PYROLYTIC FILM RESISTORS: CARBON AND BOROCARBON 



297 



there are cracks, pits, grooves or other mechanical imperfections in the 

 core, these will result in corresponding imperfections in the carbon films. 

 Various types of imperfections are shown in the schematic cross section of 

 core and film in Fig. 16. Of the imperfections there illustrated, the thinner 

 and thicker areas of the film are the result of the catalytic or chemical in- 

 fluences described earUer, and, of these, the thin areas are the more harmful. 

 If a continuous film covering the entire cylindrical core surface is employed 

 as a resistive element by making suitable electrical connections to its ends, 

 the effect of the imperfections is relatively small because each individual 

 fault is shunted by a continuous and perfect film. However, it is common 

 practice in resistor production to cut a helical groove through the film to 

 provide, in effect, a carbon ribbon wound around a cylindrical core and thus 

 to increase the resistance of the element. When this is done, the imperfec- 

 tions may become series elements in the current path or shunt elements 

 between turns, and their effects on resistor behavior are consequently greatly 



CRACK ^ 



LOOSE CONTACTS' 

 THIN FILM 



BARE SPOT 



Fig. 16 — Types of faults in carbon film resistors. 



accentuated. Figure 17 shows photomicrographs of mechanical imperfections 

 in the fihn both before and after the helixing operation. 



Carbon is deposited on the walls of cracks in the ceramic and over the 

 surfaces of unvitrified grains in porous regions, and the contacts thus formed 

 between carbon coated surfaces are similar to those in the carbon composi- 

 tion resistor or those between carbon granules in a microphone. They are 

 unstable with time, temperature and voltage; and this instabiUty is reflected 

 in the behavior of the resistor. Microscopic count of such imperfections has 

 been found to be qualitatively correlated with unfavorable effects on the 

 temperature coefficient of resistance, the voltage coefficient of resistance, 

 the noise level, and the stability of pyrolytic carbon resistors. While thor- 

 oughly vitrified ceramic cores are desirable, it is nevertheless possible to 

 employ sHghtly porous or imperfect cores under certain conditions, par- 

 ticularly for thick fihns, since the depth of penetration of carbon into 

 the ceramic can be controlled to some extent by proper choice of the pyrolyz- 

 ing conditions. 



It appears that carbon is held to the substrate by a mechanical keying 

 action, so that the surface geometry of the core is important: The scale of 



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