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High Aspect Ratio Microstructures in Flexible Printed Circuit Boards: Process and Applications
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences.
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Flexible printed circuit boards (flex PCBs) are used in a wide range of electronic devices today due to their light weight, bendability, extensive wiring possibilities, and low-cost manufacturing techniques. The general trend in the flex PCB industry is further miniaturization alongside increasing functionality per device and reduced costs. To meet these demands, a new generation of low cost manufacturing technologies is being developed to enable structures with smaller lateral dimensions and higher packing densities.

Wet etching is today the most cost-efficient method for producing a large number of through-foil structures in flex PCBs. However, conventional wet etch techniques do not allow for through-foil structures with aspect ratios over 1 – a fact that either necessitates thin and mechanically weak foils or puts severe limitations on the packing density. The fabrication techniques presented in this thesis allow for through-foil structures with higher aspect ratios and packing densities using wet etching. To achieve high aspect ratios with wet etching, the flex PCB foils are pre-treated with irradiation by swift heavy ions. Each ion that passes through the foil leaves a track of damaged material which can be subsequently etched to form highly vertical pores. By using conventional flex PCB process techniques on the porous foils, high aspect ratio metallized through-foil structures are demonstrated.

The resulting structures consist of multiple sub-micrometer sized wires. These structures are superior to their conventional counterparts when it comes to their higher aspect ratios, higher possible packing densities and low metallic cross-section. Furthermore, metallized through-foil structures with larger areas and more complicated geometries are possible without losing the mechanical stability of the foil. This in turn enables applications that are not possible using conventional techniques and structures. In this thesis, two such applications are demonstrated: flex PCB vertical thermopile sensors and substrate integrated waveguides for use in millimeter wave applications.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2008. , p. 58
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 407
Keywords [en]
Engineering physics, flexible printed circuit boards, polyimide, through-hole vias, ion track technology, thermoelectricity, thermopiles, substrate integrated waveguides, millimeter wave devices
Keywords [sv]
Teknisk fysik
National Category
Other Engineering and Technologies
Identifiers
URN: urn:nbn:se:uu:diva-8565ISBN: 978-91-554-7127-9 (print)OAI: oai:DiVA.org:uu-8565DiVA, id: diva2:171609
Public defence
2008-04-11, SiegbahnSalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:30
Opponent
Supervisors
Projects
wisenetAvailable from: 2008-03-19 Created: 2008-03-19 Last updated: 2011-01-17Bibliographically approved
List of papers
1. Reliable small via interconnects made of multiple sub-micron wires in flexible PCB boards
Open this publication in new window or tab >>Reliable small via interconnects made of multiple sub-micron wires in flexible PCB boards
2007 (English)In: Journal of Micromechanics and Microengineering, ISSN 0960-1317, E-ISSN 1361-6439, Vol. 17, no 4, p. 700-708Article in journal (Refereed) Published
Abstract [en]

A fabrication process for small through-hole vias consisting of multiple sub-micron electrodeposited wires has been developed for flexible printed circuit boards (PCB). The resistance of the vias is controlled by adjusting the number of wires per via, as well as the dimensions of the wires. The process steps include modification of the foils by irradiation with energetic ions, wet etching and metallization of the through-hole vias, double-sided surface metallization of the dielectric layer and interconnection lithography. Series of up to 360 interconnected vias of electrodeposited nickel are demonstrated in a flexible PCB foil (75 µm Kapton HN polyimide). The vias have a lateral size of 26 µm. The metal content of the demonstrated vias is 0.12% and 10% of their total volume, corresponding to a metal cross-section of 3.2 µm2 and 270 µm2, respectively. The electrical resistance per interconnected via is 2.6 Ω and 0.07 Ω, respectively. The vias can carry a current density of at least 4 × 106 A cm−2.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-96874 (URN)10.1088/0960-1317/17/4/005 (DOI)000245434200011 ()
Available from: 2008-03-19 Created: 2008-03-19 Last updated: 2017-12-14Bibliographically approved
2. Plated Through-Hole Vias in a Porous Polyimide Foil for Flexible Printed Circuit Boards
Open this publication in new window or tab >>Plated Through-Hole Vias in a Porous Polyimide Foil for Flexible Printed Circuit Boards
2008 (English)In: Journal of Micromechanics and Microengineering, ISSN 0960-1317, E-ISSN 1361-6439, Vol. 18, no 1, p. 017001-Article in journal (Refereed) Published
Abstract [en]

A fabrication process for high aspect ratio plated through-hole vias is presented for flexibleprinted circuit boards. A 75 μm thick porous Kapton foil that allows direct definition of highaspect ratio through-hole vias by dry photoresist film lithography and electrodeposition ispresented. Pretreatment with swift heavy ion irradiation and wet etching define the poredensity and porosity of the foil, similar to ion-track-etched filter membranes. Thin filmmetallization of a seed layer and lithography of a laminated dry photoresist film define the viasizes and positions. Subsequent through-hole electrodeposition produces vias consisting ofmultiple wires, where each open pore defines one wire. The via geometries are characterizedby scanning electron microscopy. The electrical properties of the vias are characterized byresistance measurements. Vias with an aspect ratio over 2 and a side length of 33 μm showhigh yield with low resistance and low variation in resistance.

