## 3 Nondestructive Testing Methods
### Scanning Acoustic Microscopy (SAM)  
SAM uses acoustic waves in the range of 15 to 300 MHz to inspect material surfaces and volumes [29]. The centerpiece of SAM is a transducer emitting a focused acoustic wave toward the specimen placed in water as a couplant. The wave penetrates the specimen and is partially absorbed by the material, scattered at fine structures, or reflected at interfaces between two materials according to their difference in acoustic impedances depending on material density and acoustic velocity. Interfaces for acoustic waves also include defects inside a single material being an interface from material to, e.g., gas for voids/pores and the reverse [30]. Reflections caused by an interface vertical to the wave propagation direction are reflected straight back, and the transducer records the echo time resolved holding several interface reflections with different signal propagation delays (referred to as time of flight) [31; 32]. A simplified set-up including a potential echo signal for a given specimen is displayed in Figure 2 with further explanations in the caption. By inspecting the pattern and time of flight inside certain time intervals of the complete echo, single layers or single interfaces can be evaluated. This process is called gating. For the given example in Figure 2, a time gate is set between R2 and R5 to inspect material 2 for defects. In the case of defects, reflections are present within the gate and no reflections are seen when no defects are present.  
The result of the described measurement in Figure 2 holds material information only in one dimension, the \(z\)-direction (propagation direction of acoustic wave and echo). Information in the \(x\) and \(y\)-directions is not included due to the focusing of the wave on a small finite area. A planar analysis is  
Figure 2: **(a)** Simplified setup of scanning acoustic microscopy (SAM). Transducer and specimen (Two different materials with a defect in material 2 are placed in water as a couplant. An acoustic wave is emitted by the transducer toward the specimen, and the potential received echo of the wave for this material stack is shown in **(b)**. The echo consists of back reflections of the wave at any material interface arriving with different propagation delays according to their depth in the stack. In this example, the reflections are caused by the interfaces from water to material 1 (reflection R1), from material 1 to material 2 (R2), from material 2 to defect (R3), from defect to material 2 (R4), and from material 2 to water (R5), and they arrive in this order at the transducer. A time gate is set between R2 and R5, which contains all reflections of defects in material 2.  
possible by raster-scanning with the transducer over the specimen and afterward combining the data to a 2D image holding the 3D information of one gate.  
In contrast to X-ray, SAM allows the detection of cracks and delaminations. Both are material separations causing a reflection due to the transition from material to air, including gas or vacuum, and back even if the gap is minute [7]. Furthermore, varieties in the porosity of sintered interconnects are detectable as many small interfaces. However, SAM is limited while measuring through complex material shapes, inhomogeneous layer structures and penetration depth Complex shapes of, e.g., bond/ribbon wires and woven fiberglass cloth of FR-4 materials scatter the wave in multiple directions and prohibit a back reflection to the transducer. Inhomogeneous layer structures, meaning that, in one planar layer, different materials with different acoustic velocity are used, impede the gating due to the nonuniform time of flight. The penetration depth strongly depends on the used frequency. Lower frequencies show a higher penetration depth, but limit the resolution due to poorer focusing.