In a typical deep-ultraviolet UVC-LED package structure, the UVC chip may be soldered on the ceramic bracket by solder paste soldering or eutectic soldering. The difference in the soldering temperature of these two packaging processes results in a greater difference in the maximum acceptable reflow temperature of the subsequent lamp bead SMT. At the same time, some special glue filling processes are also easy to cause the chip to leave the pad to form a virtual solder during the reflow process at an excessively high temperature and eventually open the circuit and die. Therefore, the SMT process of our lamp beads should strictly implement the recommended reflow curve (see Section 3, the reflow curve must be followed for details) to avoid high probability of lamp failure.
1. Reflow temperature tolerance of solder paste soldering and eutectic soldering packaged lamp beads
1.1 Soldering with solder paste: solder the chip pad and the bracket with a dot of solder paste. Compared with the traditional silver glue solid crystal thermal resistance, the thermal conductivity is greatly improved. The heat generated by the chip can be quickly dissipated. At the same time, its mechanical performance is significantly enhanced, which improves the reliability of the lamp beads.
1.2. Eutectic welding: Two different metals can form alloys in a certain weight ratio at temperatures much lower than their respective melting points. Eutectic welding has the advantages of high thermal conductivity, low resistance, fast heat transfer, strong reliability, and large shear force after bonding. The welding hole rate is relatively low, but the equipment cost is high.
At present, the melting point of the solder paste used by our company for solid crystal soldering is 217℃, and the most commonly used eutectic solder for eutectic soldering is gold-tin alloy, and its eutectic temperature is 278℃. Compared with the eutectic process, the solder paste soldering porosity is Relatively higher, its interface bonding force is relatively poor, and eutectic welding has more advantages in terms of temperature resistance.
2. Introduction to Silicone Package/Lens Package
2.1. The above are two packaging processes for the lamp beads that our company currently supplies, one is silicone filling (ie PSM lamp beads), and the other is lens packaging (QSC series lamp beads). Among them, the PSM lamp beads are directly filled with UV-resistant silicone after the die bonding is completed, and there is no colloid filling inside the lens package, and the lens is directly fixed on the bracket step by UV glue for packaging.
2.2. PSM lamp beads due to the silica gel filling process, when heated, the internal stress of the gel will inevitably increase due to the thermal barrier shrinkage, which causes the stress applied to the chip and the support interface to increase, and in severe cases, the chip and the support pad Stripping causes open-circuit dead lights, so compared to QSC series lamp beads, PSM lamp beads are more prone to open-circuit dead lights during reflow soldering.
3. Reflow temperature curve that must be observed
Silicone-filled UVC-LED lamp beads should be reflow soldered with low-temperature solder paste. The peak temperature should not be greater than 155℃, the peak temperature time should be controlled at 20 seconds, and the reflow time should not be greater than 5 minutes. Tin-bismuth solder paste is recommended .
Quartz lens package UVC-LED lamp beads
Quartz lens packaged UVC-LED lamp beads should be reflow soldered with low-temperature solder paste. The peak temperature should not be greater than 170°C, the peak time should be controlled at 20 seconds, and the reflow time should not be greater than 5 minutes. Tin-bismuth solder paste is recommended.
