The Underwriters Laboratory (UL) was set up around 120 years ago in the USA as an independent facility to test the safety of new products and new technologies. Today it has a network of sites around the world focused on product safety. It test products, certifies manufacturers, and produces and updates safety standards across a broad range of industrial and commercial sectors.
For printed circuit boards the main standards are UL 796, the specific PCB standard, and UL 94 for flammability testing of all plastics. These specify a number of performance tests to measure the long-term reliability and fire safety of the PCB. If a board is released to these standards, it is marked with the Underwriters Laboratory logo, the manufacturers logo and the board type.
When is the Underwriters Laboratory Marking Used?
UL marking is required whenever safety, especially flammability, is a critical issue. For European OEM’s (original equipment manufacturers) it is often required on any boards that go into equipment that will be imported into the USA. The requirement may be set by the OEM or by the end customer.
What does Underwriters Laboratory Marking Mean?
The base material meets the specified flammability level laid down in UL94. For FR4 the level required is UL94 V0. This means that when a vertical sample of the material is introduced into the test flame and then removed, it will self-extinguish within 10 seconds and will not drip flaming particles.
For multilayer boards we use IS400 Mid Tg 150°C (this Tg is higher than standard FR4 to ensure full compatibility with lead-free soldering).
For all high Tg (170° – 180° C) requirements we use Isola PCL-FR4-370-HR.
For one and two layer boards we use IS400 Tg 150°C or Nan Ya NP-155F Tg 150°C
The base material meets the specified level for ability to resist ignition from electrical sources. For the values see E41625 and E98983.
The base material meets the specified electrical breakdown (tracking) or Comparative Tracking Index (CTI) value. This is the voltage difference at which the insulation properties of the base material may break down, causing safety and performance issues. The Isola and Nan Ya FR4 materials meet the requirements of Class 3 (175 – 249 V).
The base material meets the performance profile levels specified for direct support of current carrying PCBs (DSR). These are specified in UL Standard ANSI/U/796A.
The boards meet the other specifications set out in the table under UL marking in PCB proto, STANDARD pool and DEFINED IMPEDANCE pool below.