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STEP 6: Component Placement
December 31, 1969 |Estimated reading time: 8 minutes
With a focus on 01005 components, a discussion of placing 0201s may seem outdated at first. But this is far from the truth. It is only now that more versatile, high-volume 0201 placement techniques are becoming a reality. This article examines the worst-case scenario of 0201 component placement.
By Scott Gerhart
The principles described in this article apply equally to any type of component being placed, but will examine the worst-case scenario of placing 0201s, as their small form factor presents special challenges for pick-and-place equipment. An 0201 component typically measures 0.6 × 0.3 × 0.2 mm), and has placement tolerances down to ±12 μm - leaving little room for positional errors on a PCB. Most placement equipment vendors claim to be accomplished in 0201 placement capability, but in the real world of electronics production, this assertion may be subject to significant limitations. Some machines must operate at reduced speeds to place 0201s, while others can place these components within a restricted area, usually in the center of the head’s placement field. For the machine owner, implications include reduced productivity as a result of slowing down the line, or the inability to populate particular assemblies, such as large backplanes. Some limitations in placement speed and accuracy can be attributed to the characteristics of the machine’s motion-control system. This may be due to partial mapping and correction of the system - often exacerbated by a lack of direct linear encoding along the motion axis - or by the method of actuator-hardware integration with the gantry-motion system.
Benefits of Linear Motion
Most machine vendors providing 0201 placement capability use either linear motors in their drive systems already, or are migrating to their use. To enhance the accuracy of the linear drive, they also may mount a linear encoder directly on the motion axis to provide closed-loop positional control. Linear actuation can deliver higher accuracy and repeatability over a larger area, but other factors, such as heat generation within the motor, are also at work. With some designs, this can be an issue: heat leads to thermal expansion, and expansion causes relative positional inaccuracy. Such inaccuracies require recalibration, a time-consuming procedure that impedes productivity.
The accuracy required to deliver consistently high 0201 placement yields depends not only on the stability and repeatability of selected motion technology, but also on the way that it is integrated into the design. For example, some manufacturers use dual-linear drives on the Y-axis gantry, driving it from both ends to eliminate possible cantilever effects that can occur when driving only one end, and relying on a bearing slide at the other. For large-board applications, this is especially beneficial for accuracy and to eliminate oscillation-settlement time between actuations.
Impact of Placement Speed De-rate
If only one end of the Y-axis is driven, the component cannot be placed until the magnitude of these oscillations has decayed to less than the maximum placement tolerance. This depends on component and pad dimensions. For 0201 components, this settle band can be about half of what can be tolerated during 0402 placement. Placement must either be delayed for an additional period until the oscillations decay to within the settle band for an 0201, or the motor speed and rate of deceleration must be reduced while reaching placement position. Either of these de-rating requirements increases component placement time - affecting throughput. Placement head technology is another contributor to de-rate. Rotary heads, picking parts at a single point, have an advantage over inline spindle systems that must take into account the mechanical offsets inherent in their designs (Figure 1).
Figure 1. Rotary heads that pick parts at a single point have an advantage over inline spindle systems.
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The Case for Rotary Placement Heads
To achieve competitive per-component placement times, inline spindles rely on gang picking. However, this ability is lost when smaller parts must be picked from the feeder accurately. With 0201s, which are sufficiently small, so that reliable picking depends on perfect alignment of the nozzle and component center, as much as 25% throughput may be lost to the inability of an inline spindle to gang-pick. Rotary-turret heads are not afflicted by this limitation, although this advantage is offset by the high cost of the multiple sensors required to achieve on-the-fly pick-height adjustment, as well as adaptive touchdown and placement. Mis-picking induces further delays in the pick-and-place process. Potential causes for this include poor component location in the feeder tape, or poor X/Y head positioning when picking the component from its detected location in the tape.
Controlling Z-height During Component Picking
Another cause of mis-picking is poor Z-height control. The height of the feeder is unlikely to change, but small differences in feeder-tape thickness, component thickness, and the offset between the top of the tape and the upper face of the component that it contains, may occur. These seemingly minor variations are sufficient to deceive a pick mechanism that cannot detect and adapt to changes in the optimum Z-height for picking. An intelligent system with the ability to automatically learn, constantly monitor, and adjust to varying pick height is essential.
