Choosing the Right Method
BY JAMES R. WILLIAMS
Companies mark products for varying reasons. Usually, marks are present for practical purposes, whether they’re serving a function, as in the case of relays and switches; indicating a safety issue or warning in compliance with legislated or mandated requests from customers like ESD, RoHS, WEEE, UL, CSA, and MIL-STD; are product identifiers, such as part numbers; performance-rating categories (ohms, farads, watts, MHz, etc); or are present for internal process or quality-related reasons (date code, batch code, inventory SKU, lot number, or tolerances). Promotional and advertising marks, such as a corporate logo, are included whenever possible. Whatever the objective, decisions to mark advanced packages are determined by many constituencies.
Choosing the appropriate marking technology depends on the amount of information there is to be printed, how much “real estate” is available, and how long and in what environments must the information be useful/legible. Figure 1 demonstrates a simplistic model. The amount of information required and the available real estate determine the critical metric of printing density in characters-per-inch (cpi).
 Figure 1. Printing density. |
In many circumstances, several rows of information are desired. The necessary maximum-print density is calculated using the row with the largest number of characters. For example, if the width of the area in the block example shown in Figure 2 is one inch, the printing density required would be 8 cpi to accommodate the longest character string-the last row. If a lower density was used, the bottom row would exceed the permissible boundaries of available space. This example is important because it dictates the choice of printing technologies.
 Figure 2. Printing density influenced by largest data string. |
The sizes of advanced packages vary; each possessing a different amount of room for product identification. Available marking areas may range from 0.094 × 0.280"-for small-outline integrated circuits (SOICs) or products such as a 150-mil, 14-pin dual in-line package (DIP)-to a 0.25 × 0.25" label for a 20- to 32-pin surface mount device (SMD), or up to 0.437 × 1.0" image for a 600-mil 24-pin DIP. For a given amount of information, the more area you have available for printing, the lower the required print density (Figure 3). The converse is true for lesser areas of available space.
 Figure 3. Flow chart demonstrates variables for deciding printing technology. |
Each specific printing technology has a maximum printing density limitation. This printing density is a function of how many dots (or pixels) per inch it can actually produce, coupled with its algorithms for character formation, including the use of barcodes, 2-D codes such as PDF417, or data-matrix code.
Whether or not the information is required to be “machine-readable” also influences printing technology choice. If “legibility” by a person is all that is necessary, low- resolution printing solutions are available to mark labels or products adequately. Typically, this type of printing is characterized as date codes, lot numbers, or part numbers with relatively small amounts of information, such as 6 or 7 characters.
 Figure 4. Printing technologies for identification/marking. |
Figure 4 shows an overview of printing technologies used today for marking components, parts, and subassemblies. Laser etching, ink-jet printing, and thermal-transfer label printing are the most commonly used marking methods.
Common Marking Technologies
Direct Marking: Ink Jet. Ink jet is a non-contact, non-abrasive alternative to product marking. Continuous ink-jet (CIJ) printers are widely used for industrial direct marking on products. They provide excellent throw distance; in many cases, up to 0.500" away from the target item. Print heights ranging from 0.030 to 0.500" in a single pass are achievable. CIJ printers can be used to mark flat, curved, and irregular-shaped surfaces. Print resolution is a primary disadvantage of CIJ printers. With a 36- to 42-µm orifice (or dot size), characters will range in height from 0.030 to 0.180".
Direct Marking: Laser Etch. All materials will, at room temperature, absorb, transmit, refract, and/or reflect light energy at a particular wavelength or range of wavelengths. The ability of a material to absorb light and react in some manner to that energy is the desired property for laser etching. For example, CO2 lasers excel at marking anodized aluminum, plastics, wood, glass, and ceramics. Ferrous metals can also be marked. A CO2 laser is a good choice when an engraved mark in plastic (without a color contrast) is desired. Selecting a laser-marking system involves choosing the proper wavelength for the material to be marked. The two major choices for most industrial applications are Nd:YAG (1064 nm) and CO2 (10.6 µm), each working best with an array of materials.
Marking speeds are difficult to quantify. The laser-beam delivery system must be physically capable of accommodating the part, and the focal point of the lens must be able to reach the required area without conflicting with the part itself.
Laser-marking systems are manufactured and operated under compliance with governmental standards. Manufacturers have to comply with CDRH (FDA) to determine the classification of the laser, either Class 1, 2, 3a, 3b, or 4 lasers. Users of the laser have to comply with ANSI Z136.1-2000.
 Figure 5. A typical 2-D (data matrix) label used today. |
Labeling: Thermal Transfer Printing. Using labels can be considered an “indirect” means of marking for SMDs and other packages. Thermal-transfer printers use a thin-film ink ribbon, which is placed in intimate contact with a suitable label material. This composite is passed under a print head comprised of a fixed array of staggered, resistive heating elements. With proper software, precise images are generated at high resolution of about 600 dpi (0.0167"). Thermal-transfer printing is thought to be the most widely used method for identification of circuit boards, components, and packages, especially whenever a high-density barcode is required.
Thermal-transfer printing of labels is a simple printing technology to implement. However, the label must still be affixed to the product, even though the mark is quickly completed on the label. Automatic-print-apply systems exist for such purposes, but, as in the case of both ink-jet and laser, require capital expenditure and fixtures to accomplish. In locations with low labor costs, labels are routinely applied manually. Label printing can be outsourced easily, whereas direct-marking technologies tend to be integrated into manufacturing process lines.
Successful marking of advanced packages requires image durability throughout the manufacturing process, regardless of the technology employed. Laser etching is thought to have the highest overall durability for abrasion and chemical resistance, although contrast of the printed area is not as good as the other two printing methods. Successful labeling requires proper matching of the ink ribbon with the label material used, as well as the adhesive required to adhere the label to the product surface. Ink-jet images exhibit the least resistance to chemicals and solvents, although UV- and thermal-curable inks show great promise.
Summary
There is no “silver bullet” for marking electronics products and packages. Laser etching, ink-jet printing, and thermal-transfer label printing each provide high print density, variable information printing, and durable images, which withstand the manufacturing environments encountered. However, each technology also requires methodical tradeoffs between specific strengths and weaknesses to optimize its respective cost/benefit.
JIM WILLIAMS, founder, chairman, and VP of research and development, may be contacted at Polyonics Inc., 867 Rte. 12, Westmoreland, NH 03467; 603/352-1415; E-mail: [email protected].