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Posted on 01 December 2019

LTCC Technology for Sensor and RF-Applications

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LTCC-ceramics has small temperature expansion

LTCC substrates manufactured to customers’ specifications are utilised in many applications. This paper highlights the specific benefits of "Low Temperature Co-fired Ceramics“ (LTCC) and provides an outlook to future developments.

By Dr. Arne Albertsen, Manager Field Application Engineering and Marketing, KOA Europe GmbH

 

The trends of miniaturization, increased reliability and high ambient operating temperatures for electronic circuits have driven the deployment of ceramic substrates and packages.

Among other technological approaches, LTCC have proven their superior performance in a variety of applications. These comprise high temperature automotive, highly reliable medical applications and RF modules for wireless communication. A strong growing segment is the manufacture of packages for Micro Electrical Mechanical Systems (MEMS).

What exactly are LTCCs and which properties make this substrate so popular? A quick glance at Wikipedia, the free encyclopedia, helps to find answers:

“Low temperature co-fired ceramic (LTCC) is a well-established process that has been in use for many years in the microelectronics packaging industry. It is similar to the thick film hybrid process employed for mutlilayer ceramic capacitors and chip inductors. LTCC technology is especially used for wireless and high-frequency applications. In RF and wireless applications, LTCC technology is also used to make multilayer hybrid integrated circuits, which can include resistors, inductors, capacitors, and active components in the same package. LTCC hybrids have a smaller initial (“non recurring”) cost as compared with ICs, making them an attractive alternative for small scale integration devices.

This technology presents advantages compared to other technologies: the ceramic could be fired below 900°C due to a special composition of the material. This permits the co-firing with high conductive materials (silver, copper and gold). LTCC also permit the ability to embed passive elements, such as resistors, capacitors and inductors into the ceramic package; hence the size of the components decreases.”

On the contrary, High Temperature Cofired Ceramics (HTCC) are made of alumina and fired at temperatures around 1600°C, such that only metals with high melting points like molybdenum or tungsten can be used as conductors. Since their electric conductivity is poor compared to silver or gold, bigger losses occur.

The in-house manufacture of the ceramic base material for the LTCC production, the „green sheets“, KOA ensures a maximum flexibility especially with regard to the mechanical properties of the substrates.

Figure 1 shows the main manufacture steps of the LTCC substrate production, starting with the unfired "green sheets“.

Process Steps of LTCC- Substrate Production

After cutting the sheets to the desired size, via holes and cavity openings are mechanically punched.

The next step is the via filling with conducting silver paste. The paste is applied through a mask with openings at the positions of the vias. Standard via diameters are 100, 150, and 200 µm. Then, the conducting traces are screenprinted. The minimum line width and the minimum line space is 60 µm, each.

The printed sheets are stacked and aligned and afterwards isostatically laminated in a water-filled pressurized tank.

The final process step is the firing at temperatures of up to 850°C. The fired material shrinks – approxímately 15% in x- and y-directions and approximately 20% in z-direction. The highly homogeneous structure of the green sheets and a precise temperature control ensure high reproducibility of the dimensional accuracy. This high accuracy allows for the realization of dimensionally accurate cavities for the mounting of semiconductor chips.

Figure 2 depicts a cross-section through a LTCC-substrate that holds a cavity for chip mounting. The cavity offers the benefit of short bond wire lengths to connect the chip to the substrate. The short wire length minimizes the influence of parasitic inductances on signal integrity, which is of use especially at high frequencies. To facilitate the heat conduction from the chip to the ambient, thermal vias can be utilized.

Cross-Section of LTCC-Substrate with "buried“ passive Components

A plot of the dielectric losses (Figure 3) of organic FR4 vs. LTCC substrate shows that LTCC have significantly lower loss at high frequencies than FR4. Another advantage of LTCC-ceramics is their small temperature coefficient of expansion: the numerical value of LTCC’s T.C.E. is in between those of Si and GaAs and thus closer than the T.C.E.s of FR4 or HTCC.

Dielectric losses of LTCC- vs. FR4-Material

Applications are for example wireless telecom equipment and "intelligent“ sensor systems. Figure 4 illustrates an example LTCC substrate with cavities.

LTCC-Substrate with Cavities

Advantages of LTCC-technology

• Multi-layer structures up to 20 layers possible
• Supports fine line / space structures down to 60 / 60 µm
• Buried components (R, L, C) can be implemented
• Low loss up to 60 GHz
• Narrow mechanical tolerance and high reproducibility
• Cavities available
• T.C.E. similar to Si, GaAs
• Robust under temperature cycling conditions
• Custom specific solutions starting from 5000 to 10000 pieces/year

Outlook

KOAs manufacture process allows for line / space of 60 / 60 µm. The main targets for the development of the technology affect the further miniaturization and the increase of the production efficiency, the latter especially for small quantities. Hence, KOA’s research activities focus on the enhancement of the methods and processes for creating conducting tracks on the multi layer substrate.

A major achievement towards highly efficient production is the application of ink-jet technology to print directly from a file onto the green sheets. In addition to the time savings, finer line /space patterns can be produced compared with traditional screen printing. In a joint-research project together with Seiko- Epson, KOA managed to demonstrate the feasibility of line / space structures with 30 / 30 µm.

The researchers prepared a special nanodispersed silver-ink that was printed onto standard green sheets by a dedicated ink-jet printer.

Enhancements for the structuring of the metallization layers (e.g. conducting traces in thin film technology) and via (e.g. laser punched fine vias) will allow for further miniaturization of the substrates. The number of applications that will become accessible to LTCC substrate solutions will grow significantly during the next years.

 

 

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