Investigation towards the optimum of power capability, ageing stability and costs effectiveness on thick film resistor pastes for AlN ceramics

The focus of this work was the optimization of a 10 Ω/□ thick film resistor (TFR) paste composition to obtain increased power capability, aging stability and minimum use of ruthenium oxide for cost savings without changing the defined narrow sheet resistance (R□) and temperature coefficient of resistance (TCR) specifications. In times of highly fluctuating precious metal costs, the use of a minimum of the precious metal ruthenium respectively ruthenium dioxide is one essential part for cost-effectiveness. The thick film paste formulation consists of the electrically conducting phase ruthenium dioxide, a lead-free glass phase and two inorganic additives for tuning thermo-mechanical and electrical properties of the formed films. A phthalate free organic vehicle with ethyl cellulose polymer was used to formulate a screen printable ceramic thick film paste. For this paper, RuO2 powders with various specific surface area values (BET) were prepared by thermal annealing of a precipitated fine ruthenium dioxide powder. All other solid and liquid components of the paste were the same as used for IKTS 10 Ω/□ TFR paste FK9611 for AlN substrates. Furthermore, the content of ruthenium dioxide in the paste compositions was changed systematically around an assumed target content to achieve the desired sheet resistivity. Concurrent to the variation of the ruthenium dioxide content the inorganic additives had to be adapted too. The influence of the variations of raw material and paste composition on the film properties were investigated by screen printing 24 resistors of 2 mm × 1 mm dimension on an 1” × 1” AlN substrate, firing at 850 °C for 10 minutes in air atmosphere and subsequently measuring R□, TCR, the stability of resistance ΔR/R0 effected by artificial aging of the resistors (stored 100 up to 1000 hours @ 200°C) and the maximum rated power dissipation (MRPD) as well as short term overload voltage (STOL). The results are discussed in regard to find an optimum between all demands of the most important electrical film properties.

Characterization of a New and Complete Lead-Free Thick Film Resistor System for the Hybrid Circuit Market

The move to lead-free thick film technology, including resistors does not necessarily mean a move away from performance. There have been many recent developments in the area of lead-free glasses, thick film dielectrics, and conductor materials. However, the development of lead-free resistor materials has been slower to enter the market. This slower evolution is due, in part, to the need to develop a series of discrete compositions that are compatible with each other and cover a wide range of resistance values. There can be 8–10 individual members in such a series. The primary goal of this development was to achieve resistor performance on par with, or superior to an existing and popular premium lead based resistor system. This paper describes a new series of lead-free thick film resistor compositions that cover a complete range of decade values from 1 Ohm/square to 10 MegOhms/square. Each member is compatible and blendable with adjacent members to cover all possible resistivity needs. This system provides excellent environmental stability of laser trimmed resistors, tight TCR gap control, a wide processing latitude, and minimal termination effects with a wide range of lead-free conductor metallurgies including Au, Ag, Ag/Pt, and Ag/Pd. In this paper, we report the results of a characterization study that includes 1000 hours of laser trim stability in 85°C/85%RH, thermal ageing at 150°C, room temperature ageing, and thermal cycle stability at −50°C +150°C. Electrical properties are presented and include Power Handling, Quan-tech Noise, and Electrostatic Discharge stability. In addition, this paper investigates and documents: resistor length effects, fire and refire sensitivity effects, thickness behavior, and the use of lead-free encapsulation.