UL approval and rigid-flex: rigid-flex is tougher and more costly to get UL-approved than rigid or flex. Finding the right supplier is key.

Often PCB designers include a note on their fabrication drawings requiring that parts meet the conditions of UL 94 VO. Because of product liability insurance requirements, designers must assure that their products meet the flammability requirements of Underwriters Laboratory. The difficulty with rigid-flex is that there are so many multilayer combinations that must be tested in order to assure that each possible combination, including yours, meets the requirements of 94 VO.

If you want to get approved on standard multilayer (rigid) materials, you must submit various samples of multilayer boards, using the thinnest and thickest dielectrics, and the thinnest and thickest copper weights, with the thinnest and thickest multilayer constructions that you wanted approved. If you want approval on flexible multilayers, you do the same thing, with the added variables in coverlayer thickness.

However, if you want approval on rigid-flex, you'll need to submit all of the possible combinations of dielectrics, copper weights and thicknesses, for both the flexible and rigid materials, with coverlayers, all in combination with one another. The simplified chart shown in TABLE 1 shows how the potential combinations increase exponentially with rigid-flex.

The number of samples required for rigid-flex increase as you try to cover the range of possibilities. As the number of samples grows, so does the cost and the testing time. A typical undertaking for a simple change or addition for a multilayer rigid board is 10 samples, with about $10,000 to test and about 10 to 14 weeks to wait for the results. By comparison, a rigid-flex application runs well over 100 samples, with an equivalent increase in cost and testing time.

So if you're a designer, how do you go about Setting a rigid-flex board certified to UL 94 VO? Usually PCB manufacturers have approval on a few basic constructions that might be made to work if you alter your design. Alternatively, your supplier can submit an application based on your specific design. Each individual application runs $8,000 to $12,000, with 10 to 14 weeks of lead time.

If you are working on a product that must meet UL 94 VO, ask your supplier to provide assurance, in writing, that your board will meet UL 94 VO. If your supplier can't do so, ask what it will require to get your product approved, or find a supplier who has approval on the build you intend to use.

You can verify a supplier's UL certification by going to www.UL.com and clicking on "Certifications," which will guide you to a file of UL-approved constructions.

BOB BURNS is national sales and marketing manager with Printed Circuits Inc., a manufacturer of multilayer rigid-flex circuit boards. He can be reached at [e-mail].

TABLE 1. Rigid-flex UL certification is more complex than for rigid
boards or flex

MATERIALS     RIGID UL APPROVAL     FLEX UL APPROVAL

Laminate      Thinnest core
              Thickest core

Prepreg       Thinnest prepreg
              Thickest prepreg

Copper        Thinnest copper
              Thickest copper

Overall       Thinnest multilayer
              Thickest multilayer

Flex                                Thinnest polyimide film
                                    Thickest polyimide film

Bondply                             Thinnest bondply
                                    Thickest bondply

Coverlayer                          Thinnest coverlayer
                                    Thickest coverlayer

Flex Copper                         Thinnest copper
                                    Thickest copper

Overall                             Thinnest multilayer
                                    Thickest multilayer

MATERIALS     RIGID FLEX UL APPROVAL

Laminate      Thinnest core
              Thickest core

Prepreg       Thinnest prepreg
              Thickest prepreg

Copper        Thinnest copper
              Thickest copper

Overall       Thinnest multilayer
              Thickest multilayer

Flex          Thinnest polyimide film
              Thickest polyimide film

Bondply       Thinnest bondply
              Thickest bondply

Coverlayer    Thinnest coverlayer
              Thickest  coverlayer

Flex Copper   Thinnest copper
              Thickest copper

Overall       Thinnest flex cable
              Thickest flex cable