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Studies on three-dimensional moulding, bonding and assembling of low-temperature-cofired ceramics for MEMS and MST applications
Materials Chemistry and Physics 89 (2005) 72–79
Studies on three-dimensional moulding, bonding and assembling of low-temperature-cofired ceramics for MEMS and MST applications P.K. Khanna∗ , B. Hornbostel, M. Burgard, W. Sch¨afer, J. Dorner Fraunhofer-Institut f¨ur Produktionstechnik und Automatisierung, Nobelstr. 12, D-70569 Stuttgart, Germany Received 14 May 2004; received in revised form 14 June 2004; accepted 24 August 2004
Abstract This paper broadly illustrates the preparation of three-dimensional shapes in low-temperature-cofired ceramics (LTCC). Test structures are fabricated by moulding single-layer green tapes into cylindrical form of different dimensions in order to investigate the penetration of cracks and the limiting value at which they appear. The factors responsible for the propagation of cracks have been analysed. Another important aspect which has been exemplified is the bonding of processed LTCC modules to metal parts with a dissimilar coefficient of thermal expansion. This is important from the point of view of attachment of LTCC-based micro-electromechanical systems (MEMS) or micro-system technology (MST) devices to a functional mechanical element. A methodology has been discussed for a smooth transition in the coefficient of thermal expansion of ceramic and metallic part in order to minimise stresses and avoid micro-cracks in the interconnections. An assembly of different post-fired modules by a fastening mechanism of individual modules for realizing a complex system are also described. Based on the above studies, a preliminary design to fabricate, shape, bond and post-process the LTCC substrate for the realization of a complex system together with the crucial processing issues is presented. Efforts have been made to implement this design and process steps on a real-time ball-bearing to prove the suitability of LTCC material for MEMS, MST and other complex systems. © 2004 Elsevier B.V. All rights reserved. Keywords: Ceramics; Multilayers; Coatings; Crack
1. Introduction Low-temperature-cofired ceramics (LTCC) is basically a mixture of recrystallized glass, ceramic powder, binder and organic solvent. This material mixture is cast in the form of tapes of variable thicknesses using doctor-blade. They are called green tapes because they are manipulated at the immature stage, i.e. before firing and sintering. The technology evolved around LTCC tapes is primarily a multilayer integration technology for electronic packaging. In this technology, the signal layers and passive elements are embedded into multilayer ceramics and active devices are buried into predesigned cavities in order to realise a functional electronic module. In the last several years, LTCC has an ever-increasing ∗
Corresponding author. Tel.: +49 711 970 1263; fax: +49 711 970 1007. E-mail address: [email protected] (P.K. Khanna).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.08.031
demand in industrial and telecommunication area. Some inherent features of this technology include: integration up to 50 layers, hermeticity, bio-compatibility, matching of coefficient of thermal expansion with Si, high thermal dissipation, hightemperature and -frequency application, and reduced size and cost. Due to the above advantages, it is a potential technology for micro-electromechanical systems (MEMS) and microsystem technology (MST) devices and their packaging. For the last few years, researchers have worked extensively [1–7] to use LTCC in the above-mentioned areas. In order to use LTCC for MEMS and MST, there is a need for developmental work for realizing special application-oriented products with micro-fluidic and electrical interfaces, inertial-element integration and also products with biochemical compatibilities. Packaging techniques have to be developed for isolation of the sensing elements such that it interacts with the measurand and simultaneously protects it from the environmental stresses
P.K. Khanna et al. / Materials Chemistry and Physics 89 (2005) 72–79
and the stresses generated from packaging. Special packaging techniques for micro-fluidic applications like micropumps, -valves, -mixers and -flowmeters have to be realized. Thermo-mechanical systems and applications, thermal and fluidic issues in MEMS and MST and advanced cooling techniques together with thermal and structural issues, their reliability and failure analysis need attention. Heat-transferenhancement techniques, high-power packaging using novel packaging technologies and advanced materials also need to be incorporated.
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All the above is in principle possible using LTCC because of its remarkable advantages mentioned above. In view of the above-stated applications, it is utmost necessary or rather mandatory at this stage to study the possibility to mould LTCC into different shapes and sizes, its attachment to a mechanical part and assembly thereafter. This work will establish the versatility of LTCC technology and the micro-systems so forth fabricated can be readily integrated into any system or sub-system. Before going into the details of shape formation, bonding and assembly, it is
Fig. 1. Cylindrical pre-laminated LTCC structures for surface lengths greater than 6 mm (a–c) and for surface lengths less than 6 mm (d–g).
