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SMT 101 Step 6 - Component Placement
December 31, 1969 |Estimated reading time: 9 minutes
By Matthew J. Tiberia
This step outlines several key issues necessary for understanding component placement using automated assembly equipment, including positioning, imaging, feeding and flexibility. It can serve as a primer or quick reference for anyone faced with specific component or application challenges, identifying the pros and cons of alternative approaches embodied in the types of equipment selected as part of an overall solution.
Electronics product manufacturers are striving to keep pace in an ever-evolving marketplace. Issues that impact day-to-day operations - processes, packaging, manufacturing, quality and, of course, products - are in a constant state of flux. And while it may be a stretch to expect automated placement equipment to alleviate these burdens, manufacturers do expect equipment that is flexible enough to assist in accommodating the market's changing demands.
Figure 1. Overhead gantry-style machines can be used for placing leaded QFPs and BGAs.
To meet this particular need for flexibility, an increasing number of assembly machines, specifically those designed for component placement, are built on a common platform, or module, that supports a series of different process requirements. Almost every leading supplier of semiconductor placement solutions markets some type of "modular" placement system to help their customers stay in step with, and even ahead of, the pace of change.
Yet even as modular placement solutions increase in popularity, the "traditional" production line - a high-speed placement machine followed by a fine-pitch placement machine - still holds a strong position on many manufacturing floors. This configuration is used for any number of reasons, from the need to maximize capital equipment expenditures already made to the need to cover inherent gaps between the capabilities of the different types of placement machines.
Deciding which equipment best suits a particular component placement application may be simplified by taking the following considerations into account.
Positioning
There are three classifications of component placement equipment: the overhead gantry-style (sometimes called Cartesian) system, the turret system, and the massively parallel system. Each positioning system presents benefits and drawbacks, depending on the application or process in which it is used, and there is generally a trade-off between speed and accuracy.
Overhead gantry-style systems offer greater flexibility and accuracy across a broad component range, but cannot match the speeds of the turret or massively parallel systems (Figure 1). As the component range becomes more concentrated on active devices, such as leaded quad flat packs (QFP), and area-array components, such as ball grid arrays (BGA), placement accuracy becomes even more critical to achieving higher yields. The turret and the massively parallel systems are usually not used for these types of components.
Overhead gantry-style positioning systems employ X- and Y-axis beams to move a placement "head" (mounted to the X-axis beam) to a specific location over a printed circuit board (PCB) that is clamped into a fixed position prior to placement. This placement head moves along the axis beams to pick components from a feeder, and then moves into position to place the components.
Because the axis beams' position and movement can control the component placement accuracy to 50 mm (0.05 mm) and below, the placement accuracy achieved by overhead gantry-style machines is the best of the three classes.
Figure 2. The turret style machine's system of identical rotating heads can reach speeds up to 50,000 cph.
Turret systems achieve their relatively higher speeds due to a series of identical rotating heads (Figure 2). Components are placed as each head reaches a fixed point above the placement location. Banks of moving feeders deliver components to these heads, which extend and retract to pick up the components and place them as the heads move over the board. The PCB moves under the rotating heads, pausing beneath the correct placement location to allow the component to be placed.
Massively parallel systems use a series of small, individual placement sections. Each section has its own lead-screw positioning system robot with a camera and placement head attached. Each placement head accesses a limited number of tape feeders and populates portions of multiple boards, which are indexed through the machine at fixed intervals. Individually, the robots perform slowly. However, their sequential or parallel operation results in high throughput.
The turret and massively parallel systems are examples of high-speed placement systems often used for small component placement. Turret systems, for example, are widely referred to as chipshooters, first for the component they most commonly place and second for the machine's rapid "shooting" ability. Because passive components, or chips, and other small leaded devices do not demand as great a placement accuracy as fine-pitch and area array-packages, chipshooters and massively parallel systems can reach remarkable throughput.
Typically, the greatest accuracy is available with an overhead gantry-style system, with placement speeds in the range of 5,000 to 20,000 components per hour (cph). Higher placement speeds can be reached with a turret system, generally running in the 20,000 to 50,000 cph range. And the fastest placement can be achieved with a massively parallel system, capable of reaching a range of 50,000 to 100,000 cph.
Imaging
A second feature of automated assembly equipment that impacts component placement is machine vision. Accuracy in automated component placement is the result of letting the machine know exactly where the PCB is and how the component is aligned in relation to the board. This is accomplished through vision systems, which generally are classified as downward-looking, upward-looking, on-the-head or laser-aligned, depending on the location or camera.
