*2.1. Prototype Modules and Measurement Euqipment*

The test modules analyzed throughout this paper contained six LEDs forming on the substrate, as presented in Figure 1a, a hexagonal circle with the diameter equal to 27 mm. The investigated white diodes of the XREWHT-L1-0000-006F8 type belonged to the XLamp ® family manufactured by Cree. Their maximal forward current value is 0.7 A and their viewing angle is 90◦. The dimensions of the MCPCB used for these modules were 50 mm × 50 mm × 1.5 mm. These substrates are dedicated for LED applications and according to the datasheet provided by the manufacturer, their thermal conductivity value is equal to 2 W/(m·K). In order to investigate the influence of the thermal pad area on the LED temperature, the modules were fabricated in two di fferent versions. The first one had the standard size of diode thermal pads, i.e., equal to 6.46 mm × 5.60 mm. Their dimensions, as visible in Figure 1b, were delimited by the LED package width and the spacing between the diode electrodes. The other version of the module had the pad width doubled to 12.92 mm. These large pads are visible in Figure 1a as the light rectangles extending outside the package perimeter.

**Figure 1.** Dimensions of the investigated multi-, light-emitting diodes (LEDs) test module: (**a**) the photograph of the circuit layout; (**b**) the LED electrode layout at the bottom side of the package.

Taking into account the prospective application of the modules, the main goal of the research was to determine the mutual influences between the devices. Looking at the circuit layout, it could be concluded that there existed three possible distances between the devices: 14.1 mm, 24.2 mm or 27.0 mm, therefore it would be enough to include only four LEDs in the thermal coupling analyses. Thus, the diode D2 was initially used as a heat source and the temperature values were measured in the diodes D1–2 and D5–6, but it soon turned out that the responses in the two latter devices were very similar. Hence, the analyses presented in the remainder of this paper will be limited only to the LEDs marked in Figure 1a with black circles. This solution allowed to keep the figures legible without any significant loss of the analysis depth.

The thermal couplings between the devices were investigated based on the results of dynamic temperature measurements, which were taken with the T3Ster ® transient thermal tester produced by Mentor ®. This piece of equipment renders possible the registration of the system's thermal responses with microsecond time resolution [27]. During the measurements, the diode cathode and anode terminals were soldered to the standard measurement cables provided together with the tester and the MCPCB was placed horizontally in thermally insulated clamps.
