Lightning adds flexibility to placement
07 March 2006
This appears to be the age of the modular placement platform, yet the use of new head technology can further increase the flexibility of this equipment.
The concept of the modular placement machine allows manufacturers to reconfigure a line or facility to optimise for a different set of products requiring different schedules and different ranges of small and large components. However, no matter how quickly this can be performed, reconfiguration consumes valuable time, demands knowledge and skills, and introduces opportunities for inefficiencies and errors. In addition, a selection of modules, such as placement heads, supporting all device types or techniques likely to be used, must be kept in stock. This alone is expensive, but even more modules - such as placement heads - may be required, in order to ensure sufficient resources for multiple lines.
A selection of head types is required, because designing a head suitable for placing large components such as fine-pitch BGAs or QFPs has traditionally demanded a different set of solutions compared to placing very small passives such as 0402 and 0201 chips. The parameters for picking large components from the feeder are very different from those for picking sub-miniature chip passives, for example, and alignment and placement performance requirements are also substantially different.
Minimise Changeovers
One alternative to frequent changing of heads is to fit different heads to several machines in the line, to ensure that the line as a whole is capable of placing all component types. However, scheduling to efficiently distribute the build across each of the machines can become difficult and time consuming. Calculating the optimum loading on each machine for each product can also be a demanding task.
Emerging motion control technologies, motors, miniature vacuum components, as well as opto-electronics and machine vision systems, now enable one head that can place virtually any surface mount component in common use or expected to enter the market within the foreseeable future. The Lightning head significantly increases utilisation by eliminating the majority of lengthy changeovers. It also relieves the complexity of scheduling and optimisation.
Key challenges to implementing such a universally-suited placement head include achieving a highly adaptive pick and place mechanism to accommodate component and substrate input variations typically seen in normal electronic assembly production environments. These variations, if not accounted for, will result in unacceptable placement defects and high pick PPM ultimately leading to poor product yield and scrap costs.
To create a placement head capable of overcoming these challenges and thereby virtually eliminating head changes, Universal Instruments has pioneered several new techniques including adopting precision servo drives in the spindle drive assemblies of its multi-nozzle Lightning head.
Multi-Spindle Efficiency
For instance, the Lightning placement head design incorporates a high precision Z-axis servo drive with patented closed loop feedback using an encoder and strain gage. This informs the placement system precisely when the spindle tip contacts the component in the feeder during pick, and again when the component contacts the PCB during placement.
The machine automatically teaches the height of each feeder at the start of production and during feeder replenishment/replacement to ensure the spindle is driven to the optimal height for pick. This minimises the impact on the component during pick. In turn, this minimises impact forces and maximises nozzle life. The machine automatically adapts to variations in components as well as tape and reel input, and ensures consistent pick performance.
To maintain both pick and place performance of small components, the nozzle must be aligned accurately to the component during pick. This is important to sustain an adequate vacuum hold force on the component in order to prevent the part from moving on the nozzle. The Lightning head system incorporates an auto-feeder teach function for small discrete components (0402, 0201, 01005) that uses the machine's downward looking vision camera to find the centre of the component pocket in the feeder prior to picking from it. This is performed at production startup and during feeder replenishment, and ensures that the nozzle tip is aligned automatically to the component for the very first pick. After the first pick, an auto pick update feature is invoked that constantly adjusts the pick position for each feeder and nozzle combination based on the real time component finds.
These two features together ensure that the nozzle is accurately aligned to the component during pick and automatically adjusts for tape input variation.
In addition, proprietary valves allow up to ten times faster switching of the vacuum for pick up and placement of the component. This allows placement of components such as CSPs, WSP, æBGA and MELFs at full speed, and combines with patented individual generation of vacuum inside each of the spindles - featuring a very short and low maintenance vacuum path - to enable practically maintenance free running of the head over long periods.
Multi-Resolution Vision
The machine vision capabilities of the Lightning head are fundamental to its ability to accurately place components with larger edge lengths. Although the camera must have a large field of view, high resolution is not so critical. On the other hand, placing small passives, especially 0201s and 01005s, demands high resolution while the field of view can be far smaller. The obvious solution to implementing dual capability on a single head is to provide a pair of cameras, each displaying the optimal combination of field of view and resolution to meet these divergent requirements. For Lightning, Universal leveraged the latest generation of digital camera technology and high-speed image recognition software to implement a compact dual resolution imaging solution, using FireWire for high-speed image data transfer.
By combining all of these technologies into one multi-spindle placement head design, Lightning is capable of placing package types ranging from 01005 SMDs up to large semiconductor devices with edge lengths as long as 30 x 30 mm without impact on the pick and place cycle time.
High-Speed, Low Maintenance
These technologies incorporated in and around the Lightning placement head have shown it is possible to combine wide-ranging component inventories on a single machine and thereby gain benefits including higher utilisation and easier scheduling. With 30 spindles, the design potentially offers high-speed placement capability in addition to the flexibility to place fine-pitch and SMD components.
One of the barriers to achieving an effective radial-spindle configuration, however, has been the challenge of developing a motion control system capable of providing sufficient acceleration, positioning precision and short settle time to make such a scheme possible. Lightning incorporates a rotary, variable reluctance motor (VR-Motor), called the "Phi-Drive", which is enabled by patented Universal Instruments technology and VR-Motor control IP. Compared to a typical servomotor, VR-Motors can maintain higher torque and efficiency at significantly higher duty cycles.
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