Wire bonding creates electrical interconnections between the die and the package. A wire bond should be strong enough to withstand application specific mechanical and thermal stresses without breaking. Bond strength is an indication of quality and reliability of a wire bond. Wire bond strength is characterised using wire pull, shear, peel and thermal testing. Generally, the highest force applied to the bond in order to pull, shear and peel the bond without breaking it is a measure of bond strength. In some cases, it may not be possible to find the exact bond strength such as when the wire breaks before the bond; the wire strength is the limiting factor and can be considered as the bond strength.
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.
It consists of applying force on the ball bond to shift it laterally. The force that laterally shifts the ball is the strength of the ball bond. The key shear test variables are the shear height which is the height at which the ball is pushed by the tool and test speed which represents the rate at which the bond is loaded. The test can generate a number of failure modes of interest out of which the most important failure is the bond failure. Generally, a bond can be weaker because of poor material quality, bond aging and incorrect bonding process. Shear testing is applicable to gold and copper balls, solder bumps and copper pillars.
Peel testing is particularly used for testing ribbon connections. It consists of using tweezers to grip one end of the ribbon or wire and moving upwards in z-axis; this pulls the ribbon upwards. The peel test produces a force vs displacement graph which indicates the strength of the ribbon interconnections. The peel test offers a flexible options for testing ribbons as it can cater for a broad range of ribbon sizes and loop geometries.
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.
The Advanced Packaging Facility has a range of wire bonding specific characterisation equipment. This includes Nordson DAGE 4000 bond tester which is able to do pull, shear and peel testing. A Wiess environmental chamber for thermal testing and multiple microscopes including Zeiss EVO scanning electron microscope for inspection. The aim of quality characterisation is to replicate the load stresses on a package so directly relevant results to the operating conditions can be achieved.