Page 10
www.us-tech.com
TechWaTch Solving UL Approval Needs of Rigid Flex Circuits By Bob Burns, National Sales & Marketing, Printed Circuits, Inc., Minneapolis, MN T
he use of rigid flex in military, avionics and aerospace applica- tions has grown steadily since
the 1990s, and it is particularly valu- able in high stress, high vibration, or thermal excursion environments that would disrupt connectivity in tradi- tional rigid boards with flex/connec- tor junctions. Rigid flex also found ac- ceptance 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 Problem The growth and acceptance of
rigid flex in the commercial sector has created a unique problem. The difficul- ty lies in getting a UL rating on rigid flex constructions due to the over- whelming number of configurations that must be represented in the test vehicles. To meet the requirement for UL recognition and rating, PWB fabri- cators manufacture and submit sam- ples 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 thick- nesses, circuitry dimensions, etc. in or- der to encompass as broad a range as possible, so that the fabricator can ap- ply their UL logo to as many parts as possible for their customers. Compli- ance 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 fabrica- tor’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 test- ing 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
JUST ANNOUNCED Now Available in Digital Format! U.S. TECH
Every Issue Delivered Directly to Your Inbox. FREE!
Great Features:
Bookmarks, highlights, notes, forward to a friend, and more.
Coming soon: U.S. TECH on your iPAD and iPhone!
Like us on Join us on Follow us on
UL logo would be a violation. It would also be a violation even if the fabricator had UL recognition on the other sup- plier’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 flexi- bility to mix and match materials within the UL qualification. Those ap- provals 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 combina- tions will pass the flame rating tests. The sample set increases exponential- ly, as does the cost and the timeline for building, submitting and testing the sample set. Mix and match qualifica- tions are not attempted very often be- cause it is hard to recoup the invest- ment needed for the original qualifica- tion. 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 sam- ples. Thus fabricators have little incen- tive to undertake the task.
Previous Options The difficulty of certifying PWBs
to UL 94 V-0 hasn’t deterred designers from incorporating rigid flex designs increasingly into medical and commer- cial applications. Its viability as a ro- bust packaging medium continues to bring value to electronics designers and users. Many rigid flex fabricators offer
to submit their customer’s construc- tions to get UL recognition one materi- al set at a time. The samples submit- ted represent only one construction; any change in materials, even adding a sheet of prepreg, automatically dis- qualify the part from UL recognition and the new build must be resubmit- ted. 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 lim- ited in scope. Few board designers/buy- ers 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
The Nation’s Hi-Tech Electronics Publication Subscribe today:
www.US-Tech.com See us at Electronics West/MDM Booth 347 and IPC/APEX Booth 3150
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 test- ing requirement, and the associated costs. However, it does not represent UL certification of the board, and would not be in compliance with insur- ance carrier agreements for product li- ability. It leaves the board buyer ex- posed to claims, should anything occur with the end product. The third attempt to fix the prob-
lem is not much of a solution. Many board fabricators, particularly the less informed and/or offshore manufactur-
ers, have little or no understanding of the testing requirements for UL recog- nition. Often they assume that if they have recognition of flexible circuit con- structions, and separately rigid board constructions, then they are legiti- mately allowed to mix and match. This is erroneous and UL’s letter of recogni- tion 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, appropri- ate action will be taken for products not in conformance with UL’s require- ments 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 quali- fied to build all rigid flex boards with that material set. That is also erro- neous. 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 some- thing go wrong.
The Solution Printed Circuits, Inc. (PCi) has
specialized in the manufacture of rigid flex boards since 1997, and had been building rigid flex since 1982 —an ear- ly 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 limit- ed applicability of UL recognition by individual constructions.
Implementation PCi undertook a two-year project
to build, submit and qualify a set of rigid flex samples that would repre- sent over 90 percent of the designs then on the market. The project re- quired 138 samples that took a year and a half to build six months of test- ing to build the largest UL 94 V-0 qual- ification of rigid flex in the world. In- terestingly 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 mate- rials alone is not enough. After receiv- ing 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 de- signers 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:
www.PrintedCircuits.com r
February, 2013
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136