The Legend of the New Marking Inks Standard Print E-mail
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Written by John Perry   
Sunday, 30 November 2008 19:00

Released in May, IPC-4781 resolves longstanding test issues.

StandardsLegend and marking inks, marking pens, and ink stamp products are integral parts of PCB fabrication, yet industry has lacked a standard for the manufacture and testing of these products, some of which are known to cause long-term reliability issues.

For years, the need for such a specification was raised in Solder Mask Performance Task Group meetings. Although solder masks and legend inks are typically intended for different purposes, some board fabricators use legend ink in place of solder mask. These inks are not subjected to the rigorous testing conducted on solder masks, which must have specific electrical properties because they bridge electrical conductors. In addition, solder masks are qualified for flammability performance as a permanent coating with Underwriters Laboratory, while legend inks typically are not. However, the breadth of any such standard for legend and marking ink qualification was beyond the scope of the task group, which was chartered to maintain the IPC-SM-840 solder mask qualification specification.

The urgency for a standard intensified as printed board geometries changed and continued to shrink. When spaces between conductive patterns were large, there weren’t many problems caused by ink coming in contact with the circuits. With smaller spaces, the ink could easily encroach on the electrical connections, causing failures. In 2004, these issues came to a head and the IPC Legend Inks Task Group was formed. And in May, IPC-4781, Qualification and Performance Specification of Permanent, Semi-Permanent and Temporary Legend and/or Marking Inks, became a reality.

IPC-4781 defines the criteria for, and method of, obtaining the maximum information about and confidence in legend and marking ink material under evaluation. This specification covers the four ink types (applications):

Type 1: Permanent legend and marking ink with some areas of direct metal contact between electrical nodes.

Type 2: Permanent legend and marking ink without direct metal contact (e.g., ink over solder mask).

Type 3: Semi-permanent legend and marking ink.

Type 4: Temporary legend and marking ink.

Recognizing the types of inks covered is significant because there is a tendency to mark printed boards with whatever marking tool is readily available, thereby raising failure rates. For example, certain color markers, such as yellow, have high ionic content, creating electromigration and, potentially, field failures. Other chemicals cause corrosion and low insulation resistance.

As with most standards, the key concerns were the properties to specify for testing and the requisite parameters. The standard had to balance what would be considered a fair amount of testing to levy against the material manufacturer with the kinds of tests that should be performed by the material user. Material compatibility was another item. The specification was intended to cover all products used for permanent, semi-permanent and temporary applications, not just legend ink products. As new technology developed, such as inkjet legend inks, the text was modified to include the latest available information, right up to the standard’s adoption.

One of the key aspects of IPC-4781 is that it is performance-based. Performance criteria are spelled out, as are the different types of legend inks, which heretofore was lacking. It gives some definition to terms loosely used in regard to legend inks. These definitions are important, as much confusion has arisen over language issues.

The heart of IPC-4781 is Table 3-1. IPC-TM-650, Test Methods Manual, is referenced for testing and measuring performance in a broad range of characteristics. No specific chemistry or application method is called out or excluded; any that meet the requirements are approved for use. Material compatibility of all inks is documented. Of major significance is assignment of responsibility between vendor and user for each specified formulation or variation of legend or marking ink material. In some cases, a designation of AABUS (as agreed between user and supplier) is given, ensuring these issues are defined in the user/supplier contract. A sample of the top of the chart, physical aspects, can be seen in Table 1. In this table, the left side defines the requirement and the right side specifies the materials and the tests to be performed. Other aspects found in the complete version of the chart include chemical resistance, electrical requirements, environmental requirements and materials. Table 2 features the top of Table 3-2, depicting who is responsible for each test and qualification.

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Military adoption? The significant issue to be decided is whether the US military will adopt the new specification. Three government specifications (MIL-I-43553, A-A-56032 and MIS-37271) predate IPC-4781, but in our opinion do not meet industry needs.

IPC-4781 will be useful for the commercial sector, but for companies that fabricate boards for the DoD, military requirements dominate legend and marking ink selection. It is hoped the Defense Department will adopt IPC-4781.

IPC-4781 mirrors other IPC specifications in terms of material qualification and conformance, such as IPC-SM-840 and IPC-CC-830 for conformal coatings, so compliance should be user friendly.

Acknowledgments
Doug Pauls of Rockwell Collins, Henry Sanftleben of Delphi Electronics & Safety and Dave Vaughan of Taiyo America contributed to this column.

John Perry is technical projects manager at IPC (ipc.org); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Last Updated on Tuesday, 02 December 2008 05:19
 

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