Essentially there are two main types of standards that are relevant to conformal coatings and which cover most common specifications. The standards can be thought of in terms of those that require independent third party testing and have published qualified or approved product lists or those that contain test specifications and requirements, but for which the burden of meeting these minimum requirements lies squarely on the manufacturer to demonstrate their acceptability for use.
The two most common standards that apply to conformal coatings that have a published qualified product list are MIL-I-46058C and UL746E.
Although declared inactive for new designs in November 1998, this standard remains the de-facto industry standard due to the requirement for independent third party certification and being the only published standard with an associated Qualified Product List (QPL), the latest version is available from www.dscc.dla.mil.
Products that have been successfully tested to this standard and appear on the QPL can be expected to be of high performance, and the consumer can rest assured that the material has been thoroughly tested by an independent laboratory to demonstrate performance. Materials that claim to ‘meet the requirements’ of MIL-I-46058C should be treated with caution and the consumer should understand that these have probably not been tested to the rigorous standard required to obtain qualification. This is an important distinction that should be understood by the end user.
Underwriters Laboratories (UL) are perhaps the premier global safety certification body for consumer electronics. With regards to conformal coating, this standard can be largely thought of as being composed of two parts; electrical safety and flammability (UL94).
The electrical safety tests are conducted on test coupons coated with materials applied at their manufacturer’s recommended MINIMUM thickness. The coupons are preconditioned before electrical testing to determine the maximum operating temperature (Thermal Index). In addition, panels are subjected to various combinations of humidity and temperature cycling to determine suitability for internal or external use. After conditioning, the coupons are subjected to a series of voltage spikes, peaking at 10kV to ensure the dielectric breakdown, resistance of the coating material is sufficient to prevent arcing between tracks of a nominal and known spacing.
With respect to flammability, coupons of a UL approved V-0 laminate are coated at the manufacturer’s MAXIMUM recommended thickness. These coupons are then subjected to a flammability test per UL94, which can either be conducted with the coupon mounted in the same plane as the applied flame (Vertical Burn) or perpendicular to the applied flame (Horizontal Burn). For each test, the time to extinguish the flame and whether any flaming debris is produced will correspond to a particular performance designation, with 0 being the best designation. Per UL94, the material will have a designation such as V-0, signifying it was tested in the vertical plane (the harder to pass of the two types of test) and that it was self-extinguishing. V-0 materials are widely available, and there is no rationale for choosing a product that is not rated V-0.
In addition to these two specifications with associated qualified products lists (QPLs) there are a number of other international and trade association specifications that relate specifically to conformal coatings. The major difference between these specifications and those listed previously, lays in the onus for performing testing and reporting results lies solely upon the manufacturer of the material, with no requirement to use a certified laboratory and without maintenance of an independent QPL.
IPC-CC-830 Rev B
This specification was intended to provide an alternative to MIL-I-46058C when it was announced inactive for new designs in 1998. The specification has found widespread use in many industries and in many countries, despite being a trade association specification. This specification mirrors many of the performance and acceptance criteria, as well as the test methods as laid out in the MIL spec, hence any material appearing on QPL-46058C is automatically considered to meet the requirements of IPC-CC-830.
The most significant difference between IPC-CC-830 and MIL-I-46058C is the ability to self-certify compliance and the lack of an independently maintained QPL. The onus is on the supplier to perform and document performance data in the form of a test report when requested by a consumer. If the customer has any doubt, there are independent laboratories that can conduct material performance testing to demonstrate that the material meets the requirements of the IPC-CC-830 document.
In addition to these published specifications, there are customer specific specifications, with acceptance criteria and qualified product lists, but these are generally available only to materials suppliers and are generally considered to be Original Equipment Manufacturer (OEM) intellectual property.