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P.K. Khanna et al. / Materials Chemistry and Physics 89 (2005) 72–79
important to understand the basic steps of LTCC manufacturing. 2. Low-temperature-cofired ceramics fabrication process The LTCC technology is a well-defined method to produce complex multilayer circuits with the help of single tapes. Conductive, dielectric and resistive formulations are screenprinted on each tape. The single sheets are then laminated together and fired in one step resulting to a miniaturized module. Another important advantage is that every single layer can be inspected and in the case of inaccuracy or damage, the same is replaced before firing, thus improving the yield of production. The low firing temperature of about 850 ◦ C makes possible to use low-resistive materials (silver and gold) instead of molybdenum and tungsten (which have to be used in conjunction with high-temperature-cofired ceramics). Resistors are processed with the help of special resistive pastes. These pastes are printed on top or bottom side, fired and later laser-trimmed to obtain precise values. Additionally, hightolerance-buried resistors can also be produced. Capacitors and inductors are build-up only with the help of special predesigned structures. It is possible to involve photo-processes in the LTCC process sequence, but screen-printing of the formulations on green sheets with the help of a conventional thick-film technique is more common and easily adoptable. Standard thick-film pastes are not used except for several post-fire processes; this is due to the shrinkage of tape in x-, y- and z-axis during the firing cycle. The major process steps for LTCC fabrication are as follows [8,9].
2.1. Slitting, preconditioning and blanking The green sheets are normally shipped on rolls of different widths and upon its arrival the tape has to be unrolled onto a clean stainless steel table. The sheets are cut to a pre-decided size with a razor blade, laser or a punch system. These parts have to be a little larger than the blank size, if the material needs to be pre-conditioned. If a laser is used, it is very important to control the power to avoid firing of the green sheets. Some tapes need to be pre-conditioned, which means that the green sheet has to be baked in an oven for about 30 min at 120 ◦ C (depending on the manufacturer and the material). Normally the tapes are shipped with an applied thin plastic sheet called Mylar, which has to be removed at the latest before lamination process. Mylar is used as a filling mask for the vias by some manufacturers. Orientation marks, lamination tooling holes and the final working dimension in case of to-be-pre-conditioned tapes are obtained using a blanking die or laser. In order to compensate for the difference in shrinkage in x- and y-directions, individual parts are rotated by an angle of 90◦ . 2.2. Via formation and filling Vias may be punched or drilled with a low-power laser or via a punching system. Vias can be filled with a dedicated via filler equipment or a conventional thick-film screen printer. In the latter case, the tape has to be placed on a sheet of paper which in turn lies on a porous stone. The tape is held at its place with the help of a vacuum pump and this vacuum also assists the filling of vias. In this process, the vias must have a larger diameter than tape thickness and the smallest possible
Fig. 2. LTCC metal bond (a) and cross-sections of the same (b–d).
P.K. Khanna et al. / Materials Chemistry and Physics 89 (2005) 72–79
size of vias to be filled depends on the viscosity of the paste. Both methods require a mask and this should be made on a 150–200 m thick stainless steel plate. Alternatively, a Mylar foil discussed above can also be used for mask making. 2.3. Printing Conductor tracks are printed on the green sheet using a conventional thick-film screen printer. The screens to be utilised are standard emulsion-type thick-film screens. Similar to via filling process, a porous stone is used to hold the tape in place. Printing of the conductor tracks tends to be easier and of higher resolution as compared to the standard thick film on alumina. This is attributed to the flatness and solvent absorption of the tape. After printing, the vias and conductors have to be dried in an oven at 80–120 ◦ C for 5–30 min depending on the material system used. Certain materials need to be levelled at room temperature for a few minutes before drying. Resistors are printed with the same screen printer but with a different screen. Screen-printing by photo-imageable paste for better line resolutions is used for high-frequency applications. High line resolutions can also be obtained with special fine-line screen printers. 2.4. Stacking and lamination Individual processed layers are placed – one on top of the other – over the tooling pins of a stacking jig. Hot soldering iron or heat pliers are used to tack the sheets at the edges. The lamination process can be carried out by two distinct methods. In the first method called uniaxial lamination, the
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tape assembly is pressed between heated plates at a temperature around 70 ◦ C and a pressure of approximately 200 bar for 10 min. This method requires a 180◦ rotation of the LTCC assembly after half the time. The uniaxial lamination could cause problems with cavities and windows. This method also causes higher shrinking tolerances than the isostatic lamination process mentioned below. A major difficulty is the flowing of the tape especially at the edge of the part during the firing cycle. The second method is to use an isostatic press. The stacked tapes are vacuum-packaged in a foil and pressed in hot water. The temperature and time are similar to uniaxial lamination process and the pressure is about 350 bar. Deep cavities and windows need to have an inlay during the process of lamination. 2.5. Cofiring and post-firing Laminates are fired in a single step on a smooth, flat setter tile, e.g. a ceramic substrate. The firing should follow a specific time–temperature profile and is done in a programmable box kiln. A typical firing profile consists of slow rise in temperature (around 2–4 ◦ C min−1 ) up to about 450 ◦ C with a dwell time of about 1–2 h, where the organic burnout takes place. Then the temperature rises to 850–875 ◦ C with a dwell time of about 10–15 min. The whole firing cycle lasts between 3 and 8 h depending on the material. Large and thick parts may cause the need of a modification in the firing profile. Resistor pastes need to have a defined firing condition and profile else their value may vary enormously. Certain materials need to be post-fired, which means that the paste is to be applied after firing the tape and then it has to
Fig. 3. LTCC substrate and mechanical screws and nuts (a) and mechanical fastening of the substrates (b, c).
