UL is abbreviated as Underwriter Laboratories lnc. and is one of the most trusted qualified assessment provider by many manufacturers.
To printed circuit boards, the standard are UL 796, specifical PCB standard, and UL 94, which are used for flammability test of all plastics. These specify many performance tests to measure the long-term reliability and fire safety of PCBs. UL logo, manufacturer logo, and circuit boards types will be marked if the PCB follows these standards.
The UL certification mark is the RU symbol, which is very common to see on the circuit boards.
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As long as the safety (especially inflammability) is the key issue, the UL mark is required. For Europen OEM, any boards of the equipment that shipped into the United States need the UL mark. The requirements can be set by OEM or the ultimate customer.
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How to add the UL mark on my circuit boards?
Click "Additional Options" on the page of " PCB Instant Quote -> PCB Manufacturing ", Find "UL Marking" in the expaneded list of special requirements, and choose where you want to add the UL mark.