Taking it to the next level
BY SCOTT LUTZOW
The tape-and-reel process is typically the final step in semiconductor packaging, and it can substantially affect yield and impact a manufacturer's bottom line. Tape-and-reel technology has stabilized in recent years, however, and system performance has reached a peak. As global demand drives continuous growth in semiconductor device production, many manufacturers require more speed and functionality from their tape-and-reel equipment to keep pace with high-volume manufacturing operations. To meet this demand, recently developed tape-and-reel systems are offering improvements in performance, reliability and cost-efficiency.
The Packaging Standard
The packaging process for electronic components varies with package type, as shown in Figure 1, but all electronic components must be packaged into some form of transfer media before they can be sent for assembly into a final product. Almost all leaded, non-leaded and grid-array devices can be packaged safely and reliably via tape-and-reel, and printed circuit board (PCB) assemblers readily use reels of components on their placement machines. As a result, tape-and-reel technology has become the most common means of processing, handling, storing and transporting semiconductor devices. Other packaging methods, such as tube- or tray-based media, are less common because they are not as easy for PCB assemblers to use.
Figure 1. Configuration of a typical packaging line, with the tape-and-reel process as part of the final packing step.
In addition to packaging devices, tape-and-reel equipment is a useful tool for quality assurance and quality control (QA/QC). As part of the tape-and-reel process, devices can be visually inspected for mark integrity and pin-one orientation, and most systems also perform three-dimensional lead analysis. Following inspection, rejected devices are sorted out while good components are packaged into tape.
As the semiconductor market has matured, tape-and-reel technology has stabilized, with many system suppliers taking similar approaches to the process. The most common tape-and-reel systems are gravity-fed and based on pick-and-place technology. They include some degree of vision inspection and, occasionally, some basic electrical testing for QA/QC. Such systems have remained largely unchanged since the early 1990s and have been shown to be effective for a wide range of packaging applications. However, the fundamental design of these machines creates inherent thresholds of performance and reliability that limit overall efficiency and have a negative impact on cost-of-ownership, particularly for manufacturers who rely on tape-and-reel for high-throughput, high-volume device packaging.
Figure 2. Diagram showing forces acting on a device using a traditional pick-and-place transfer method.
Traditional tape-and-reel systems process common small outline (SO) package types using one of two pick-and-place methods for device transfer – methods initially developed by the first two suppliers of tape-and-reel equipment. The first method, typically found on medium-throughput tape-and-reel systems, employs a single-arm pick-and-place process. The device is removed from an infeed track by a vacuum head mounted on a single swing arm and immediately placed into carrier tape (Figure 2). The second method relies on a more complex rotary pick-and-place mechanism and is often used with higher-throughput tape-and-reel systems. In this method, the device is removed from the infeed track by a single head mounted on a rotary turret. The turret then moves the device to subsequent stations, where inspection nests and tape-and-reel modules reside. These rotary systems can have anywhere from 4 to 32 positions in the rotation cycle, depending upon the number of inspections or checkpoints required. Often, every other position is used as a verification station to detect dropped devices.
Both of these methods have become widely accepted because they have offered satisfactory results. They fulfill the basic requirements of the user by inspecting components and transferring them from input media to tape-and-reel, usually without damaging them in the process. However, in response to increasing production demands, a greater emphasis has been placed recently on developing tape-and-reel systems with higher cycle rates in an effort to increase production throughput. Unfortunately, these developments have yielded few benefits for manufacturers because typical single-head pick-and-place systems become unstable at rates of more than 6,500 units per hour (uph).
Figure 3. Inside view of the tooling track and taping mechanism on a tape-and-reel machine.
Traditional tape-and-reel systems suffer device jams at a rate of one in every 1,000 to 2,000 devices. When these systems are pushed beyond 6,5000 uph, reliability suffers dramatically because these jams cause the tape-and-reel system to go down every 15 to 20 minutes, thus requiring the constant attention of an operator. Furthermore, vision systems that perform adequately at normal speeds can be overburdened by these severely reduced cycle times, introducing a greater incidence of over-rejection (good parts declared bad) and under-rejection (bad parts declared good). The results are a decrease in overall yields and an increased number of potentially bad parts shipped to customers.
In addition, higher speeds can magnify unsafe handling characteristics of both pick-and-place designs. During the pick-and-place operation, these systems subject each device to pressure, as well as torsional forces such as inertia and skid or shear force (Figure 2). These forces can cause the device to become misaligned on the pick-and-place nozzle. When the misaligned device is pushed into the tape pocket, the fragile leads are usually severely damaged and a useless component is sealed into the reel. Camera-over-tape vision systems or 100-percent human inspection are often required to compensate for the increased incidence of bent leads, and detaping entire reels to remove bad product is often the only solution.
