PCB Technical - PCB circuit board welding quality control method

1. The design of PCB circuit board

1, pad design

(1) When designing plug-in component pads, the size of the pad should be designed appropriately. If the pad is too large, the spreading area of the solder is larger, and the solder joint formed is not full, while the surface tension of the copper foil of the smaller pad is too small, and the solder joint formed is a non-wetting solder joint. The matching gap between the aperture and the component lead 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 welding condition.

(2) When designing the pads of SMD components, the following points should be considered: In order to remove the "shadow effect" as much as possible, the solder ends or pins of the SMD should be facing the direction of the tin flow to facilitate contact with the tin flow. Reduce false welding and missing welding. Smaller components should not be arranged after larger ones, lest the larger ones prevent the tin flow from contacting the pads of the smaller ones and cause solder leakage.

2, PCB proofing flatness control

Wave soldering requires a high degree of flatness of the printed board. Generally, the warpage is required to be less than 0.5mm. 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 their warpage requirements are even higher, otherwise the welding quality cannot be guaranteed. The following matters should be paid attention to:

(1) Properly store printed boards and components, and shorten the storage period as much as possible. During soldering, dust, grease, oxide-free copper foil and component leads are conducive to the formation of qualified solder joints. Therefore, printed boards and components should be stored in a dry place., In a clean environment, and try to shorten the storage period.

(2) For printed boards that have been placed for a long time, the surface should generally be cleaned, which can improve solderability, reduce false soldering and bridging, and remove the surface of component pins that have a certain degree of oxidation on the surface. Oxide layer.

2. Quality control of craft materials

In wave soldering, the main process materials used are: flux and solder.

1. The application of flux can remove oxides on the soldering surface, prevent re-oxidation of the solder and soldering surface during soldering, reduce the surface tension of the solder, and help heat transfer to the soldering area. Flux plays an important role in the control of soldering quality. At present, most of the fluxes used in wave soldering are no-clean fluxes. When selecting fluxes, there are the following requirements:

(1) The melting point is lower than that of solder;

(2) Wetting and spreading faster than molten solder;

(3) Viscosity and specific gravity are smaller than solder;

(4) It is stable in storage at room temperature.

2, quality control of solder

The tin-lead solder is continuously oxidized at high temperature (250 degree Celsius), which causes the tin content of the tin-lead solder in the tin pot to decrease continuously, deviating from the eutectic point, resulting in poor fluidity, and quality problems such as continuous soldering, virtual soldering, insufficient solder joint strength, etc. . The following methods can be used to solve this problem:

(1) Add redox agent to reduce the oxidized SnO to Sn and reduce the generation of tin dross.

(2) Add a certain amount of tin before each soldering.

(3) Use solder containing antioxidant phosphorus.

(4) Nitrogen protection is used to insulate the solder from the air and replace the ordinary gas, thus avoiding the generation of scum.

The current method is to use phosphorus-containing solder under a nitrogen atmosphere, which can control the scum rate to a low level, with fewer soldering defects and better process control.

PCB board proofing

3. Welding process parameter control

The influence of welding process parameters on the quality of the welding surface is more complicated, and the main points are as follows:

1. Control of preheating temperature

The role 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 suffer Thermal shock produces warpage and deformation. According to our experience, the general preheating temperature is controlled at 180 to 200 degree Celsius, and the preheating time is 1-3 minutes.

2, welding PCB track inclination

The orbital inclination has a more obvious influence on the welding effect, especially when welding high-density SMT devices. When the inclination angle is too small, bridging is more likely to occur, especially during soldering, the "shaded area" of the SMT device is more likely to be bridging; while the inclination angle is too large, although it is conducive to the elimination of bridging, but the amount of tin in the solder joint is too small, and it is easy to produce false weld. The orbital inclination should be controlled between 5°-7°.

3, wave crest height

The height of the wave crest will change due to the passage of the soldering work time. Appropriate corrections should be made during the soldering process to ensure the ideal height for the soldering wave crest height. The soldering depth is 1/2-1/3 of the PCB thickness. allow.

4, welding 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 will become poor, so that the solder pads or the solder ends of the components cannot be fully wetted, resulting in defects such as false soldering, sharpening, and bridging; when the soldering temperature is too high, It accelerates the oxidation of the pads, component pins and solder, and it is easy to produce false soldering. Generally speaking, the welding temperature should be controlled at 250+5 degree Celsius.