Solder paste is the most important material in the surface mount process. Due to the special chemical properties of solder paste. It must be stored and used under specific environmental conditions to play its role, otherwise it will seriously affect the quality of soldering. The main control points of solder paste are:
A. Preservation of solder paste: The solder paste should be sealed and stored in an environment of 2-10 ℃. It is recommended that the temperature of the refrigerator be kept at about 5 ℃. Because solder paste has an expiration date limit, it must follow the “first in first out” principle.
B. Defrosting and stirring of solder paste: The solder paste must be thawed before use, so-called reheating. The temperature recovery time is about 4 hours. When the solder paste is removed from the refrigerator, the start time and the end time of the temperature recovery must be recorded immediately. The tin paste can only be used after the warm-up time has expired. The tin paste must be stirred before use. The stirring time is 3-5 minutes.
C. Use time of solder paste on the steel mesh: If the dwell time of the solder paste on the steel mesh will exceed 2 hours, the solder paste must be recovered into the solder paste tank and tightened to save the solder paste. If the continuous printing time exceeds 8 hours, the remaining solder paste must be disposed of.
A. Wave solder flux coating amount
It is required to have a thin layer of flux on the bottom surface of the printed board, to be uniform, not too thick, and pay special attention to no excess for the no-clean process. The flux coating method is a quantitative spray method, the flux is sealed in the container, will not volatilize, will not absorb moisture in the air, and will not be contaminated, so the flux composition can remain unchanged. The key requirement is that the spray head can control the spray volume, and the spray hole of the spray head should be cleaned frequently and cannot be blocked
B. Preheating temperature of electronic product circuit board
The role of preheating is to fully evaporate the solvent in the flux, 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; make the printed board reach a certain temperature before welding to avoid thermal shock Warpage deformation occurs. Generally, the preheating temperature is controlled at 180-200 ° C, and the preheating time is 1 to 3 minutes.
C. Track inclination of wave soldering track
The influence of the orbital inclination angle on the welding effect is more 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 shielding 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 joint eats too little tin, which is easy to produce virtual welding. Therefore, the inclination of the track should be controlled between 5 ~ 7.
D. Wave height of wave soldering
The height of the wave crest will have some changes due to the welding work time. It should be appropriately corrected during the welding process to ensure the ideal height for soldering, and the depth of the soldering tin is 1 / 2-1 / 3 of the thickness of the PCB.