1. Design of PCB circuit board
1.1. Pad design
a. When designing the plug-in component pads, the size of the pads should be designed appropriately. If the pad is too large, the solder spreading area is large, and the formed solder joint is not full, while the surface tension of the copper foil of the smaller pad is too small, and the formed solder joint is a non-wetting solder joint. The matching gap between the aperture and the lead of the component is too large, and it is easy to solder. When the aperture is 0.05 - 0.2mm wider than the lead, and the diameter of the pad is 2 - 2.5 times the aperture, it is an ideal condition for welding.
b. When designing the SMD component pads, the following points should be considered: In order to eliminate the "shadow effect" as much as possible, the solder ends or pins of the SMD should be facing the direction of the tin flow, so as to facilitate the contact with the tin flow and reduce the Welding and missing welding. The smaller components should not be arranged after the larger components, so as to prevent the larger components from hindering the solder flow from contacting the pads of the smaller components and causing leakage soldering.
1.2, PCB proofing flatness control
Wave soldering has high requirements on the flatness of the printed board. Generally, the warpage is required to be less than 0.5mm, and if it is greater than 0.5mm, it needs to be flattened. In particular, the thickness of some printed boards is only about 1.5mm, and the warpage requirements are higher, otherwise the welding quality cannot be guaranteed, and the following matters should be noted:
a. Properly store the printed board and components, and try to shorten the storage period. During welding, copper foil and component leads without dust, grease, and oxide are conducive to the formation of qualified solder joints. Therefore, the printed board and components should be stored in a dry, dry place. In a clean environment, and keep the storage period as short as possible.
b. For printed boards that have been placed for a long time, the surface should generally be cleaned, which can improve solderability, reduce virtual soldering and bridging, and remove the surface oxidation of component pins with a certain degree of surface oxidation. Floor.
2. Quality control of process materials
In wave soldering, the main process materials used are: flux and solder.
2.1. The application of flux can remove oxides on the soldering surface, prevent re-oxidation of the solder and the soldering surface during soldering, reduce the surface tension of the solder, and help the heat transfer to the soldering area. Flux plays an important role in the control of soldering quality. At present, most of the no-clean fluxes used in wave soldering have the following requirements when selecting fluxes:
a. The melting point is lower than that of solder;
b. The speed of wetting and diffusion is faster than that of melting solder;
c. The viscosity and specific gravity are smaller than those of solder;
d. It is stable in storage at room temperature.
2.2. Quality control of solder
The tin-lead solder is continuously oxidized at high temperature (250°C), so that the tin content of the tin-lead solder in the tin pot continues to decrease, which deviates from the eutectic point, resulting in poor fluidity and quality problems such as continuous soldering, virtual soldering, and insufficient solder joint strength. . The following methods can be used to solve this problem:
a. Add a redox agent to reduce the oxidized SnO to Sn and reduce the generation of tin dross.
b. Add a certain amount of tin before each welding.
c. Use solder containing antioxidant phosphorus.
d. Use nitrogen protection to isolate the solder from the air and replace the ordinary gas, so as to avoid the generation of scum.
The current method is to use phosphorus-containing solder in a nitrogen-protected atmosphere, which can control the dross rate to a low level, with fewer welding defects and better process control.
3. Welding process parameter control
The influence of welding process parameters on the quality of the welding surface is complex, and there are the following major points:
3.1. Control of preheating temperature
The function of preheating: ① Make the solvent in the flux fully volatilize, so as not to affect the wetting of the printed board and the formation of solder joints when the printed board passes through the solder; ② Make the printed board reach a certain temperature before soldering, so as not to be affected by the solder. Thermal shock produces warping deformation. According to our experience, the general preheating temperature is controlled at 180-200℃, and the preheating time is 1-3 minutes.
3.2. Inclination of welding track
The influence of the track inclination on the welding effect is obvious, especially when welding high-density SMT devices. When the inclination angle is too small, bridging is more likely to occur, especially in soldering, the "shielded area" of the SMT device is more prone to bridging; while the inclination angle is too large, although it is conducive to the elimination of bridging, but the solder joints eat too little tin, which is prone to virtual voids weld. The orbit inclination should be controlled between 5°-7°.
3.3. Peak height
The height of the wave crest will have some changes due to the passage of the soldering work time. It should be properly corrected during the soldering process to ensure the ideal height for the soldering wave crest height. The tinning depth is 1/2 - 1/3 of the PCB thickness. allow.
3.4. Soldering temperature
Welding temperature is an important process parameter that affects welding quality. When the soldering temperature is too low, the expansion rate and wetting performance of the solder deteriorate, so that the soldering pads or component soldering ends cannot be sufficiently wetted, resulting in defects such as virtual soldering, sharpening, and bridging; when the soldering temperature is too high, It accelerates the oxidation of pads, component pins and solder, and is prone to virtual soldering. Generally speaking, the soldering temperature should be controlled at 250+5℃.
Regarding the quality control method of PCB board sampling, I will introduce it here today. Welding is an important process step in the pcb proofing process. In order to ensure the welding quality of the circuit board, you should master the quality control methods and welding skills.
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