FINAL PROJECT REPORT FORM

Transcription

1 FINAL PROJECT REPORT FORM IMCRC Project number 202 Budget code J11423 INVESTIGATOR DETAILS Details Principal Investigator Co-investigator 1 Co-investigator 2 Title Dr Dr Prof. Forename(s) David Arthur Dennis Patrick Changqing Surname Hutt Webb Liu Organisation Loughborough University Loughborough University Loughborough University Division or Department Dept. of Mech. & Man. Eng. Dept. of Mech. & Man. Eng. Dept. of Mech. & Man. Eng. PROJECT DETAILS Title of Research Project Surface Preservation Coatings to Enable Versatile Metal Particle Deposition and Sintering Funds Awarded ( ) 125K - 250K (Medium) Start Date: November 2006 Staff ( ) End Date: January 2011 OBJECTIVES AND RESEARCH SUMMARY Original objectives as presented in the grant proposal The aim of this research proposal is to carry out an experimental study of the use of self-assembled monolayer (SAM) coatings to act as oxidation preventing sintering aids for metallic particles with sizes ranging from the micro to the nanoscale. The application of this approach in areas ranging from the production of printable metallic inks for decorative packing and functional applications, to larger scale powder metallurgy and structural materials applications will also be investigated. To achieve this aim the following objectives have been identified: Extension of an existing SAM preservation method for fluxless soldering of copper to powders in the micron size range. Investigation of strategies to form SAM coatings on oxide free nanoparticles, either through solution treatment of commercially available nanoparticles, or the application of combined nanoparticle formation / monolayer production synthesis Analysis of the sintering behaviour of micron and nano-sized SAM preserved particles; including optimisation of sintering conditions, investigation of the sintered materials bulk properties and detailed materials analysis of the interparticulate bonding mechanism Attempt to extend the SAM protection method to other suitable metallic powders such as nickel Demonstration of the potential applications of micron and nano sized particles and powders developed during the research program. These objectives remained valid throughout the research (tick box), or These objectives were changed (tick box). X (If so, please explain how/why in your report)

2 New objectives As this feasibility study progressed it became apparent that it would not be possible within the time available to meet all of the objectives as some were more challenging than others. Below is a summary of how these changes occurred: Extension of an existing SAM preservation method for fluxless soldering of copper to powders in the micron size range. this remained valid throughout Investigation of strategies to form SAM coatings on oxide free nanoparticles, either through solution treatment of commercially available nanoparticles, or the application of combined nanoparticle formation / monolayer production synthesis this remained valid, but experiments to treat commercially available copper nanoparticles to remove oxides demonstrated that this was very difficult. An alternative approach was identified, and this was further explored within another IMCRC feasibility study. Analysis of the sintering behaviour of micron and nano-sized SAM preserved particles; including optimisation of sintering conditions, investigation of the sintered materials bulk properties and detailed materials analysis of the interparticulate bonding mechanism this remained valid throughout the project, but focussed towards the application of preserved micron scale particles in electrically conductive adhesives Attempt to extend the SAM protection method to other suitable metallic powders such as nickel this objective remained valid, but within such a limited project, it was found that there was not sufficient time to investigate this in detail. Some solder powders were tested and nickel foils were also examined. Demonstration of the potential applications of micron and nano sized particles and powders developed during the research program this remained valid Project abstract at time of proposal Solid state sintering produces net shape or near net shape parts from a metal powder without melting occurring. Advantages include complex shape capability, low energy consumption, high material usage, low capital costs, a unique degree of control over microstructures and composition and ability to process refractory metals. Typical sintering process conditions of high temperatures and inert or reducing atmospheres are partly dictated by the presence of oxide layers on the surfaces of the metal particles. The initial stages of sintering are driven by diffusive surface material flow, assisted by the release of surface energy that occurs as a consequence of the reduction in the total surface area of the sintering particles. Metal oxides have a reduced surface energy and therefore reduce the driving force for sintering to take place. Recently, the move towards digital, data driven manufacturing routes has led to the extension of sintering methods to laser-scanning rapid manufacturing and metal nanoparticle ink jet technologies. Sub-100nm diameter nanoparticle colloidal inks are particularly attractive because the large particle surface area to volume ratio drives the sintering temperature to as low as 100 C. However, particle oxidation issues restrict current applications to relatively inert, costly metals such as silver or gold. A potential solution to this problem is to pre-treat the particles to remove the oxides from the surfaces and apply a protective coating. This coating would then be displaced during the sintering process to enable metal to metal contact without barrier layers. Self-assembled monolayers have been shown to act in just this way when applied to micro-etched copper surfaces on printed circuit boards, enabling soldering without the use of flux. The aim of this research proposal is to extend the use of SAMs as an anti-oxidation coating to sintering, initially for copper particles. Approaches will include scale-down : applying existing deposition techniques to increasingly smaller particles; or scale-up where nano particles of increasing size are synthesised with SAM coatings already applied. The potential benefits include not only the ability to build a variety of metal structures on temperature sensitive substrates using nanoparticle inks, but also process and efficiency improvements in conventional and laser assisted sintering, through enabling the use of cheaper ambient atmospheres and reduced laser powers.

