pacitors [3] the LTCC (Low Temperature Co-fired Ceramic) technology, which is developed for almost three decades, is very well suited to manufacture MCM ...
Hybrid LTCC temperature and humidity sensor (Hybrydowy LTCC czujnik temperatury i wilgotności)
MATEUSZ CZOK, mgr inż. DOMINIK JURKÓW, prof. dr hab. inż. LESZEK GOLONKA Wroclaw University of Technology, Faculty of Microsystem Electronics and Photonics
New technologies allow manufacturing of smaller and faster electronic devices, with high packaging density. In a single package, many different applications such as sensors, actuators, wireless communication, RF and microwave systems [1,2], etc. can be combined in a multichip ceramic module (MCM-C). Nowadays 3D devices with passive and active components are fabricated. Because of the possibility of manufacturing embedded elements such as buried resistors, capacitors [3] the LTCC (Low Temperature Co-fired Ceramic) technology, which is developed for almost three decades, is very well suited to manufacture MCM modules. Additionally the fabrication of devices in all three dimensions reduces the size and increases packaging density [1,3] and enables fabrication of buried chambers and channels inside the LTCC packages. Therefore integration of sensors with electronic circuits might be easily achieved. Forming of vertical interconnections (vias) between different metallization levels in multilayer (up to 100 layers) LTCC modules allows manufacturing of high density interconnections (HDI) substrates for electrical circuits. Fabrication of vias consists of two processes: holes formation in the LTCC tape and via filling with electrically conductive material. Mechanical punching and laser milling are two common methods of holes fabrication, which are commonly used in LTCC technology. Advantages and disadvantages of these methods are discussed in [4]. There are also two popular methods of via filling: first which is based on filling through a Mylar™ stencil or a stencil made of thin metal foil and second is based on vacuum assisted technique. In this article another method of via filling is presented. To ensure good chambers geometry of buried 3D structures, sacrificial volume materials (SVM) are used. This technique enables fabrication of closed chambers, which are commonly found in microsystems technology, e.g. membranes, channels, reaction chambers and actuators. Chambers are filled with SVM like carbon-black paste, cetyl alcohol, strontium carbonate [5-8] or laser cut carbon pieces which are intended to disappear during a sintering process. The choosing of a proper sacrificial material is crucial for the whole burnout profile (e.g. process length) and the chamber filling ease. A multichip module which combines vias, thick film resistors, passive SMT devices, a microcontroller (ATmega88) and a humidity and temperature sensor (SHT11) is presented in this paper. The temperature influence on the SHT11 measurements is very high. Therefore to improve ambient temperature measurement, heat transfer from other electronic components needs to be reduced [9]. So a closed chamber was designed below the sensing element for its thermal insulation. A sacrificial layer of cetyl alcohol was used to manufacture the chamber. At the end the X-ray inspection was used to examine the fabricated LTCC module and its electrical vertical connections. ELEKTRONIKA 12/2009
Design
The designed LTCC module consists of three DuPont 951PX green tape layers which layouts are shown in Fig. 1. Elements are placed both on the bottom (Fig. 1a) and the top (Fig. 1b) layers. A middle interconnecting layer with vias (Fig. 1c) is used to provide essential electrical connection. a)
b)
c)
Fig. 1. Module design: a) bottom layer, b) top layer, c) vias Rys. 1. Projekt układu: a) wartwa dolna, b) warstwa górna, c) przelotki
Technology
First conductive paths and resistors films, which are presented in Fig. 1, were screen-printed on LTCC green tape surface (DP 951PX - 256 µm before sintering) using the DP 6146 (Pd/Ag) and the CF-041 (10 kΩ/sq.) pastes, respectively. Then vias were fabricated. Then the cavity for the closed chamber was formed with laser in the internal layer of the module. The chamber below the sensing component reduces thermal inertia of the sensor. Next the tapes were laminated in isostatic press at 70°C and 20 MPa for 10 minutes. The cross-section of the closed chamber is presented in Fig. 2. The device was sintered in a box furnace at the temperature cycle presented in Fig. 3.
