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MARCH 2020 I PCB007 MAGAZINE 65 External Compliance Requirements Because the solder mask becomes a perma- nent part of the PCB dur- ing its service life, it is re- quired to meet a number of external specifications on performance. UL 94 UL94 is a flammability specification by U.S.- based Underwriters Laboratories. UL94 ap- plies, in some form or another, to all electri- cal and electronic devices sold in the USA. For solder mask, UL94 specifies that the solder mask should not increase the flammability of the bare circuit board laminate by more than a permitted level, as measured under specif- ic test conditions. Materials are rated V2, V1, or V0, with V0 being the highest-performing (least flammable) material. Achieving a V0 grade can be more difficult for an inkjet solder mask—particularly on thin laminates—because of the lack of fillers. These can help to inhibit flammability, and their absence may require the incorporation of flame retardants in the ink to achieve it. The range of flame retardants suitable for use in solder mask inks is quite restricted, and patents exist to protect their use in this ap- plication. ROHS Compliant ROHS compliance requires that the solder mask contain none of the heavy metals listed in the standard. This is usually not a problem, and products are easily certified to be compli- ant by an external test laboratory. IPC SM-840E IPC SM-840 is an "industry standard" collec- tion of solder mask performance and property- related tests covering temperature, chemical and electrical resistance, adhesion, etc. NASA Outgassing Compliance with NASA specification SP-R- 0022A/ASTM E 595 is required for polymeric materials—like solder mask—going into space. The specification requires <1% loss of weight over 24 hours at 125°C in a vacuum. Automotive Standards There are many automotive specifications, but some of the three most important, covering thermal cycling/storage of solder mask materi- als are shown in Table 1. Test pieces are cycled between low and high temperatures for a spec- ified number of cycles and are then examined. Test pieces should not exhibit any cracks, blis- tering, or loss of adhesion. Once again, the lack of filler in inkjet solder masks, which helps to prevent crack propa- gation, makes these specifications more chal- lenging. Next Time In the second part of this article, I will dis- cuss the advantages of inkjet solder mask pro- cesses and present testing data. References 1. G.H. McKinley & M. Renardy, "Wolfgang von Ohnes- orge," Physics of Fluids, 23, 127101, 2011. 2. B. Derby, "Inkjet Printing of Functional and Structural Materials: Fluid Property Requirements, Feature Stability, and Resolution," Annual Review of Materials Research, 2010. Chris Wall is technical director and general manager for Electra Poymers Ltd. Wall is responsible for all aspects of the day-to-day operations of the company and for development activities including equipment, materials and processes. Table 1: Specification and test condition comparisons between two manufacturers.

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