Wire pull testing
It consists of pulling a wire upwards, in z-axis, using a metal hook. The test can be set up to be non-destructive where a specified force is applied on the bond or destructive where the applied force is increased till the bond/wire breaks. The test leads to a number of failure mechanisms, the results of which are analysed to determine the bond strength. The test parameters such as test speed and tool positioning are a critical part of the test as variation in the parameters can produce a variety of failure mechanisms. Wire pull testing can also be used for ribbon bonding. In a wire pull test common practice is to pull the centre of the wire for the tests. It is called the mid span pull. Alternatively if the failure mode of interest is to test the bond at one of the ends the hook can be moved close to that end. When testing multiple samples it is important to pull the wires at the same position for comparison.
Thermal testing consists of either heating a wire bonded die assembly to a specific temperature or to expose it to thermal cycling. The aim is to put the wire bond under thermal to determine if it can survive application requirement. Thermal stress can be caused by the application of the electronics or the fabrication processes such as curing an encapsulation.
In addition to the standard wire bond tests, the wire bond structures can also be observed using SEM or the optical profiler to identify any potential issues with the wire bonds such as bond break due to metallization lift off, structural deformity of the ball bond or bump, soft metallization, poor heel stick and heel cracking. The visual inspection can be used in conjunction with the wire bonder to identify and rectify recurring issues which lead to poor bond strength.