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A NOVEL METHOD OF CREATING RESISTORS IN PRINTED WIRING BOARDS

IP.com Disclosure Number: IPCOM000008812D
Original Publication Date: 1998-Sep-01
Included in the Prior Art Database: 2002-Jul-16
Document File: 3 page(s) / 152K

Publishing Venue

Motorola

Related People

Steven M. Scheifers: AUTHOR

Abstract

A method of creating resistors in primed wiring boards was developed. The method consists of creat- ing a suitable size hole in the substrate and filling the aperture with an appropriate resistive ink. The sub- strate may be organic or ceramic and the ink used a common polymer thick film version or a fireable ver- sion depending on the substrate type. Excess ink is removed from the top and bottom surface of the sub- strate by a suitable method such as abrading. It is preferable to remove the polymer thick film inks after the curing step. However, it is preferable to remove the ceramic resistive material prior to firing. Metal ter- minals to the resistor are applied subsequent to the for- mation of the final resistor by any commonly used methods of applying metal in printed wire board con- structions The cross section area of the hole, substrate thickness, and ink resistivity are used to control the final value of the resistor.

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68 MOTOROLA

Technical Developments

A NOVEL METHOD OF CREATING RESISTORS IN PRINTED WIRING BOARDS

by Steven M. Scheifers

ABSTRACT

step concludes the attachment process of the resistor to the board.

DISCUSSION

   A new method for forming resistors has been devised that permits the use of critical board surface area and that attains accurate resistor values. Commonly used commercial, inexpensive, and high volume manufacturing steps can be employed to reduce the cost of manufacturing. The method can be used for ceramic or organic substrates. Fireable resis- tive inks are used with the former, and polymer thick film inks are typically used in the latter.

  Since resistance is proportional to the resistivity of the material and the length of the resistor but inversely proportional to the cross sectional area of the resistor, the cross sectional area of the apertures parallel to the substrate surface, the substrate thickness, and the resis- tivity of the ink will define the value of the resistor. The accuracy of the formed resistor depends on the dimensional control of the aperture forming process, the variation in substrate thickness, and the variation in the resistivity of the formed resistor material. The variation in substrate thickness and the typical methods

used to form the apertures can be reproducibly con- trolled to tolerances that are better than those of the tra- ditional screen priming processes used to form resis- tors on the surface of the substrates. This eliminates most of the variability that necessitates the laser trim- ming process. Controlling processing conditions and the incoming specifications on the resistive inks will minimize variation of the resistivity of the formed resistor material.

  The method involves creating apertures in the sub- strate material as shown in Figure 1. This might be accomplished by punching the green precursor material of the ceramic prior to firing or by drilling an organic substrate. It is generally necessary that the holes be slightly overfilled as depicted in Figure 2. Screen

  A method of creating resistors in primed wiring boards was developed. The method consists of creat- ing a suitable size hole in the substrate and filling the aperture with an appropriate resistive ink. The sub- strate may be organic or ceramic and the ink used a common polymer thick film version or a fireable ver- sion depending on the substrate type. Excess ink is removed from the top and bottom surface of the sub- strate by a suitable method such as abrading. It is preferable to remove the polymer thick film inks after the curing step. However, it is preferable to remove the ceramic resistive material prior to firing. Metal ter- minals to the resistor are applied subsequent to the for- mation of the final resistor by any commonly used methods of applying metal in printed wire board con- structions The cross section area of the hole, substrate thickness, and ink resistivity are used to control the final value of the r...