3 Summary of Outcomes: In simple terms describe your work in such a way that it could be publicised to a general audience. Metal powders are used in a range of important manufacturing processes from powder metallurgy and laser sintering to construct 3D parts, to fillers in electrically conductive adhesives. In many of these applications, metal to metal contact of the powder particles is required in order to allow sintering (metallic bonding and diffusion) to take place, or to enable a low resistance electrical path to be established from one particle to another. However, the presence of oxides that form on the surface of many metals when exposed to air can hinder the contact between the metal particles and prevent or reduce sintering and increase electrical resistance. The aim of this research was to investigate ways to remove the oxide from metal particles and apply a protective coating that would limit the re-oxidation of the powders such that they could be used more effectively. The research focussed on the use of copper powders as these are of great interest in the field of printed electronics where pastes or inks composed of metal particles are printed onto a surface and subsequently heat treated to make contact between them to produce an electrically conductive track. At present most of the printed electronics are carried out using silver based materials due to the low oxidation of silver that facilitates good electrical contact, however silver is expensive and in relatively low abundance such that there is a strong demand for copper instead. However, copper forms a significant oxide layer and this must therefore be addressed before it can meet the requirements of the applications. The research developed and characterised a method for the removal of oxide from readily available copper powders (particle sizes in the range 1 to 50 m) and applied an organic coating before reoxidation could occur. This coating could reduce the rate of oxidation of the particles when exposed to air and almost stopped it completely when stored at low temperatures. A key feature of the coating was its ability to be removed when heated up so that the underlying copper was exposed and able to interact with neighbouring particles. Trials of the sintering of the copper powders using heat treatments including laser heating were carried out, but showed only partial sintering of the particles. Much greater success was achieved when the treated copper powders were used to prepare an electrically conductive adhesive by mixing them with a resin: when the resin cures, it shrinks, pressing the copper particles together making an electrically conductive pathway. Such materials using silver as the filler are widely available due to the favourable properties of this metal mentioned above, but are far less common for copper. The adhesives formulated using copper in this research were found to have comparable electrical performance to silver based ones, although further work is needed to improve their long term reliability. By printing the paste and placing electrical components, fully functioning electrical circuits with copper interconnects were constructed and future work is looking at ways to exploit the technology. Web address with further details if you wish: BENEFICIARIES Beneficiaries of the research The research led to the development of a technique for the preservation of copper powders to enable them to be used as fillers in electrically conductive adhesives, replacing the higher cost, less abundant silver fillers that are traditionally used. This may open up new opportunities for lower cost printed electronic devices to be manufactured, bringing economic and social benefits through a wider range of applications and markets including electronic tags, consumer goods and healthcare. The opportunity to exploit the technology is now being explored through initial discussions with a number of companies. The project provided benefits to the researchers through experience gained in the handling of copper powders together with their analysis. One researcher gained valuable practice in writing research proposals and later moved to another University where he was successful in being awarded an EPSRC Career Acceleration Fellowship. The background research skills were also transferred to a new PhD student who is now continuing the research.

4 (a) Publications (1) Please list up to five significant publications that arose principally as a result of the research funded through this grant and indicate the type (journal, conference, book, patent, software, other); whether there was an international or industrial co-author and whether the publication was refereed. AUTHOR(S) TITLE REFERENCE Journal/Conf. year vol. page TYPE Refereed? Industrial co-author? International co-author? S.J. Ebbens, D.A. Hutt, C. Liu Patterning of Copper using Inkjet Printing of Self- Assembled Monolayers Electronics and Packaging Technology Conference (EPTC) Dec Conference paper X R.E. Litchfield, D.A. Hutt, C. Liu and D.P. Webb Preservation of copper powders for electrical interconnect applications Journal paper in preparation In preparation

5 (c) Follow-on Support Please indicate the level of further research support that has arisen principally as a result of the work supported under this grant. Please do not include any contributions already listed under the section on collaborators on the current project. FUNDING SOURCE DETAILS SUPPORT ( ) EPSRC Other UK Research Council Other UK Government IMCRC feasibility study to apply the technology to the functionalisation of nanoparticles IeMRC feasibility study 78k 12k UK Industrial UK Other European Commission Other Industrial Supply of materials from potential industrial collaborators. Est. 500 Other Non-Industrial Internally funded (Loughborough University Materials Research School) PhD studentship to investigate curing of adhesives based on functionalised copper powders. 45k

