Thick-Film Metallizations

Frmctions of a thick-film conductor include conductor interconnections, soldered lead and device attachment, thick-film resistor terminations, crossover connections, capacitor electrodes, chip and die bonding, wire bonding, low-value resistors, and packaging of thick-film circuits. The factors considered in 

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Acetylcholineesterase and choline oxidase Electrode was developed by co-immobilization of AChE with ChO H202. Disposable sensors constructed by co-immobilization in a gelatin membrane on Pt electrode or by immobilizing AChE in polyurethane on a thick-film metallized Pt electrode. A kinetically controlled bioenzyme sensor was also used at a low activity of ChE for determining inhibitors. [75]... 

The thick-film metallization process requires modest capital equipment and the process is relatively simple. Typical capital equipment for thick-film metallization includes a silk screening printer and a furnace for annealing and removal of the organic binder. This equipment is relatively modest in cost. In addition to silk screening on a flat substrate, thick-film techniques can be applied by dipping, spraying, or roller coating the non-flat surface [4], Thick-film techniques can also be applied in a continuous operation which can be cost effective. 

Micromachined and microfabricated electrochemical sensors have been used either per se, or as part of a sensor system, in many practical applications. This includes various biosensors and chemical sensors reported in research literature. An example of a practical electrochemical sensor is the yttria-stabilized zirconium dioxide potentiometric oxygen sensor used for fuel-air control in the automotive industry. Thick-film metallization is used in the manufacture of this sensor. Even though the sensor is not microsize, this solid electrolyte oxygen sensor has proven to be reliable in a relatively hostile environment. It is reasonable to anticipate that a smaller sensor based on the same potentiometric or the voltammetric principle can be developed using advanced microfabrication and micromachining techniques. 

Resistance and capacitance monitoring. A thick film metallic pattern on a ceramic substrate consisting of two combs separated by 100/ m was covered with a globe top. During Irradiation capacitance and conductance were monitored with applied alternating current of 300 Hz and 3 kHz. As shown In Figure 4 capacitance And resistance reach a constant value after 3 min. respectively after 5 min. 

It then addresses the micro-hotplates concept that has led to the development of different types of micromachined gas sensor devices. The different reahzations of micromachined semiconductor gas sensors are presented thin- and thick-film metal-oxide, field effect, and those using complementary metal-oxide semiconductors (CMOSs) and silicon-on-insulator (SOI) technologies. Finally, recent developments based on gas sensitive nanostructures, polymers, printing and foil-based technologies are highlighted. 

A second technique widely used for the deposition of thick-film metal-oxides on alumina substrates is screen-printing. Looking at the success met by the thick drop-coated films, screen-printing was reconsidered. Vincenzi et al (2001) screen-printed Pd-doped Sn02 paste onto... 

Vinod PN (2009) Specific contact resistance and carrier tuimeling properties of the silver metal/ porous silicon/p-Si ohmic contact structure. J Alloys Compd 470 393-396 Vinod PN (2013) The fire-through processed screen-printed Ag thick film metal contacts formed on an electrochemically etched porous silicon antireflection coating of silicon solar cells. RSC Adv 3 3618-3622... 

It is common to refer to thick-film metallizations as "gold," or "silver," or "copper" — the conducting metal component in the paste. It is important to keep in mind that the typical conductors for ceramics are compositions of glasses, ceramic powders, and conducting metal particles. As a result, the conductivity of typical gold conductors is 30-50% that of bulk copper and that of typical silver conductors, 70-90% that of bulk copper. The conductivity of plated thick-film and DBC approach that of bulk copper. Table 2.1 summarizes the properties of typical conductors for ceramic application. 

A very useful test is specified in the document called Resistance to Dissolution of Metallization. Parts are tested by immersing them in solder at a higher temperature (-260°C) for an extended period of time and once removed, examination of the deposit is undertaken. This test will certainly catch the thin deposits applied over non-solderable basis metals and will also catch poor surface preparation of solderable basis metals.This test should be used on all non-solderable basis metal components and for all thick film metallization parts and should be used before basic solderabUity testing is begun. 

Below, we consider alumina substrates, developed in advance of LTCC metallization technology, as the basis for thick film metallization and metallization for HTCC, and establish the differences from LTCC metallization. 

As shown below, thick film metallization for alumina substrates has two further types, a high temperature process type and low temperature process type. 

Figure 3-3 Schema of low temperature type thick film metallization of alumina substrates (1) Frit bond type and (2) chemical bond type.

Thick-film metallizations for solar cells are processed in infrared-heated furnaces. These furnaces utilize infrared lamps as the source of heat. With infrared furnaces, it is possible to maintain very short dwell times at peak firing temperatures. 

Figure 41 Schematic representation of microdevices attached to thick film metallization pads printed on a ceramic carrier in a multichip module carrying both flip chip 1 and surface-mounted 2 components. Enlarged views show the bonding area (a) silver-filled isotropic adhesive with possible irreversible changes due to thermal cycling (b) anisotropic adhesive made of conducting spheres embedded in thermoplastic or...

Soldering to thick film metallizations (e.g., Au-Pd, Au-Pt, Ag-Pd) Pb-lOSn, Pb-50In... 

Thick-film metal compositions are elemental metals Ag, Pt, Au as well as alloy compositions Au-Pd, Au-Pt, Au-Pt-P4 Ag-Pd, and Ag-Pt. (A limited number of HMC assemblies use Cu-and Ni-based, thick-film systems.) Lead-tin solder joints made to these metal compositions present several challenges. First, the elemental hlms, Ag and Au, exhibit relatively fast dissolution rates in Pb-Sn solder. Excessive dissolution can risk complete consumption of the thick film and loss of the conductor pad. Second, rapid solid-state intermetallic compound layer growth occurs between the thick-film compositions and Pb-Sn solders, even at modest service temperatures [54-57]. The consequences of solid-state aging are a thick, brittle intermetallic compound layer and loss of the thick film layer. 

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