PCBA Tech - SMT placement machine turret structure

1. The advantages and disadvantages of the turret structure

1) Up to 6 nozzles of different sizes are suitable for any patch head on the turret, and there is no need to replace the nozzles during work. That is to say, in smt patch proofing or processing and production, any nozzle can pick up components at any time, eliminating excessive and unnecessary operation steps.

2) The vision system is installed between the pick-up point and the patch point of the turret mounter, and can be used for image processing on the fly.

3) The simple mechanical gear-driven turret design can keep the power supply cables, servo systems, encoders, sensors and cameras stationary, so that there will be no electrical problems due to constant bending stress caused by motion to break the line ( For example, when the stage is constantly moving from the feeder to the camera, the circuit board may malfunction during operation).

4) Now the placement speed of the turret placement machine can reach more than 54500 points/h (such as HT122). The best single-head mounter can only mount about 15,000 points/h. In order for the bench mounter to achieve the same speed, multiple machines are required, which means that there will be more parts that need to be maintained, lubricated, calibrated and repaired.

5) Using a turret design, only install the feeder on one side of the placement machine. In this way, the front and rear width of the placement machine can be reduced by about 1m. Therefore, there is no need to install additional feeders and component storage devices on the other side of the smt production line, thereby reducing the area of the entire operation area.

2. Disadvantages of turret structure

1) Due to the limitation of the mechanical structure, the mounting speed has reached the limit and cannot be increased significantly, and it occupies too much space and is noisy.

2) The turret mounter can only mount components in tape packaging or bulk packaging, while tube materials and reels cannot be mounted.

3) The turret type placement machine has a fixed placement head position relative to the PCB placement position. The PCB is accurately positioned at the specified placement position by running in the X-Y direction, and the vibration caused by high-speed movement, Will cause the position of the already pasted components to shift, and ultimately affect the placement accuracy.

4) The range of components that can be mounted is limited to chip components and is not suitable for mounting IC devices.

3. Limitations of turret structure

When the turret structure is in motion, the mounting speed is limited due to the limitation of the maximum allowable value of the radial acceleration on the vacuum nozzle. The device on the suction nozzle bears the action of the acceleration vector force. This vector is the resultant force of the centrifugal force and the tangential force. The resultant force acting on the device and the frictional force between the device and the end face of the suction nozzle are balanced. This frictional force is restricted by the suction force on the end face of the suction nozzle and the friction coefficient between the device and the suction nozzle.

According to the calculation of mechanical motion engineering mechanics, the maximum allowable acceleration between the device and the suction nozzle is conventionally not more than 50m/s2. Therefore, when high mounting output is required, the diameter of the turret should be reduced. Another factor that restricts the mounting speed of the turret structure mounter is the maximum allowable acceleration of the device first on the PCB. Usually this value is 7～10m/s2.

The turret time has 3 time determination cycles: turret displacement movement acceleration limit, PCB movement acceleration limit and feeder displacement movement limit. Among these 3 time periods, the longest period is the placement output of the turret structure Constraints. In addition, the distance between the two mounting positions on the PCB board is more than 30mm, which will reduce the mounting output by 50%, and the conversion time between the two feeders will also affect the production capacity.