Disassembly and recycling - technology to meet the manufacturing revolution

Norman Stockham is Technology Manager of the Microtechnology Centre and Head of the Electronics and Sensors Industry Team. He has over 20 years experience in the research and development of microelectronic and sensor packaging and assembly and is currently involved in lead-free soldering and environmentally sustainable manufacturing technology.

Impending environmental legislation from around the globe is causing a total rethink of manufacturing processes in a range of industrial sectors including electronics goods and automotive. Roger Wise and Norman Stockham summarise some of the areas where TWI is moving to meet the challenge that product 'take-back' legislation is imposing.

Traditionally, the manufacture of a product can be thought of as a linear process, starting with manufacture and ending with disposal of the product when its useful life has expired.

This approach is resource intensive, and leaves the problem of the disposal of the product at the end of its life. Many such products are currently buried in landfill sites, but this activity will be strongly discouraged in future by impending legislation. 'Take-back' legislation requires that products are repossessed by the companies which manufactured them so that they can be recycled or disposed of appropriately.

Alternative approaches therefore need to be considered and technologies required to enable change must also be developed. To minimise or eliminate scrap materials for landfill, better use must be made of existing materials. The implication is that products in future will need to be designed with maintenance, refurbishment, re-use and recycling in mind. This is one aspect of a movement called 'sustainable development'. One possible view of future product manufacture and life cycle management is shown in Fig.1.

The new technology indicated on the graph will include the means to disassemble some or all of the components, and the means to add value to the components by sorting, testing, re-using, refurbishing and, if necessary, recycling the materials used to make components. It is not currently possible to predict the ownership of the product at each point on the graph shown in Fig.1 but it is likely that companies manufacturing the product may wish to (or must) take ownership of it again at the disassembly and recycling stages.

Product design

To accomplish the type of product life cycle envisaged in Fig.1, the design of the product must incorporate certain key features such as:

• Some facility to allow for upgrades or product evolution.
• The means to disassemble and inspect the product thoroughly and in a cost effective manner.
• The ability to disassemble and recycle as much of the product as possible.

This is a stringent set of criteria which suggests some degree of modularisation of component parts by function. In this way, product development and evolution can be handled one module at a time. Eventually of course, a completely new product will probably be required and at this point the 'old' product will have to be refurbished or recycled.

In Japan, the concept of design for disassembly has been given a name, 'inverse manufacturing', ^[1] and is being developed rapidly to minimise land-filling over a range of industrial sectors, such as automotive, electronics, textiles and building products. In the EU, Directives for the disposal of electrical and electronics goods are proposed to come into operation in 2004 and those applying to other industries including automotive are under discussion.

One example of a Japanese company which has already fully embraced the inverse manufacturing concept is Fuji Xerox. This company manufactures a photocopier in accordance with a zero landfill policy where redundant components are refurbished or fully recycled. ^[2]

Joining for disassembly

The design of joints which can be disassembled several times requires a thorough knowledge of the specification of the product, the materials involved and the in-service environment. Having assessed the requirements for the joint, the next issue is the selection of the most appropriate joining/disassembly technologies.

There are four main categories of joining:

• welding
• adhesive bonding
• brazing and soldering
• mechanical fastening

Of these, techniques in the categories welding, adhesive bonding and brazing, all involve the formation of chemical bonds (with a few exceptions such as welding of thermoplastic polymers).

Disassembly techniques

There are many techniques available for disassembling joints and components, some of which allow reassembly with or without some rework and others which break the component down for material recovery.

Techniques for disassembly are shown in the Table.

Table: Some techniques for disassembly

Of these the biological and chemical techniques are of special interest because they can involve material transformation and re-use, leading to products which are truly sustainable. TWI is involved with one example of this, the biodegradable circuit board.

Mechanical methods for disassembly

For structures where sealing and mechanical strength are important, joints are often welded. These structures will have a lifetime which can be calculated based on the properties of the materials involved and the in-service loads. At the end of the useful product life, recycling can be envisaged and often a degree of disassembly is required.

In such cases, one option is to load a defect which has been deliberately introduced to permit disassembly at the appropriate time. It is of course of vital importance that defects added for the purpose of disassembly at the end of the life of the product do not cause premature failure of the product in-service. For this reason it is envisaged that defects can be introduced by a number of means including during manufacture in a region of negligible mechanical stress. A simple example of the type of defect envisaged is the can often used to store fizzy drinks. The defect is at the ring pull - it will not fail during manufacture and transportation, but will fail in a controlled manner when the can is to be opened ( Fig.2).

It should be noted that all of the techniques so far devised for the disassembly of welded structures in metal mean that the parts cannot be welded back together again to produce an identical item without first adding material to the joint. There is a need to develop welding technology for metals which can be made and broken repeatedly without the addition of any other material between joining operations.

New developments at TWI

TWI with its extensive knowledge of materials, assembly technology and environmental conditions/requirements has found itself ideally placed to assist companies in assessing the impact of environmental legislation on current product lines and the design of the next generation of products. It is also in the process of developing certain joining and disassembly techniques in readiness for the perceived need and a few examples of these are given below.

Lead free solder for circuit boards

Connections for electronic devices to circuit boards generally involve the use of solder. Lead is due to be banned from solder in the EU and this has precipitated much development and testing of lead free solders.

