CircuiTree - January 2009 - (Page 33) Neither type of barcode is reprogrammable, which is a major limitation for today’s flexible manufacturing systems. Manufacturers generally create multiple versions of a basic product that differ by only a few components. Often these components are installed after the device has been put in inventory or possibly even reworked from existing finished stock. In these cases, the barcode has to be manually replaced to indicate the new finished part number or serial number with little, if any, history saved for those units. Barcode throughput is limited by scanners that only read one label at a time. If multiple simultaneous reads are needed, multiple scanners must be installed, and even with inexpensive linear barcode scanners, multiple scanners represent an overhead and maintenance headache. tocol intended for high volume inventory control. It is now an ISO standard and operates in the 840 to 960 MHz frequency band worldwide. Large corporate retailers in North America, Europe, and Asia are using it to track products throughout their supply chain from their suppliers all the way to the store stock rooms. PCB manufacturers can benefit from the hardware and software developed for these retailers with readily available building blocks to incorporate into their manufacturing systems. Implementing RFID on PCBs Using RFID for tracking electronics is not new; earlier versions used low frequency (LF) and high frequency (HF) tags. Those trials either adhered a label-based tag to the PCB or etched an antenna and directly mounted the RFID chip. Unfortunately, both HF and LF require The need for high-quality manufacturing, outsourcing, flexibility, and legal requirements are pushing the traceability of manufacturing assemblies to the limit. There are other possible problems with barcodes that may or may not apply to a given application: oil, grease, or other chemicals on the label can render it unreadable; less physical durability means labels can be burnt or torn and become unreadable; and manual reading speeds are hindered if bar codes are difficult to find or at odd orientations. RFID and Gen2 RFID is a technology that allows the contactless electronic identification of goods. The RFID tags are essentially rewritable memory chips with a radio. Tags that use a battery to boost the distance they can communicate are called active tags. Passive tags harvest power from the reader’s energy field and do not require a battery. There are many varieties with various levels of sophistication, memory size, read range, and security. This application lends itself well to one of the newest types of passive tags, Gen2 UHF. The Gen 2 specification, which was finalized in late 2004, is a simple, low-cost pro- relatively large loop antennae, so those solutions did not reduce the real estate required; in fact, in many instances, they require more space than barcodes. And the benefits of those systems did not justify the cost. UHF tags, in contrast, only need a small dipole or slot antenna (see Figure 1). A simple dipole would look like a small copper trace approximately 3 in (75 mm) long. A slot antenna would be a slim void, in the ground planes approximately 1.5 in (35 mm). The design of the antenna is flexible and can meander around other features on the board. It is even possible to design the antenna to use the entire area of the PCB to collect energy from the reader field. Historically, RFID solutions also lacked a suitable package type that was compatible with surface mount manufacturing. To keep the device size as small and the cost as low as possible, RFID makers typically use bare die assembled with flip chip techniques. This is impractical because the vast majority of manufacturing lines do not incorporate flip chip equipment. The advent of a small, surface mount package that is compatible with conventional pick-and-place assembly lines is crucial. The surface mount technology package for the UHF die can be less than 1.5 x 1 mm (60 x 40 mil), which is similar in size to small ceramic resistors and capacitors, ideal for high density electronics. A major advantage of RFID is that it does not require a line of sight to read the tags. As long as the PCB is in the reader field it can be read. This means that as long as a reader antenna is set above the assembly line with sufficient transmit power, the position and orientation of the RFID chip is not critical. And as long as the PCB is not in a shielded enclosure or package, the tag may be readable. Read ranges for Gen2 tags can be as long as 10 m (30 ft). If too much read range is an issue, as it can be in certain cases, then it is an easy matter to reduce the range by changing the tag antenna design. And the Gen 2 specification requires that tags are disabled with a kill command; this renders the tags permanently inoperative if a manufacturers desires. While 96 bits, or 12 bytes, of information may not sound like enough memory for encoding, it is actually quite flexible. This amount of memory was selected to provide enough numbering space to give billions of products their model number and serial number. Ninety-six bits provides a total of 296, or 79 x 1027, unique numbers. While manufacturers will typically have their own method of encoding tags, a simplistic assignment method could use 32 bits each for a model number, serial number, and a set of manufacturing codes. Each 32-bit segment would have approximately 4.3 billion unique numbers available. The flexibility this memory offers can be extremely useful for contract manufacturers (CMs) as they face special problems trying to interface with dozens or hundreds of disparate customer systems and methods. In their own factories, they can use the encoding system that works best for them and then simply reprogram the tags when shipping to their OEM customers. If a CM has a customer that does not want RFID-enabled tracking, the device can either be killed or mounted to the scrap portion of the PCB and taken off before shipping. One available manufacturing line flow allows information to be programmed at various steps and logged in the manufacturing circuitree.com • January 2009 33 http://www.circuitree.com
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