courtesy : Computer Placement
Placement input
- Flexible placer, chip shooter, and other specialized machines.
- PWB with solder print.
- Components supplied by feeders.
- Computer files: computer program controls location of each component on the PWB (X, Y and angular theta), feeder inventory levels, placement machine vacuum holder capability, automatic component realignment, placement accuracy, vision systems, and transportation of PCBs through the line.
Placement process
Basic placement sequence generally includes: board indexing, board registration, fiducial vision alignment, component pick-up, component centering/vision inspection, component placement and board indexing. Component pick-up, component centering/vision inspection, component placement are repeated for each component. Sometimes, adhesive dispensing and on-line electrical verification are also included in the sequence.
Through the process of board indexing, the stencil-printed PWB is loaded to the appropriate position. Fiducial marks, also known as fiducial markers, provide common measurable points for all steps in the assembly process. There are many types of fiducials. Global fiducials are used to locate the position of all features on an individual printed circuit board. When multiple boards are processed as a panel, the global fiducials may also be referred to as panel fiducials if used to locate the circuits from the panel datum. Local fiducials are used to locate the position of an individual land pattern or component that may require more precise location, such as a 0.02 in (0.51 mm) pitch QFP.
Board is located by identify global fiducials on the PWB. Then the feeders pick up and center the components at a known distance from the component. Higher placement accuracy requires help from local fiducials visualized by optical or laser sensors. Vacuum pickup head removes components from feeders. In the end, the component is placed at the correct X, Y and theta location with all leads ion the correct pads in contact with solder paste. The PWBs with all components correctly placed will then move to the reflow process.
There are three primary attributes that shall be considered in the component placement system: accuracy, speed and flexibility. Accuracy involves the aspects of resolution, placement accuracy and repeatability. Speed involves the aspects of equipment placement rate, de-rating strategy and production through-put. Placement rate is determined by machine type and the distance between components on a board. Flexibility involves the aspects of component variety, number of feeders and PCB size range.
Types of pick and placement machines
A pick and placement machine is a robotic style machine that places some variety of types of components. It includes features such as: component pickup feeder locations, vacuum pickup, vision system, automatic component realignment, repeatable placement accuracy, and transportation system for PCBs.
The pick and place machine is often the most important piece of manufacturing equipment for placing components reliably and accurately enough to meet throughput requirements in a cost-effective manner. Typically, surface mount pick and place equipment, including a full complement of feeders constitute about 50% of the total capital investment required for a medium volume surface mount manufacturing line.