To achieve an adequate strength bond between material interfaces it is necessary to establish intimate contact across the joint interface. This is achieved in adhesives, soldering and brazing by adding a third filler material which can fill the interfacial gaps. For disassembly or reuse, this can be advantageous as the joint can potentially be reheated/mechanically stressed and broken without damaging the product. If necessary ( eg replacing an electronic package on a PCB) it may then be possible to remake the joint. However, it should be noted that in some cases ( eg plastic components) an adhesive may be detrimental to the recycling process as it effectively acts as a contaminant on the plastic surface should it be remelted. This problem may be eased by the development of adhesives with a closer chemical match to the components being joined.

To achieve intimate interfacial contact in welding it is necessary to use fusion or plastic deformation techniques. In both cases, joint disassembly for reuse is more complex than soldering, brazing or adhesives because it will probably require some form of machining to cut through the interface. Re-assembly then relies on the design of the component in terms of having sufficient material to enable re-welding. However, in terms of recycling, welding has a major benefit in that it does not introduce additional potentially contaminating materials which could interfere with the remelting/reuse of base component materials. Mechanical fastenings, at first sight, appear very attractive for disassembly as components can be taken apart many times without damage. However, in the case of screw fixings they are also considered to be a problem due to the length of time (predominantly manual dismantling) it takes to remove a range of screw/bolt connections.

It can be seen that product design engineers need to consider carefully the end-of-life reuse/recycling requirements prior to establishing a joining technology.

TWI is involved in establishing the wetting characteristics of new solder alloys in conjunction with a Cambridge University PhD programme on lead-free solder joint (primary ball grid array/flip chip) modelling. This and other studies are highlighting that the most promising replacements are Sn-Cu, Sn-Ag and SnAgCu with small additions of other elements. A solder 'production line' is being set up at TWI North (Middlesbrough) to enable companies to try out some of these alloys on their components.

Local solder joint heating techniques such as lasers ( eg diode, Nd-YAG) are being investigated for use on cheap/low temperature recyclable boards. Conductive adhesives and welding/brazing are also now being actively considered for specific boards and interconnections.

Other means of connection can also be envisaged such as the increased use of plugs/sockets and novel mechanical attachments, some using materials such as shape memory alloys.

Biodegradable circuit boards

The biodegradable circuit board offers a method for disassembly and 'closed loop' recycling ie the products of the biodegradation can be fed back into the food chain of the organisms used to create the next circuit boards. An early prototype, based on starch is shown in Fig.4. TWI, in partnership with BTTG, is currently seeking partners to develop this technology for exploitation.

Thermal methods

In the early 1990s, TWI invented a technology for the rapid joining of structures involving dissimilar materials which it called the polymer coated material (PCM) joining technique. The basic concept was that components to be joined were coated with a thin layer of a thermoplastic polymer before being joined using a polymer welding technique to another coated component ( Fig.5). It was claimed that strong joints could be produced very rapidly using this technique ^[4] and that, more importantly, the joints could be disassembled on the application of sufficient heat energy to re-melt the thermoplastic. Welding of polymers involves the entanglement of polymer chains at the interface and so is strictly a mechanical attachment technique, albeit on a very small scale. The fact that these mechanical attachments can potentially be made and remade repeatedly is therefore to be expected.

Until recently the weakness in the technology has been the need to use solvents to coat materials to be joined with a thin layer of thermoplastic polymer. Recently, some trials have been conducted which suggest that coatings of thermoplastic polymer can be produced by in-situ polymerisation at the surface of the components to be joined. This approach would eliminate the need for solvents and could also be carried out locally therefore avoiding the need for a large capital investment.

Friction extrusion - an emerging recycling technology

As a solid phase technique, friction heat and pressure caused by relative motion and the initial restriction in extrusion flow, allows a plasticised layer to form without the need for an external heat source. The plasticised layer is of limited thickness and forms in close contact to the plunger. A relatively localised heat affected transition zone separates the plasticised layer from the compressed powder which remains stagnant within the cartridge. With continued generation of the plasticised layer and progressive consumption of the material, a solid rod of new material is hydrostatically extruded. This approach compares favourably to other recycling techniques where the aluminium is actually melted. For example, techniques involving melting often result in the generation of dross due to the oxidation of some of the aluminium. They also consume a lot of energy since all of the aluminium has to be melted. Friction extrusion on the other hand, operates with aluminium in the solid phase, resulting in virtually no oxidation, and consuming less energy than the fusion techniques.

In trials, TWI has been using aluminium 6082 (H30) machine swarf and the resulting extrusion has displayed good ductility in a 180° bend test ( Fig.7) and excellent consolidation. This technology is at a relatively early stage of development but the work to date has clearly demonstrated its feasibility.

General recommendations

• Assess the relevance/impact of the approaching environmental legislation on your current products and assembly technology.
• Wherever possible avoid the use of environmentally toxic materials. When their use is unavoidable, ensure that they are clearly marked and easy to separate.
• Design new products for reuse, recycling and energy recover.
• Make products simple and cheap to take apart.
• Wherever possible use the minimum number of different materials.
• Consider the environmental legislation as a business opportunity.

Conclusions

• Legislation will drive a change in manufacturing industry which will force companies to reassess the value chain for products such that minimal or zero landfill is involved.
• Joining, disassembly and recycling concepts will have to be developed before products embracing the 'zero-landfill' concept can be finally designed and manufactured.
• TWI is in the process of developing certain key technologies and skills to help its Industrial Member Companies to meet the impending changes with the confidence to exploit the opportunities which will be presented.

References