Murata’s solution “LTCC”
LTCC substrate plays a major part at the forefront of “electronification”
A momentum for market expansion is picking up with a backdrop of mechatronics integration
Mr. Kozo Kagawa
Chief of planning and sales promotion section, Functional substrate products division, Module business headquarters, Murata Manufacturing Co., Ltd.
As the number of electrical components installed into one automobile increases, a momentum for miniaturization of ECU (Electronic Control Unit), or mechatronics integration that integrates ECU into mechanical structure, is building up. One of the products Murata Manufacturing Co., Ltd. focuses on to respond such trend is LTCC (Low Temperature Co-fired Ceramics) substrate “LFC® series”. The company has realized a LTCC substrate with high reliability to tolerate an extremely severe usage environment. The company further intends to contribute to miniaturization thorough making packaging density higher by miniaturization of circuit patterns and other means.
One notable feature of a LTCC substrate is that, in order to use conductors with smaller conductor resistance, such as Ag and Cu, it uses a ceramic material that can sinter materials at 1000°C and lower, which is lower than a melting point of these metals. Since a LTCC substrate has excellent electrical property, it has been used in super-computers that seek high performance or wireless circuits that treat high-frequency from more than 30 years ago. Automotive application started around the middle of 1990s. Robert Bosch GmbH of Germany, a world’s major electrical component manufacturer, started considering the introduction of LTCC as a next-generation multilayer substrate for ECU in 1980s, and started commercial production of ECU using a LTCC multilayer substrate since 1995.
Actually, the “LFC® series” of LTCC Murata has been actively developing for the in-vehicle use started from Bosch’s LTCC. In January 2004, Murata took over the LTCC substrate business from Ogaki Ceramics, an affiliate of Sumitomo Metal Electronics Devices, which was co-developing in-vehicle LTCC with Robert Bosch. By absorbing this business, Murata took over several projects along with the co-development project with Bosch. This made Murata one of the leading companies in the area of in-vehicle LTCC substrate technology together with Bosch.
Fig-1. Example of in-vehicle module on which LFC® series of LTCC multilayer substrate was mounted (click to enlarge)
Murata’s LTCC multilayer substrates are already adopted by European electric component manufacturers and ECUs attached with their LTCC substrate are playing an active part in automotive electronification. “We are proud to be the world’s top supplier in the LTCC market.” (Mr. Kozo Kagawa, Chief of planning and sales promotion section, Functional substrate products division, Module business headquarters, Murata Manufacturing Co., Ltd.) For example, Murata's LFC® series is used in control ECU, such as transmission control, electric power steering, ESC or electronic stability control, or dual-clutch transmission. (See Fig-1) “The series is also used in the ECUs for the latest 7-speed automatic transmission and electric power steering system for European luxury cars.” (Mr. Kagawa)
Mounting components on both surfaces of a substrate
Fig-2. Basic structure of LFC® series for in-vehicle use(click to enlarge)
The material of LFC® series is a mixture of glass (CaO-Al2O3-B2O3-SiO2) and Alumina (Al2O3). Compared with a conventional ceramic substrate by Alumina, the sintering temperature of the LFC® series substrate is lower. The “LFC® series substrate does not include materials such as lead (Pb) or cadmium (Cd) restricted by environmental protection laws.” (Mr. Yoshimasa Kugutsu, Assistant manager of planning and sales promotion section, Functional substrate products division, Module business headquarters, Murata Manufacturing Co., Ltd.) Since the glass transition temperature at which the material property changes is as high as over 600°C, the series can be used in a high temperature environment of 140°C to 150°C. In case of high-frequency applications such as front-end module of mobile phone, passive components such as capacitors, inductors, and filters are frequently integrated into the substrate. So, the number of layers can become as many as 20. However, in case of in-vehicle applications such as ECU module, substrate is not much complex with the number of layers limited just around 4 to 6 layers.
Fig-3. Effect of thermal via (thermal conductivity)(click to enlarge)
Chip components, flip-chip IC or bare chip IC parts can be mounted on the surface. (See Fig-2) To connect between layers, blind vias are used. Since a printed resistor can be mounted on the reverse side, the density of the top surface can be increased, thus enabling the miniaturization of the entire circuit board. “For a printed resistor, we are using a high-precision “HTF series” resistor with resistance accuracy as low as less than ±1%. It has excellent anti-plating durability and does not include Pb and Cd.” (Mr. Kugutsu) For inner wiring and surface conductor, low-resistivity Ag is used with Ni-Pd-Au plating for higher reliability in connection. It can also mount bare ICs with wire-bonding or form a thermal via extending from one side to the other side of substrate surface as preventive measures against heat generation.
