Active Disassembly and Reversible Interconnection

Transcription

1 Active Disassembly and Reversible Interconnection T. Suga and N. Hosoda The University of Tokyo, RCAST Komaba , Meguro-ku, Tokyo , Japan URL: Abstract Disassembly, dismantling, debonding, detachment or disconnecting of an assembly with ease is the critical issue to realize D E effectively. However current joining technology especially used for electronic products, such as soldering, wire bonding, thermo-compression bonding, adhesive bonding, are not used to be designed for dismantling, even though rework or repair is indispensable in practical applications. In this paper, the concept of active disassembly and reversible interconnection is critically reviewed by considering the bonding and debonding mechanism of the materials interfaces. Disassembly process is characterized both by disassembly energy and disassembly entropy. Disassembly energy is supplied indirectly but efficiently by a certain mechanism built-in the product or by releasing a potential energy builtin the assembly which may be triggered by a smaller energy than the actual debonding energy. Active disassembly is defined as such disassembly operations using an active mechanism which is build-in the assembly macroscopically or microscopically, or even in the material itself to reduce the necessary energy and/or entropy for disassembly. The reversible interconnection is a concept for interconnection method which enables the products assembled and disassembled reversibly. Active assembly can be used for realizing a reversible interconnection. Several new attempts to develop a method for reversible interconnection are presented. The bonded interface is weaken by segregation of alloying elements, or by hidridation of hydrogen storage alloy used as bonding intermediate which becomes very brittle or pulverized by absorbing hydrogen. Also selectivity of disassembly can be achieved by using temperature dependence of the phenomena of hydrogen absorption. Some examples will be shown to demonstrate how the concept is applied to electronic products or dismantling PWB assemblies. Introduction The main idea behind the 'Design for Disassembly', one of the key technologies of the Inverse Manufacturing, has been to design products which can be easily dismantled and disconnected at the ultimate stage of their lives with the external force provided by some tools. Now, however, a new concept is being added to it: to equip the products with potential energy or a mechanism inside that can actuate themselves to come apart when we want to disassemble them. Compared with the conventional 'passive' approach, the new concept may be called 'Design for Active Disassembly (DAD)' The term 'Active Disassembly' appeared first in a report about future disassembly and recycling technology of the Delft University of Technology, written in In their report, the active disassembly is defined as "the principle that products takes themselves apart or split open at the end of their useful life cycle. This can be accomplished by using a drastic change in temperature or an electrical charge. In order for this to happen, the product would require an actuator inside it, or less desirably, applied to it after its useful life cycle...." With regard to it, the Design for Active Disassembly should not be limited to this additional mechanism of the actuators only on a macroscopic basis. The defmition of this concept should be much Eiroader and general one, extending to micro-actuators which can be produced by means of micro-fabrication technology, materials which can separate on their own in their end-oflife, and even debonding of materials interface on atomic scale. This paper at first reviews the processes of the assembly and disassembly by categorizing the 'energies' required for the processes and clarifies conditions for the Active Disassembly. Reviewing the current technology for assembly, it is noticed that little consideration has been taken for the disassembly and debonding especially in the bonding technology. For instance, conventional metallurgical joining methods at elevated temperatures are usually accompanied by uncontrollable chemical reactions. These processes result in formation of residual thermal stresses and brittle reaction products at the bonded interfaces, both of which may cause undesirable debonding at the interface, and reduce the reliability of the inter /00/$ IEEE 330