Controlling Z-height During Component Placement
After achieving a successful pick, the challenges for Z-height control continue with component placement. Small variations in board height that would not affect larger component placement can prevent successful 0201 placement, the height of which is typically 0.15-0.25 mm. Other factors, such as board warpage caused by reflow of underside components, can also induce Z-height changes that are significant to 0201 placement on the top of the board. If board height is lower than expected, it is possible that the component may be released above the solder paste deposit, rather than being placed into the paste - risking a poor solder joint after reflow. Conversely, if board height is higher than expected, placement may occur with excessive force, potentially damaging the component. Displacement of the solder paste deposit from the pad site is also likely. This may not only impair electrical and mechanical performance of the reflowed solder joint, but may cause other faults, such as bridging or solder balling on the board due to solder paste dispersal around the pad.
Equipment designers have developed several solutions for the correct calculation of Z-height. Those based on dead-reckoning techniques do not perform any actual sensing of board height, but calculate Z-height for each placement on the assumption that the board is flat, and that component-thickness data are correct and consistent. While this may prove satisfactory for larger components, correct Z-height calculation for 0201 placement is a challenge, and commonly gives rise to a missing part defects on machines.
Closed-loop Z-height adjustments can be achieved using a light source and optical-scanning encoder positioned at either side of the board, an arrangement that allows accurate calculation of placement height to within a few microns. Unfortunately, this solution cannot be applied effectively to pick-height adjustment, so it solves only half of the pick-and-place problem. Another design approach is monitoring the current flow through the placement head’s Z-axis, although this requires a larger impact force to enable the system to detect the change on current draw that indicates touchdown. A more effective method is to implement on-head sensing; for example, using a strain gauge that will provide adaptive Z-height calculations for both pick-and-place operations. A strain gauge provides a continuously variable output proportional to nozzle force, enabling real-time control of the nozzle’s Z-position. While this gives more responsive and adaptive height control, implementation costs for each nozzle of a turret head is considerable.
Radial Flexibility
A radial head combines the adaptability of the turret-head to pick 0201s reliably at high speeds, and the flexible placement accuracy of an inline head - supporting a wide component range. A multi-nozzle radial head equipped with a single Z-drive mechanism can deliver a cost-effective, fast, and accurate solution for placing 0201 components (or smaller), up to large, fine-pitch quad-flat pack (QFP) semiconductor devices.
One manufacturer’s radial head, for example, has 30 nozzles and achieves chip-shooter placement rates, but implements continuous Z-height monitoring and optimization with a single Z-drive and sensing mechanism that can adjust to pick and board height.
Shorten the Vacuum Path
Vacuum-generation and reversal techniques used to pick-and-place 0201s are at their most responsive when using a short vacuum path and high-speed valves, reducing hysteresis effects. This principle is not restricted to small components, but the low height of an 0201 results in close proximity of the nozzle with feeder tape and board surfaces. Experience indicates that on touchdown of an 0201 component, even a small static charge can be sufficient to prevent the device separating from the nozzle. A responsive, fast-acting vacuum path aids separation by allowing the vacuum to be reversed momentarily. This applies a short-duration separation force, while fast-acting valves prevent solder paste displacement during separation.
Conclusion
Given the small dimensions of 0201 components, the fact that they can be placed at all using conventional pick-and-place motion control is impressive. For the 0201 form factor to deliver its full potential, further evolution is needed to achieve speed and yields equivalent to larger and more-established outlines. Assemblers also must be able to place 0201s anywhere on a board confidently. Only then will it be possible to bring the potential savings of the 0201 footprint to a wider range of applications, including premium-value assemblies. At this point, manufacturing with 0201s will become a less-demanding process; and increasing demand is bound to drive up production volumes. However, even then it would be wise not to assume that all pick-and-place machines can cope with small form factors. To secure the speed and yields they will need to remain competitive, board assemblers should take careful scrutiny of machine capabilities.
Scott Gerhart, Genesis platform director, Universal Instruments, may be contacted at (607) 779-7205; e-mail: gerharts@uic.com.