Downward-looking cameras look for marks on the PCB called fiducials and are used primarily for registering the PCB to a correct position prior to component placement. Upward-looking cameras are used to inspect components from a fixed location, so the part must be moved over the camera for visual processing prior to placement. Chipshooters do this as the component is moving around the turret.
Overhead gantry-style machines usually move the component over a separate camera station as part of the placement routine. At first glance, this may seem time consuming. However, the head must travel to a feeder to collect components anyway, so if the camera is positioned between the pick position (from the feeder) and the placement location (on the board), image acquisition and processing may be performed during travel, essentially making this a seamless part of the operation.
Recognition time generally increases as the components become larger or more complex. To offset this additional time, placement equipment can incorporate two upward-looking cameras - one on each side of the machine - to minimize travel distances.
The upward-looking cameras on chipshooters can determine accuracy within approximately 100 µm (0.1 mm). While this is very good, it may not be good enough to maintain higher yields for products using leaded and area-array components. The upward-looking cameras used on overhead gantry-style machines tend to have more sophisticated image processing capabilities and are able to determine placement accuracy to an even tougher standard.
Some equipment suppliers are building vision capabilities directly on the placement heads of overhead gantry-style systems for some smaller devices. This is often referred to as on-the-head imaging. Component imaging and adjustment are then performed while the head travels to its placement position. While it does not match the speeds of the turret systems, it can reach speeds adequate for many applications.
Laser-aligned vision is significantly different in comparison to the other vision systems. A laser light is shined from a transmitter to a receptor, where a spindle essentially "dips" the component into the laser's beam. What makes this system so different is that the receptor is not looking for an image of the component itself, but rather the pattern of light that is broken - the component's shadow - as the component passes through it.
From this shadow, the component is rotated to the correct alignment. This is a reliable system for parts up to 10 mm, but mostly is ineffective for larger parts and area-array components because of an inherent inability to ascertain the necessary detail level to process these package types. Area-array components, for example, use solder balls on their underside to complete the electrical connection with the PCB. Solder balls are not visible to a laser-aligned vision system because of their location.
Depending on the accuracy requirements of a given application, machine vision capabilities may be more important than ever.
Feeding
Overhead gantry-style machines can support different feeder types, including bulk, tape-and-reel, tray, tube, odd-form and other custom designs. In contrast, high-speed turret and massively parallel systems are entirely fed by bulk cassette or tape-and-reel packaging.
When components are packaged in a form other than bulk or tape-and-reel, the overhead gantry-style machine may be the only alternative: high-speed machines are eliminated from consideration because they cannot feed these components automatically.
From a logistical standpoint, manufacturers should look for component placement equipment that expands their available placement options. For instance, some manufacturers' equipment may require feeders designed specifically for a particular machine, limiting the use of the feeder.
This feeder limitation requires resources devoted to tracking and logging which feeders can handle which components. With feeders dedicated to a particular machine, storage of replacement or idle feeders can take up significant floor space. And in an age where operations are measured using metrics such as throughput per unit of floor space, potential manufacturing space used to house items peripheral to component placement or other board-critical functions is seen as wasteful. This is why modular systems are gaining in popularity. A more efficient approach to component placement and product manufacturing is promoted when items like feeders can be used with similar machines.
Again, however, it pays to fully examine and understand the numbers and component types that may be used in a given PCB design. While the manufacture of electronics products may shift to modular assembly lines, it is important to remember that the traditional configuration - a high-speed placement machine followed by a fine-pitch placement machine - is still a choice for some high-chip-count applications, such as cellular phones and computer motherboards.
Flexibility
Beyond "micro" flexibility in positioning, imaging and feeding components, "macro" flexibility in adjusting manufacturing volume and accommodating product changeover has become key to making decisions about new assembly equipment.
High-volume lines that are expected to perform at maximum throughput ad infinitum may be best achieved with a chipshooter, a massively parallel system or both. High product changeover environments, on the other hand, may be better served with modular lines made up of similar, if not identical, overhead gantry-style machines. This configuration provides the flexibility to adapt to a volatile market.
Making the Choice
It should be evident that there is more to automating electronics assembly than just buying a given machine or even a line of equipment. Indeed, the automated assembly equipment itself is just part of a total solution that also includes technical expertise, systems integration and product support.
But as a starting point, successfully automating component placement hinges on understanding several key equipment issues: component positioning, imaging, feeding, and micro and macro flexibility. With this basic understanding, anyone faced with specific component or application challenges can make more intelligent decisions in identifying the pros and cons of alternative approaches embodied in the different types of equipment.
MATTHEW J. TIBERIA is surface mount technology analyst for Universal Instruments Corp., P.O. Box 825, Binghamton, NY 13902-0825; (607) 779-5819; Fax: (607) 779-7968; E-mail: tiberia@uic.com.