2. Project objective(s)

The SHIFT vision

In order to realize the “ambient intelligence” vision it is clear that more and more electronics systems will accompany the citizen. These electronics systems, moving along with the owner, and present near the body (e.g. portable communication systems), on the body (e.g. smart textiles), or even inside the body (implants), will have to communicate with each other and with systems, which are fixed in the ambient. It is clear that the “carry-along” electronic systems may not hamper the comfort of the human “carrier” too much, even must almost be non-noticeable to the user. Therefore they must be lightweight and compact, must preferably take the shape of the object in which they are integrated, must be highly functional at a reasonable price. For this reason the logic evolution will be such that present rigid electronic substrates and assemblies will be replaced by flexible counterparts. This trend is just starting and will become stronger in the coming years. To boost the combination of high functionality and compactness / comfort, there is a strong need for substantial technology developments on flex technologies. This is exactly what SHIFT will aim to do.

State of the art

Flexible materials are being used already today as base substrates for electronic assembly. Typical materials include polyimides for high-end applications down to cheap paper-like or plastic materials like PET for very cost-sensitive applications. In this project we will concentrate on this “high-end” type flex substrate materials like polyimide, LCP (liquid crystal polymer) etc. Manufacturers of these types of flex circuits detect an increasing interest in a wide variety of fields for flex technology.

The development of high density and high performance flexes is mainly driven by applications as LCD, HDD, DVD and consumer products as camcorders or foldable mobile phones. The needs for the interconnection of LCDs, especially color LCDs and the increasing demands on the resolution are pushing the limits of the technology for single and double sided flexes. High volume production reaches a pitch of 50 µm for single sided and 70 µm for double sided flexes. An example of a LCD interconnection flex with a pitch of 50 µm is given in the picture below. Multilayers are mainly required in high end consumer products as camcorders and the leading edge of technology was used for the motherboard of the Playstation 2 which was a 10 layer flex build up with a 6 layer core and two sequential build ups. All features known from rigid boards as surface via, via on pad or staggered vias are possible. Only a very restricted number of manufacturers in the Far East can provide these technologies and the European manufacturers are far behind the capabilities of the Far East located companies especially if high volume production is required.

Although enjoying an increasing popularity current flex substrate and assembly technologies still have serious restrictions :

Normally a flex substrate is a 3-layer laminate where a Cu sheet (layer 1, thickness typically minimum 12µm) is laminated onto a polyimide carrier (layer 2) using an adhesive (layer 3). Thinner Cu is not possible because of mechanical constraints (danger for breaking of the thin Cu layer during the lamination process). Although widely used these 3-layer laminates have following limitations :

• The conductor pitch is limited to minimum 40-50m, because the underetching of the Cu during wet etching process is proportional (and about equal) to Cu thickness
• The Cu thickness puts a lower limit to bending radius of the substrate
• The presence of an adhesive has negative influence on feasibility and reliability of certain advanced assembly technologies, like e.g. adhesive flip-chip (melting of laminate adhesive during flip-chip thermocompression step)
• Only a limited number of sources for adhesives are available; which increases the price for 3-layer laminates

Also 2-layer laminates are offered on the market, where the Cu is deposited onto the polyimide without the use of adhesive. These laminates are expensive because of the complex production process. The problem is the adhesion of the Cu (or any other metal) onto the polyimide surface. The adhesion strength of a metal onto a polyimide is normally very low. What is currently done in production environment to produce 2-layer laminates with sufficient adhesion strength, is first modifying the polyimide surface by e.g. plasma treatment, followed by a thin-film deposition (sputtering) of Cu onto the modified (more reactive) polyimide, followed by further increasing the Cu thickness by electrolytic plating. In this case thin Cu layers with very fine pitch capability are possible, but it is clear that the use of vacuum processes (plasma treatment, sputtering) increases the price of the product considerably. Another production method for 2-layer laminates, used in Japan, is the deposition of liquid polyimide layers onto Cu films followed by an imidization process at high temperatures in a nitrogen atmosphere. This is a cheaper production process, but the restriction here is also the minimum required Cu thickness for mechanical reasons, thus limiting the minimum achievable pitch. The lamination of two single sided materials using a thermoplastic polyimide adhesive is used to get double sided materials. Today such 2-layer laminates have a minimum Cu thickness of 12 µm.

