Assembly: Where Speed and Flexibility Meet
December 31, 1969 |Estimated reading time: 11 minutes
In any conversation concerning SMT assembly, achieving speed and flexibility simultaneously will almost always be cited. It’s no wonder why this area of technology is characterized by continued growth and change. This article focuses on the ability to handle a wide range of end-product assembly variations while maintaining high utilization and economic output.
By Scott Gerhart
The concept of speed in SMT assembly needs little introduction. Most equipment suppliers specify speed in terms of thousands of components per hour (cph), while most SMT assemblers think in terms of completed assemblies per hour. More sophisticated measurements consider process yield and take into account the effects of print and reflow process variables, as well as pick-and-place precision. Speed is easy to understand and measure. On the other hand, the term “flexibility” can be harder to pin down. This article discusses the ability to handle a wide range of end-product assembly variations while maintaining high utilization and economic output.
A flexible line can build three variants of cell phones for profit today, and build a high-end server board tomorrow. While this may seem like a strange proposition to an OEM, it is completely normal in the world of EMS and original design manufacture (ODM) assembly, where customer loyalty can be measured in pennies.
In component placement, achieving both speed and flexibility reliably and consistently has proven to be a challenge for manufacturers and equipment designers. The result typically has been to opt for speed, at the expense of flexibility. This is natural because speed is easy to understand, engineer, market, and sell. Building-in flexibility requires thinking about what will come next. It isn’t easy to predict the future, so we tend to revert to what we know. However, as more assemblers look to diversify customer portfolios, this shortsighted approach has led to unintended consequences.
Does Speed Mean the Same to Everyone?
Pioneering vendors in the assembly-equipment arena were quick to identify the “speed with flexibility” issue, and began developing various strategies to meet the changing needs of assemblers. These first logical steps centered on improving the speed of an inherently flexible line configuration, or introducing flexibility to a line known for fast throughput. But how can you judge speed improvements related to placement? While a basic measure of throughput per machine speed is cph, it also can be viewed as the throughput of the placement section of the production line. Perhaps it is better expressed by the ability to change from one product to another efficiently, while maintaining high utilization of all equipment in the line. In this case, throughput should be measured across a range of products coming off the end of the line. That’s not simply throughput, but also productivity. With any interpretation, everyone’s perception is different, but these factors may affect the choices an assembler must make in pursuit of productivity.
Characterizing Flexibility
Flexibility means being able to handle a range of end-product assembly variations while maintaining high utilization and economic output. But there are other flexibility considerations that can be a hedge against technology obsolescence. The current leading-edge, but increasingly popular, package-on-package (PoP) assembly process demands sufficient flexibility from a placement head to position a high-density logic device, dip the top-mating memory package in flux, and return precisely to the original position to place it on top of the first device (Figure 1). Is your assembly solution flexible enough to accommodate these demands, or more importantly, to cope with what the next generation of mobile technologies brings?
Figure 1. Package-on-package (PoP) assembly process puts demands on the placement head.
Flexibility can also be determined as the ability to handle a wide range of SMT component technologies - be it leaded, bumped, chip-scale packages (CSPs), or pin-in-paste - or to handle any lot size from small to several hundreds of thousands, maintaining high utilization of all equipment modules in the line (Figure 2). There also is the type of flexibility that allows new products to be introduced into manufacturing at a small scale - defining and testing new components, boards, and processes - and ramping to volume production rapidly. Flexibility is crucial for short batch runs and frequent product changes, where downtime for changeover is more critical to overall productivity than assembly line throughput during operation.
Figure 2. Flexibility means being able to place a range of components on a variety of boards.
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Changing Needs
With the advent of mass customization, needs converge. Even stable products are not immune to the requirements of different geographic sectors, which dictate small but important changes to the base product that must be met during manufacture. Emerging consumer markets also demand better technologies quickly, leading to shorter product life cycles. OEMs and their manufacturing partners who cannot rush a new product, or an enhanced variant of an existing product, will miss the boat as competitors carve into their share.
What assemblers need is maximum throughput speed at line-level while they are running, plus the flexibility to rapidly implement frequent product changeovers. In addition to SMT assembly capability, the implications of this extend to process and logistic procedures with issues such as broader component inventories, board-size changes, line-balancing between machines, software upload, and variant control.
Right-sizing Your Line
Selecting the right assembly equipment is part of the answer. To deliver optimum gain, a line- or factory-wide perspective of productivity proves more beneficial than simply considering individual machine throughput. Right-sizing lines, the manufacturing model in which a small line is configured with sufficient capacity for each particular build, can be advantageous. This process is sometimes called a fixed-family setup. A multi-cell approach comprising many small lines together can make sense by reducing the likelihood of product changeovers affecting factory-wide productivity. As a manufacturing philosophy, this contrasts with a monolithic line that turns out product at high speeds for a short time, before enduring a comparatively long changeover routine.
Smaller cells (lines) can be used to produce lower volumes without changeover, or used in parallel to produce higher volumes of a particular product. One benefit is maintaining higher utilization by enabling changeover of a particular cell while keeping the other cells productive. This approach wastes less time in product setup and tear-down. From a capital-expenditure perspective, the only disadvantage of a multi-cell approach is that you need extra infrastructure in the form of printers, conveyors, and ovens to ensure that each line is supplied with product. Falling somewhere between the two are modular platform lines configured to accommodate a number of products and variations. These will feature two, three, or four gantry-style platform placement machines (essentially identical, but with potentially differing capabilities) with enough on-machine component inventory between them to deliver a diverse range of device types. This eliminates the need to change feeders, reels, or trays to supply components for a new product coming on the line. This is a typical manifestation of a highly flexible line, known for its ability to handle a range of parts and balance the workload between machines. While immensely flexible, accurate, reliable, and productive, these types of lines are not known for high speed.
