## 2 Investigated LED and Soldering Process  
Tests were carried out for power LEDs of the XMLBWT-02-0000-000 HT20E7 type produced by Wolfspeed Inc. [40] (Durham, NC, USA). The LEDs were used with a maximum admissible forward current I\({}_{\text{Fmax}}\) = 3 A and a maximum admissible junction temperature T\({}_{\text{jmax}}\) = 150 \({}^{\circ}\)C. Thermal resistance between the junction of this LED and the soldering point was R\({}_{\text{thj}}\) = 2.5 K/W [40]. The diode was characterized by its correlated color temperature CCT = 3000 K. In contrast, the generated luminous flux is equal to \(\Phi_{\text{v}}\) = 200 lm at the forward current I\({}_{\text{F}}\) = 700 mA and junction temperature T\({}_{\text{j}}\) = 85 \({}^{\circ}\)C.  
The tested devices were soldered to MC PCB (Metal Core Printed Circuit Board) of the type XM-NT-15B2-F [41]. In the soldering process, three types of solder pastes were used: the classical SACX0307 alloy (REF) and composite ones, SACX-TiO\({}_{2}\) (containing grains of TiO\({}_{2}\) of a diameter of 200 nm) and SACX-TiO\({}_{2}\) nano (containing grains of TiO\({}_{2}\) of a diameter of 20 nm).  
In the soldering process of the samples, a convection reflow oven SMT 460C (Essemtec AG, Aesch, Switzerland) was used. The temperature profile of the soldering process is presented in Figure 1. The linear temperature profile contains 3 phases: pre-heating (duration equal to 60 s, temperature range of 160-180 \({}^{\circ}\)C), reflowing (duration equal to 40 s, temperature range of 213-247 \({}^{\circ}\)C), and cooling (duration equal to 70 s, temperature decreases from 247 \({}^{\circ}\)C to 150 \({}^{\circ}\)C). The convection reflow was performed in an air atmosphere.  
The tested LEDs were assembled using each of the mentioned solder pastes. Three of the tested LEDs were assembled in each group with each kind of tested paste. Alumina substrates, Star MCPCB board with a 22 mm diameter and 2.4 mm thickness, were used. A Cu layer covered these substrates with a 60 \(\upmu\)m thickness. The galvanic Ni/Au layer with a 2.5/0.25 \(\upmu\)m thickness was used as a surface finish [41].  
During the test, the LEDs mounted on the MCPCB were situated on the alumina heat sink. A view of the sample is shown in Figure 2a, and samples assembled for the test are shown in Figure 2b. The dimensions of the used heat sink were as follows: 250 \(\times\) 125 \(\times\) 35 mm.  
Composite soldering alloys were made based on SACX0307 (Sn99 Ag0.3 Cu0.7) solder paste produced by Alpha Industries (Chantilly, VA, USA). To this paste, 1.0 wt. % of TiO\({}_{2}\) reinforcements produced by Sigma-Aldrich (St. Louis, MI, USA) were added. These reinforcements have two sizes: 20 nm (nanoparticles) and 200 nm (submicron particles). The paste and reinforcements were mixed homogeneously using the ball milling process. This process was carried out using a planetary ball mill Pulveristette 5 produced by Fritsch GmbH (Idar-Oberstein, Germany) for 10 min at 300 rpm.  
Figure 1: Temperature profile of convection reflow oven SMT 460C.  
The solder paste SACX0307 without any reinforcements was used as the reference sample. Stencil printing with a stencil thickness of 125 \(\upmu\)m was used to prepare the solder joints. The stencil aperture was narrowed by 10% over the thermal pad to avoid a shortcut during the soldering process (Figure 3).