By Robert Bertz, RVSI Systemation, New Berlin, Wis.
The recent rise in the popularity of leadless packages, such as the Leadless Chip Carrier (LCC), the Bumped Chip Carrier (BCC) and the Quad Flat No-Lead package (QFN) has gained the attention of package inspection equipment suppliers. Although still in its infancy, the leadless package appears to be the clear successor to current surface mount technology (SMT) for small packages. With a small footprint and an extremely thin profile, the leadless package is based on traditional wire bonding and leadframe processes, enabling an extremely attractive cost structure and price parity with small outline packages. By eliminating external leads, the package footprint can be decreased by nearly half. In addition, the exposed die attach pad can be soldered directly to the circuit board to provide enhanced thermal performance. The leadless package is very well suited to devices with less than 100 I/O, particularly in the cost-conscious, consumer market. Leadless Advantages The advantages of leadless devices are many, but along with the new technology come new challenges-primarily in the areas of media transfer and inspection. As the final stage in semiconductor manufacturing, the tape-and-reel process can substantially affect yield and significantly impact a manufacturer's bottom line. Yet, in recent years, tape-and-reel technology has stabilized while system performance has reached a peak. As global demand drives continuous growth in semiconductor device production, and new packages-such as leadless ones-are adopted, many manufacturers require more speed and functionality from their tape-and-reel equipment. To meet this demand, recently developed tape-and-reel systems are offering new performance and inspection capabilities. Transfer Media Before electronic components can be sent for final assembly, they must be packaged into some form of transfer media. Almost all leaded, non-leaded and grid-array devices can be packaged safely and reliably on tape-and-reel. (PWB assemblers readily use reels of components on their placement machines.) As a result, tape-and-reel has become the most common means of processing, handling, storing and transporting semiconductor devices. Tape-and-reel is the standard across many industry segments, including high-end manufacturing, contract assembly, commodity and fabless manufacturing. Other packaging methods, such as tube- or tray-based media, are less common and not as easily employed by PWB assemblers. Screening for Cosmetic Defects Suppliers of leadless packages see a need for back-end inspection between electrical test and packaging. Their customers are typically assembly houses that are employing increasing quantities of leadless devices. Cautious with new technology, assemblers are demanding that even cosmetic defects be screened. Many suppliers have responded by providing either 100 percent or sampling inspection performed by inspectors armed with microscopes. The large volumes of leadless components make it difficult to maintain consistency. In addition to its use for 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. Most systems also perform three-dimensional lead analysis. Following inspection, rejected devices are sorted out while good components are packaged into tape. High-Performance Vision Inspection To further enhance performance, next-generation tape-and-reel systems feature high-performance vision inspection rivaling dedicated inspection systems of the recent past. Traditional camera-based vision systems, with accuracy and repeatability of 0.4 to 0.5 mils, cannot always meet the inspection requirements of today's packages. With the latest high-resolution vision technology, fail-safe optics and powerful imaging software, next-generation systems reliably inspect devices with an accuracy of 0.3 mils and with a repeatability of 0.3 mils at 3 sigma. This high performance guarantees GR&R (gage repeatability and reproducibility) that never exceeds 10 percent. As the volume of leadless devices climbs and their defects become understood, manufacturers are beginning to replace manual inspection with automated post-test inspection. This change is partially in response to quality demands from board assemblers. With the scaling down of components, less material protects the die, which focuses more attention on superficial surface damage. With leaded and bumped interconnects literally disappearing, other obstacles interfering with the flush mating of board-on-board must be considered. Saw-singulated devices are at risk of electrical shorting caused by copper smear. Punch singulated devices are prone to stress fractures resulting in microcracks or the more severe (but more easily detected) chip out. All leadless devices can suffer from defective or improperly dimensioned lands, or excess solder, resulting in electrical shorting. The accuracy and repeatability of vision systems have rarely been important selection criteria for tape-and-reel equipment, but high-performance vision has brought added benefits to the inspection process.
This caliber of vision inspection enables significant reductions in over-rejection compared to previous-generation systems. Over-rejection brings about the most immediate impact on a tape-and-reel system's cost-of-ownership. It results from guardbanding, a common practice in which the inherent measurement uncertainty of the vision system is subtracted from the device rejection threshold. By virtue of their superior measurement certainty, advanced vision systems permit less guardbanding. The impact of guardbanding on system yield is easily understood. 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, however, will determine how conservatively the guardbanding levels must be set. When large guardbands are applied to accommodate less accurate and less repeatable vision systems, more potentially good devices are rejected. 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 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 perform sufficiently well at lower volumes, they become unstable, unreliable and inefficient when asked to do more. 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. Increased Throughput Innovative device-transfer techniques and leading-edge vision systems enable these next-generation systems to offer increased throughput, more accurate inspection and significantly reduced downtime over previous systems. Additionally, some systems take advantage of a dedicated platform design to further enhance performance, while maintaining enough versatility to accommodate changeovers and allow easy conversion to future product requirements. Ultimately, by assuring increases in yield and overall product quality, next-generation tape-and-reel systems can provide substantial improvements in cost-of-ownership, offering manufacturers an efficient and safe vehicle to move their product packaging to the next higher level.
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