In an effort to overcome the limitations of traditional tape-and-reel equipment for high-speed, high-volume manufacturing, next-generation systems are being introduced. To reliably attain throughput levels of 9,000 uph or more (a desirable rate for high-volume production), many of these systems are configured to handle specific families of devices. While retooling is possible with these systems, frequent changeover between different device types can reduce overall performance. These next-generation systems take advantage of new device handling methods and advancements in vision inspection technology to achieve higher speeds than traditional systems while improving overall yields and significantly reducing system downtime.
One of the most common forms of device damage, bent leads, is a major concern among device manufacturers and their customers. One of the first next-generation tape-and-reel systems to be introduced features an innovative method of transferring devices into carrier tape, shown in Figures 3 and 4, which was specifically designed to eliminate bent leads. In this method, device transfer begins after the vision inspection process has verified the integrity of the device. The device is then immediately singulated to the transfer area, where a single-pivot swing arm head rests above. Vacuum is applied, which engages the device to the transfer head, applying no pressure to the top of the device. Once the device's position is verified, the base of the tooling track slides away, opening a direct path to the carrier tape below. The swing arm then lowers the device into carrier tape, moving in only one direction.
In contrast to traditional pick-and-place systems, this method eliminates all opposing torsional forces and their effects on device positioning. The system does not work to counteract the force of gravity, but rather uses gravity to assist the process. The resulting placement into carrier tape is more accurate and repeatable, eliminating damage to fragile device leads and avoiding costly system jams.
This new type of device transfer is suited to handle most two-sided components. While traditional pick-and-place technology was designed for large plastic leaded chip carrier (PLCC) and small-outline integrated circuit (SOIC) packages, this method was developed to safely handle fragile thin shrink small outline packages (TSSOPs). The handler applies no pressure to the topside of the device, eliminating the possibility of invisible micro-cracks in the die or mold compound that pick-and-place systems can introduce. Additionally, the smooth, controlled movement of the swing arm and servo-controlled tape indexing help ensure the integrity of the device during placement into carrier tape.
Improvements in Vision Inspection
Next-generation tape-and-reel systems can also feature high-performance vision inspection similar to that of dedicated inspection systems. Traditional camera-based vision systems, with accuracy and repeatability of 10 to 12 microns, cannot always meet the inspection requirements of today's packages. With the latest high-resolution vision technology, improved optics and imaging software, next-generation systems can reliably inspect devices with an accuracy of 7.5 microns and repeatability of 7.5 microns at 3 sigma. This high performance guarantees gage repeatability and reproducibility that never exceeds 10 percent.
Figure 4. A tape-and-reel machine with a new device transfer method.
Over-rejection has the largest, most immediate impact on a tape-and-reel system's cost-of-ownership. The accuracy and repeatability of vision systems have rarely been important selection criteria for tape-and-reel equipment, but high-performance visual inspection enables significant reductions in over-rejection compared to previous-generation systems. Over-rejection results from guardbanding, a common practice in which the inherent measurement uncertainty of the vision system is subtracted from the device rejection threshold.
The impact of guardbanding on system yields is easy to understand. While the measurements of the majority of inspected devices will be less than the rejection threshold, a portion of the measured devices will encroach upon that threshold. The accuracy and repeatability of the vision inspection process will determine how conservatively the guardbanding levels must be set. When large guardbands are applied to accommodate less accurate and repeatable vision systems, more potentially good devices are rejected. The improved measurement accuracy and repeatability of advanced vision systems permit less guardbanding, minimizing over-rejection and allowing more known good devices to be processed to tape-and-reel.
Tape-and-reel is a widely accepted standard for packaging electronic components for safe transfer to customers for final assembly. As electronics production continues its rapid growth and more devices are packaged into tape-and-reel media, the performance of tape-and-reel equipment is becoming more vital to the manufacturing bottom line. High-end, subcontract and commodity manufacturers, in particular, require higher levels of performance than traditional tape-and-reel systems can accommodate. While traditional systems can perform sufficiently at lower speeds, they become unstable, unreliable and inefficient as throughput requirements increase. To meet the demand for reliable, high-volume tape-and-reel solutions, equipment suppliers have developed next-generation systems with a number of key technological advancements.
Innovative device-transfer techniques and leading-edge vision systems enable these next-generation systems to offer increased throughput, more accurate and repeatable inspection, and significantly reduced downtime over previous systems, reducing cost-of-ownership. Furthermore, some systems take advantage of a dedicated platform design to further enhance performance, while maintaining enough versatility to accommodate changeovers and allow conversion to meet future product requirements.
Scott Lutzow, product manager, can be contacted at RVSI Systemation, 5400 S. Westridge Drive, New Berlin, WI 53151; 262-860-7700; Fax: 262-760-7725; E-mail: firstname.lastname@example.org.