Avoiding Slow-down
What is it that causes a placement machine to slow down? Component size is probably the most obvious reason - the larger the component, the higher the inertial and momentum forces when traversing the board on the spindle of a placement head - the more likely the component is to move on the nozzle due to excessive acceleration, deceleration, or directional changes. On placement machines that use a moving table, such as conventional turret chipshooters, the same principles apply even after the component has been placed on the board. A slight movement of a fine-pitch, multi-leaded device can cause lead-to-pad misalignment, which could render the board useless after reflow.
The latest generation of gantry-style platform machines that feature fixed tables offer significantly higher confidence in this regard, routinely achieving full-speed operation, even with components up to 30 × 30 mm. Equipment design plays its part. For example, the possible cantilever effects caused by driving gantries from only one end can also lead to de-rating of placement speed because time must be allowed for oscillation to settle between motion actuations before the component can be placed accurately. Therefore, operators must drive it from both ends simultaneously.
Similarly, nozzle design can help. A large QFP part benefits from a larger nozzle, holding it securely during the motion phase between pick-and-place operations. But you wouldn’t want to use that nozzle for a 0201 device. How many different nozzle types do you need? More importantly, what is the capability of the placement head in terms of component range, and how much flexibility exists with the nozzle selection?
Ultimate Speed Gains
How can speed be improved? There is no doubt that linear motors are the key to achieving the highest speed ratings while ensuring precision placement. They offer levels of motion performance that cannot be matched by ball-screw technology. The basis for most installed placement equipment was established on earlier ball-screw technology, which presently limits maximum throughput. However, all placement equipment manufacturers accept that linear motors are the future, and it seems that they are incorporating these motors into their latest generation or high-end equipment designs.
Figure 3. Internal view of placement head technology.
Maximizing the number of spindles in the rotary placement head is another way to improve speed (Figure 3). Every time you have to move the placement head, you are not actually placing parts, so the goal is to reduce the number of long-motion arcs to improve throughput. The more spindles (nozzles) you have, the more components you can load up during the pick cycle over feeders, and the more you can place on the board before having to return to the feeder bank. The current maximum available on one head is 30 spindles with resulting placement rates in the domain of turret chipshooters. However, the number of spindles is not the only factor that can affect throughput. The range of component sizes that can be picked and taken to the board by any given placement head can also affect throughput. Many placement systems using mini-turret-style heads will need to bypass individual spindles once a part exceeds dimensions of 3-5 mm2. The result is more trips to the board based on part size or, in some cases, a requirement for different heads. In such instances, having more spindles can limit speed over a broad mix of parts where handling range is not extensive enough for the board.
Head Technologies
Central to the high-speed, fully flexible concept is the placement head itself. Inline spindles rely on gang-picking, but the loss of this ability where small components are involved can reduce throughput by as much as 25%. Rotary heads overcome this and reduce the potential for mis-picking - combining the adaptability of a turret-head to pick small components reliably at high speeds with the placement accuracy of an inline head. A multi-nozzle radial head equipped with a single Z-drive mechanism can deliver a cost-effective, fast, and accurate solution capable of placing components from small 0201s and 01005s up to large, fine-pitch QFPs.
A decision must also be made regarding rotary head type. If changing the component mix also means changing the radial head to suit, the user must be prepared for a considerable investment of time in recalibrating the head - even if this is performed off the machine. This means that many users will choose not to change the head, and will instead run the machine or line in a non-ideal configuration, consequentially reducing its speed. The result is a bottleneck in the line that leads to poor asset utilization. An alternative is to specify a system that allows all heads to handle all component types, controlling the changeover of component mix using software.
Designing the Ideal Solution
Having established what can slow component-placement operations, identifying how higher speeds can be achieved, and attempting to define the major facets of flexibility, how do we relax the boundaries between traditional high-speed placement and flexible fine-pitch to create the ideal equipment solution? As a basis, the modular-platform concept that has been adopted by most manufacturers and endorsed by OEMs, ODMs, contract electronic manufacturers (CEMs), and EMS providers, is a starting point to ensure scalability and future-proofing. Module functionality is designed to overlap to assist line-balancing - new functionality can be added easily, and new machines can be added in the line as required to boost productivity. A key differentiator to add speed while maintaining flexibility is placement head technology.
Figure 4. Linear motors are key to achieving the highest speed ratings while ensuring precision placement.
A fixed-table system will eliminate reasons for slowing a machine down from its potential maximum. Linear motors will deliver sustainable speed, accuracy, and repeatability (Figure 4). Using dual motors driving each end of the gantry may also eliminate possible cantilever effects, and any associated settle delay or reduction in motor speed and acceleration. They also require less maintenance; so reduced scheduled downtime is an additional benefit to factory-wide productivity.
Conclusion
When defining what flexibility means, it becomes apparent that an equipment designer’s remit must also be flexible if we are to fully recognize and serve the requirements of the end-user. It also is clear that in the quest to deliver speed and flexibility, equipment designers and vendors coming at the challenge from a background of inherent flexibility are ahead of the curve. They are finding decisive ways to add speed - not simply as throughput at machine level, but as productivity at the end of the line and across the factory.
Scott Gerhart, director, Genesis Platform, Universal Instruments, may be contacted via e-mail: gerhart@uic.com.