Fig. 2. The cross-scetion of the closed chamber Rys. 2. Przekrój porzprzeczny komory
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Fig. 3. Sintering profile Rys. 3. Cykl wypalania
Via fabrication
First cylindrically shaped holes (120 µm diameter) were formed in the LTCC tape with mechanical punching. The advantages over the laser milling of holes are regular shapes of formed vias and no thermal damages of the LTCC tape. For via filling purposes a special construction was used. The main part of the device is a head which consists of a reservoir containing the filling paste, a capillary and a needle. Applying vertical force onto the top of the head causes its movement downwards till the LTCC surface. Further force applying makes a small volume of paste injection into a hole out of the capillary in the final stage of the needle’s movement. This novel method of via filling gives fine results with a failure ratio (the number of improperly filled vias to the total filled vias number) as low as 0.1%. X-ray inspection was per-
Fig. 4. X-ray picture of a filled vias formed by injection filling method Rys. 4. Zdjęcie rentgenowskie przelotek wykonanych metodą iniekcji
formed in order to qualitatively evaluate the quality of vias obtained by the method described above. A test structure’s cross-section image is shown in Fig. 4. The filled via appears as a uniform dark region. This indicates uniform via filling with a conductive material and fine reliability of interconnections can be expected.
Electrical circuit
The electrical circuit, which consists of the ATmega88 microcontroller, an integrated temperature and humidity sensor SHT11, a LCD display (DEM 16101 SYH), a constant voltage regulator (LP2980) and passive components (capacitors and resistors), is presented in Fig. 5. The measured values of temperature and humidity are internally digitized and stored in
Fig. 5. Electrical circuit Rys. 5. Schemat elektryczny
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SHT11 registers. The data is transferred to the microcontroller via the two-wire serial interface with a thick film pull up resistor on the data line. Then the microcontroller process the data and the results are shown on the LCD display. To achieve proper display a voltage divider, which was based on screenprinted resistors, was used. Low circuit energy consumption was achieved by using low energy consuming electrical components, disabling unused microcontroller peripherals and enabling microcona)
b)
troller’s and SHT11’s sleep modes. In result a 12 times power consumption reduction was obtained what enabled battery supplying. The circuit current and the power consumption vs. time are presented in Fig. 6a and 6b, respectively. Working cycle to sleeping time ratio equals 17%. The electrical circuit with screen-printed and soldered passive components is shown in Fig. 7. Flexible connection between the LTCC module and the LCD display was used to ensure stability and durability of soldered wire pins. The package’s and the LCD display’s dimensions are similar.
Conclusion
A technology of LTCC based electronic device is presented in the paper. Electronic components such as passive SMT devices, surface thick-film resistors, a microcontroller and a digital sensing element are soldered onto the LTCC module. The temperature and humidity measurements are visualized on a LCD display. The power consumption of the device was reduced with using sleep modes of the ATmega88 and of the SHT11 elements. Vertical electrical connections between layers were fabricated with a novel via fabrication technique. The chamber below the SHT11, which reduces thermal inertia, was manufactured with fugitive phase technique. The solution enables to achieve good quality chambers. Moreover the injection via filling method is presented. X-ray via inspection was performed and results are presented. In further work the module will be miniaturized significantly using 0603 passive components and more precise conductive paths layout. In addition, the next module will integrate a RF communication component. The device can be used to monitor an air-conditioning system. The authors wish to thank the Polish Ministry of Science and Higher Education (grant no. R02 017 02) for financial support and Mr. Andrzej Bochenek for the manufacturing of the via filling device. Also authors wish to thank the Tele & Radio Research Institute for X-ray inspection.
Fig. 6. Electrical measurments: a) current consumption vs. time, b) power consumption Rys. 6. Pomiary elektryczne: a) pobór prądu, b) pobór mocy
References [1] [2] [3] [4] [5] [6] [7]
Fig. 7. The final device with screen-printed and soldered passive component Rys. 7. Gotowa struktura z nadrukowanymi elementami pasywnymi
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[8]
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