The main causes of failure of the Au-AI bonding system include the growth of Au-Al intercompound related to high temperature, and the formation of Kirkendall voids at the bonding interface. Both of these causes can lead to a decrease in the bonding strength of the device's leads, an increase in the contact resistance at the bonding point, or even an open circuit failure.
Figure 1. Schematic diagram of an Au-Al bonding system
First, we introduce the first cause of failure: the growth of Au-Al compounds. Due to the different chemical potentials of gold and aluminum, after the Au-Al bonding system is stored at high temperature (above 150 °C, 10 hours), five intermetallic compounds will be produced between them: Au4Al, Au5Al2, Au2Al, AuAl2, AuAl, their lattice constant, expansion coefficient, volume change during formation are different, and the conductivity is low. In the process of temperature change, there is a large internal stress at the bonding point, which is easy to produce cracks at the phase interface, resulting in an increase in contact resistance, which is manifested as a good and bad phenomenon when used, and finally leads to performance degradation or open circuit. AuAl2 is purple, commonly known as purple spot; Au2Al is white, it is called white spots, white spots are brittle and low conductivity, and cracks are easily produced; Au4Al, Au5Al2, AuAl are light yellow.
Figure 2 and Figure 3. Metallographic morphology of Au-Al compounds
Figure 4 and Figure 5. Au-Al compound SEM morphology
Let's look at another cause of failure of Au-Al bonding systems: the Kirkendall effect of the bonding interface. At high temperature, the atoms between gold and aluminum diffuse each other, the diffusion coefficient of the two is different, Au diffusion is faster than Al diffusion, and voids are generated when continuous aging at 300 °C, resulting in Kirkendall voids. During the continuous aging process, small voids gradually join together, forming bonding point interfaces or peripheral voids, causing poor contact or lead breakoff, resulting in open circuit failure.
Figure 6.Morphology of the Kirkendall void SEM
Case study
Below I will give a practical case to give you a better understanding of the failure principle of the Au-Al bonding system. Failed molded integrated circuit, many pins are open, and after opening, it is found that all the open-pin bonding wires have been disengaged from the pads.
Figure 7.Failed sample bonding wire disengagement topography
Observe the surface topography of the pad and the bond alloy sphere, and a yellow, loose substance can be seen at the pad (Figure 8). A pit is visible on the surface of the bond alloy ball, that is, a Kokendal void (Figure 9).
Figure 8.Pad topography of a failed sample
Figure 9.Spherical surface topography of bonded alloys for failed samples
prophylaxis
Failure of the Au-Al bonding system can lead to poor contact or detachment of leads, degrading and failure of device performance, which must be taken seriously. Depending on what can lead to the Au-Al compound and the Kirkendall void, several precautions are suggested:
☑ Select the optimal bonding conditions in thermosonic welding, replace the conditions of pure thermal bonding in thermo-hot welding with ultrasonic power and pressure, and carefully adjust the appropriate temperature, power and pressure to minimize the possibility of intermetallic compounds.
☑Au-Al bonding failure shows sensitivity to temperature, the aging temperature of the finished product should be strictly controlled, and the temperature rise should be strictly controlled when the device is used, especially for devices with high power density and poor external heat dissipation conditions.
☑ A barrier layer is used to prevent interaction.
☑ To prevent manual errors, careful tuning of bonding pressure, bonding ultrasonic power, and bonding temperature must be carefully debugged.
☑ Strengthen screening and visual inspection.
Au-Al bonding has good thermal fatigue resistance under warm punch conditions, and the bonding tension is in
Within the qualified range, the bonding resistance increases with the increase of the test cycle; High temperature stress causes
Au_Al bonding interface forms compounds with high resistivity, causing degradation of bonding electrical properties. at
In the 150°C and 175°C high temperature tests, the Au-AI bonding resistance gradually increases with the storage time
increased, and some of the bonding resistance increased sharply in the 200°C high temperature test
Al_Au bonding resistance remains largely unchanged in the corresponding high-temperature test. Strong bonding of Au-Al bonding points
The degree decreases with the increase of the test time and decreases with the increase of the test temperature
The bond strength of the A1_Au external bonding point remains basically unchanged.
Physicochemical performance testing and analysis of failed samples showed that Au5Al was generated. , Aunl and
Au's metals such as L ask compounds and Kerkendall voids, the formation of compounds that lead to bonding electricity
The resistance gradually increases, the appearance of voids will cause the bond resistance to increase sharply, and the presence of the bonding groove will make the bonding groove empty
The hole first forms at the periphery of the bond, and increasing the temperature can be fast between Au and Au-rich compounds
Rapid formation of the Kerkendal l void.
Therefore, when heterogeneous bonding is required, priority is given to aluminum-gold bonding, and in military devices, gold-aluminum bonding is reduced as much as possible.