Single-digit defect rates
Published on 6-Jan-2009
Veldhoven, April 25th
To reach single-digit defect rates in SMD assembly, the Pick & Place cycle must be as close as possible to a closed loop process. Defects can include misplaced, on-edge, extra or missing components. For defect-free placement, the pick-up heads should continuously monitor whether components are present on the nozzle tip from pick right through to place. They should also give extremely accurate placement, and placement force control.
Conventional machine design
It is the machine design that defines whether the Pick & Place process is a closed loop. Multiple nozzle heads often share a single camera system, either using vision recognition as a stop-and-go action, or on the fly by checking the nozzle positions one by one. Both before and after camera alignment, a component’s presence is then monitored only by vacuum sensors (when present, and when activated).
Today’s components are getting smaller, though, and so the reliability of vacuum detection is decreasing. Nozzle tips need to have similar or smaller size than the pick-up surface of small 0201 and 01005 (0.4 x 0.2 mm) capacitors and resistors, meaning that the vacuum air flow through these nozzles is limited. So, the force holding the component is reduced and the acceleration forces become more critical. In addition, pollution from paper tape or solder paste particles can and will influence the vacuum sensor reading.
That affects the reliability of the component presence check during component transport from the pick to the final placement coordinates. The process time becomes very limited for high placement rates, because the available sensors and camera systems have to be shared.
To reduce the number of missing components, some machine suppliers offer a presence check sensor as an additional option. On multi nozzle heads, though, this sensor can only be located at the pick and placement positions. When moving components from pick to camera alignment to placement, only the vacuum sensor can monitor the component presence. If a missing component is detected just before placement, it is uncertain where the component has been dropped. For some machine designs, the optional presence sensor also reduces the output and so increases placement costs.
A-Series has built in reliability
The Assembléon A-Series machines use inherently more reliable component presence check and placement control. Since the AX-501 and AX-301 robots only pick one component at a time, all process sensors are available to this single component.
Every AX robot comes standard with both a vacuum sensor and a laser system. This combination makes any re-pick action as short as possible because the non-presence detection is performed directly above the pick position. That means no extra servo movements are needed to start a re-pick action. Component alignment is performed on the fly by rotating the component in the laser beam, and the same laser sensor monitors the component presence from pick to place. After placement, the laser and vacuum sensor can check the absence of the component on the nozzle tip again to ensure the component has been placed on the circuit.
Parallel placement process gives robots more time per component
The parallel machine concept of Assembléon’s A-Series machines allows each robot to devote much more process time to each individual Pick & Place action. The output of 165,000 components per hour comes from the parallel process of multiple A-Series robots, instead of the fast individual servo movements required on multi nozzle heads.
The combination of a longer process time with vacuum and laser presence checks on the A-Series robots creates a complete closed-loop presence check process over the total Pick & Place cycle. Each robot can place a component with an accuracy of 40 micron @ Cpk>1.00, and each component placement is controlled by the impact and static force control process. These sensors and processes are available as standard and don’t slow down the placement cycle.
All this combines to give the Assembléon A-Series machines the lowest cost per placement, combined with best-in-class placement defects.
