PCBs
Stay firmon approvalS
Getting UL approval on rigid flex constructions is a nightmare due to the number of configurations that must be tested. Printed Circuits offers a solution explains Bob Burns
The growth and acceptance of rigid flex has created a problem. The difficulty lies in getting a UL rating on rigid flex constructions due to the overwhelming number of configurations thatmust be represented in the test vehicles.
Many electronics designers andmanufacturers are required to
have their products recognized and rated by UL, usuallymandated by their insurance carriers. This rating signifies that the product can sustain a certain level of exposure to flame, without igniting. Products thatmeet this requirement havemuch lower chances of starting a fire and thus reduce chances of product liability claims.
Tomeet this requirement, printed wiring
board (PWB) fabricators submit samples of their boards to UL for a rating approval. The most popular approval rating is 94 V-0 andmost fabricators have a file with UL that designates what constructions can carry the UL logo.
The sample requirements call out dielectric
thicknesses, copper thicknesses and circuitry dimensions in order to encompass as broad a range as possible, so that the fabricator can apply their UL logo to asmany parts as possible. Compliance is verified by UL auditors and any violations can lead to immediate revocation of the fabricator’s UL recognition.
For a fabricator to obtain UL recognition, they
typically start with one supplier’smaterials and build their test sample base fromthere. A typical test submissionmight be 10 to 20 test coupons, cost $10,000 to $14,000 and take 12 to 16 weeks for testing and approval. Fabricators can then put their UL logo on boards manufactured with that supplier'smaterial, as long as it falls within the parameters of
18 | January/February 2013
the coupons originally submitted.
Don'tmix and match
Once approved, the fabricatormust use the samematerials as were submitted in the original sample. To use another supplier's laminate or prepreg and apply their UL logo would be a violation, even if the fabricator had UL recognition on the other supplier’s material. Further UL recognitionmust be sought by submittingmix andmatch samples.
Bob Burns, national sales andmarketing, Printed Circuits
These approvals can get large and expensive due to the larger sample sets required. UL needs to verify that regardless ofmaterial combinations, thicknesses, or copper
constructions, all will pass their flame rating tests. The sample set increases exponentially, as does the cost and the timeline for building, submitting and testing the sample set.
Double the difficulty These constraints apply to both rigid board and flexible circuit fabricators, however, things getmuchmore difficultwith rigid flex. Rigid flex fabricators usematerials fromboth rigid and flex
Most popular constructions can be certified immediately, at no cost to the buyer
processes. This necessitates a combinedmaterial library that is so large, it can be hard to cover all the possiblematerial combinations. Products can incorporate rigid laminate, flex laminate, prepreg, flexible coverlayer and bondply in all thickness variations and copper configurations. A basic UL qualification would therefore require well over 100 sample coupons,months of building and submitting for testing and an exponential increase in cost. Thus
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