Workflow of SMT grape ball mounting machine

Grape ball phenomenon usually refers to the phenomenon that solder pastes are not completely melted and welded together in the reflow soldering process, but are gathered into single solder beads and stacked together to form grape clusters

Causing SMT grape ball phenomenon

1. Solder paste oxidized due to moisture

The water oxidation of solder paste is the main reason for the grape ball phenomenon. The oxidation of solder paste can be divided into many types, such as operator carelessness, expired solder paste or improper storage, reheating/poor mixing of solder paste, resulting in moisture absorption of solder paste. This may be the reason for the oxidation of the solder paste. One possible reason is that some steel plates (nets) do not completely dry after solvent cleaning.

2. Solder paste and flux volatilization

The flux in solder paste is an important factor affecting the melting of solder paste. The role of flux is to remove the oxides on the metal surface and reduce the solder metal surface. Actually, there is another purpose to protect tin powder. Avoid contact with air. If the flux volatilizes in advance, the effect of removing oxides on the metal surface cannot be achieved. In this case, the solder paste must be used up within a period of time after unpacking, otherwise the flux will volatilize and the solder paste will deteriorate. In addition, the preheating area of reflow soldering welding is too long, and the flux volatilizes too much, so the possibility of grape ball phenomenon is large.

3. Insufficient return water temperature

When the reflow soldering temperature is not enough to provide conditions for complete melting of solder paste, solder paste may also appear grape ball phenomenon. If less solder is printed, the probability of solder paste oxidation and flux volatilization will be higher. The reason is that the smaller the amount of solder paste printed, the higher the proportion of solder paste in contact with air, and the easier it is to produce grape ball phenomenon. This is why the 0201 component is more prone to grape ball phenomenon than the 0603 component.

Solution to improve SMT grape ball phenomenon:

1. Use more active solder paste.

2. Increase the printing volume of solder paste.

3. Increase the width or thickness of the steel plate opening, increase the printing volume of flux and solder paste, and improve the oxidation resistance of solder paste.

4. Shorten the preheating time in the reflow soldering curve, increase the temperature rise slope, and reduce the volatility of flux.

5. Adding nitrogen gas reduces the oxidation rate of solder paste.

The working process of the SMT machine

The working process of the SMT machine roughly includes the following processes: board feeding, PCB fixation, nozzle selection, board feeder selection, component picking, component detection, displacement positioning, component placement, and plate out placement

1. The PCB board to be installed enters the working area of the SMT machine and is fixed in the predetermined position;

2. The component is installed at the picking position of the tip of the SMT machine according to the programmed position through the feeder

3. Move the adhesive head to the suction position, activate the vacuum, and the suction nozzle sucks the corresponding parts set in the program through negative pressure, then detect whether the parts are being sucked in by the sensor;

4. The mounting head uses a visual recognition positioning system to read the component features of the component library and compare the extracted components (such as appearance, size, etc.). Calculate the position and angle;

5. According to the set program, move the mounting head to the position set in the program, so that the center of the component coincides with the mounting position of the PCB board;

6. The mounting head lowers the suction nozzle to the programmed height (the mounting height of the component is preset in the library), turns off the vacuum, and places the component on the corresponding PCB pad;

7. Afterwards, the PCB board components have been installed, the suction nozzle has returned to its original position, and the PCB is transported through the track to the next SMT process to repeat the installation of more PCBs.

The above is the explanation provided by the editor of PCB circuit board company.

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