Lifetime prediction of thick aluminium wire bonds for mechanical cyclic loads
Introduction
The configuration of electronic components in control units is playing an increasingly important role due to stringent safety requirements [1]. The function of control units for brakes, airbags, and driver assistance systems must be reliably guaranteed over the course of the vehicle’s lifetime. Thick wire bond connections are used in modern control units to conduct the strong electrical currents in control units, which are often bonded between separate mechanical components, such as a ceramic substrate and a lead frame (see Fig. 1).
This means that vibrations or temperature fluctuations can cause relative motion between these components. Two failure mechanisms appear:
First: The lifting of the bond base from the metallisation caused by the different expansion coefficients of aluminium and silicon in semi-conductor structures (cf. [2], [3], [4], [5], [6]).
Second: Wire fracture at the crossing of the bond base and the loop (cf. [7], [8], [9]). There are a number of causes:
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Wire bonds between different components with different expansion coefficients (e.g. between a lead frame on one plastic and one ceramic substrate) experience relative motion at their bonding base when temperature fluctuates.
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The expansion of the wire itself causes a bending motion in its heel area due to the difference between the thermal expansion coefficients of the wire material and the assembly plate.
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Vibrations or mechanical shock loads cause relative motion between components [10].
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Vibrations lead to natural oscillation of the wire [1].
The amount of relative motion depends on the load (induced thermally or mechanically), the location of the control device, and many other parameters. It must be determined experimentally for the given application. Once the direction and amount of relative motion has been measured, a lifetime prediction can be derived from load analysis such as the finite element method (FEM) performed on the wire bond. This paper will describe the procedure from load analysis to lifetime estimation for low-cycle fatigue in thick aluminium wire bonds consisting of 99.999% aluminium under purely mechanical load. The experimentally determined lifetime for different geometries and loads will be predicted on the basis of damage parameters which can be calculated by means of FEM simulation. The model accounts for the influence of geometrical parameters and material changes occurring during the bonding process. The model presented here allows calculation of the lifetime of various geometric shapes of aluminium thick wire bonds with a wire diameters between 300 and 500 microns. Special attention is given to the influence of the bond process.
Section snippets
Material of bond wire aluminium
Creating a lifetime model using finite element simulation requires both geometrical data for the wire bonds and values large enough to sufficiently characterise material behaviour. During work done at the Fraunhofer Institute for Materials Mechanics in Halle, Germany, the material parameters were determined by means of single-axis tensile tests on non-bonded wires [11] with a Young’s modulus of 64 GPa measured at 21 °C. Pieces of wire about 30 mm long with diameters of 125, 200, 300, 400, and 500
Bond test bench for end-of-life measurements
A test bench capable of moving the bond bases relative to each other was constructed for wire bond cyclic fatigue testing (Fig. 3). Twenty equal bonds were then bonded to the metallisation of two ceramic substrates, one fixed and the other in cyclic motion. The lifetime tests almost exclusively showed breakage in the first bond base. (Additionally overlaid motions such as torsion are not taken into account here). All endurance runs mentioned in this paper – unless otherwise indicated – were
Simulation results
The installed wire bonds were microscopically measured and the geometry compiled in Ansys Workbench 11® with the “Design Modeler” CAD tool. All geometric parameters were obtained from the microscope image and transferred to the CAD model. The material properties obtained from the tensile test (see Fig. 2) were assumed to apply to the entire geometry model. An E-module of 64 GPa and a kinetic material model was assumed [12]. For load analysis, a bond pad was fixed, and the one opposite executed
Discussion
The S–N fatigue curve of wires with diameters of 300, 400, and 500 μm are very similar, but different from wire diameters of 125 and 200 μm. Wires with 300–500 μm exhibit a very similar stress strain behaviour during single-axis tensile tests. One the one hand, because of their low thickness, 125 and 200 μm wires must be loaded at a higher yield point in the annealing process during manufacture; otherwise they would be too soft for the bonding process. On the other, their different behaviour can
Conclusion
The methodology introduced here allows the lifetime of thick aluminium wire bonds to be calculated for cyclic mechanical loads. Damage is calculated using the damage parameter calculated in simulation.
During fatigue of wire bonds, a constant electric resistance value is observed over the lifetime, with a spike in resistance occurring shortly before the breaking point. Stress strain curves for thick wires with various diameters could be determined by means of single-axis tensile tests. A
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