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be fired again. The post-firing profile depends on the material used and varies in a wide range.
3. Experimental procedure and discussions 3.1. Crack penetration in moulded structures
2.6. Singulation and post-processing The singulation process consists of cutting of fired parts into smaller pieces or shapes. There are three possible ways to realize it. The first approach is to use a dicing saw, which is a common method and works quite well for rectangular shapes. It provides tight outside dimensional tolerances and allows high-quality edges. The second method uses an ultrasonic cutter. The final part shows low tolerances, may have unusual shapes and the process is very slow and expensive. The third possibility is to use a laser to cut the fired tape. The tolerances are tight, the quality of the edges is bad and the process is expensive. A number of post-processing steps for the LTCC manufacturing like surface mount technology, wire bonding, flip-chip, and inspection and testing of the LTCC module have to be carried out.
Cylindrical steel pieces (diameters of 1, 2, 3, 4, 5, 6, 8 and 10 mm and approximately 20 mm long) were utilised for rolling the LTCC green tapes. The steel cylinders were polished before use. Heratape CT-700 of thickness around 203 m was used for this study. Small pieces of LTCC tapes of 11 mm length were rolled on steel rods of above-stated diameter. After rolling, the tape was cut and adhered to the rod with a mild glue at the edge. No further processing like lamination or firing was done. The samples were inspected under an optical microscope to examine the rolled surface and find out the limiting point at which the cracks appear. Fig. 1a–c depicts the rolled LTCC structures on steel cylinders of different dimensions. Till the surface length of the tape was more than 6 mm, no cracks were observed and the surface was smooth and flawless. Below this length, the crack starts penetrating (see Fig. 1d–g), and smaller the surface
Fig. 4. Schematic representation: (a) front view of steel cylinder, (b) top view of the same, (c) multilayer LTCC structure, (d) front view of moulding fixture of LTCC into cylindrical shape and (e) top view of the same.
P.K. Khanna et al. / Materials Chemistry and Physics 89 (2005) 72–79
length, more dominant is the crack. The above experiment was repeated for different tape thicknesses and tapes at different periods of their useful life. It was observed that both the above parameters also have a significant effect on the penetration of cracks. 3.2. Adhesive bonding of LTCC to metal Test structures were fabricated using CT-700 Heratape of a thickness of around 129 m. The LTCC assembly consisted of three tapes and the fabrication process followed is already explained in detail in Section 2, so only specific fabrication parameters are briefly described here. The slitting and blanking processes were performed using a Lambda Physik KrF excimer laser. After stacking, the lamination was performed in a uniaxial system at a temperature of 70 ◦ C and a pressure of 25 MPa for 10 min. This was followed by burnout and firing which was done in a programmable box furnace at 850 ◦ C with a dwell time of 25 min. After firing, the LTCC substrates
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of different dimensions were singulated. The metallic parts electro-plated with Ni were bonded with the LTCC substrate using Loctite 460 adhesive and cured overnight at room temperature (see Fig. 2a). The cross-section of these samples is presented in Fig. 2b–d. No significant deformities or cracks were observed in the joining region. A factor of major concern in this bonding process is the mismatch in the coefficient of thermal expansion of metallic part and LTCC. Although, the stresses caused by this mismatch are to some extent absorbed in the adhesive but a better approach for this has been discussed in the later sections. 3.3. Mechanical assembly of LTCC parts LTCC substrates fabricated for adhesive bonding were utilized for this purpose. A circular hole of ≈1 mm diameter was drilled into some of the substrates using the excimer laser system (see Fig. 3a). Tapes with holes drilled at the green stage followed by lamination and firing were also used in
Fig. 5. (a) Methodology for CTE match between LTCC and steel part, (b) attachment of LTCC to steel part and (c) attachment of devices and lid sealing procedure.
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a parallel approach. Assembly of three to four LTCC substrates is demonstrated in Fig. 3b and c using a conventional mechanical screw and nut assembly. 3.4. Approach for moulding of LTCC The basic approach for the fabrication of cylindrical LTCC parts is shown in Fig. 4. Fig. 4a and b shows the schematic representation of the steel part. It is cylindrical in shape with a diameter of 100 mm and a height of 20 mm. In order to prepare the steel for attachment to LTCC, it should be first coated with a modifier to match its coefficient of thermal expansion with that of LTCC. The modifier has been described in the next section. Then a coating of Ni is provided to it to make it solderable. In a parallel approach, multilayer LTCC structures with cavities for device attachment should be prepared till the lamination stage (Fig. 4c). The burnout and firing of the same should be carried out between two concentric cylinders (Fig. 4d and e) in which the inner cylinder should be solid and the outer one meshed. The reason being that during burnout, the outer meshed cylinder provides an ease in the evaporation of organics.