2 connections. Just when they are needed to be disassembled, they can never be debonded and disassembled exactly at the bonded interface because of degradation of the interconnection and the consitutents of the assembly. It means that the disassembly process is no other than the failure of joints, and then the disassembly process is regarded as irreversible. Thus the technological development for interconnection should aim to enable constituents of assembly not only to be bonded but also to be debonded reversibly. The concept of the reversible interconnection is introduced to overcome the problems in the current interconnection technology which takes care of only one way of interconnections. In the reversible interconnection, it is important not only to achieve a reliable interface but also to debond and separate the joints, so that any material could be bonded and debonded reversibly. If the structure and nature of the bonded interface are controlled at an atomic scale and the assembly is separated ideally at the bonded interface, then a closed loop assembly system can be established. Such reversible interconnections are not necessarily thermodynamically reversible. However the concept may provide a key technology of active disassembly. Assembly and Disassembly Processes How components are assembled should be considered in order to analyze how to disassemble them. The assembly consists of two processes: A) interconnection between components and B) geometrical constraint. The interconnection is made by AI) either chemical bonding or A2) potential energies of force fields, and the geometrical constraint is gained by SI) screws, snap fits, and other constraining parts or B2) geometrical interlocking of parts. As for the chemical bonding, it comprises Al-a) bonding through an intermediate layer and Al-b) direct bonding. The intermediate layer includes structural intermediate layer such as stress relaxation layer, bonding layer like braze, solder and adhesive, and reaction layer such as reaction products and difision layer, while the direct bonding is composed of solid state bonding, diffusion bonding, ultrasonic bonding, fictional bonding, and specifically surface activated bonding (SAB) at room temperature. Meanwhile, the potential energy of force fields includes A2-a) magnetic, A2-b) electrostatic, A2-c) gravity, and A2-d) elastic forces (spring, screw, snap fit), usually used together with frictional forces. Therefore, it is clear that the disassembly also consists of two processes: release of A) interconnection between components and B) geometrical constraint. The geometrical constraint includes B- 1) deformation or removal of the key components such as screws and snap fits and B- 2) fracture or elimination of the key components. In addition, the actual disassembling process requires the removal of the disassembled components through a certain path. The complexity of the path decides the trickiness of the removal. It should be also noted that we need some tool to remove them from the main body. Considering from all those things, we can evaluate the 'disassemblablity' quantitatively based on 1) energy for the disassembly, specifically for disconnecting the bonded parts and releasing the constrained parts, and 2) entropy for the disassembly which indicates the degree of difficulty of the disassembly judging from how may methods are used to make the interconnections as well as the number of paths to take in the process of disassembling. [2] Another parameter which affects the disassemblability is the number of the components assembled all together. For instance, which is more difficult to disassemble either a very complicated structure like a jigsaw even if made of a small number of components or a simple one with countless parts each of which is systematically assembled with screws? Generally speaking, the more components are used, the more energy it takes to disassembly them. However it is also true that it will take much longer time if labors in disassembly plants are not informed how to do with the jigsaw. Disassembly Energy Fig. 1 shows the disassemblability analyzed from the energetic point of view The disassembly energy. can be classified into energies for 1) disconnecting the interconnected components, 2) releasing the constraints, 3) removing the disconnected components, and 4) operating tools necessary for the disassembly work. Of these, 1) and 2) are the intrinsic energies that the components themselves have. The sources of those energies are 1) extemal forces and 2) field forces (magnetic, electrostatic, gravity, and elastic potential forces).

3 Assembly Disassembly Debonding energy Components - Key component - Mutual wnstraint Disassembly energy Release energy Removal of components Operation energy for tools I / Field forces - Electronagnetic forc - Gravitation potentia External forces I I Enegy source Tools Figure 1. Energies required for disassembly process The disassembly energy can be classified into energies for 1) disconnecting the interconnected components, 2) releasing the constraints, 3) removing the disconnected components, and 4) operating tools necessary for the disassembly work. Of these, 1) and 2) are the intrinsic energies that the components themselves have. The sources of those energies are 1) external forces and 2) field forces (magnetic, electrostatic, gravity, and elastic potential forces). And 1) contact tools and 2) non-contact tools are used, for the disassembly. These non-contact tools are one - of the essentials for the smart disassembly. It consists of the use of 1) force fields including magnetic, electrostatic, gravity and elastic potential energies, and electromagnetic field (for instance, laser or microwave), and 2) media such as reactive media (hydrogen, organic solvent), and thermal media (heat bath, coolant). The conditions for high disassemblability are 1) lower disassembly energy, 2) lower disassembly entropy, 3) smaller number of interconnections and parts, 4) shorter disassembly paths, 5) smaller and lighter parts, 6) thorough communication of the disassembly information, and 7) the active disassembly. Active Disassembly interpreted as the potential energy and the energy transducer built in the products, and this is why they were named 'active'. The main examples of the energy transducer are motors, bi-metals, water evaporation, mechanical links, and other built-in transducers or actuators. The built-in potential energy is sometimes larger than the interconnecting energy, and vice versa. In the former case, the excessive energy is used remove the disassembled components (Figure 2), whde in the latter case, the energy Trigger or retease ot potentsl energy 1 - \ pig5ggq Polentlal energy buildin Figure 2. Active disassembly in case of the potential energy larger than the debonding energy. In a very limited sense, the active disassembly can be 332

