Testing power electronics in minutes instead of months
The high demands on automotive electronics - e.g. a lifetime of at least 20 years - are a challenge for semiconductor chips and their external connections. Conventional tests take a long time. A practical problem that is now solved by a new test procedure.
The standard reliability tests for heavy wire bonds simulate typical load profiles by running hundreds of thousands of power-up and power-down cycles in rapid succession until the wire bonds fail. Such an accelerated test is the power cycling test (PC test), but it can still take several months. It is therefore hardly suitable for ensuring ongoing production quality; moreover, the necessary test stands are expensive. For a long time there been a desire for a supplementary rapid test in which it would be sufficient to obtain semi-quantitative measurements.
It would be valuable to quickly see whether the bond quality had gone up or down after a change in materials or process details.
"Minutes instead of months - that was the target for the new tester", Siegfried Seidl specifies the requirements for the test, which was then developed at the Vienna University of Technology.
Fig.1: Clamping pliers after lift-off of wire bond (Fig.: F&S BONDTEC Semiconductor GmbH)
This test was brought to market by F&S Bondtec in the form of the fully automatic BAMFIT tester (Bondtec Accelerated Mechanical Fatigue Interconnect Test) and a patent application is pending. First prizes have already been won with this system.
The basic principle of the new test method is simple: During operation, a wire bond is thermally stressed at each switching cycle because the semiconductor chip heats up due to the current flow. As a result, the aluminium bonding wire and silicon chip expand very differently - by a factor of 8. This creates a mechanical stress load at the bond point, which over time leads to fatigue cracks in the bond interface. The BAMFIT tester models precisely this mechanical stress. For this purpose, a test device was realized in which a clamp similar to a pair of pliers grips the bond foot over almost its entire length and at a defined height of about 30 µm above the bond surface (Fig. 1). This clamp is based on a thick wire bonder and is constructed in such a way that it is excited by an ultrasonic transducer in the same way as a bond wedge and then performs a longitudinal oscillation which pulls back and forth in the wire direction at the bond foot. The pull amplitude is adjustable by the ultrasonic level and is about 1 µm, which is comparable to the thermal expansion. However, instead of a cycle of 5 seconds as in the PC test, typically 60,000 cycles/s are performed as in wire bonding. In addition, a small upward tensile load of a few cN is applied via this wire clamp to facilitate the lifting of the bond. After failure of the bond, the wire clamp continues to lift off the bond, which is detected by the test device, and the number of cycles passed is recorded as a measure of quality. Even for a few million load cycles, the tester needs only a few minutes and not months like the PC test. Different load scenarios are generated in the PC test by different temperature strokes, among other things. These can also be easily simulated with the BAMFIT method, because a higher ultrasonic level also generates a larger mechanical vibration amplitude at the clamping tool and thus at the bond interface. The BAMFIT tester is based on an automatic bonder and tester of the 56XX family from F&S Bondtec. Numerous functions such as programming of any number of bonding points, image recognition, touchdown for determining the bond height, generation and monitoring of the ultrasound etc. are therefore already integrated. Existing bonders can be converted to the BAMFIT method simply by exchanging the bond head. In the meantime, it has also been shown that this tester is also superior to the classic shear test and provides better differentiated statements about the bond quality, especially for bonds of very high quality.
"This opens up completely different possibilities for quality assurance and process monitoring, initially for power electronics, which are also the basis for further developments that we are currently working on."
