## 3 Nondestructive Testing Methods
### Transient Thermal Analysis (TTA)
Equation (3) is used on all, and the normalization is performed in an interval from 100 to 1000 \(\upmu\)s. The region of interest is the highest peak, where an increase of 0.05 can be interpreted as a failure [43; 44; 45]. Figure 4: Evaluation methods in TTA: (**a**) \(Z_{th}\) evaluation and (**b**) normalized logarithmic derivative. Data are generated by an optimized FEM model in a transient thermal simulation with a crack growth of 10% or 30% in the solder joint compared to its area described in [44]. An increase of 0.63 K/W or 2.37 K/W was observed in \(Z_{th}(t)\) and 0.012 or 0.044 in peak increase for B(z). Figure 3: Measurement sequence for transient thermal analysis (TTA) with course of power \(P\), junction temperature \(T_{I}\), and voltage of temperature-sensitive parameter \(V_{TSP}\). In addition to the normalized logarithmic derivative, the structure function is a common evaluation method for TTA data. A detailed explanation would be beyond the scope of this paper and can be found in [46; 47; 48]. Independent of the chosen type of evaluation, the signal-to-noise ratio (SNR) is always a critical parameter for TTA [11]. A simple but time-intensive way to increase SNR is averaging over several measurement repetitions. In that way, a measurement of a single semiconductor can last from several seconds up to minutes depending on time required for the set-up to reach thermal equilibrium and the repetition count. An in-line implementation is, therefore, not feasible, and high-volume studies on a laboratory level are also not practical. Currently, commercial TTA equipment is only available in manual operation mode, including electrical contacting of the semiconductor and starting the measurement process, being time-intensive for the operator and prone to failures. For this reason, we developed an automatized thermal impedance measurement equipment (ATIM), shown in Figure 5c, controlled by a central software solution and including several subsystems. The ATIM allows successive measurement of semiconductors on the panel level and reduces the time effort for the operator to a minimum [49; 50]. For measurements with the ATIM, the device under test (DUT) has to be attached to a substrate with probing pads for the electrical connection with spring probes. An exemplary substrate with two LEDs and three probing pads is depicted in Figure 5b. The anode and cathode of each LED are routed separately to probing pads, whereby the central pad is shared between the cathode of one LED and the anode of the other for this substrate. The probing head with four spring probes, shown in Figure 5a, drives down to the pads in the \(z\)-direction and allows a four-wire connection. Three additional harder springs are used to increase the clamping force of substrate to a temperature-stable plate. For an automatized measurement, one or more substrates, each holding several semiconductors, are placed on the temperature-stable plate with a TIM in between for better thermal connection, and the position and orientation are detected by a camera. The software calculates the movement of the \(XY\)-stage and  
Figure 5: (**a**) Probing adapter with four spring probes for electrical connection and three harder springs to increase clamping force. (**b**) Example of a substrate with two devices under test (DUTs) and three probing pads. (**c**) Automatized thermal impedance measurement equipment (ATIM) with subsystems (1: signal column; 2: heat/sense power source; 3: rotation axis with integrated force sensor and probing adapter; 4: \(XY\)-stage; 5: XYZA-axis controller; 6: data acquisition (DAQ) unit; 7: \(Z\)-axis; 8: camera; 9: temperature-stable base plate; 10: power supply). rotation axis, and it successively contacts all semiconductors. The actual TTA equipment consisting of a heat/sense power source and a data acquisition unit (DAQ) performs the TTA measurement with the required settings and automatically saves the data. In that way, the effort for the operator is reduced to placement of the substrate and starting the process.