Providing a Critical Link
BY DAVID BERNARD
Potential growth in radio frequency identification (RFID) applications is limitless; constrained in the near-term only by the cost of the RFID tags themselves. Any improvements in the production yield of high-volume RFID tags will be beneficial. Innovation in the assembly process of RFID tags abounds, with manufacturers using conventional die-bonding technology as well as newer innovations to reduce chip-attachment cost and increase assembly process speed. Some of these newer interconnection technologies include the use of conductive ink, integrated chip technologies, and fluidic assembly using either non-conductive adhesives or anisotropic conductive adhesives.
Manufacturing Efficiency
Reducing RFID manufacturing costs and implementing efficient flip chip assembly processes to meet the target cost of high volume end-users, such as Wal-Mart, agribusiness, and the Department of Defense, is the goal of RFID tag manufacturers. To meet these demands for RFID, and the potential use of ultra-high frequency (UHF) passive RFID tags, manufacturers have developed patented and proprietary methods for embedding a flip chip onto a plastic substrate or inlay that functions as a transponder antenna.
Some manufacturers build their RFID inlays using a conventional flip chip assembly process; precisely placing the flip chip on the antenna substrate using robotic assembly and conventional die-bonding technology. Other RFID manufacturers use a printable process, whereby alternate layers of conductive, insulating, and semiconductor materials are deposited on a substrate to form an electronic circuit, including the antenna.
To ensure reliable fabrication of an UHF passive RFID tag, there must be assurance of proper orientation and placement of the flip chip on the embossed inlay; complete interconnection of flip chip pads to the antenna; and sufficient connection between flip chip pads to the antenna and intervening conductive layers.
The use of suitable flip chip assembly processing equipment presents a serious challenge in high-volume production of RFID tags. Issues of market pressure to lower manufacturing costs, combined with the use of new materials and new, potentially variable bonding technologies, can mitigate against production efficiency. One question is whether using X-ray inspection can assist in driving down the cost of RFID assembly and improve manufacturing yields. X-ray inspection can analyze the processes for best practice and contribute to production yield by developing methodologies to further the reliability and quality of RFID tag interconnections.
Inspection Methods
The integrity of the electrical connection from the flip chip pad to the antenna is a critical factor in the quality of an RFID tag. Being able to inspect the electrical connection when the conductive connection is made has a significant impact on overall yield, and the continuous improvement of the RFID assembly process.
Several inspection methods are available to monitor and control the quality of the RFID assembly process. Optical imaging detects planar objectivity of the flip chip and other surface layers, while 3-D interferometry can be used for advanced surface characterization of the inlay embossed sites. However, neither of these inspection technologies can detect delamination, cracks, or breaks in the buried inner layers of the completed RFID tag. This has lead to the use of X-ray inspection to monitor the RFID assembly process. With the influx and evolution of substrate and interconnect materials used, X-ray inspection provides one of the few inspection options currently available to reliably ensure that a high-quality product is delivered to the end-user.
 Figure 1. X-ray image of flip chip and antenna interconnection. |
The use of X-ray inspection assures proper orientation and alignment of the flip chip relative to the embossed inlay of the RFID antenna (Figure 1). X-ray imaging also verifies variations in the consistency – or voiding level – of the interconnections and/or attachments between the flip chip and the antenna. Current X-ray systems offer substantially enhanced image grayscale sensitivity, and the ability to make oblique angle views of tiny planar objects without compromising the available magnification. This means that X-ray inspection can image extremely small devices for quality and process control, and detect subtle variations in the sample and substrate density, even when very little material is present in the actual device.
 Figure 2. X-ray image of an RFID tag. |
Using X-ray inspection during the assembly process ensures that adequate contact and adhesion between dissimilar materials and adequate filling of via holes, for example, is achieved to provide good contact with the flip chip pads (Figure 2). As a result, detection of potential delamination, cracks, or breaks in the inner layers prevents conduction of the electronic signals, inadequate contact, and adhesion due to coefficient of thermal expansion (CTE) mismatch, and/or achieves inadequate filling of via holes.
Conclusion
Demand will stimulate growth as the vision for RFID applications and requirements expands exponentially. The driving force behind RFID growth is currently manufacturing cost vs. production yield of RFID tags. The present consensus is to reduce the manufacturing cost of RFID tags to below 5 cents or less. This figure includes the cost of the assembly line, depreciation, materials, and yield loss. With an abundance of companies conducting research on a wide variety of interconnection and inlay materials used in the manufacturing of RFID tags, X-ray inspection may provide a critical link that can detect defects during the assembly process.
X-ray inspection accommodates both the conventional methods for assembling RFID tags, as well as other evolving visionary methods as manufacturers refine methods to reduce manufacturing costs. This will be critical for military or defense requirements, where high-end RFID tags need to be reliable under extreme environments and usage. Integration of X-ray inspection may play an increasing role in the overall cost reduction and yield improvement of RFID tag assembly.
References
For a complete list of references, please contact the author.
DAVID BERNARD, product manager, X-ray systems, may be contacted at Dage Precision Industries, 4024 Clipper Court, Fremont, CA 94538; 510/683-3930; E-mail: [email protected].