National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-96875 (URN)10.1088/0960-1317/18/1/017001 (DOI)000252965900030 ()
Projects
wisenet
Available from: 2008-03-19 Created: 2008-03-19 Last updated: 2017-12-14Bibliographically approved
3. Ion Track Enabled Multiple Wire Microvia Interconnects in Printed Circuit Boards
Open this publication in new window or tab >>Ion Track Enabled Multiple Wire Microvia Interconnects in Printed Circuit Boards
2008 (English)In: Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, ISSN 0168-583X, E-ISSN 1872-9584, Vol. 266, no 8, p. 1659-1665Article in journal (Refereed) Published
Abstract [en]

As the call for higher wiring density in packaging and vertical microvia interconnections (microvias) rapidly evolves, the need for smaller lateral dimensions in printed circuit boards (PCB) microvias must be met. The ion track lithography described in this paper allows for high throughput micromachining of small, deep, vertical microvias in flexible PCB and all-polymer laminates.

Ion track lithography makes use of swift heavy ion irradiation to enhance the selectivity and directionality of chemical etching. Within the areas exposed to the ion irradiation, small sub-micron pores (capillaries) are created, one for every ion. If etching is prolonged, the pores become merged. Electrodeposition from a metallic seed layer is used to fill these structures with metal. The lithography masks define either the areas where the ion tracks are developed or where the tracks are metallized. The smallest achievable size of the microvias is only limited by the resolution of the mask; microvias below 10 μm in diameter can also be achieved also in thick polyimide foils.

Since each impinging ion forms one track, the foil’s porosity can be controlled by adjusting the irradiation dose, as well as by etching the pores to a suitable size. Depending on the porosity and material, the resultant metallized microvia consists of either individual or interlaced wires (like strands in a bundle wire), or is a solid. As an individual sub-micron wire may have an aspect ratio of several hundreds, this allows for the fabrication of truly vertical microvia structures, allowing ultra-high density microvia batch production.

Demonstrator microstructures with highly vertical microvias have been fabricated in foils up to 125 μm thickness. Several components integrated in flexible PCB have been presented by us, e.g. magnetoresistive sensors, thermopile IR-sensors and microwave components like inductor elements.

Keywords
Printed circuit board, Polyimide, Foil, Lithography, Ion track, Template
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-96876 (URN)10.1016/j.nimb.2007.11.014 (DOI)000256677600105 ()
Projects
wisenet
Available from: 2008-03-19 Created: 2008-03-19 Last updated: 2017-12-14Bibliographically approved
4. Vertical Thermopiles Embedded in a Polyimide-Based Flexible Circuit Board
Open this publication in new window or tab >>Vertical Thermopiles Embedded in a Polyimide-Based Flexible Circuit Board
2007 (English)In: Journal of microelectromechanical systems, ISSN 1057-7157, E-ISSN 1941-0158, Vol. 16, no 6, p. 1341-1348Article in journal (Refereed) Published
Abstract [en]

A fabrication process for vertical thermopiles embedded in a 75-mu m-thick polyimide foil has been developed for flexible printed circuit boards (flex PCBs). The vertical connections consist of electrodeposited antimony- and nickel-plated through-hole vias. The plated through-hole vias consist of multiple wires, with a total metal content that is 1% of the total via volume. The via fabrication technique is similar to standard flex PCB wet etch and metallization processes. The main difference is that the foils are pretreated with ion irradiation to induce highly selective vertical etch rates. The thermopiles were characterized by measuring their voltage response to an applied temperature difference across the foil thickness.

