Why do PCB pads have defects after wave soldering? The following SMT processing plant will give you an analysis of the causes and solutions.
①The PCB design is unreasonable, and the pad spacing is too narrow.
②The pins of the plug-in components are irregular or the plug-in is skewed, and the pins are close to or have been touched before welding.
③PCB preheating temperature is too low, components and PCB absorb heat during soldering, which reduces the actual soldering temperature.
④The soldering temperature is too low or the conveyor speed is too fast, which reduces the viscosity of the molten solder.
⑤The flux activity is poor.
Therefore, our solutions are:
The first is to design in accordance with PCB design specifications.
The pins of the second plug-in component should be shaped according to the PCB hole spacing and assembly requirements.
Third, set the preheating temperature according to the PCB size, board layer, number of components, and whether there are components to be mounted, etc. The temperature of the bottom surface of the PCB is 90-130°C.
The fourth tin wave temperature is (250±5)℃, and the welding time is 3～5s. When the temperature is slightly lower, the speed of the conveyor belt should be slowed down.
Fifth, replace the flux.
The common defects of PCB pads during wave soldering are as follows:
(1) The board surface is dirty. This is mainly due to the high solid content of the flux, too much coating amount, too high or too low preheating temperature, or too dirty conveyor claws, excessive oxides and tin dross in the solder pot, etc.
(2) PCB deformation. Generally occurs on large-size PCBs, due to the large quality of the large-size PCB or the uneven quality of the components caused by the uneven layout. This requires the PCB design to make the components distributed as evenly as possible, and design the process side in the middle of the large-size PCB.
(3) Dropped film (dropped film). The quality of the patch adhesive is poor, or the curing temperature of the patch adhesive is incorrect. The curing temperature is too high or too low will reduce the bonding strength. The wave soldering cannot withstand the effects of high temperature shock and wave shear force, causing the mounted components to fall on In the pot.
(4) Other hidden defects. The grain size of the solder joints, the internal stress of the solder joints, the internal cracks of the solder joints, the brittleness of the solder joints, and the poor strength of the solder joints require X-ray, solder joint fatigue testing and other inspections. These defects are mainly related to factors such as the soldering material, the adhesion of the PCB pads, the solderability of the solder ends or pins of the components, and the temperature profile.
