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.
The wire breaks in the middle along its length. This result does not indicate the bond strength as the bonds on both ends of the wire remain intact. The result may be acceptable if the real life load of the bond is likely to be similar to the force required to break the bond.
Heat affected zone (HAZ) in a wire is the part of wire closest to the ball. HAZ is not melted but due to the application of heat has its microstructure disturbed. The pull test that results in the wire breaking from the heat affected zone shows that HAZ is the weakest point in a wire bond.
The wire breaks at the heel of the loop which shows the heel is the weakest part of the bond. The test highlights the heel strength, if it is unacceptable the bonding parameters such as the bond head force and the loop shape can be varied.
The bond lifts off from the neck or the heel due to failure of the intermetallic connections. This test shows the strength of the bond which may or may not meet the application requirements.
The test causes part of the metallization on the die to lift off whilst being attached to the wire. The test shows the bond pad strength however the bond strength is unknown.
A pull test may result in bond pad on the substrate or package being lifted off with the bond. This shows that the bond is stronger than the bond pad.
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.
The test causes the bond between the ball/bump and the bond pad to break. This outcome shows the bond strength. The results may be acceptable if the bond strength is higher than the expected load on the bond.
A shear test can cause the ball/bump be broken into two parts. This outcome does not show the bond strength. The acceptability of the bond arrangement depends on the application requirements.
The test causes the bond pad or die metallization to break into two parts. This outcome presents the pad strength which is the weakest point in the wire bond.
The test causes parts of both the bond pad and die to break producing a carter as a result of it. The crater runs through to the bond pad to the die. This outcome reveals the pad strength which may be the weakest part of the bond
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 testing and characterisation equipment. This includes Nordson DAGE 4000 bond tester, Alicona infinite focus non-contact surface profiler, Tencor P-11 contact profiler, Zeiss EVO scanning electron microscope and Wiess environmental chamber.
We aim to replicate the load stresses on a package in every quality assessment to ensure meaningful characterisation of the packaging process and directly relevant results to the operating conditions.