terlayer on steel or LTCC substrate to make their coefficients of thermal expansions compatible. The modifiers can also be termed as stress-relief interlayers. The coefficient of thermal expansion of steel (≈10.8 ppm K−1 ) is approximately twice as that of LTCC (≈5.4 ppm K−1 ) and the matching of their coefficients is utmost important in order to avoid undue stresses on either of them when they expand on heating or contract on cooling. These residual stresses are very harmful and responsible for a low-strength and reliability of the joint. In order to match their coefficients with close tolerances, one or more layers of intermediate thermal expansion may be incorporated into the LTCC-steel joint. In the past, special solutions based on interlayer material selection for minimizing these residual stresses and for effective improvement in joint strength have been reported by researchers [10]. The attachment of the LTCC part with built-in cavities to the steel ring is shown in Fig. 5b. The attachment of devices in the cavities by a wire-bonding, flip-chip and surface mount technology attachment process together with the attachment of lids to the cavities for hermetic sealing are presented in Fig. 5c. 3.6. Fabrication of moulded LTCC structures
3.5. Approach for attachment of LTCC to steel The approach for attachment of LTCC to steel is shown in Fig. 5a. The prepared steel part is attached to the prefabricated LTCC part by a conventional soldering process. The speciality of the process lies in the matching of the coefficient of thermal expansion of LTCC and steel. This matching is done by providing a coating of a modifier. A LTCC-steel modifier is a material composition which can be coated as in-
A scheme of the fixture for moulding of the LTCC is presented in Fig. 6a and b. It consists of three parts: the inner cylinder which was a real-time ball bearing in this case, an outer meshed cylinder with a cut, and a holder for proper positioning of both the cylinders. For the preparation of the LTCC structures, five CT-700 Heratapes of ≈315 mm length, ≈11 mm width and ≈203 m thickness were stacked, and laminated to form a planar and flexible substrate. It was then
Fig. 6. Schematic of the fixture for moulding of LTCC (a, b) and fabricated cylindrical LTCC structures (c, d).
P.K. Khanna et al. / Materials Chemistry and Physics 89 (2005) 72–79
fired in the moulding fixture described above. The process parameters were similar to that reported in Section 3.2. The fabricated moulded LTCC structures are shown in Fig. 6c and d. In Fig. 6c, the moulded LTCC lies between the outer meshed cylinder and ball bearing. In Fig. 6d, the moulded LTCC part lies on the outer surface of the ball bearing after removal of the outer meshed cylinder. The change in the dimension of the LTCC can be attributed to the shrinkage of the tape assembly during the firing process.
4. Conclusions • The penetration of cracks in moulded LTCC structures has been examined. It was found that in addition to the surface length, the thickness of the tape and the ageing factor of LTCC green tapes play a critical role. • Bonding of LTCC to steel by adhesive means was possible and stresses due to difference in their coefficient of thermal expansion could be relieved to some extent by the adhesive. The thickness of the adhesive has to be optimised taking into account the required mechanical strength of the joint and the stress relief parameter. The mechanical assembly of LTCC has also been briefly described. • A process for moulding of LTTC and its attachment to steel part taking into account the fixtures required for the process and the mismatch in coefficient of thermal expansion between them has been proposed. Some samples were fabricated using the same to demonstrate the process. • The integration of a sensor in an LTCC structure has already been done in parallel. Investigations for incorporation of devices in the buried cavities and their sealing for realizing an MST device is being carried out. Surface nonlinearity and curvatures are critical factors which need
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attention from the point of view of device bonding and lid sealing.
Acknowledgements The authors are thankful to Mr. R. Grimme, Mr. T. J¨ager, Mr. F. Konkol and all the members of our group and Mr. F. Gora, Heraeus, Hanau for their help and discussions. The authors are also thankful to Dr. G. Rixecker, Stuttgart, and Mr. J. Werner, Stuttgart, for accessing their facilities. The support from Institut f¨ur Industrielle Fertigung und Fabrikbetrieb, Stuttgart, is highly acknowledged. One of the authors (P.K.K.) is thankful to the Director, CEERI, Pilani, India, for his guidance and permission to pursue this work.
References [1] M.R. Gongora-Rubio, P. Espinoza-Vallejos, L. Sola-Laguna, J.J. Santiago-Avil´es, Sens. Actuators A 89 (2001) 222. [2] N.L.A. Braga, S. Bampi, K. Shenai (Eds.), Proceedings of the XV SBMicro International Conference on Microelectronics and Packaging (SB), S˜ao Paulo, 2000, p. 189. [3] M. Gongora-Rubio, L.M. Sol´a-Laguna, P.J. Moffett, J.J. SantiaagoAvil´es, Sens. Actuators 73 (1999) 215. [4] H. Teterycz, J. Kita, R. Bauer, L.J. Golonka, B.W. Licznerski, K. Nitsch, K. Wi´sniewski, Sens. Actuators B 47 (1998) 100. [5] S. Slosarˇcı k, J. Bansky, M. Dovica, M. Tk´acˇ , J. Electrical Eng. 48 (1997) 250. [6] J. Zhong, M. Yi, H.H. Bau, Sens. Actuators A 96 (2002) 59. [7] J. Li, G.K. Ananthasuresh, J. Micromech. Microeng. 12 (2002) 198. [8] IMST GmbH: LTCC process, 2000. http://www.ltcc.de. [9] DuPont Microcircuit Materials: LTCC process, 2004. http://www.dupont.com/mcm/gtapesys/part3.html. [10] Reducing CTE mismatch between coatings and Si-based ceramics, Lewis Research Center. http://www.nasatech.com/Briefs/Jun98.
Studies on three-dimensional moulding, bonding and assembling of low-temperature-cofired ceramics for MEMS and MST applications P.K. Khanna∗ , B. Hornbostel, M. Burgard, W. Sch¨afer, J. Dorner Fraunhofer-Institut f¨ur Produktionstechnik und Automatisierung, Nobelstr. 12, D-70569 Stuttgart, Germany Received 14 May 2004; received in revised form 14 June 2004; accepted 24 August 2004
Abstract This paper broadly illustrates the preparation of three-dimensional shapes in low-temperature-cofired ceramics (LTCC). Test structures are fabricated by moulding single-layer green tapes into cylindrical form of different dimensions in order to investigate the penetration of cracks and the limiting value at which they appear. The factors responsible for the propagation of cracks have been analysed. Another important aspect which has been exemplified is the bonding of processed LTCC modules to metal parts with a dissimilar coefficient of thermal expansion. This is important from the point of view of attachment of LTCC-based micro-electromechanical systems (MEMS) or micro-system technology (MST) devices to a functional mechanical element. A methodology has been discussed for a smooth transition in the coefficient of thermal expansion of ceramic and metallic part in order to minimise stresses and avoid micro-cracks in the interconnections. An assembly of different post-fired modules by a fastening mechanism of individual modules for realizing a complex system are also described. Based on the above studies, a preliminary design to fabricate, shape, bond and post-process the LTCC substrate for the realization of a complex system together with the crucial processing issues is presented. Efforts have been made to implement this design and process steps on a real-time ball-bearing to prove the suitability of LTCC material for MEMS, MST and other complex systems. © 2004 Elsevier B.V. All rights reserved. Keywords: Ceramics; Multilayers; Coatings; Crack
1. Introduction Low-temperature-cofired ceramics (LTCC) is basically a mixture of recrystallized glass, ceramic powder, binder and organic solvent. This material mixture is cast in the form of tapes of variable thicknesses using doctor-blade. They are called green tapes because they are manipulated at the immature stage, i.e. before firing and sintering. The technology evolved around LTCC tapes is primarily a multilayer integration technology for electronic packaging. In this technology, the signal layers and passive elements are embedded into multilayer ceramics and active devices are buried into predesigned cavities in order to realise a functional electronic module. In the last several years, LTCC has an ever-increasing ∗
Corresponding author. Tel.: +49 711 970 1263; fax: +49 711 970 1007. E-mail address: [email protected] (P.K. Khanna).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.08.031
demand in industrial and telecommunication area. Some inherent features of this technology include: integration up to 50 layers, hermeticity, bio-compatibility, matching of coefficient of thermal expansion with Si, high thermal dissipation, hightemperature and -frequency application, and reduced size and cost. Due to the above advantages, it is a potential technology for micro-electromechanical systems (MEMS) and microsystem technology (MST) devices and their packaging. For the last few years, researchers have worked extensively [1–7] to use LTCC in the above-mentioned areas. In order to use LTCC for MEMS and MST, there is a need for developmental work for realizing special application-oriented products with micro-fluidic and electrical interfaces, inertial-element integration and also products with biochemical compatibilities. Packaging techniques have to be developed for isolation of the sensing elements such that it interacts with the measurand and simultaneously protects it from the environmental stresses
P.K. Khanna et al. / Materials Chemistry and Physics 89 (2005) 72–79
and the stresses generated from packaging. Special packaging techniques for micro-fluidic applications like micropumps, -valves, -mixers and -flowmeters have to be realized. Thermo-mechanical systems and applications, thermal and fluidic issues in MEMS and MST and advanced cooling techniques together with thermal and structural issues, their reliability and failure analysis need attention. Heat-transferenhancement techniques, high-power packaging using novel packaging technologies and advanced materials also need to be incorporated.
73
All the above is in principle possible using LTCC because of its remarkable advantages mentioned above. In view of the above-stated applications, it is utmost necessary or rather mandatory at this stage to study the possibility to mould LTCC into different shapes and sizes, its attachment to a mechanical part and assembly thereafter. This work will establish the versatility of LTCC technology and the micro-systems so forth fabricated can be readily integrated into any system or sub-system. Before going into the details of shape formation, bonding and assembly, it is
Fig. 1. Cylindrical pre-laminated LTCC structures for surface lengths greater than 6 mm (a–c) and for surface lengths less than 6 mm (d–g).
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P.K. Khanna et al. / Materials Chemistry and Physics 89 (2005) 72–79
important to understand the basic steps of LTCC manufacturing. 2. Low-temperature-cofired ceramics fabrication process The LTCC technology is a well-defined method to produce complex multilayer circuits with the help of single tapes. Conductive, dielectric and resistive formulations are screenprinted on each tape. The single sheets are then laminated together and fired in one step resulting to a miniaturized module. Another important advantage is that every single layer can be inspected and in the case of inaccuracy or damage, the same is replaced before firing, thus improving the yield of production. The low firing temperature of about 850 ◦ C makes possible to use low-resistive materials (silver and gold) instead of molybdenum and tungsten (which have to be used in conjunction with high-temperature-cofired ceramics). Resistors are processed with the help of special resistive pastes. These pastes are printed on top or bottom side, fired and later laser-trimmed to obtain precise values. Additionally, hightolerance-buried resistors can also be produced. Capacitors and inductors are build-up only with the help of special predesigned structures. It is possible to involve photo-processes in the LTCC process sequence, but screen-printing of the formulations on green sheets with the help of a conventional thick-film technique is more common and easily adoptable. Standard thick-film pastes are not used except for several post-fire processes; this is due to the shrinkage of tape in x-, y- and z-axis during the firing cycle. The major process steps for LTCC fabrication are as follows [8,9].
2.1. Slitting, preconditioning and blanking The green sheets are normally shipped on rolls of different widths and upon its arrival the tape has to be unrolled onto a clean stainless steel table. The sheets are cut to a pre-decided size with a razor blade, laser or a punch system. These parts have to be a little larger than the blank size, if the material needs to be pre-conditioned. If a laser is used, it is very important to control the power to avoid firing of the green sheets. Some tapes need to be pre-conditioned, which means that the green sheet has to be baked in an oven for about 30 min at 120 ◦ C (depending on the manufacturer and the material). Normally the tapes are shipped with an applied thin plastic sheet called Mylar, which has to be removed at the latest before lamination process. Mylar is used as a filling mask for the vias by some manufacturers. Orientation marks, lamination tooling holes and the final working dimension in case of to-be-pre-conditioned tapes are obtained using a blanking die or laser. In order to compensate for the difference in shrinkage in x- and y-directions, individual parts are rotated by an angle of 90◦ . 2.2. Via formation and filling Vias may be punched or drilled with a low-power laser or via a punching system. Vias can be filled with a dedicated via filler equipment or a conventional thick-film screen printer. In the latter case, the tape has to be placed on a sheet of paper which in turn lies on a porous stone. The tape is held at its place with the help of a vacuum pump and this vacuum also assists the filling of vias. In this process, the vias must have a larger diameter than tape thickness and the smallest possible
Fig. 2. LTCC metal bond (a) and cross-sections of the same (b–d).
P.K. Khanna et al. / Materials Chemistry and Physics 89 (2005) 72–79
size of vias to be filled depends on the viscosity of the paste. Both methods require a mask and this should be made on a 150–200 m thick stainless steel plate. Alternatively, a Mylar foil discussed above can also be used for mask making. 2.3. Printing Conductor tracks are printed on the green sheet using a conventional thick-film screen printer. The screens to be utilised are standard emulsion-type thick-film screens. Similar to via filling process, a porous stone is used to hold the tape in place. Printing of the conductor tracks tends to be easier and of higher resolution as compared to the standard thick film on alumina. This is attributed to the flatness and solvent absorption of the tape. After printing, the vias and conductors have to be dried in an oven at 80–120 ◦ C for 5–30 min depending on the material system used. Certain materials need to be levelled at room temperature for a few minutes before drying. Resistors are printed with the same screen printer but with a different screen. Screen-printing by photo-imageable paste for better line resolutions is used for high-frequency applications. High line resolutions can also be obtained with special fine-line screen printers. 2.4. Stacking and lamination Individual processed layers are placed – one on top of the other – over the tooling pins of a stacking jig. Hot soldering iron or heat pliers are used to tack the sheets at the edges. The lamination process can be carried out by two distinct methods. In the first method called uniaxial lamination, the
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tape assembly is pressed between heated plates at a temperature around 70 ◦ C and a pressure of approximately 200 bar for 10 min. This method requires a 180◦ rotation of the LTCC assembly after half the time. The uniaxial lamination could cause problems with cavities and windows. This method also causes higher shrinking tolerances than the isostatic lamination process mentioned below. A major difficulty is the flowing of the tape especially at the edge of the part during the firing cycle. The second method is to use an isostatic press. The stacked tapes are vacuum-packaged in a foil and pressed in hot water. The temperature and time are similar to uniaxial lamination process and the pressure is about 350 bar. Deep cavities and windows need to have an inlay during the process of lamination. 2.5. Cofiring and post-firing Laminates are fired in a single step on a smooth, flat setter tile, e.g. a ceramic substrate. The firing should follow a specific time–temperature profile and is done in a programmable box kiln. A typical firing profile consists of slow rise in temperature (around 2–4 ◦ C min−1 ) up to about 450 ◦ C with a dwell time of about 1–2 h, where the organic burnout takes place. Then the temperature rises to 850–875 ◦ C with a dwell time of about 10–15 min. The whole firing cycle lasts between 3 and 8 h depending on the material. Large and thick parts may cause the need of a modification in the firing profile. Resistor pastes need to have a defined firing condition and profile else their value may vary enormously. Certain materials need to be post-fired, which means that the paste is to be applied after firing the tape and then it has to
Fig. 3. LTCC substrate and mechanical screws and nuts (a) and mechanical fastening of the substrates (b, c).
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be fired again. The post-firing profile depends on the material used and varies in a wide range.
3. Experimental procedure and discussions 3.1. Crack penetration in moulded structures
2.6. Singulation and post-processing The singulation process consists of cutting of fired parts into smaller pieces or shapes. There are three possible ways to realize it. The first approach is to use a dicing saw, which is a common method and works quite well for rectangular shapes. It provides tight outside dimensional tolerances and allows high-quality edges. The second method uses an ultrasonic cutter. The final part shows low tolerances, may have unusual shapes and the process is very slow and expensive. The third possibility is to use a laser to cut the fired tape. The tolerances are tight, the quality of the edges is bad and the process is expensive. A number of post-processing steps for the LTCC manufacturing like surface mount technology, wire bonding, flip-chip, and inspection and testing of the LTCC module have to be carried out.
Cylindrical steel pieces (diameters of 1, 2, 3, 4, 5, 6, 8 and 10 mm and approximately 20 mm long) were utilised for rolling the LTCC green tapes. The steel cylinders were polished before use. Heratape CT-700 of thickness around 203 m was used for this study. Small pieces of LTCC tapes of 11 mm length were rolled on steel rods of above-stated diameter. After rolling, the tape was cut and adhered to the rod with a mild glue at the edge. No further processing like lamination or firing was done. The samples were inspected under an optical microscope to examine the rolled surface and find out the limiting point at which the cracks appear. Fig. 1a–c depicts the rolled LTCC structures on steel cylinders of different dimensions. Till the surface length of the tape was more than 6 mm, no cracks were observed and the surface was smooth and flawless. Below this length, the crack starts penetrating (see Fig. 1d–g), and smaller the surface
Fig. 4. Schematic representation: (a) front view of steel cylinder, (b) top view of the same, (c) multilayer LTCC structure, (d) front view of moulding fixture of LTCC into cylindrical shape and (e) top view of the same.
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length, more dominant is the crack. The above experiment was repeated for different tape thicknesses and tapes at different periods of their useful life. It was observed that both the above parameters also have a significant effect on the penetration of cracks. 3.2. Adhesive bonding of LTCC to metal Test structures were fabricated using CT-700 Heratape of a thickness of around 129 m. The LTCC assembly consisted of three tapes and the fabrication process followed is already explained in detail in Section 2, so only specific fabrication parameters are briefly described here. The slitting and blanking processes were performed using a Lambda Physik KrF excimer laser. After stacking, the lamination was performed in a uniaxial system at a temperature of 70 ◦ C and a pressure of 25 MPa for 10 min. This was followed by burnout and firing which was done in a programmable box furnace at 850 ◦ C with a dwell time of 25 min. After firing, the LTCC substrates
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of different dimensions were singulated. The metallic parts electro-plated with Ni were bonded with the LTCC substrate using Loctite 460 adhesive and cured overnight at room temperature (see Fig. 2a). The cross-section of these samples is presented in Fig. 2b–d. No significant deformities or cracks were observed in the joining region. A factor of major concern in this bonding process is the mismatch in the coefficient of thermal expansion of metallic part and LTCC. Although, the stresses caused by this mismatch are to some extent absorbed in the adhesive but a better approach for this has been discussed in the later sections. 3.3. Mechanical assembly of LTCC parts LTCC substrates fabricated for adhesive bonding were utilized for this purpose. A circular hole of ≈1 mm diameter was drilled into some of the substrates using the excimer laser system (see Fig. 3a). Tapes with holes drilled at the green stage followed by lamination and firing were also used in
Fig. 5. (a) Methodology for CTE match between LTCC and steel part, (b) attachment of LTCC to steel part and (c) attachment of devices and lid sealing procedure.
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a parallel approach. Assembly of three to four LTCC substrates is demonstrated in Fig. 3b and c using a conventional mechanical screw and nut assembly. 3.4. Approach for moulding of LTCC The basic approach for the fabrication of cylindrical LTCC parts is shown in Fig. 4. Fig. 4a and b shows the schematic representation of the steel part. It is cylindrical in shape with a diameter of 100 mm and a height of 20 mm. In order to prepare the steel for attachment to LTCC, it should be first coated with a modifier to match its coefficient of thermal expansion with that of LTCC. The modifier has been described in the next section. Then a coating of Ni is provided to it to make it solderable. In a parallel approach, multilayer LTCC structures with cavities for device attachment should be prepared till the lamination stage (Fig. 4c). The burnout and firing of the same should be carried out between two concentric cylinders (Fig. 4d and e) in which the inner cylinder should be solid and the outer one meshed. The reason being that during burnout, the outer meshed cylinder provides an ease in the evaporation of organics.
terlayer on steel or LTCC substrate to make their coefficients of thermal expansions compatible. The modifiers can also be termed as stress-relief interlayers. The coefficient of thermal expansion of steel (≈10.8 ppm K−1 ) is approximately twice as that of LTCC (≈5.4 ppm K−1 ) and the matching of their coefficients is utmost important in order to avoid undue stresses on either of them when they expand on heating or contract on cooling. These residual stresses are very harmful and responsible for a low-strength and reliability of the joint. In order to match their coefficients with close tolerances, one or more layers of intermediate thermal expansion may be incorporated into the LTCC-steel joint. In the past, special solutions based on interlayer material selection for minimizing these residual stresses and for effective improvement in joint strength have been reported by researchers [10]. The attachment of the LTCC part with built-in cavities to the steel ring is shown in Fig. 5b. The attachment of devices in the cavities by a wire-bonding, flip-chip and surface mount technology attachment process together with the attachment of lids to the cavities for hermetic sealing are presented in Fig. 5c. 3.6. Fabrication of moulded LTCC structures
3.5. Approach for attachment of LTCC to steel The approach for attachment of LTCC to steel is shown in Fig. 5a. The prepared steel part is attached to the prefabricated LTCC part by a conventional soldering process. The speciality of the process lies in the matching of the coefficient of thermal expansion of LTCC and steel. This matching is done by providing a coating of a modifier. A LTCC-steel modifier is a material composition which can be coated as in-
A scheme of the fixture for moulding of the LTCC is presented in Fig. 6a and b. It consists of three parts: the inner cylinder which was a real-time ball bearing in this case, an outer meshed cylinder with a cut, and a holder for proper positioning of both the cylinders. For the preparation of the LTCC structures, five CT-700 Heratapes of ≈315 mm length, ≈11 mm width and ≈203 m thickness were stacked, and laminated to form a planar and flexible substrate. It was then
Fig. 6. Schematic of the fixture for moulding of LTCC (a, b) and fabricated cylindrical LTCC structures (c, d).
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fired in the moulding fixture described above. The process parameters were similar to that reported in Section 3.2. The fabricated moulded LTCC structures are shown in Fig. 6c and d. In Fig. 6c, the moulded LTCC lies between the outer meshed cylinder and ball bearing. In Fig. 6d, the moulded LTCC part lies on the outer surface of the ball bearing after removal of the outer meshed cylinder. The change in the dimension of the LTCC can be attributed to the shrinkage of the tape assembly during the firing process.
4. Conclusions • The penetration of cracks in moulded LTCC structures has been examined. It was found that in addition to the surface length, the thickness of the tape and the ageing factor of LTCC green tapes play a critical role. • Bonding of LTCC to steel by adhesive means was possible and stresses due to difference in their coefficient of thermal expansion could be relieved to some extent by the adhesive. The thickness of the adhesive has to be optimised taking into account the required mechanical strength of the joint and the stress relief parameter. The mechanical assembly of LTCC has also been briefly described. • A process for moulding of LTTC and its attachment to steel part taking into account the fixtures required for the process and the mismatch in coefficient of thermal expansion between them has been proposed. Some samples were fabricated using the same to demonstrate the process. • The integration of a sensor in an LTCC structure has already been done in parallel. Investigations for incorporation of devices in the buried cavities and their sealing for realizing an MST device is being carried out. Surface nonlinearity and curvatures are critical factors which need
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attention from the point of view of device bonding and lid sealing.
Acknowledgements The authors are thankful to Mr. R. Grimme, Mr. T. J¨ager, Mr. F. Konkol and all the members of our group and Mr. F. Gora, Heraeus, Hanau for their help and discussions. The authors are also thankful to Dr. G. Rixecker, Stuttgart, and Mr. J. Werner, Stuttgart, for accessing their facilities. The support from Institut f¨ur Industrielle Fertigung und Fabrikbetrieb, Stuttgart, is highly acknowledged. One of the authors (P.K.K.) is thankful to the Director, CEERI, Pilani, India, for his guidance and permission to pursue this work.
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