“Although the thermal conductivity of LFC® substrate itself is superior to a resin substrate, it is not necessarily higher than that of Alumina substrate. However, by forming a thermal via under the IC and heightening the ratio of via area, the conductivity improves many times more than that of Alumina substrates.” (Mr. kagawa)
Highly reliable substrate realized by unique manufacturing process
Fig-4. Processing flow of pressure-assisted Zero-shrinkage sintering method(click to enlarge)
Among many features that LFC® series possesses, in-vehicle equipment designers should particularly pay attention to the fact that it realized an extremely high dimensional accuracy and minimized the warp caused in the process of manufacturing LTCC substrates. (See Fig-4) It enhances the quality in substrate manufacturing, as well as the reliability in mounting components.
In general, the material of ceramic substrate shrinks during a sintering process, making substrate dimension 15% to 20% smaller. Since the shrink rate is not constant, the shrinkage not only reduces the usable area for design, but very much influences the process management including the control of printing pattern according to the shrinkage percentage. Non-shrinking substrates that does not change the size before and after the sintering process solved these problems. In addition, Murata also developed a unique pressure-assisted Zero-shrinkage Sintering method that sinters the substrate while applying a pressure to the layered material both from the top and the bottom. To be specific, they first make a hole that will later become a via hole or a through hole on the material sheet and print a circuit pattern. Then they stack these sheets and sinter them while adding an even pressure from the top and the bottom. “With this method, the substrate shrinks in a vertical direction, but hardly in a horizontal direction. Moreover, the surface becomes extremely flat by being pressed uniformly. (Mr. Kagawa)
In case of Murata’s largest substrate (202.0 x 202.0mm), dimensional accuracy after sintering is normally ±0.05%. Even at the final processing stage, it remains ±0.1%. In addition, flatness is as high as 5µm/4mm□. While, in case of conventional Alumina substrates or HTCC (High Temperature Co-fired Ceramics) used in vehicles, it is as large as around ±1.0% at the final stage of manufacturing process. “If dimensional accuracy remains ±0.1%, when a substrate measures 101 x 101mm, the error will be 0.101mm at the periphery of the substrate where the error could become maximum. That much displacement may not create problems thanks to the self-alignment effect during the soldering process. However, if the error becomes more than 10 times larger, it may create problems depending on the size of the component or the land.” (Mr. Kagawa) Bumpy surface also results in mounting failure. In other words, LFC® series with high dimensional accuracy and flat surface has an advantage in terms of securing the reliability of module substrates equipped with components.
Fig-5. Conductor flattened by simultaneous sintering method (click to enlarge)
Another point that should not be overlooked is that Murata has achieved the flatness as high as 5µm/4mm□ including the conductor on the surface for LFC® series made with Pressure-assisted zero-shrinkage Sintering method. (See Fig-5) By simultaneously sintering the conductor layer and substrate material while applying a pressure to conductor, a conductor layer gets buried into the substrate and the surface becomes flat. In case that a conductor pattern is printed on the surface, the pattern does rise from the surface without exception. “If the surface is flat, it will allow maximum area for wire bonding and stable bonding.” (Mr. Kagawa) In other words, this flattening technology enhances the reliability at the time of mounting to the next level.
Growing needs for highly reliable LTCC
Mr. Yoshimasa Kugutsu
Assistant manager of planning and sales promotion section, Functional substrate products division, Module business headquarters, Murata Manufacturing Co., Ltd.
Recently, a momentum of demand for LFC® series substrate is picking up in the in-vehicle application field. It is because the number of electronic equipment installed in one automobile is increasing as so-called electronification progresses. In order to effectively use the limited space in one automobile, each and every piece of electric equipment is required to become compact. For the same reason, there is a tendency to combine electric equipment, such as ECU, with mechanical systems, such as engine and transmission. This is so-called mechatronics integration. LFC® is excellent in terms of reliability and suitable for miniaturization as it can be packaged in high density. For the purpose of further integration of mechatronics, substrates tolerating environment with temperatures around 140°C to 150°C will be needed. LFC® whose glass transition temperature is as high as 600°C has no problem in this respect. In sum, it responds to the two needs: “There are other substrates such as in HTCC or Alumina that tolerate heat better. However, LFC® excels them in terms of dimensional accuracy. Alumina substrates, for the moment, are not suited to be made multi-layered or into a larger panel, so it will be difficult to heighten its density over the level of LFC®.
With the growing momentum for LFC® demand, Murata is steadily stepping up its support system for designers. “For example, we now have a system to accept design data processed by major CAD tools so that we can cope with any developing tools our customers might be using. (Mr. Kugutsu) In addition, Murata streamlined the prototype-making processes. The original method entailed a mold to punch holes in prototype substrate. Three years ago, laser processing equipment of substrates was introduced for the in-vehicle use. “By doing away with the mold, we could significantly reduce the cost and the time necessary for prototype-making, and we believe that the flexibility to respond to the customers’ design changes has been improved as well.” (Mr. Kugutsu)
In the future, Murata is going to advance technologies used in LFC® series further to meet increasingly sophisticated demand from the market. “We will make our pitch pattern even finer. That will deal with the increase of pin numbers in LSI and enable higher mounting densities at the same time. With higher densities, we can better contribute to miniaturization of substrates.” (Mr. Kagawa)
* LFC® is the trademark of Murata Manufacturing Co., Ltd.
