pcb is widely used in modern electronics. With the rapid development of electronic technology, PCB density is getting higher and higher, and more and more welding process requirements are also increasing. Therefore, it is necessary to analyze and judge what factors affect PCB welding quality and find out the causes of welding defects, so as to improve the overall quality of PCB board. So, what are the factors that affect PCB board welding?
The warping of circuit boards and components occurs in the welding process, and defects such as virtual welding and short circuit occur due to stress deformation. Warping is often caused by an imbalance in temperature between the upper and lower parts of the board. Large PCBS will also warp when they fall due to the weight of the board itself. Ordinary PBGA devices are about 0.5mm away from the printed circuit board. If the device on the circuit board is large, the solder joint will be under stress for a long time as the circuit board returns to normal shape after cooling. If the device is raised by 0.1mm, it will be enough to cause the virtual welding open circuit.
Two, circuit board design
In the layout, the size of the circuit board is too large, although the welding is easier to control, but the printing line is long, the impedance increases, the anti-noise ability decreases, the cost increases; After hours, the heat dissipation decreases, welding is not easy to control, easy to appear adjacent lines interference with each other, such as the electromagnetic interference of the circuit board. Therefore, PCB board design must be optimized:
(1) Shorten the connection between high-frequency components and reduce EMI interference.
(2) Large weight (such as more than 20g) components, should be fixed with a bracket, and then welded.
(3) Heat dissipation of the heating element should be considered to prevent defects and rework caused by large ΔT on the surface of the element. The thermal sensitive element should be far away from the heating source.
(4) the arrangement of components as parallel as possible, so that not only beautiful and easy to weld, suitable for mass production. 4∶3 rectangular circuit board design is most preferable. Do not change the width of the wire to avoid discontinuities in the wiring. When the circuit board is heated for a long time, the copper foil is easy to expand and fall off. Therefore, the use of large area copper foil should be avoided.
Three, the solderability of the circuit board hole
The poor solderability of circuit board holes will lead to virtual welding defects, which will affect the parameters of components in the circuit, lead to the instability of multilayer components and inner wire conduction, and cause the functional failure of the entire circuit.
The main factors affecting the solderability of printed circuit boards are:
(1) the composition of the solder and the nature of the solder. Solder is an important part of welding chemical treatment process, it is composed of chemical materials containing flux, common low melting point eutectic metal for Sn-Pb or SN-PB-Ag. The content of impurities should have a certain proportion control, to prevent the oxides produced by impurities to be dissolved by the flux. The function of flux is to help solder wet the circuit surface of the plate to be soldered by transferring heat and removing rust. White rosin and isopropyl alcohol solvents are commonly used.
(2) Welding temperature and metal surface cleanliness will also affect the weldability. The temperature is too high, the solder diffusion speed is accelerated, at this time has a high activity, will make the circuit board and solder melt surface rapid oxidation, welding defects, circuit board surface pollution will also affect the solderability resulting in defects, these defects include tin beads, tin balls, open circuit, poor gloss, etc.
In a word, in order to ensure the quality of PCB board, in the process of making PCB board, to choose excellent solder, improve the solderability of PCB board, and prevent warping, prevent the generation of defects.
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