PCBA Tech - SMT grape ball and placement machine workflow

Grape ball phenomenon is generally that the solder paste is not completely melted and welded together during reflow, but instead aggregates into individual tin beads and stacked together to form a phenomenon similar to grape bunches.

Causes of SMT grape ball phenomenon

1. The solder paste is damp and oxidized

Moisture oxidation of solder paste is the main cause of the grape ball phenomenon. The oxidation of solder paste can be divided into many categories, such as operator careless operation, solder paste expired or improper storage, solder paste rewarming / poor mixing, causing the solder paste to absorb moisture and damp. It may be the cause of the oxidation of the solder paste. Some steel plates (mesh) are used on-line without being completely dried after solvent cleaning is also one of the possible reasons.

2. Solder paste flux volatilizes

The flux in the solder paste is an important factor that affects the melting of the solder paste. The purpose of the flux is to remove oxides on the metal surface and reduce the surface of the solder metal. In fact, there is another purpose to cover the tin powder to protect it. Avoid contact with air. If the flux is volatilized in advance, the effect of removing oxides on the metal surface cannot be achieved. Therefore, the solder paste must be used up within a certain period of time after unpacking, otherwise the flux will volatilize and the solder paste will change. Dry. In addition, if the preheating zone in reflow is too long, the flux will evaporate excessively, and the more chances of grape ball phenomenon will occur.

3. Insufficient reflow temperature

When the reflow temperature is not enough to provide the conditions for the solder paste to completely melt, the solder paste may also have a grape ball phenomenon. If the amount of solder printed on the PCB of the solder paste is smaller, the chance of the solder paste oxidation and flux volatilization will be higher. This is because the smaller the amount of solder paste printed, the higher the ratio of solder paste to air contact, and the easier it is to cause grape balls. So this is the reason why 0201 components are more prone to grape ball phenomenon than 0603 components.

Solution to improve SMT grape ball phenomenon:

1. Use solder paste with better activity.

2. Increase the printing volume of solder paste.

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

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

5. Turn on nitrogen to reduce the oxidation rate of solder paste.

Smt placement machine workflow

The work process of the placement machine roughly includes the following processes: board feeding PCB board fixing-suction nozzle selection-feeder selection-component picking-component detection-displacement positioning-component placement and placement-out of the board

1. The PCB board to be mounted enters the work area of the mounter and is fixed in a predetermined position;

2. The component passes through the feeder and is installed to the pick-up position of the placement head of the placement machine according to the position set by the program

3. The placement head moves to the suction position, opens the vacuum, the suction nozzle sucks the corresponding component set by the program through negative pressure, and then detects whether the component is sucked by the sensor;

4. Through the visual recognition and positioning system, the placement head reads the component characteristics of the component library and compares the absorbed components (such as appearance, size, etc.). Calculation of position and angle;

5. According to the set program, the placement head moves to the position set by the program to coincide the center of the component with the placement position of the PCB board;

6. The placement head lowers the nozzle to the height set by the program (the placement height of the component is set in the program library in advance), closes the vacuum, and the component is placed on the corresponding pad of the PCB;

7. After the PCB board components are all mounted, the suction nozzle returns to its position, and the PCB is transferred to the next smt process through the track. Repeatedly, more PCB board placement work is completed