Keywords
flexible printed circuit boards (flex PCBs), plated through-hole vias, polyimide foils, thermoelectricity
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-96877 (URN)10.1109/JMEMS.2007.907783 (DOI)000252012800006 ()
Projects
wisenet
Available from: 2008-03-19 Created: 2008-03-19 Last updated: 2017-12-14
5. A PCB-like Process for Vertically Configured Thermopiles
Open this publication in new window or tab >>A PCB-like Process for Vertically Configured Thermopiles
Show others...
2008 (English)In: Journal of Micromechanics and Microengineering, ISSN 0960-1317, E-ISSN 1361-6439, Vol. 18, no 6, p. 065021-Article in journal (Refereed) Published
Abstract [en]

Thermopiles are important components in infrared thermal detectors, thermoelectric generators and thermoelectric coolers. We present a thermopile structure with up to 224 vertically arranged thermocouple legs in a polyimide flex material. The thermopile is optimized for infrared thermal radiation detection and is fabricated using printed circuit board-like (PCB-like) processing. Each thermoelectric leg consists of a bundle of a few hundred sub-micrometre-sized strands of either antimony or nickel. These metal wire bundles were achieved by employing ion track technology on the polyimide foil, resulting in a porous dielectric material. Electrochemical methods were used to grow the thermoelectric materials in the pores. The plating mask was produced in a laminated dry photoresist. A small metal cross section, 20 mu m(2) (1 vol%), ensured a low heat exchange between the two surfaces of the flex. The typical resistance per thermocouple was 34 Omega. A responsivity to irradiance of 4.3 V mm(2) W(-1) was measured when heating with a white light source (irradiance 1 mW mm(-2)).

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-96878 (URN)10.1088/0960-1317/18/6/065021 (DOI)000256619600039 ()
Projects
wisenet
Available from: 2008-03-19 Created: 2008-03-19 Last updated: 2017-12-14Bibliographically approved
6. Substrate Integrated Waveguides (SIW) in a Flexible Printed Circuit Board for Millimeter Wave Applications
Open this publication in new window or tab >>Substrate Integrated Waveguides (SIW) in a Flexible Printed Circuit Board for Millimeter Wave Applications
2009 (English)In: Journal of microelectromechanical systems, ISSN 1057-7157, E-ISSN 1941-0158, Vol. 18, no 1, p. 154-162Article in journal (Refereed) Published
Abstract [en]

Substrate integrated waveguides (SIWs) are presented and demonstrated in a flexible printed circuit board (flex PCB) for application in the 77-81 GHz range. The vertical walls of the SIWs presented in this paper consist of multiple electrodeposited metallic wires. The diameters of these wires and the spacing between them are on the order of hundreds of nanometers. Hence, the walls can be seen as continuous metallic walls, and the leakage losses through them become negligible. In turn, the SIWs presented in this paper can operate at higher frequencies compared with previously presented structures that are realized with PCB fabrication processes. The attenuation of the SIWs is comparable to that of microstrip lines on the same sample. The SIWs are successfully demonstrated in a SIW-based slot antenna. The antenna gain along the z-axis (normal-to-plane) was found to be around 2.8 dBi at 78 GHz which is in agreement with the simulated values. [2008-0047]

Keywords
Flexible printed circuit boards (flex PCB), millimeter-wave (mm-W) antennas, millimeter waves, polyimide foils, substrate integrated waveguides (SIWs)
National Category
Signal Processing
Research subject
Engineering Science with specialization in Microwave Technology
Identifiers
urn:nbn:se:uu:diva-96879 (URN)10.1109/JMEMS.2008.2009799 (DOI)000263123100016 ()
Available from: 2008-03-19 Created: 2008-03-19 Last updated: 2017-12-14Bibliographically approved
7. 79 GHz Slot Antennas Based on Substrate Integrated Waveguides (SIW) in a Flexible Printed Circuit Board
Open this publication in new window or tab >>79 GHz Slot Antennas Based on Substrate Integrated Waveguides (SIW) in a Flexible Printed Circuit Board
2009 (English)In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 57, no 1, p. 64-71Article in journal (Refereed) Published
Abstract [en]

The design, fabrication and characterization of 79 GHz slot antennas based on substrate integrated waveguides (SIW) are presented in this paper. All the prototypes are fabricated in a polyimide flex foil using printed circuit board (PCB) fabrication processes. A novel concept is used to minimize the leakage losses of the SlWs at millimeter wave frequencies. Different losses in the SlWs are analyzed. SIW-based single slot antenna, longitudinal and four-by-four slot array antennas are numerically and experimentally studied. Measurements of the   antennas show approximately 4.7%, 5.4% and 10.7% impedance bandwidth (S-11 = -10 dB) with 2.8 dBi, 6.0 dBi and 11.0 dBi maximum antenna gain around 79 GHz, respectively. The measured results are in good agreement with the numerical simulations.

Keywords
Flexible printed circuit boards, microstrip-to-SIW transition, millimeter wave, slot array antenna, substrate integrated waveguides (SIW)
National Category
Signal Processing
Research subject
Engineering Science with specialization in Microwave Technology
Identifiers
urn:nbn:se:uu:diva-96880 (URN)10.1109/TAP.2008.2009708 (DOI)000264246700008 ()
Available from: 2008-03-19 Created: 2008-03-19 Last updated: 2017-12-14Bibliographically approved

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