Circuits Assembly - December 2008 - (Page 40) Standards The Legend of the New Marking Inks Standard Released in May, IPC-4781 resolves longstanding test issues. Type 2: Permanent legend and marking ink without egend and marking inks, marking pens, and ink direct metal contact (e.g., ink over solder mask). stamp products are integral parts of PCB fabricaType 3: Semi-permanent legend and marking ink. tion, yet industry has lacked a standard for the Type 4: Temporary legend and marking ink. manufacture and testing of these products, some of which Recognizing the types of inks covered is significant are known to cause long-term reliability issues. because there is a tendency to mark printed boards For years, the need for such a specification was raised in with whatever marking tool is readily available, thereby Solder Mask Performance Task Group meetings. Although raising failure rates. For example, certain color marksolder masks and legend inks are typically intended for ers, such as yellow, have high ionic content, creating different purposes, some board fabricators use legend ink electromigration and, potentially, field failures. Other in place of solder mask. These inks are not subjected to the chemicals cause corrosion and low insulation resisrigorous testing conducted on solder masks, which must tance. have specific electrical properties because they bridge elecAs with most standards, the key concerns were the trical conductors. In addition, solder masks are qualified properties to specify for testing and the requisite paramfor flammability performance as a permanent coating with eters. The standard had to balance what would be conUnderwriters Laboratory, while legend inks typically are sidered a fair amount of testing to levy against the matenot. However, the breadth of any such standard for legend rial manufacturer with the kinds of tests that should be and marking ink qualification was beyond the scope of performed by the material user. Material compatibility the task group, which was chartered to maintain the IPCwas another item. The specification was intended to SM-840 solder mask qualification specification. cover all products used for permanent, semi-permanent The urgency for a standard intensified as printed board and temporary applications, not just legend ink prodgeometries changed and continued to shrink. When spaces ucts. As new technology developed, such as inkjet legend between conductive patterns were large, there weren’t inks, the text was modified to include the latest available many problems caused by ink coming in contact with the information, right up to the standard’s adoption. circuits. With smaller spaces, the ink could easily encroach One of the key aspects of IPC-4781 is that it is perforon the electrical connections, causing failures. In 2004, mance-based. Performance criteria are spelled out, as are these issues came to a head and the IPC Legend Inks Task the different types of legend inks, which heretofore was Group was formed. And in May, IPC-4781, Qualification lacking. It gives some definition to terms loosely used in and Performance Specification of Permanent, Semi-Permanent and Temporary Legend and/or Marking Table 1. IPC-4781, Table 3-1, Requirements of Qualification Inks, became a reality. Requirement for Marking Type Testing and Inspection IPC-4781 defines Type 1 Permanent the criteria for, and Permanent No Type 3 IPCType 4 Other Test Requirement Direct Electrical SemiParagraph TM-650 method of, obtaining Temporary Methods Metal Metal Permanent Method the maximum inforContact Contact mation about and conPhysical ASTM fidence in legend and D3363, marking ink material Hardness Yes Yes Yes No 3.5.1 None Scratch Hardness under evaluation. This Test specification covers the Adhesion to four ink types (appliSolder Mask Yes Yes Yes AABUS* 3.5.2.1 2.4.28.1 – Materials cations): Conformal Type 1: Permanent Coating AABUS AABUS AABUS No – None AABUS Adhesion legend and marking Flammability Yes Yes Yes No 3.6.3 None UL94 ink with some areas FTIR Yes Yes Yes Yes 3.2.3.1 None – of direct metal conEnvironmental tact between electrical AABUS AABUS AABUS AABUS 3.12 None – Compliance nodes. *AABUS: As agreed between user and supplier. Circuits Assembly DECEMBER 2008 circuitsassembly.com L John Perry is technical projects manager at IPC (ipc.org); johnperry@ipc.org. 40 http://www.ipc.org http://www.circuitsassembly.com
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