## 1 Introduction  
Soldering is still one of the most important joining technologies in microelectronics. Almost all electronic devices contain components that are fixed to the circuit board by soldering. Compared to other joining technologies (such as adhesives), the solder joints form mechanically strong, electrically conductive and reliable joints. In the electronics industry, the most typical is convection reflow soldering; however, in some applications, vapour phase soldering is used. Soldering methods differ in thermal profiles and soldering atmospheres. A consequence of the mandatory implementation of lead-free soldering technology in the EU (under RoHS Directive 2002/95/EC and RoHS 2011/65/EU) was that, since 2012, almost no electronic equipment has contained lead. Until 2006, tin-lead alloys (e.g., 63 Sn37 Pb or 60 Sn40 Pb) were mainly used in the electronics industry. As a result of the transition to lead-free technology, the widespread use of tin-silver-copper (SnAgCu, SAC) solder alloys, e.g., 96.5 Sn3 Ag0.5 Cu (SAC305) or 95.5 Sn4 Ag0.5 Cu (SAC405) was introduced [1]. There is an increasing demand for high-quality joints and reliability relating to miniaturization, but also in the application of high-temperature power electronics [2]. The insulated gate bipolar transistor (IGBT) is a fully controllable power semiconductor device that allows one to switch voltages and currents of high value [3]. They are commonly used in switch-mode power converters [4]. Their applications include wind turbines, aircraft, and electric vehicles [5; 6]. [PERSON] et al. analyzed the main failure mechanism of IGBT modules. They found that the key factor affecting the highest junction temperature and reliability of IGBT modules was the bond-wire lift-off, but just as often it was the cracking and fatigue of the solder layer [7]. Moreover, at elevated temperatures, irreversible plastic deformation can occur in the solder layer, resulting in the formation of internal crack damage [5]. In a discrete IGBT, the high current causes significant heat flow in the structure, so materials with good thermal and electrical conductivity are required. The properties of IGBTs are strongly influenced by temperature. It affects both the electrical properties of this device and its reliability. The junction temperature of the device rises above ambient conditions as the power dissipated increases. From the point of view of an electronics systems designer, information about the value of junction temperature is critical for choosing the appropriate cooling system and for estimating the lifetime of the device. The efficiency of a cooling system is typically evaluated using junction-ambient transient thermal impedance waveform (\(Z_{th}(t)\)). Its value at a steady state is called junction-ambient thermal resistance (\(R_{th}\)). Its value is related mainly to the cooling system used, but also to the type of device and the surface of the solder pads [8; 9]. In such structures, the substrate acts as a heat sink. Figure 1 shows the theoretical heat flow through the component to the substrate. Heat is generated when the current flows through the component to the substrate. Heat flows from the hotter to cooler areas--the arrows in Figure 1. The heat from the inside of the element reaches the substrate through the soldered joints. From Sixty to ninety-five percent of the thermal energy can be dissipated by the FR4 substrate. In the whole structure, only the \(R_{th}\) value of the solder joint can change according to the applied soldering technology and thermal profile. Therefore, a properly designed assembly increases the reliability of the device [10]. The quality of the solder joints is very important for proper heat transfer in the soldered structure [2]. It was proven that degradation of the solder joints leads to an increase in thermal impedance and thus accelerates the failure possibility of the device [11].