The use of rigid flex in military, avionics and aerospace applications has grown steadily since the 1990s, and it is particularly valuable in high stress, high vibration, or thermal excursion environments that would disrupt connectivity in traditional rigid boards with flex/connector junctions. Rigid flex also found acceptance in high reliability medical applications in implantable devices, and dynamic flex applications that require never fail reliability. It has also proven invaluable in packaging a lot of functionality in very small spaces, such as digital cameras, video cameras and miniaturized medical electronics.
The growth and acceptance of rigid flex in the commercial sector has created a unique problem. The difficulty lies in getting a UL rating on rigid flex constructions due to the overwhelming number of configurations that must be represented in the test vehicles. To meet the requirement for UL recognition and rating, PWB fabricators manufacture and submit samples of their boards to UL for a rating approval. The most popular approval rating is 94 V-0. Most fabricators have a file with UL that designates what constructions they may put their UL logo on, to designate that a board has met UL's 94 V-0 test requirements.
The sample requirements call out dielectric thicknesses, copper thicknesses, circuitry dimensions, etc. in order to encompass as broad a range as possible, so that the fabricator can apply their UL logo to as many parts as possible for their customers. Compliance is verified by UL auditors that visit fabricators, looking for violations. UL letters of recognition specifically state that 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's materials and build the test sample base from there. A typical test submission might be 10 to 20 test coupons — costing $10,000 to $14,000 — and can take 12 to 16 weeks for testing and approval. UL recognition gives the fabricator the ability to put its UL logo on PWBs manufactured with that supplier's material, as long as it falls within the parameters represented in the coupons originally submitted.
Using the Same Materials
Once approved, the fabricator must use the same materials as were submitted in the original sample. To use another supplier's laminate or prepreg within a PWB, and apply the UL logo would be a violation. It would also be a violation even if the fabricator had UL recognition on the other supplier's material. The fabricator cannot mix and match (even if both materials were approved by UL separately) two material suppliers without specifically achieving a recognition from U.L. by submitting mix and match samples. Mix and match approvals are obtained whenever a fabricator wants the flexibility to mix and match materials within the UL qualification. Those approvals can get large and expensive due to the larger sample sets required. UL needs to verify that regardless of material combinations, thicknesses, or copper constructions that all combinations will pass the flame rating tests. The sample set increases exponentially, as does the cost and the timeline for building, submitting and testing the sample set. Mix and match qualifications are not attempted very often because it is hard to recoup the investment needed for the original qualification. These same constraints are true for both rigid board fabricators as well as flexible circuit fabricators. However, with rigid flex things get much more difficult.
The cost alone is difficult to ever recoup, let alone the manufacturing time and expense of building the samples. Thus fabricators have little incentive to undertake the task.
The difficulty of certifying PWBs to UL 94 V-0 hasn't deterred designers from incorporating rigid flex designs increasingly into medical and commercial applications. Its viability as a robust packaging medium continues to bring value to electronics designers and users.
Many rigid flex fabricators offer to submit their customer's constructions to get UL recognition one material set at a time. The samples submitted represent only one construction; any change in materials, even adding a sheet of prepreg, automatically disqualify the part from UL recognition and the new build must be resubmitted. The problem with this approach is that it is still expensive in its own right, and still a lengthy process to get qualified, and in the end it is very limited in scope. Few board designers/buyers want to pay the cost to get UL recognition on their parts at all, let alone on a board-by-board basis.
Some designers have worked around the requirement by specifying that the board be built with materials that are recognized by UL to 94 V-0. This eliminates the sampling and testing requirement, and the associated costs. However, it does not represent UL certification of the board, and would not be in compliance with insurance carrier agreements for product liability. It leaves the board buyer exposed to claims, should anything occur with the end product.
The third attempt to fix the problem is not much of a solution. Many board fabricators, particularly the less informed and/or offshore manufacturers, have little or no understanding of the testing requirements for UL recognition. Often they assume that if they have recognition of flexible circuit constructions, and separately rigid board constructions, then they are legitimately allowed to mix and match. This is erroneous and UL's letter of recognition states directly:
Products that bear the UL Mark shall be identical to those that were evaluated by UL and found to comply with UL's requirements. If changes in construction are discovered, appropriate action will be taken for products not in conformance with UL's requirements and continued use of the UL Mark may be withdrawn.
Some fabricators think that if they have UL recognition on a rigid flex submission, then they are qualified to build all rigid flex boards with that material set. That is also erroneous. The error, if discovered by an auditor, leaves the board fabricator vulnerable to losing UL recognition, and leaves the board buyer exposed to product liability claims should something go wrong.
Printed Circuits, Inc. (PCi) has specialized in the manufacture of rigid flex boards since 1997, and had been building rigid flex since 1982 — an early pioneer in this technology. PCi saw that many customers were coming to rigid flex from either a hard board or flex background and could not accept the cost, testing requirements or limited applicability of UL recognition by individual constructions.
PCi undertook a two-year project to build, submit and qualify a set of rigid flex samples that would represent over 90 percent of the designs then on the market. The project required 138 samples that took a year and a half to build six months of testing to build the largest UL 94 V-0 qualification of rigid flex in the world. Interestingly in the first round of testing, a couple of the samples failed, even though they were manufactured with UL 94 V-0 approved materials. This serves to validate UL's requirement that each construction must be tested before UL certification is awarded, and that specifying and using 94 V-0 materials alone is not enough. After receiving the UL Qualification, PCi also added color coded travelers for all manufacturing lots that required UL certification. The color coded travelers allow the UL Auditor to easily verify which lots require UL certification, and to quickly determine compliance.
Most popular constructions can be certified immediately at no cost to the board designer or buyer. PWB designers and buyers now have a source for fully compliant UL 94 V-0 rated boards, and can be assured that their boards are rated 94 V-0.
Contact: Printed Circuits, Inc., 1200 West 96th St., Minneapolis, MN 55431 952-888-7900 Web: http://www.PrintedCircuits.com