Legend ink
14 June 2010
After the application of solder resist as part of the board manufacturing process legend is applied by screen printing and then a curing operation.
Legend can be applied as a flood process then imaged and developed, and more recently there has been a growth in the use of direct write legend markings using jet printing. The legend, or nomenclature as it is also known, is in the form of letters and numbers which aid the assembly and identification of component positions during inspection and test.
Traditionally the ink used is white in colour as it is the best contrast on a green solder mask, however some design teams have used other colours. Yellow has been used to indicate a product that contains static sensitive devices. To aid identification, some companies also changed both the solder mask and the legend colour when a product was being produced with lead-free solder as opposed to tin/lead.
As a basic design guide, so as to ensure legibility, the legend width used should not be any smaller than 0.175mm for screen printing and 0.125mm for ink jet printing. The smaller the font the more likely that 0, 6, 8, and 9 numerals will fill with ink and cause unnecessary cosmetic rejection of the product. The separation from copper lands of solder mask apertures should be no less than 0.25mm. However this is a printed board manufacturing process so should be confirmed with the board supplier.
The minimum use of component legend is recommended due to the inaccuracy of the printing process and the variation of the printed ink thickness. In most cases, the device part number, orientation or pin one locator are all that is required. You don’t need a box around each part; where there are sequential numbers in a row just mark the first and last parts. They may be etched into the surface of the copper during PCB manufacture which eliminates the ink printing process and reduces the board cost. Alternatively they could be located in the resist surface in a position not covering copper circuitry.
All previous subjects can be seen on the Jargon Buster section of www.EMTWorldWide.com.
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