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Fine Tuning
Wave configurations for soldering conveyor angles.
The 1960s saw the rise of wave soldering systems with horizontal conveyors as well as angled conveyors. Ultimately, the angled conveyor provided the best soldering results. From a process point of view, issues such as conveyor angle, contact length, dwell time, soldering speed and drainage conditions were all key factors.
The goal in machine soldering is to get a high output rate combined with a high yield. A process should involve minimum maintenance and minimal use of consumables. It should be able to form good, sound solder connections, without solder remaining in between the joints after the soldering process is complete. We also do not want to overheat components such that their reliability will be compromised as a result of the soldering process. This column will first consider the effect of wave configuration in combination with conveyor angle.
Since the thermal load to the PCB must be limited to prevent thermal damage to the PCB or components, wave temperature and dwell time in the wave must also be limited. Another reason to limit dwell time is that the flux activity will be exhausted if the dwell time is too long. Also, a strong washing action by the wave might remove too much flux, resulting in solder failures.
Flux. Flux must perform two main functions during the solder process. First, it must make the joint area and the solder surface sufficiently clean, to promote solder wetting for good joint formation. Second, as the assembly leaves the solder wave, the flux must create conditions whereby solder will drain off without excessive oxide formation. Oxide formation at this part of the process will create solder bridging between joints, or even solder webbing. If the flux is unable to fulfill these functions, solder problems will be created that cannot be fixed by any wave setting.
Dwell time. The dwell time in the solder is a combination of contact length in the solder wave and conveyor speed, but speed has its limitations. It takes a certain amount of time for the solder to separate from the PCB at the wave exit zone. How much time depends a great deal on the joint configuration on the solder side. The drainage conditions will often be improved when we solder under a steeper angle. At a steeper angle, the contact length in the solder wave will be reduced. As a result, we must reduce conveyor speed to establish the desired dwell time.
Contact length. For high conveyor speeds, one needs a longer contact length to get the desired dwell time, which means a larger wave area. This can only be achieved when soldering under a small angle. The solder angle is directly related to the contact length in the solder wave. The depth of the PCB in the solder wave is limited by board thickness because the solder must not flood the PCB during its entrance into the wave.
General settings. For most practical situations, the optimal setting of the conveyor angle, with respect to contact length, dwell time, and drainage conditions, is 7°. This is based on a system using SnPb40 solder, RMA-type fluxes and a bath temperature of 240° to 260°C.
Because the type of PCB varies, the wave machine should be capable of adjusting the conveyor angle. This also means that the machine must have a nozzle that can create a long wave area, which is needed when soldering under a small angle. Due to its larger surface area, such a nozzle may create more dross.
The tuning of the adjustable backplate is more difficult with longer nozzles when soldering at a steep angle. The solder rise and the velocity of the solder at the wave surface area in front of the PCB is less on a machine with a long wave area than it is on a smaller wave area. Why? The PCB, upon its entry into the wave, will partially block the nozzle, so the solder at the free wave area will rise and move at a higher speed. The smaller this free area is, the higher the solder will rise, and the greater the speed of the solder in the free area. With a higher solder rise, it is easier to tune the backplate for a correct solder overflow during the passage of the PCB. This overflow will flood off remaining oxides from the wave surface and thus facilitate bridge-free soldering.
The ideal process, from the manufacturer’s point of view, would be to have the nozzle length unique to the conveyor angle. This is not very practical, however. Instead, a good compromise is to permit the user to choose between nozzles of 70 to 120 mm, in view of the process desired.
Gert Schouten is a senior engineer at Vitronics Soltec (vitronicssoltec.com); gschouten@vsww.com. His column appears monthly.
