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August, 2016


Advances in Thick-Film Ceramic Technology


By Sebastian Löeffler, Günter Reppe, Angela Rebs, Marcus Herrmann, Christopher Mauermann, Alexander Kaiser, and Karl-Heinz Fritz, Cicor Advanced Microelectronics & Substrates GmbH


T


hick-film, hybrid ceramic substrates have provided a base for high-reliability circuits in the space, avionics, medical, and industrial


markets for years. Along with new interposer sub- strates for mass production based on polymers, sil- icon and glass, thick-film ceramic can be a solution in many areas. Often, however, the disadvantages of the


technology, which include broad lines, spaces, and oversized vias, prohibit its application in high-density substrates. The combination of thick-film with


advanced technologies for fine-line structuring can deliver quality base substrates, especially for packaging with high demands on tempera- ture and lifetime. Combined with state-of-the- art assembly technologies, the thick-film hybrid is ready to be used for new applications. The technologies currently available are etched thick-film, laser structuring of thick-film inks, special solutions for improvement of via-resolu- tion in multilayer hybrids, and thin-film coated surfaces for soldering pads. In recent decades, thick-film technology has


not kept up with other technologies in miniaturiza- tion. Now, after many improvements, it is possible to create structures with the necessary high density that allow the technology to be used in many appli- cations. It has enabled the production of multilayer- chip wire connections, including packaging in a small area. Thick-film is a very complex technology with many versatile elements, including preparation of the layout, applicable materials, temperature range and endurance. It has come to play an impor- tant role especially in power applications connected with other technologies and materials.


Standard Thick-Film The standard thick-film technology has


worked so well for so long, due to its comparative- ly inexpensive production. All technologies with additional photolithographic steps (thin-film tech- nology, etched thick-film, photolithographic thick- film, and others) require much more effort and cost significantly more. It is possible to reach a better structure resolution with standard thick-film by improving the print technology. It is important to note the interaction of


screen and thick-film ink. This includes both con- ductor inks and dielectric inks. Gold inks are suitable for fine-line prints.


During printing the inks have natural limits according to their rheological properties. There is also the problem of material thickness decreasing at the line edges in the case of fine-line print, which is not significantly improved by using differ- ent screens. In our tests, we used 350-400 mesh screens


for high structure resolution and accuracy. Such screens are necessary at conductor lines as well as


small vias. For our own investigations all screens were deposited with 15 µm photoresist.


Gold Fine-Line Printing Achieving high-quality and accurate fine-line


prints can be accomplished by using high-density


where the structure is manufactured by laser, or for dielectric layers, by the reactivation of the dif- fusion patterning process.


Etched Thick-film In principle, it is possible to use etched thick-


film technology with every standard gold ink. First, the ink is printed all over. After drying at 150°C (302°F), the gold ink will be fired with the thick-film profile. Then photolithography is car- ried out as in a thin-film process. The thick-film substrate will be deposited with photoresist, exposed with a chrome mask, developed, and finally chemically wet-etched. It is possible to create widths and spaces between 20 and 30 µm with this method. It is also possible to apply this etch technol-


Fine-line print with high-density gold ink.


gold-conductor compositions. In general, these inks are not aluminum-wire-bondable. To decide on a suitable ink for an application,


consider the following: the size of the substrate, the substrate material (alumina, AIM, LTCC), the den- sity of the fine-line areas, specification of inks, spec- ification of a fired thickness, bond technology and geometries, and the requirements of the par- ticular application — industrial, high-fre- quency, etc. The size of the vias greatly influences


conductor density. Usually, dielectric inks with an excellent via resolution have the problem of bad flow. In this case an uneven surface will occur. This sort of surface can result in problems for multilayer circuits. A compromise between via resolution and sur- face evenness must be found. Reducing the via sizes by using thinner


screens is possible, but it requires more prints per isolation level to reach the required thickness. There are realizable via sizes down to a diameter of 200 µm, using a special ink with high via-resolution and a 400 mesh screen. The actual specified via diame- ters in the layout depend on the density. If vias are dense (e.g. 20-50 vias per cm2) and the substrate area is very large, the specified diameters are between 300 and 350 µm. Increasing conductor and via density with


standard print technology is not possible. But, by using special screen deposition, fine-line printing can be improved considerably. The problem of decreased line edge thickness during printing will grow as the line width shrinks. This problem is solved only with more expensive technologies and provides a significant advancement in structure quality. This technology includes photolithograph- ic steps, such as etched thick-film, applications


ogy on alumina and aluminum-nitride sub- strates, as well as on fired LTCC. In the future it will be important to develop this technology on fired dielectric. This variant is needed above all


for multilayer substrates. However, more prob- lems at the deposition and exposition must be dealt with because of its uneven surface. Also, the chemical stability of fired thick-film inks must be investigated. After solving this problem, we can realize fine-line structures not only on ceramics but also on second or third conductor layers on


Etched thick-film gold fine-line structures.


dielectric. The technology guarantees that the thickness of the layer is constant along the line width. It is extremely important for narrow con- ductor lines, such as 20 to 50 µm.


Laser Structuring Laser structuring is becoming an increasing-


ly popular method. The handling takes place while dry, as opposed to the etched thick-film process.


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