Integrated components for flexible substrats

The technology of embedded active and passive components in flexible PCB is one possibility to enter the third dimension. Before using these options a good knowledge about the requirements and specifications is necessary. A range of target values and target tolerances should be provided, that have to be realized for the functional demonstrators of the users.

The knowledge of the enabling technologies will give an overview of technologies and materials available for the fabrication of embedded/integrated components.

Integrated resistors

• Sputtering of thin films layers e.g. NiCr: Sputtered resistors are being characterised and the process parameters are under further optimisation (IMEC and Hightec)
• Processing and performance of printed resistors
• Optimization of process technologies and performance of electroless Ni(P) resistors
• Laminate resistors

   

Hightec MC AG has done research using the proprietary HiCoFlex technology:
Ultra-thin, highly flexible multilayer foil elements with integrated thin film components like resistors,  RF lines and structures are produced by the HiCoFlex technology. The foils can be laminated into conventional printed flex-boards as local high-resolution parts and connected to the wiring of the print. By this method expensive features can be limited to the area where they are needed.

HiCoFlex is a technology for producing flexible multilayer substrates. The structuring technique allows track widths of 15 mm, spacing of 10 mm and vias of 30 mm. Actually circuits with up to 4 metal layers are realized on temporary rigid substrates and released from the carrier afterwards using a special release layer. The total thickness of such a foil is around 50 mm. The minimum bending radius is smaller than 0.5 mm. The resulting highly flexible foil-like circuits have excellent mechanical and electrical properties.

Recently the integration of thin film resistors have been investigated. Resistor sputtering, laser trimming, annealing, temperature dependence, thermal ageing and humidity tests will be discussed.

Advantages of foil elements with integrated components are higher miniaturizations, new ways of 3D packaging and more design possibilities.

Details can be seen in the presentation as held by Hans Burkard during the 2006 "be flexible" Workshop in Munich: HiCoFlex 

 

Printed resistors

• Thermo setting and thermo curing resistor pastes printed with tolerances +/- 20%
• Lamination of printed resistors: value shift observed
• Reliability test: aging, cycling and humidity: in the latter dramatic degradation

Electroless Ni(P)

• A variety of test patterns realized
• Measurement and trimming of resistors without problems
• Reliability testing at TU Berlin: aging, cycling and humidity: in the latter dramatic degradation - corrective action: use of humidity protectant over the resistors
• Two types of NiP resistor foils have been evaluated (from OHMEGA Technology and GOULD), some contacts have been taken with other suppliers and samples are under study

Integrated capacitors

Integration of capacitors into build up layers of the flex materials

• established contact with “Energenius” for high-C materials on the base of ferroelectric; preliminary results;
• Experiments with C-Ply from 3M;
• A survey of available solutions in respect to embedded capacitors and the procurement of some materials have been done.
• Preliminary design of high frequency capacitors have been done and first test vehicles have been manufactured and tested.

Integrated RF structures

A build-up (combination of integrated resistors, integrated capacitors and RF structures, by using thin-film techniques) was to be made, starting from substrates delivered by Hightec.

• Find polymer material and material combinations suitable for RF with special attention to the idea of PI-BCB-PI sandwich structures
• Realisation of flexible RF structures (cables, interconnections, passive RF elements) on HiCoFlex basis. Measure RF properties of such structures to evaluate suitable materials
• Use of commercially available polymer foils for the realisation of flexible RF structures (cables, interconnections, passive RF elements)
• Different methods for temporary attachment and release of foils evaluation were tested: Best method with lowest release forces with the thermal release sheet was REALPHA, NITTO DENKO Corp.
• Test structure for PI-BCB Multilayer and PI-BCB-PI Sandwiches designed and test series produced
• RF characterisation of PI-BCB Multilayer and PI-BCB-PI Sandwiches. Preliminary RF measurements for 5 mm line length sample measured

In order to make RF-structures based on spin-on polyimides and sputtered metal layers, a lot of technology development was needed. These preparatory experiments include:

• substrate preparation: after processing the base PI layer has to be released
• metallisation of polyimide
• drilling of the vias through polyimide layers and stop on a 1µm thin metal layer
• adhesion of next polyimide on a cured polyimide

Embedded active components

• Embedding of chips into build up layers of flex PCBs (TUBerlin).
• Thinning of wafers to a residual thickness of 20-30µm and allocation of thinned wafers to the  project partners by FhG (IMEC and Fraunhofer IZM need wafers with standard pad metallization of Ni/Au, TUB with immersion bumps)

A technology for embedding ultrathin dies in flexible substrates has been developed by IMEC Ghent: the UTCP (ultra-thin chip package). Ultrathin chips (thickness 20-30 µm) are packaged in between 2 polyimide layers. The result is a very thin chip package, with a total thickness of only 50 - 60 µm. Chip, PI layers and metal are so thin that the whole package is bendable.

• The base substrate is a uniform polyimide layer, applied (spin coated and cured) on a rigid carrier.
• A dispensed BCB is used as adhesive layer for fixation of the chip.
• A second spin-on polyimide layer is covering the chip.
• Vias to the contacts of the chip are laser drilled;
• Contact metal layer is sputtered and photolithographically patterned; This metal layer is providing a fan out to the contacts of the chips.
• The whole package can be released from the rigid substrate.

Good test chips are now provided by Fraunhofer IZM, which will be packaged and tested.

• No flow underfiller caused problems in polluting the bond tool. Problem could be solved by using a protective (Teflon based) layer on the tool and Teflon tape.
• Thin chips were partially contaminated with residues from the dicing tape and / or water glass. Improvements have been achieved in the meantime.
• Embedding of thin chips will be done by die bonding/lamination via drilling by laser and metallization will be investigated at TU Berlin.