6 PROJECT PARTNERS Details Project Partner 1 Name of partner organisation Address Line 1 Town/City Administrative Area/County Postal Code Country Xaar PLC Science Park Cambridge Cambridgeshire CB4 0XR UK

7 FINAL SUCCESS STORY FORM IMCRC Project Number: Summary of Case Study/Research Project This project aimed to develop methods to remove oxides from metal particle surfaces and coat them with self-assembled monolayers to restrict their re-oxidation. By preventing the re-oxidation of particles it would be possible to use them for a range of applications that require direct metal to metal bonding, e.g. sintering, without intermediate layers of oxides preventing this taking place. The research was conducted through a number of work packages, largely focussing on copper as the base metal due to its importance in electronics manufacture. Initially, the research aimed to use a chemical solution based method: to do this, techniques to transfer powders between liquids needed to be developed and many alternatives were investigated. A highly efficient process was identified and the coating of the SAM on the copper surface with almost no oxide present, was confirmed by XPS analysis. In addition, the ability of the powders to be preserved during storage in air at different temperatures was measured using XPS and it was found that storage in a freezer enabled the particles to remain largely oxide free for many months. Extension of the coating technique to increasingly smaller particle size powders was investigated to determine its applicability across a range of materials and applications: it was found to be highly successful for 10 m sized materials and could be applied to 1-2 m sized powders. Throughout this development, approaches to scaling up the technology to meet the requirements of industrial processes were considered. Extension of the technique to nanopowders was not possible and an alternative approach was investigated through another IMCRC feasibility study. A number of methods to sinter the powders under an inert atmosphere were investigated including the use of a vacuum oven and CO 2 laser sintering. Using the higher temperature laser method, some evidence of sintering was observed, although it was difficult to control the heating to achieve a uniform treatment. As such, efforts were directed towards using the preserved particles as metallic fillers in electrically conductive adhesive pastes where conductivity is achieved by direct contact of one metal particle with another within the matrix. This was very successful and enabled circuits to be printed. Electrically conductive adhesives usually consist of a metallic filler with an epoxy resin. During cure, the epoxy resin shrinks forcing the particles together, leading to electrical contact with low resistance. Traditionally, silver has been widely used as the filler due to its good conductivity, but more importantly its ability to resist oxidation such that the silver particles can provide low resistance when in contact with each other. However, silver is very high in cost and relatively scarce and there has been much interest in developing materials based on copper as an alternative. In this work, preserved copper powder was mixed with an epoxy adhesive, printed onto a substrate and cured at 100 to 150 o C under an argon atmosphere. Tracks with conductivity comparable to that obtained with silver were produced and by combining the process with the placement of components, fully working electrical circuits were produced. As far as we are aware, this is the first time that this has been achieved in this way and opportunities to exploit the technology with companies are being explored. Evidence of Academic Impact, including impact on the research area The project led to the development of a copper based conductive adhesive with electrical performance comparable to that obtained with silver based materials. This is expected to be lower cost and more materials efficient than the conventional silver filled materials. Demonstrator circuits have been fabricated by printing the copper adhesive, for which there appear to be no comparable examples in the literature, and these have been used to discuss future collaboration and exploitation with industry. As a result of the research and input from Xaar, an opportunity to apply the SAM technology to the functionalisation of copper nanoparticles was identified and further IMCRC funding was secured to carry out another feasibility study in collaboration with a new industrial partner. The issues identified with sintering of the powders and curing of adhesives led to discussions with the Materials Department at Loughborough University. A joint application to the Materials Research School, Loughborough University was successful and a PhD student commenced his study in October 2010 to investigate alternative methods of sintering and curing of copper based inks and pastes. A journal paper and conference paper are now in preparation based on the conductive adhesive paste research.

8 Evidence of Impact on the Economy and/or Society The project was successful in developing a coating strategy for copper powder that would enable it to be used as a filler in conductive adhesives. The ability to replace the silver fillers in traditional conductive adhesives with lower cost, more abundant copper, offers an opportunity to increase the application of these materials in the area of printed electronics which is a rapidly growing field with applications in diverse markets such as electronic tags, consumer goods and healthcare. The potential to patent the process has been investigated and discussions have been held with several companies to investigate the potential for exploitation. Benefits to Researchers, Students, or Collaborators The project provided benefits to a number researchers. Dr Stephen Ebbens, the original Research Associate, gained experience of the handling of copper powders together with their analysis. He also gained valuable experience of writing research proposals and has just recently been awarded an EPSRC Career Acceleration Fellowship at Sheffield University. Dr Robert Litchfield also gained valuable research skills and was able to pass on his experience of materials preparation to a new PhD student who is continuing the work investigating the curing of conductive adhesives. Background information and relevant website(s)