Understanding The UL94 Vertical Burn Test

Background  

Underwriter’s Laboratories (UL) is a global safety certification company that performs testing on products across many industries to determine if they are fit-for-use. It is commonplace to see the UL label on consumer goods throughout the world, including on plastics used for electronics. Specifically, these materials are tested for flame retardancy – the ability to self-extinguish when exposed to fire. Flame retardant materials are becoming a critical component in modern circuitry as designers implement safeguards to ensure that fire will not spread in the case of ignition.

Testing Specifics

This test can be performed on either conformal coatings or plastics used in electronics.

When testing conformal coatings, strips of FR4 laminate (which contain bromine to make the strip itself flame retardant) are used as a substrate. Both sides of the strip are coated to a desired thickness, with most manufactures aiming to achieve flame retardancy over a thickness range (reported by UL as min. and max. coating thickness in microns).

For plastics, the strip itself is fabricated from the material being tested, with thickness ranging between 1.5 to 3.5 mm.

For this test, the substrate is clamped in the vertical position and set on fire using a 20 mm blue flame held at a 45° angle, 10 cm from the bottom end of the test strip. Cotton balls are placed 30 cm below the bottom end of the specimen to check for flaming particles that might drip from the test surface. The test is performed in two stages. First, the flame is held up to the specimen for 10 seconds, removed, and allowed to self-extinguish. The flame is then re-introduced to the specimen for an additional 10 seconds, then removed. Figure 1 (below) illustrates the test setup.

Figure 1: Illustration of stages in the UL94 Vertical Burn Test. Stage One comprises a 10-second ignition phase followed by flame removal until the material self-extinguishes, followed by Stage Two, a 10-second re-ignition phase.

Should the flame reach all the way up to the holding clamp, the specimen is deemed a failure and no designation is awarded. Successful designation of the material depends on the extent of burning after the flame has been removed following the second ignition phase, tested on five prepared samples. Table 1 (below) outlines the criteria required to achieve specific UL94 designations:

V-0V-1V-2
Burning time of sample after flame application (secs)≤10≤30≤30
Total burning time for 5 samples (secs)≤50≤250≤250
Afterglow time following second ignition (secs)≤30≤60≤60
Ignition of cotton ballsNoNoYes
Combustion up to holding clampNoNoNo
Table 1: Test criteria for achieving UL94 V-0, V-1 and V-2 designation

Certification

Products that pass the standard are listed on the UL database either as coatings or plastics. These products also bear the UL symbol on the product label for easy recognition:


Below products either are certified by UL for passing UL94V-0 vertical burn test or meet the testing specifications.

UL 94V-0 Certified products (File# E334302):

Meets UL 94V-0: