LTCC track

1.1    Module H1: LTCC Technology in Sensor Applications

This module has been organized on 25^th September immediately following the IMAPS-CPMT Poland 2013 conference, at the conference venue. The module was intended to provide students with an overview of LTCC technology with particular focus on sensor applications.  The module included two lectures:

Dr. Karol Malecha, Wrocław University of Technology: LTCC technology in sensor applications Dr. Thomas Maeder, Ecole Polytechnique Federale de Lausanne, LTCC for integrated sensor-device applications On the left: Dr Thomas Maeder (EPFL) during lecture on LTCC sensor systems.

1.2    Module L1: LTCC Fabrication

The goal of the laboratory exercise was to familiarize the participants of the SENSEIVER Summer School with the whole LTCC process beginning from fabrication of a green tape up to the final product in a form of an electronic device.

The training comprised a few LTCC processing steps: tape casting, dicing, screen printing, stacking, isostatic lamination, inspection, firing, assembly, soldering and testing.

The first step (tape casting) was based on the green tape developed at ITE, while in all other steps a commercial Du Pont 951 dielectric tape was used for fabrication of a DC/DC converter.

The exercise spanned 4 days (1-4 October). Instructors were:

• Prof. Dorota Szwagierczak (ITE)
• Dr. Jan Kulawik (ITE)
• Dr. Dominik Jurków (Wrocław University of Technology)
• Ms. Beata Synkiewicz (ITE)
• Mr. Andrzej Kapusta (ITE)

1.2.1    Tape casting

The slurry for tape casting comprises inorganic and organic components. The organic part contains binder, plasticizer, dispersant and solvent. Final properties of the dried tape are determined by the ceramic powder. Solvent should not react with the ceramic powder or solve it, but it should solve other components of the slurry. Binder forms thin adhering layers around inorganic particles which after solvent evaporation provide bonding of these particles. It is responsible for high density and strength of the green tape. Dispersant and plasticizer facilitate uniform dispersion of the binder. These components can be adsorbed onto the ceramic particles surfaces. Dispersant lowers the viscosity of the slurry. Plasticizer effects flexibility of the green tape. For training purpose, the glass-ceramic material developed at ITE was chosen as the inorganic component.  This material exhibits good compatibility with commercial conductive pastes. The organic part of the prepared slurry contained polyvinyl butyral as a binder, fish oil as a dispersant, polyethylene glycol and dibutyl phthalate as plasticizers, toluene and isopropyl alcohol as solvents.
Two-step procedure of weighing and subsequent ball-milling was applied in the exercise. In the first step, the glass-ceramic powder, solvents, dispersant and plasticizers are weighted in appropriate proportions and ball-milled to break soft agglomerates and to ensure uniform adsorption of dispersant on the surfaces of ceramic powder particles. Then the binder was added and further milling was performed to ensure good homogenization of the slurry.
Two-step mixing of the inorganic and organic components of the slurry was carried out by milling in a planetary Fritsch ball-mill (Pulverisette 5, Germany).
Tape casting of the slurry prepared by the participants was carried out using R. Mistler TTC-1200 tape caster. The slurry is cast onto polyester carrier tape coated with silicone. Polished granite block in the initial part of the machine ensures very good flatness of casting surface and lack of any vibrations. The gap in the doctor blade can be precisely adjusted by the use of micrometer screws. Casting speed, temperature of heaters and air flow can be also regulated.
Drying is one of the most important steps in tape casting process. Thick tapes require close control of drying conditions. For each slurry formulation, there is a critical thickness at which the tape starts to crack spontaneously during drying due to shrinkage stresses. Green tapes exhibit anisotropic shrinkage behavior. The shrinkage in the casting direction is normally smaller than that of transverse direction. The obtained tapes were smooth, flexible, with good mechanical strength, almost defect-free.

Below: A produced LTCC tape.

1.2.2    Laser cutting

First step of a DC/DC convertor fabrication in LTCC technology was cutting by a laser green sheets (commercial DP 951 dielectric tapes 100 um thick) into desired dimensions, followed by drilling of vias and holes for positioning. The E-355-3-G-OA device (Oxford Lasers, UK) includes a pulsed laser, operating in the ultraviolet range.
After the converted G-code file was loaded into machine software, the participants could observe the cutting process. Students learned also about the basic presets possible to be modified in the Oxford Laser Machine and their influence on depth and quality of cutting.

1.2.3    Screen printing 

The operation of the MT-320TVC screen-printer (Micro-tec, Japan) was explained. This is a high precision auto alignment type screen-printer. Using this machine, patterns can be printed with a high degree of reproducibility, under low pressure. The printing conditions can be adjusted digitally.
During the exercise, the set of screens prepared earlier at ITE for fabrication of the DC/DC converter was utilized. Stainless steel 325 mesh and Murakami capillary film 30 mm thick were used for fabrication of these screens.

1.2.4    Filling of vias The MT-320TVC screen-printer was used for vias filling and deposition of thick film conductive paths (in the next step). DuPont 6141 Ag conductive paste was utilized for filling of vias. Each green sheet contained 2-3 vias. After screen printing, the conductive layers were dried. 1.2.5    Screen printing of conductive paths Conductive paths were screen printed from Ag ESL 9916 paste. After screen printing, the conductive layers were dried.

1.2.6    Stacking of green sheets Four blank sheets (with filled vias) and three sheets with the screen printed conductive paths were stacked using a manual stacker. 12 stacks were prepared for further lamination. Above: ESR Mitar Simic inspecting a green tape sheet. On the left: Radosław Wójcik (student from AGH-UST) during layer stacking operation.

1.2.7    Isostatic lamination

The stacked green sheets were vacuum closed in plastic bags using vacuum sealing machine. A IL 4008PC Pacific Trinetics Corporation laminator was used for isostatic pressing of the stacks.

1.2.8    Process control by heating and optical microscopes

Possibility of determination of optimal firing profiles and control of detrimental effects like delamination during thermal treatment by the observation in a heating microscope was demonstrated to the participants. A Leitz high temperature microscope equipped with a camera and system for continuous registration of images was used for tracking of changes in the shape and dimensions of a small sample of the investigated material during continuous heating up. Such observations enable the determination of softening and melting points and optimal temperature ranges for sintering. It is also possible to reveal the temperature range in which undesired phenomena can occur.
The participants had also the possibility to watch images of cross-sections of the green laminates of multilayer capacitors and multilayer multiferroic composites in a Hirox digital optical microscope with magnification up to 3500. This microscope can be helpful for control of quality of several LTCC processing steps, such as laser cutting, vias filling, screen printing, lamination, channel and cavities formation, etc.

1.2.9    Firing Firing of the fabricated green laminates was carried out in a chamber furnace. The temperature profile was set carefully according to the producer recommendation. During the first step, burnout of organic components from the green sheets and conductive pastes took place. The second step results in cosintering of ceramic and metallic layers. On the right: One of the fired substrates.

1.2.10    Inspection of vias and conductive paths

The electrical conductivity of vias and conductive paths was checked. The quality of the paths was also controlled in an optical microscope.

1.2.11    Assembly

The instructor showed two working pick and place machines (Quadra) for SMT printed circuit board assembly and explained their operation rules. Deposition of solder paste using a stencil was also demonstrated.
For economic reasons, the participants carried out assembly of the fabricated device manually. They deposited the solder paste and placed electronic elements (resistors, capacitors, integrated circuit) according to the design of the DC/DC converter.

1.2.12    Soldering Three types of soldering machines were shown – a conventional belt three-zone convection oven for reflow soldering, an equipment for wave soldering (with standing wave), and two furnaces for vapor phase soldering. After the assembly step, soldering of the fabricated devices was performed using the vapor phase soldering oven (Asscon, Germany). On the right: A correctly assembled device.

1.2.13    Testing

As the result of the exercise, a properly working DC/DC converter was fabricated using LTCC technology. The output voltage of the device was measured and it was consistent with the designed value.

Group photo at LTCC laboratory, left-to-right: D. Szwagierczak (instructor, ITE), K. Cvejin (ESR), G. Miskovic (ESR), M. Zawadzka (ER), D. Jurków (instructor, Wrocław University of Technology), J. Kulawik (instructor, ITE), L. Manjakkal (ESR), I. Kianpour (ESR), M. Simic (ESR), B. Hussein (ESR), S. Ajkalo (ESR).