AT&S Fehring

Citation

Ling, J. (2005), "AT&S Fehring", Circuit World, Vol. 31 No. 2. https://doi.org/10.1108/cw.2005.21731baf.002

AT&S Fehring

AT&S Fehring

Keywords: Printed circuits, Electronics industry

• •

  A driven departure, an imminent arrival

Some figures, first. The world market for flexible circuits is valued at $3.8 billion, of which the figure of $2.8 billion represents polyimide. The CAAGR for flexible circuits over the next 5 years is 12 per cent. Europe accounts for only 9 per cent of the world production of flexible circuits, but 25 per cent of that market is for flexible circuits for telecommunications and automotive, and another 25 per cent is for computers and office equipment.

It comes as no surprise therefore to find that at Fehring, in the rolling wine- growing Styrian countryside of southern Austria, AT&S have embarked upon an ambitious and capital intensive project to meet customer demands. It also completes the circle on product availability from the AT&S family; many of their new flexible printed circuit (FPC) customers also buy rigid boards from them, so the spectrum is now full.

Circuit World met with those who have the responsibility for the project, which is less than 1 year old. Dipl.- Ing. Markus Leitgeb, Senior Project Leader, takes care of materials qualification and test, and Ing. Josef Ladenhaufen, who is looking after the production engineering. Enthusiasm shines through, and there is an air of expectation as the commissioning of the new 5 million Euro production line at Fehring draws close. In the next 12 months they will be producing flexible circuits at the rate of 100,000 m² per annum, under the experienced eye of Ing. Erwin Kurta on the factory floor.

Flex at AT&S is not concentrated in one place. Single-and double-sided flex will be produced at Fehring, whilst rigid-flex boards will emanate from Fohnsdorf, as well as prototyping. Flex-multilayers, a subject of even further challenge, will be undertaken at Leoben.

So why flex, and why now? It is the automotive industry that is the driver, a somewhat apt phrase. Given the staggering electronic content cost of any new car, and the consequent demands for the reduction in both weight and use of space, and one begins to see where flexible circuitry fits in. Flexible circuits can be used in dashboards, mirror controls, engine controls, in gear selection systems, and in other applications such as in cameras, inkjet printers, clamshell mobile telephones. Flexible circuits are light, they can be folded to conform to a 3D shape; they are highly reliable; they can be used for highly reliable dynamic flexing, and offer a high degree of design freedom. They are shock and vibration resistant, and eliminate the need for connectors. They can drive up technology as well as drive down costs. A cost accountant's dream come true.

Flex is not a simple subject. Nothing to do with circuits is ever simple, and flex is no exception. The permutations that exist for the selection of the right substrate for a particular end use are myriad, and great care has to be exercised with the choice. But as this industry loves TLAs, we will start with one, FPCs.

FPCs come in various forms – single- sided not-plated-through – barebacked flex – double-sided not-plated-through and double-sided plated-through. FPCs will also come in multilayer format using laser drilling and buried vias, but more of that in later issues of Circuit World.

There are three main substrates. Polyimide, which is solderable, but expensive (three times the price of polyester), and suffers from a current supply problem. Polyester is not solderable, but offers low cost FPCs. There is polyethylenenaphtalene (PEN) which is solderable under certain conditions, and there are some alternative films such as PES, PPS, PEEK, PBTP and liquid crystal polymers (LCPs) which offer some interesting characteristics.

In the polyimide world there are three major players – Du Pont (USA) with Kapton, Ube and Kanika Fuji (Japan). With polyimide films there are two ways in which the copper can be bonded – adhesive, and adhesiveless. With the former the copper foil is bonded to one or both sides of the polyimide film with either an acrylic or an epoxy adhesive. This is referred to as electro-deposited (ED) copper, and is good for the flex-to-install and static applications.

The adhesive process is low cost, but disadvantages include adhesive smear during drilling, a potential for high temperature cycling failure, dynamic properties are reduced, and the dielectric constant is raised by the adhesive. Of the two adhesives, epoxy has the better chemical resistance and is superior for moisture resistance.

With adhesiveless the copper can be vacuum sputtered onto the polyimide, or the polyimide resin can be cast onto the copper foil, or you can laminate. This is known as rolled annealed (RA) copper, and is best for dynamic flex, albeit at twice the cost of ED copper. It comes in half, one and two-ounce copper thicknesses (18/35/70 µm). As an in-between there is galvanic copper which comes in some new types which offer high ductility for various dynamic applications.

Adhesiveless has many advantages – these being high temperature reliability, improved chemical resistance and electrical performance, fine line and hole capabilities (with the best performance coming from the vacuum sputtered type) and there is excellent dynamic behaviour. RA copper has quite the best surface for subsequent processing especially after chemical etching.

In the soldermask department suppliers have been able to supply both a screenprintable flexible solder mask, epoxy or acrylic based, and suitable for use on polyimide (Lackwerke Peters) and there is now a screen printable photoimageable flexible solder mask available from electra polymers.

One of the advantages for AT&S has been the establishment of their own group buying department in Hong Kong. With both RA and ED polyimide being difficult to obtain in Europe nowadays, they have no problems at all.

One of the developments at AT&S on FPCs has been the production of boards with stiffeners, designed to provide a stable base for soldering operations later on. AT&S claim to be unique in that they can provide a stiffener not only on a single card but also on a complete panel. Stiffeners can be made up from layers of FR4, CEM-1, or FR2, as well as polyimide and polyester, using thermal set adhesives or pressure sensitive adhesives.

AT&S have written the definitive book on design guidelines, with some good recommendations on pad or land hold-down techniques. They have also produced an FPC Specification Manual, which goes into minute detail on the products and their specifications; it looks at the various combinations of dielectric materials, types of copper foil, and permutations thereof. There are 19 standard combinations and a further 64 available on request. For instance, if a customer wanted a double-sided plated-through FPC based on 50 mm polyimide laminate with 70 µm ED copper, that would be a standard, whereas the same product with on 75 µm polyimide with 70 µm RA copper would be a special. Interesting to note that on single- and double- sided plated-through FPCs they will be offering 75 µm gaps and tracks.

A tour of the new production line reveals the care that has gone into equipment selection. The process sequence varies for single- and double- sided FPCs. Drilling tooling holes is done 10-up, but for interconnects panels have to be drilled singly on carrier plates. Hole sizes are between 0.15 and 0.25 mm; then comes reverse pulse panel plating on a new Atotech line running at 1 m/min. Imaging takes place on an imposing new line comprising Kuttler in-feed to a Schmid chemical pre-clean line, from which the panel enters a Class 1000 clean room. Here there is a dry film laminator, a stainless steel double-sided 5 kw exposure unit (registration by four cameras onto fiducials), then panels exit to a DES line.

After that remains a choice of flexible screen-printable solder mask or coverlay (DuPont or Krempel). Contour routing by laser is available at their Fohnsdorf plant as a further step, and then panels can be chemically tinned, or given an OSP (Entek) or ENIG (Atotech) finish before electrical test (new ATG horizontal flying 8-probe) and punching.

Fehring operates to TS16949 standards and were up and running in serious production late last year. They aim to be No. 1 in Europe, they say. Given the investment (5 million Euros) that has taken place, and given the depth of knowledge, experience and size of scale at AT&S there is little room to doubt that this will be so.

Plate 1 The flex DES line at Fehring

Plate 2 The drill shop

Plate 3 The flex DFL line

Plate 4 Storage at Fehring

Plate 5 Fehring's uniplate line

Plates (1)-(5) show the production lines and facilities at AT&S Fehring.

John LingAssociate Editor