Abstract:
A surface mount chip resistor for increasing power handling capabilities of radio frequency (RF) circuits and for minimizing parasitic capacitance and inductance effects, the chip resistor includes a ceramic substrate having a main portion and an outrigger. A resistor element is between an input contact and an output contact on a top surface of the main portion. A ground plane attachment area is on a top surface of the outrigger. The ground plane attachment area is mounted to a ground plane of a circuit board to provide a heat pathway for dissipating heat generated by the resistor element.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims the benefit of U.S. Provisional Application No. 61/695,193, filed on Aug. 30, 2012, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     1. Field 
     The present disclosure describes chip resistors and other resistive devices with improved power handling capability while maintaining low capacitance essential for circuits to operate efficiently in the microwave region of the radio frequency (RF) spectrum. 
     2. Description of the Related Art 
     Chip resistors are a common element in RF and Microwave circuits. The various uses for chip resistors generally include one or more of the following: change the amplitude of an RF signal, limit the current in an RF transmission line, or alter the impedance of the transmission line. 
     As circuit board sizes decrease, designers are faced with the challenge of fitting or positioning more and more circuitry into smaller spaces. Also, as frequencies increase, designers are trending toward smaller and smaller components to minimize parasitic affects which are detrimental to the functioning of circuits. However, decreasing board sizes and smaller components present challenges to heat dissipation in order to prevent premature failure. 
     Physically small devices tend to have better performance in the microwave region of the RF spectrum because they constitute a small fraction of a wavelength at the frequencies of interest. This reduces detrimental parasitic effects due to excessive inductance and capacitance. As the size of the component decreases, the power handling capability also decreases. 
     Heat dissipation can be improved by using advanced materials having good thermal conductivity, such as Diamond, Beryllium Oxide, or Aluminum Nitride. Because these materials are either very expensive or toxic, they fail to provide a viable commercial solution. In addition, when using a higher thermally conductive material, the heat generated in a conventional surface mount chip resistor is dissipated through the circuit traces or through the air, which fail to provide satisfactory thermal paths. 
     Therefore, there exists a need for a chip resistor that combines small physical size, high power handling capability, low capacitance, and low cost. For example, a typical, commercially available, surface mount resistor, in the 0402 size, measures 0.040×0.020 inches (1.0×0.5 mm) and has a power rating of about 0.063 watts. This is insufficient for many applications. If higher power is forced through the surface mount resistor, it will heat to the point that the solder connections will melt leading to a catastrophic failure of the circuit. 
     Using a physically larger surface mount resistor with a higher power rating can overcome these drawbacks. However, because current design trends favor smaller circuit boards, circuit boards may lack the physical space for the inclusion of a larger surface mount resistor. Even if sufficient space was available, a larger surface mount resistor will have more capacitance and/or inductance, requiring more elaborate tuning measures to minimize these effects. Circuitry necessary to tune out these parasitics takes up valuable board space and reduces the bandwidth of the circuit, an unsatisfactory side effect. In addition, many boards are already designed for optimal RF performance and the dimensions of the circuit traces cannot be changed without a substantial performance, financial, and delivery impact. Designers must use the layout as it is already manufactured. 
     There is a need for a chip resistor which matches the current industry-standard pad size and configuration, which drastically improves the power handling capability, and at the same time, maintains the low capacitance essential for circuits to operate efficiently in the microwave region of the RF spectrum. 
     SUMMARY 
     The present disclosure relates to a chip resistor with an extended portion or outrigger attached to a main portion of a substrate. The outrigger provides a heat pathway to a ground plane of a circuit board to dissipate heat generated by the chip resistor. 
     In one implementation, the chip resistor includes a substrate having a main portion with a top surface and a first extended portion with a top surface. First and second contacts are mounted on the top surface of the main portion, and a resistor element is mounted on the top surface of the main portion between the first and second contacts. A first conductor is positioned on the top surface of the first extended portion, spaced apart from the resistor element, and configured for mounting to a ground plane of a circuit board for dissipating heat generated by the resistor element. 
     In another implementation, a surface mount chip resistor includes a ceramic substrate having a main portion, and first and second extended portions, with the main portion located between the first and second extended portions. An input contact and an output contact are mounted on the top surface of the main portion. A resistor element is mounted on the top surface of the main portion between the input and output contacts. A first conductor is spaced apart from the resistor element and positioned on the top surface of the first extended portion. A second conductor is spaced apart from the resistor element and positioned on the top surface of the second extended portion. The first and second conductors are configured for mounting to a ground plane of a circuit board to provide two pathways to dissipate heat generated by the resistor element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features, obstacles, and advantages of the present application will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, wherein: 
         FIG. 1  is an isometric view of a surface mount chip resistor according to an implementation of the present disclosure; 
         FIG. 2A  is a top view of a chip resistor according to an implementation of the present disclosure; 
         FIG. 2B  is a top view of an array of chip resistors according to an implementation of the present disclosure; 
         FIG. 3A  is a chip resistor with a single outrigger prior to assembly according to an implementation of the present disclosure; 
         FIG. 3B  is a chip resistor with a single outrigger after assembly according to an implementation of the present disclosure; 
         FIG. 4A  is a chip resistor with double outriggers prior to assembly according to an implementation of the present disclosure; 
         FIG. 4B  is a chip resistor with double outriggers after assembly according to an implementation of the present disclosure; 
         FIG. 5  is a graph of performance data according to an implementation of the present disclosure; 
         FIG. 6  is a table of performance data according to an implementation of the present disclosure; 
         FIG. 7  is a table of specifications according to an implementation of the present disclosure; and 
         FIG. 8  is a table of materials according to an implementation of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Apparatus, systems and methods that implement the implementations of the various features of the present application will now be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate some implementations of the present application and not to limit the scope of the present application. Throughout the drawings, reference numbers are re-used to indicate correspondence between referenced elements. 
