B Printed circuit board SMT placement equipment
SMT placement equipment
As the part is carried from the part feeders on either side of the conveyor belt to the PCB, it is photographed from below. Its silhouette is inspected to see if it is damaged or missing (was not picked up), and the inevitable registration errors in pickup are measured and compensated for when the part is placed. For example, if the part was shifted 0.25 mm and rotated 10° when picked up, the pickup head will adjust the placement position to place the part in the correct location. Some machines have these optical systems on the robot arm and can carry out the optical calculations without losing time, thereby achieving a lower derating factor. The high end optical systems mounted on the heads can also be used to capture details of the non-standard type components and save them to a database for future use. In addition to this, advanced software is available for monitoring the production and interconnect database — of the production floor to that of supply chain — in real time.
A separate camera on the pick-and-place head photographs fiducial marks on the PCB to measure its position on the conveyor belt accurately. Two fiducial marks, measured in two dimensions each, usually placed diagonally, let the PCB's orientation and thermal expansion be measured and compensated for as well. Some machines are also able to measure the PCB shear by measuring a third fiducial mark on the PCB.
To minimize the distance the pickup gantry must travel, it is common to have multiple nozzles with separate vertical motion on a single gantry. This can pick up multiple parts with one trip to the feeders. Also, advanced software in the newer generation machines allows different robotic heads to work independently of each other to further increase the throughput.
The components may be temporarily adhered to the PCB using the wet solder paste itself, or by using small blobs of a separate adhesive, applied by a glue-dispensing machine.