When you need to squeeze a bar coded label onto an already-packed circuit board,only diminutive 2-D symbols will do.

In the consumer-driven world of pocket computers and bookshelf stereo systems, it's hard to imagine design engineers as philosophers on a par with St. Thomas Aquinas. Yet engineers today are cramming more and more features into rapidly shrinking electronic products—thanks to multiple integrated chips. Finding unused space for extra items such as labels on the printed circuit boards (PCBs) inside today's electronic equipment is akin to determining the number of angels that can dance on the head of a pin, a classic theological problem. Fortunately, as major electronics manufacturers like Panasonic are discovering, two-dimensitonal (2-D) symbologies such as Data Matrix have transformed our ability to present considerable information in tight spaces.

The Old Days
Earlier generations of consumer electronics products incorporated traditional linear bar code labels or bar coded parts to help manufacturers track production, monitor quality, account for inventory, and match customers with valid warranties. But what about all these new miniature systems? Must manufacturers forgo the considerable benefits of AIDC (automatic identification and data capture) simply because they're short on real estate? Today's cell phones, for example, are packed with electronic components that claim almost all the space on a PCB, leaving barely an eighth of an inch square available for any kind of label. So what does a philosopher-engineer do when the government mandates serialized accounting of all cell phone production within its boundaries?

A Rational Approach
Such was the situation confronting Panasonic UK Ltd., a world supplier of cell phones. Bill Smyth, a senior engineer at Panasonic, was assigned the task of figuring out a solution to the labeling problem created by the British authorities' mandate. He summed up the ideal symbol this way: "[as] small as possible, yet able to contain ten alphanumeric characters, and be read every time, first time, so that the production line [is not] disrupted." Mr. Smyth revisited bar code layouts, common small label sizes (such as 0.5 by 0.2 inch), and widely used symbologies (such as Code 39 and Code 128) and confirmed that this PCB application's requirements outdistanced the capabilities of what was available. How was he going to comply with the British government's demands by encoding up to ten characters in a 0.125-inch square? Panasonic had attempted in-house printing of traditional bar code labels, but determined that the perceived benefits did not meet the company's expectations. According to Mr. Smyth, "The labels were not of a quality that would allow a 100 percent first read rate, [and] numerous labels came off the PCB during the assembly process, which not only [caused us to lose] track of the PCBs but [also] clogged up the equipment's filters. There was also the added problem of duplicate numbers printed by the operators. We needed to have a better solution, as the planned increase in production would not tolerate these problems." Consequently, all bar coded parts were outsourced.

Panasonic's label supplier, Dernier Ltd., an AIDC products reseller, introduced samples of Data Matrix, one of more than 30 two-dimensional (2-D) symbologies currently available. The samples were prepared for Dernier by one of the company's principals, DATA2, a photocomposed bar code label manufacturer. Dernier recognized the value of this symbology's high density and worked with DATA2 to be production ready.

Currently, Data Matrix is one of the AIDC (automatic identification and data capture) industry's more accepted 2-D symbologies. It is supported by industry organizations such as AIM (the Automatic Identification Manufacturers association) and the EIA (the Electronics Industry Association) for marking silicon wafers and components. The Automotive Industry Action Group (AIAG) recommends Data Matrix for small parts identification; the Semiconductor Equipment Manufacturers Institute (SEMI) selected Data Matrix as its standard for wafer marking. Data Matrix can encode up to 2335 characters per symbol—400 to 500 characters per square inch.

One recognizable aspect of 2-D matrix symbologies is their checkerboard appearance and square shape. (See the accompanying sidebar, "Inside Data Matrix," for details.) In addition, each symbology includes a "finder code" that distinguishes it from the others and provides a decoding path for scanners.

Another major attribute of these symbologies is their ability to correct errors resulting from print image imperfections. An inherent benefit of the 2-D design is that it allows for accurate scanning despite partial image damage or poor print contrast—as poor as 2 percent, whereas linear bar codes require a print contrast of at least 75 percent for reliable scanning.

This attribute of 2-D symbology serves electronic equipment manufacturing applications particularly well, as temperatures encountered on production lines may reduce print contrast, rendering traditional bar codes difficult to read.

Although many printers are capable of producing Data Matrix, none could meet Panasonic's 0.125-inch square requirement for up to ten characters. Labels from Dernier and DATA2 fit the bill: a 0.125-inch square encoded with six numeric characters. The dimension of the checkerboard foundation, the "element," was 6 mils. Because of the extremely small element, these symbols have been referred to as "Micro Data Matrix," although existing standards do not currently list that name.

Photocomposition, a precise bar code printing technology based on computer-controlled photography, was used to generate the image. More characters could be encoded, if desired, within the same space by further reducing the element's size, a benefit of photocomposition. And, these miniature symbols could be easily read with a charge-coupled device (CCD) scanner or a 2-D camera. Panasonic tested Data Matrix compatible scanners from Welch Allyn and Label Vision Systems, confirming that the Micro Data Matrix label's 6-mil element scanned.

The bar coded labels were delivered in roll format, leaving the challenge of how to affix them to the PCB. The short-term solution was to apply them manually, with tweezers, thus enabling Panasonic to meet the British government's serialization mandate. Explained Mr. Smyth, "The use of tweezers was a common solution, as this method is already used with a number of other components. We intend to use manual methods for now until we find the time to look at automated methods. The task of [label] application is just an added step to an already existing position, so there is no increase in personnel."

Keep the Faith
According to Mr. Smyth, Panasonic intends "sometime into the future" to check out methods of applying these Micro Data Matrix labels automatically to cell phone PCBs—a challenge akin to pulling a camel through the eye of a needle. It may seem impossible, but the increasing miniaturization of bar coded labels and parts would appear to indicate that just about anything is achievable in this business—if you approach the problem rationally, of course.

Product Information:
LVS 3200 Data Sheet (requires Adobe Acrobat Reader).