Issue link: https://iconnect007.uberflip.com/i/1437606
32 PCB007 MAGAZINE I DECEMBER 2021 (depends on the voltage to which capacitor banks are charged and the length of time for which they are discharged) and the frequency at which light pulses are incident on the mate- rial system. Average power is the critical con- trol over the temperature ramp rate achiev- able for the device stack. While certain device structures can be soldered with extremely high ramp rate, others require a slower ramp rate to preserve device structure and prevent uncon- trollable outgassing. e temperature range can be controlled by spatial selectivity for: • ermally-sensitive substrates like LED arrays on PET • ermally-sensitive components like batteries or displays using an aluminum mask with laser-cut openings • ermally-sensitive regions like the reflow under a BGA with conductive heating • ermally-sensitive solder joints like SAC305 reflow in 0.375 seconds • Reliability-sensitive lead-free solder joints where the short heating time minimizes intermetallics • When low voids are required, <3% voids are achievable For the same structure, the peak tempera- tures can be manipulated either by increasing the ramp rate, exposure time, or a combina- tion of both (Figure 4). Like standard reflow mechanism, each change provides a differ- ent opportunity in optimizing the solder joint quality. For the explored device structure, at an average incident power density of 16 W/cm 2 , reflow of the solder can be observed starting at 1.5 seconds but will improve the joint qual- ity up to three seconds as reflected through improved fillet shape and intermetallic for- mation. At five seconds of exposure, we start to observe mechanical failure and buckling of the flex circuit. Figure 5 shows an interior of the photonic soldering equipment. is equip- ment (Figure 6) comes in a batch and convey- orized unit. A complete automated assembly line of paste-inspection-placement-soldering would be only 21 feet long and have a process- ing time of only three to four minutes. Figure 4: a) Temperature profile of SAC305 solder joint on a 0603 resistor with medium power for three seconds; b) Temperature profile of the solder joint with different pulse trains at high power; c) Temperature profile of a solder joint with different power settings. (Source: NovaCentrix)