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30 PCB007 MAGAZINE I DECEMBER 2021 fiberglass. e flash tube's white light is not drying with temperature; it is drying with the high energy density of electromagnetic radia- tion (ER). e ER rays penetrate deep into the layer to be dried and excite the molecules there. e absorption of the energy happens in a few tenths of a second. e molecular activa- tion vaporizes the solvent or water. Because the radiation energy penetrates deep into the layer, it couples to the pigments and dries them from the inside out, and the undesirable effects on the surface such as bubble formation or blistering do not occur. In addition, ER light can be dosed and applied in a very targeted manner, right to the point. With this innovative technology, highly conductive patterns, components (resistors/ capacitors) and insulators can be cured on substrates like papers, fabrics, or plastics, all in less than a few hundred milliseconds. 2 Printing technologies as well as ink types are detailed in Chapter 11 of the free 007ebook, Flexible Circuit Technology, 4 th Edition. 3 Photonic Soldering and Sintering Table 1 provides an outline of different selec- tive soldering techniques organized by how the thermal load is delivered. In these conventional cases, the exposure is confined to the heating area to ensure lower thermal load on the temper- ature-sensitive parts of the device. Since the heating medium is confined and will need to be moved from one area to another, the speed of processing with these tech- niques is typically slow. L i g h t - g e n e r a t e d ( E R ) heating, as in laser solder- ing, can also be performed by flash tubes, but with the advantage of soldering com- ponents not in the line-of- sight. e first of these new soldering systems is PulseForge, developed by NovaCentrix. With the use of 500-volt power supplies of 30 to 40 KW capability attached to banks of high- voltage storage capacitors and controlled by high-voltage circuits, specially designed flash tubes can now perform the standard lead-free reflow in just seconds and with power usage just 10% of standard reflow ovens. Figure 2 shows that light-absorbing materials will heat during the light pulse, and cool imme- diately when the light is removed. Although this is possible to solder with just one pulse, the superior process is to use a pulse-train of pulses (with differing pulse duration and power) so as not to overheat the substrate. Figure 2: PulseForge soldering, differentiating from normal reflow with broad-spectrum white light. (Source: NovaCentrix) Table 1: An outline of different selective soldering techniques arranged by thermal heating source thermal profile. (Source: NovaCentrix 4 )