Patent GB-1370041-A (GB)
Title: Thick film resistors
IPC: H01C17/232
Filed: 1972-03-02
Abstract: 1370041 Resistors HONEYWELL Inc 2 March 1972 [2 March 1971] 9735/72 Heading H1S A method of making a thick film resistor comprises the steps of applying to an insulating substrate 32 (Fig. 6) a resistor ink 10 allowed to dry on the substrate prior to firing at the sintering temperature of the ink, e.g. 1000 C. for fifteen minutes, applying electrically conductive layers 16 and 22 and firing these at a sintering temperature lower than that of the resistor ink, e.g. 550 C. such that substantially none of the conductive material diffuses into the resistor ink. The substrate 32 is preferably of 96% pure aluminium oxide ultrasonically cleaned in ethyl or methyl alcohol for between 30 seconds and 2 minutes and fired at 1000 C. for 15-20 minutes. A desired temperature coefficient of resistivity (T.C.R.) for the resistor ink may be obtained in the following manner. Metal, e.g. gold powder with a particle size between 3 and 20 microns or a metal oxide powder of Group VIII mixed with lead borosilicate glass powder of the same particle size, when mixed with a liquid agent, e.g. " decanol " provide a suspension that increases the T.C.R. of a resistor ink. A metal oxide powder, e.g. ruthenium oxide, 325 mesh, or a metal oxide powder mixed with boro lead silicate glass and further mixed with a liquid agent, e.g. decanol provide a suspension that decreases the T.C.R. of a resistor ink. Using the above suspensions, resistor inks of positive and negative T.C.R. greater than 20 p.p.m. may be produced and blended to form an ink having a desired T.C.R. The proportions to be used are determined from the empirical equation where P = per cent of positive T.C.R. ink in blend for manufacture of zero T.C.R. resistors, A = ln (100 + s),B = ln s, s = 3, for ruthenium system inks, T 100  = T.C.R. of positive T.C.R. ink. Alternatively, To and T 100  may be plotted on a graph (Fig. 1), and connected by a straight line representing the above equation, P then being determined from the intersection of this straight line and the horizontal zero T.C.R. line. P may have to be corrected due to the approximate nature of the above equation and this is done by calculating the exact T.C.R. of the resistor ink made with the proportion P of positive T.C.R. ink, using the formula where is the change in resistance corresponding to a change in temperature of #T, and plotting the resultant high or low value, TCR 1  or TCR<SP>1</SP> 1  respectively. The appropriate line is then drawn either between TCR 1  and To or between TCR<SP>1</SP> 1  and T 100  and the corrected value of P is determined from the intersection of this line with the zero T.C.R. line. The correction can be repeated by always connecting the last calculated and plotted T.C.R. value and connecting it with the nearest T.C.R. point of opposite sign. Normally two corrections are sufficient to obtain a desired T.C.R. value within 20 p.p.m. A specific example of the complete above process is included. A buffer layer 34, to protect the resistor against atmospheric agents comprises a silicon polymer filled with magnesium oxide such as dimethylpolysiloxene the layer being printed or brushed over the resistor 10. The layer is polymerized and heat cycled, details of which are given, and the resistor is finally trimmed and cleaned with a spray of nitrogen. A second layer 36 consisting of a mixture of iron oxide and magnesium silicate suspended in xyeline, e.g. glyptal 1201B paint, is sprayed over the entire substrate, the whole then being exposed to 100 C. for 4 hours. Graphs are included in the Specification to demonstrate comparisons of resistors made by the method of the invention and prior methods.