4 A Tflgger of release of potential energy pzzziq \T Potential energy build-in 1 Debonding energj j-anerdisassemblyl Figure 3. Active disassembly in case of the potential energy smaller than the debonding energy. shortcoming is made up for by the extemal forces and others ( Figure. 3). The release of those potential energies needs a trigger. And what matters in the active disassembly is that the energy for the trigger should be small. The trigger is 1) thermal activation energy (gunpowder, chemical reaction), 2) entropy change (mixing of reactive substances chemical reaction), release of constraint (force field of potential energies). Other requirements in the active disassembly are as follows: 1) to control the bonding state at the interface in order to reduce the interconnection either by chemical bonding or field forces, 2) to release the constraints with the self-deformation or self-removal of components deformation of snap fits, bimetal or shape memory alloys), and failure of elimination of key components (dissolution of screws or snap fits, removal of screw heads), 3) to use the field forces for the removal of the disassembled components (gravity: removal of keys by inclination, electromagnetic field: removal of keys). Reversible Interconnection deformation of the component itself like snap-fit made of the shape memory alloy. However the most inovative method could be found in development of the reversible interconnection. There are several ideas for debonding method for the reversible interconnection compiled in Table 1. It is known that the silicon terminated by hydrogen is very inactive and difficult to be bonded to Al. This might be due to the fact that the formation energy of AI hydride is low. Similar phenomena are enbrittlement of grain boundaries of certain alloys by segregation of hydrogen at the interface. These phenomena suggest a possibility to control the interface bond strength by hydrogen radical or hydroxyl group. Another idea is to use the hydrogen storage alloy as a bonding intermediate which is pulverized or produce a large expansion strain when it absorbs hydrogen as illustrated in Figure 4. It has been demonstrated that the method works for Al- LaNiAl' alloy joint which was bonded by Surface Activated Bonding (SAB) method and debonded by the suggested method, ie. when the joint is places in hydrogen atmosphere, then the joint failed by the pulverization of the LaNiAl layer without applying any external force. It was demonstrated that a construction using LaNiA1 intermediate collapses in hydrogen atmosphere of about 3 MPa at room temperature. Figure 5 shows a peel off of a thin Cu film from glass substrate, between which a thin film of LaNiAl alloy was inserted Because no enough stress could be accumulated in the film of a thickness of 1 'pm, it is not pulverized. The SAB described above is a bonding method which has been developed recently intend to produce an atomically clean and sound interface between materials [3]. The bonding procedure consists of the surface activation process using ion bombardment or plasma irradiation for creating clean and activated surfaces followed by a mechanical contact in clean atmosphere such as vacuum or inert gases. This apparently timeconsuming method, however, is promising for certain applications in next generation of micro-system integration. The fact that no intermediate layer is formed by Table 1 Examples of reversible interconnection Debonding by interfacereactions with hydrogen-radicalor hydroxyl group; A1-Si Hydride formation and pulverization of hydrogen storage alloy at the bonded interface; A1-LaNA1 Enbrittlement of the bonded interface by segregation of alloying elements or formation of brittle intermetallic compounds; A1-Fe Interfacialstresses caused by the thermal expansion mismatch; Cu-A1203 There are several ideas for the active diassembly in removing the constraint between the components such as 333 the SAB provides also another possibility of separating joints by interfacial stresses produced by the thermal

5 expansion mismatch between the bonded materials. Such combinations include for instance Cu - Al2O3 or Cu - Si02. Raw material c Hydriding I ration Figure 4. Concept of the reversible interconnection using hydrogen storage alloy. Enbrittlement of the bonded interface by formation of brittle intermetallic compounds provides also another possibility of debonding. A typical example is Cr-Ni stainless steel and A1 joint. It is difficult to bond both materials directly by conventional diffision bonding tecbques, because a very brittle intermetallic compound is formed during the bonding process. To avoid such formation of the intermetallic compound, a &ffision barrier coating such as TiN is used. However, the direct bonding of such materials is possible by the SAB method. The joint then is heated up to 800 C, high temperature reactions take place between the A1 and Cr-Ni steel, and then it is observed that the joint fails without applying any external force due to the formation of the intermediate layer with an amount of voids. Concluding remarks &fore nydrsgen exposure I" I After hydrwen exposure Figure 5. Cu circuits on a glass substrate with an intermediate layer of hydrogen storage alloy, and their peel-off from the substrate after exposure in hydrogen atmosphere. The most critical issue of the active disassembly to be considered is obviously its cost and safety in operations of the assembly. Also selectivity in disassembly is to be considered in order to dismantle a specific part of specific parts. The selective disassembly includes such as 1) to use solders of different melting temperature, 2) to use hydrogen storage alloy of different activation temperature, and 3) materials of different critical strain rate of rapture. [ 1 J C. B. Boks, E. TempelmanF i nal report of the Delphi study on hture disassembly and recycling technology for the electronics and automotive industry, TU Delft, April 1997 [2] T. Suga, K. Saneshige, J. FujimotoQ uantitative Disassembly Evaluation, ISEE 1996, [3] T. Suga, Y. Takahashi, H. Takagi: Surface activated. bonding -an approach to joining at-room temperature. Ceram. Trans., 35 (1993)