JEDEC JESD22-B109.
Partial coring may be used, if necessary, to eliminate surface skin effects.[3][4]
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The quality of a crimp joint depends on the mechanical strength of the joint as well as its electrical conductivity. If results from pull force tests are within an allowed range, it assures that the proper crimp force has been applied during the crimping process.
This is crucial as enough force must be applied to break down the layer of non-conductive oxides that may build up on the stripped conductor and the plating on the inside of the terminal. This is necessary to provide a good metal-to-metal contact. If this does not occur, electrical resistance can increase.
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Overcrimping a crimp termination will reduce the circular area of the conductor and thereby increase electrical resistance.
Crimp joint testing (commonly known as "pull testing") ensures the integrity of the final product but, more importantly, it ensures the effectiveness of the crimping tool itself since it can be calibrated according to test results. Calibration is best carried out in response to Statistical Process Control (SPC) analysis which can be gathered from data obtained by pull testers.
There are many standards to adhere to and each manufacturer or user might have different requirements to fulfil. But one thing in common is that international and industry standards all prescribe procedures for testing the tensile strength of wire terminals in a repeatable manner.
It is essential that there is no jerking or sudden application of force. This is the reason why standards state that there should be a 'constant rate of pull' and most define a pull rate at which tests should be performed. The only practical way to achieve repeatability for pull force tests is to set a speed by using a motorised pull tester such as the WTST range of testers.
Although standards define a minimum acceptance-value for the pull force according to the wire diameter and AWG conductor size, it is of greater importance to the manufacturer and user to know that joints can definitely sustain higher loads. So an increasing axial force is applied to the connection until either the terminal and wire separate or the wire breaks.
Directly pulling the wire joint until destruction at a constant rate (typically 25 to 50 mm/min) is the most commonly adopted test method and is known as 'Pull and Break'. It is performed by using a motorised pull tester in which the peak force to separate the terminal from the wire is measured. This value is then compared to the table of minimum pull forces outlined in the standard in order to determine a pass or fail of the connection.
Other test methods call for more in-depth procedures:
When carrying out a pull test most companies do not remove the insulation sleeve around the crimp. However, the IPC/WHMA, UL, USCAR and VW standards specifically require that the insulation support is rendered mechanically ineffective by opening it up. Certain other standards (e.g. VW) provide a higher pull force value if the insulation support is left in place around the crimp.
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