     In the following detailed description, numerous specific details are set forth to provide an understanding of the present invention. It will be apparent, however, to one ordinarily skilled in the art that elements of the present invention may be practiced without some of these specific details. In other instances, well-known structures and techniques have not been shown in detail to avoid unnecessarily obscuring the present invention. 
     The drawings show a chip resistor which has an industry-standard footprint but with improved power handling capability while maintaining very low capacitance and inductance. As described above, a typical, commercially available, surface mount resistor in the 0402 size has a power rating of about 0.063 watts. The chip resistor described herein has a power rating of 1 watt, more than 15 times greater than currently available products, while maintaining the same footprint and low capacitance and inductance. Although the 0402 size is used here as an example, the concept is scalable to larger and smaller size chips. In addition, the frequencies discussed by the present disclosure range from about 100 KHz to 60 GHz or any portion thereof. However, in other implementations, other frequencies may be utilized. 
       FIG. 1  presents a chip resistor  100  according to an implementation of the present disclosure. The chip resistor  100  includes a substrate  110 . A resistor element  140  and two contacts  130  lie on a top surface of a main portion  112  of the substrate  110 . A ground plane attachment area  120  lies on a top surface of an extended portion  114  of the substrate  110 . The substrate  110  may be made of a ceramic material. 
     The resistor element  140 , the contacts  130 , and the ground plane attachment area  120  may be screen printed or otherwise deposited onto the substrate  110 . For example, the resistor element  140  may be formed using a resistive ink or a conductive ink in screen and stencil printing processes, sometimes referred to as “thick film” resistors, which generally have a thickness between about 200 micro-inches to about 1000 micro-inches. Alternatively, the resistor element  140  can be formed using sputtering, evaporation or other vacuum deposition processes of a resistive material and etched, sometimes referred to as “thin film” resistors, which generally have a thickness between about 300 Angstroms to 25,500 Angstroms (1.2 micro-inches to 100.4 micro-inches). 
     The chip resistor  100  exhibits improved thermal performance by utilizing ground plane material that is already available on a printed circuit board (PCB). Ground planes are generally made of copper, which is an excellent heat conductor. Ground planes are also highly solderable which facilitates attachment of components. The ground plane material provides for the correct functioning of microwave circuits and is necessary for optimal circuit operation, but is generally unused to attach other components. 
     The chip resistor  100  incorporates the addition of an extended piece of ceramic material, the extended portion  114 , attached to the resistor substrate, the main portion  112 , which is then soldered down to the existing ground plane on the circuit board. Heat is drawn away from the resistor element  140 , where the heat is generated, and dissipating it safely into the surrounding metal ground plane. The chip resistor  100  has one “outrigger,” or extra piece of ceramic material (i.e. the extended portion  114 ) attached for applications having only one available ground plane. As will be discussed below, other implementations may have more outriggers. 
     With respect to RF performance, the additional piece of ceramic, the extended portion  114 , has minimal or no effect on the response of the circuit, thus retuning is not necessary. Adding the additional piece of ceramic adds almost no cost to the manufacturing of the chip resistor  100 , but results in considerably higher performance, mechanically, thermally, and electrically. 
       FIGS. 2A and 2B  illustrate exemplary dimensions for manufacturing chip resistors similar to the chip resistor  100 . However, in alternative implementations the dimensions and thicknesses may vary based on the application. A chip resistor  200  in  FIG. 2A  may be similar to the chip resistor  100 . The chip resistor  200  has contacts  230 , a resistor element  240 , and a ground plane attachment area  220  over a substrate  210 . 
     The substrate  210  has a width  216 , which may be about 0.040 inches, and a height  214 , which may be about 0.060 inches. Each contact  230  has a width  232 , which may be about 0.010 inches, and a height  234 , which may be about 0.020 inches. Each contact  230  is a distance  212 , which may be about 0.005 inches, offset from an edge of the substrate  210 , while aligned against side edges of the substrate  210 , as seen in  FIG. 2A . The resistor element  240  is between and touches the contacts  230 . The resistor element  240  has a height  242 , which may be about 0.010 inches, and a width  244 , which may be about 0.020 inches. Note that the widths  232  and  244  approximately equal the width  216 . The ground plane attachment area  220  also has the width  216 . The ground plane attachment area  220  also has a height  222 , which may be about 0.020 inches. 