So a decrease of the price of the base substrate is a vital issue for wider use of high-end flex, and especially cheap high-end flex base substrates with very fine pitch capabilities (thin Cu conductor layers) do not exist today.

On this type of flex only pure electronic assembly is performed up to flip-chip components. Embedding of passive or active components in flexible printed circuits is at least not state of the art and is, to the knowledge of the consortium, not performed anywhere in the world so far, certainly not on a commercial basis. The limitation to two component layers (front and back side of the flex laminate) limits the compactness of the circuit.

Objective

The objective of the project is the development of smart, high-integration, mechanically flexible electronic systems, for a wide variety of applications. “Smart” means that the flexible multilayer laminate has embedded components, and that the different flex layers in the multilayer structure can have different functions, meaning that it might be necessary to combine layers of different base material in the laminate. The principle of such smart flex, including assembled components, is shown in the figure below. Compactness of the resulting circuit will be boosted in two ways:

• By using the third dimension for electronic component integration (not only on front and back side, but potentially on every conductive layer)
• By drastically increasing the wiring density through the introduction of new flex manufacturing and lamination techniques

In order to achieve the final goal of smart, compact flex assemblies a logical build-up of activities is foreseen in the project :

Development of a number of generic base technologies :

• New cheap high density flex base substrate manufacturing process through electroless Cu deposition technology on polyimide (“eCuflex”); goal is to reach a conductor pitch of 20µm
• Process for PTH (plated through holes) and via holes in high-density flex, and for production of double-sided flex substrates
• New multilayer flex lamination technology using solid state diffusion technology, allowing to obtain highly reliable compact multilayer flex substrates
• Extremely bendable multilayer high-density interconnection substrates (HiCoFlex) with narrow pitch and small via holes, and its extension to Large Area Panels.
• Development of technologies for embedding electronic components : passives (R and C), RF structures, and ultrathin (20µm) foldable chips
• Assembly (lead-free) of standard components and of ultrathin (20µm) foldable chips with very small gap (10µm) onto flex laminates with embedded components, preserving mechanical flexibility, even of a completely assembled circuit
• Combination and integration of several functional layers, which may be produced on different types of base substrate materials, e.g. the combination of a high density digital circuit on a polyimide substrate, together with an RF circuit, produced on an LCP substrate, thus creating the possibility for complex, high-functionality flex circuits.

Modelling and testing

The technological developments will be supported by modelling activities (electrical, mechanical, thermal and thermal-mechanical) and testing (especially reliability testing) in order to qualify the technologies, and facilitate design of multilayer, embedded flex circuitry. These supporting activities will be carried out throughout the project.

Cost effective production processes

To make price competitive products not only the base materials should be cheap, but high volume products should be manufactured in a cost effective way. Therefore during the development of base technologies and flex integration processes the possibility to implement these technologies in a reel-to-reel production system should be kept in mind. After feasibility demonstration actual development and implementation of the necessary reel-to-reel processes for multilayer flex manufacturing, component embedding and assembly, is programmed. When reel-to-reel processing is not possible for a developed technology, a second way for cost-effective manufacturing will investigated, namely large area production (LAP).

Demonstration activities

During the second phase of the project the developed generic base flex technologies and the integration of these technologies into smart flex systems will be demonstrated by designing and producing functional prototypes/demonstrators.

From the first phase of the project on (generic basic technologies development) the activities will be triggered by the end-users in the consortium.

It is strongly believed that for each degree of flex complexity, from ‘simple’ single-sided high density flex up to the most complex multilayer flex systems, potential applications can be found. In this way, bringing together the right basic blocks of flex substrate production, component integration and assembly, the technologies developed in the proposed SHIFT project, will lead to applications in a very wide variety of industrial sectors.

The proposed technological objectives are clearly beyond the current state of the art world-wide : component embedding in flex does not exist today, proposed cheap, high density eCuflex flex manufacturing and solid state diffusion lamination techniques are original, embedding and assembly of ultra-thin (20µm) chips on embedded flex is an innovative technology, and so on. Successful developments in SHIFT will truly open up a vast number of opportunities to come closer to the ambient intelligence vision.