Tin beads are sometimes produced during PCBA processing, which is a defect of electronic processing and is generally prone to appear in production processes such as SMT chip processing. For a processing-oriented enterprise dedicated to providing quality services, all processing defects need to be resolved. To solve a problem, we must first know the cause of its occurrence. So what is the reason for the tin beads? Let me briefly share with you the reason why tin beads are produced during SMT patch processing.
1. Selection of solder paste
1. Metal content
Generally, the metal content and mass ratio in solder paste are about 88% to 92%, and the volume ratio is about 50%. When the metal content increases, the viscosity of the solder paste increases, which can effectively resist the force generated by vaporization during the welding preheating process of SMT chip processing. The increase in metal content makes the metal powder closely arranged, making it easier to combine without being blown away when melting.
2. Oxidation degree of metal powder
The higher the degree of oxidation of the metal powder in the solder paste, the greater the bonding resistance of the metal powder during soldering, and the solder paste will not be easily wetted between the PCBA pad and the chip component, resulting in reduced solderability.
3. Metal powder size
The smaller the particle size of the metal powder in the solder paste, the larger the overall surface area of the solder paste, which leads to a higher oxidation degree of the finer powder, and the phenomenon of solder beads is intensified.
4. Amount and activity of flux
Too much flux will cause local collapse of the solder paste and lead to tin beads. When the flux is not active enough, the oxidized part cannot be completely removed, which will also lead to tin beads in PCBA processing.
5. Other matters needing attention
If the solder paste is not reheated, splashing will occur during the preheating stage of the SMT patch to produce tin beads. The PCBA substrate is damp, the indoor humidity is too heavy, the wind blows the solder paste, and the solder paste adds excessive thinner , Machine stirring time is too long, etc. will promote the production of tin beads.
2. Production and opening of steel mesh
1. Opening
In the process of opening the steel mesh, the opening is opened according to the size of the direct pad, so that the solder paste may be printed on the solder layer during the solder paste printing process of the SMT chip, resulting in the appearance of solder beads.
2. Thickness
Steel mesh Baidu is generally between 0.12 ~ 0.17mm, too thick will cause the solder paste to collapse, resulting in tin beads.
3. Mounting pressure of the placement machine
If the pressure is too high during mounting, the solder paste will be easily squeezed onto the solder mask layer under the component. During the reflow soldering, the solder paste melts and runs around the component to form solder beads.
4. Setting of furnace temperature curve
Generally, tin beads are produced in the reflow soldering process of PCBA processing. During the preheating stage, the temperature of the solder paste, PCBA and chip components rise to between 120 and 150 ° C. It is necessary to reduce the components during reflow Thermal shock, at this stage, the flux in the solder paste begins to evaporate, so that the small particles of metal powder run under the component separately, and run around the component to form tin beads during the current flow.