       FIG. 2B  depicts a wafer  250  having an array of chip resistors  200 . The wafer  250  has a width  252 , which may be about 3.000 inches, and a height  256 , which may be about 3.000 inches. In  FIG. 2B , the wafer  250  is a 3 inch square wafer, but in other implementations the dimensions may vary as needed. 
     The wafer  250  may hold an array of 40×30 chip resistors  200 , although in other implementations the size of the array may vary. The array is offset a distance  254 , which may be about 0.505 inches, from a side edge of the wafer  250 , and offset a distance  258 , which may be about 0.455 inches, from a top edge of the wafer  250 , as seen in  FIG. 2B . 
     The chip resistors  200  further include a protective coating  245  covering the resistor element  240 . The protective coating  245  shares the width  244  of the resistor element  240 , and the height  234  of the contacts  230 . Within the array, each chip resistor  200  is spaced a distance  264 , which may be about 0.010 inches, from neighboring chip resistors  200  in the same row, and spaced a distance  262 , which may be about 0.010 inches, from adjacent rows. Thus, multiple chip resistors  200  may be fabricated on a single wafer  250 . 
       FIG. 3A  depicts a pre-assembled state  300  of a chip resistor  305 . The chip resistor  305  may be similar to the chip resistors  100  and  200 . The chip resistor  305  includes a substrate  310  which includes a main portion  312  and an outrigger  314 , a ground plane attachment area  320 , an input contact  330 , an output contact  335 , and a resistor element  340 . The chip resistor  305  is configured to attach to a coplanar wave guide structure  350 . 
     The coplanar wave guide structure  350  includes RF traces  360 , and via holes  370 , which connect to a bottom ground plane not seen in  FIG. 3A . A heat sink area  315  corresponds to an attachment area for the chip resistor  305 . As seen in  FIG. 3A , when the chip resistor  305  is attached to the heat sink area  315 , the input contact  330  aligns with and contacts the RF trace  360 , and the output contact  335  aligns with and contacts the other RF trace  360 . The ground plane attachment area  320  aligns with and contacts several via holes  370  for connection to the ground plane. An assembled state  302  is depicted in  FIG. 3B . 
     Another implementation may incorporate two outriggers, rather than one. With two outriggers, the resistor element is mounted centrally between two thermal paths to the ground plane. Two outriggers advantageously doubles the power handling capability of the chip resistor and works particularly well for RF circuits using Coplanar Ground Planes (CPWs), where there is copper material already available on both sides of the RF transmission line, resulting in a very efficient heat sink. 
       FIGS. 4A and 4B  show a double outrigger arrangement according to an implementation of the present disclosure.  FIG. 4A  illustrates a pre-assembled state  400  of a chip resistor  405 . The chip resistor  405  includes a substrate  410  which includes a main portion  412 . The chip resistor  405  also includes an input contact  430 , an output contact  435 , and a resistor element  440 . Unlike the chip resistors  100 ,  200 , or  305 , the chip resistor  405  includes a first extended portion or outrigger  414  and a second extended portion or outrigger  416 . Accordingly, the chip resistor  405  further includes a first ground plane attachment area  420  over the first extended portion  414  and a second ground plane attachment area  425  over the second extended portion  416 . 
     The chip resistor  405  is configured to attach to a coplanar wave guide structure  450 , which may be similar to the coplanar wave guide structure  350 . The coplanar wave guide structure  450  includes RF traces  460 , and via holes  470 , which may connect to a bottom ground plane not seen in  FIG. 4A . A heat sink area  415  corresponds to an attachment area for the chip resistor  405 . As compared to the heat sink area  315  in  FIG. 3A , the heat sink area  415  is larger, encompassing two rows of via holes  470  rather than just one. 
     As seen in  FIG. 4A  when the chip resistor  405  is attached to the heat sink area  415 , the input contact  430  aligns with and contacts the RF trace  460 , and the output contact  435  aligns with and contacts the other RF trace  460 . The ground plane attachment area  420  aligns with and contacts a first row of via holes  470  for connection to the ground plane. The ground plane attachment area  425  aligns with and contacts a second row of via holes  470  for connection to the ground plane. An assembled state  402  is depicted in  FIG. 4B . 
       FIG. 5  shows a graph  500  of performance results of a chip resistor, such as the chip resistor  100 ,  200 ,  305 , or  405 , listed in table  600  of  FIG. 6 . As the chip resistor handles more power, the temperature rises. As seen in  FIGS. 5 and 6 , the chip resistor of the present disclosure reaches a temperature of about 75.1 degrees C. at 1 watt, which generally outperforms conventional chip resistors. 
       FIG. 7  presents a table  700  of typical specifications of a chip resistor according to an implementation of the present disclosure. 
       FIG. 8  presents a table  800  of exemplary materials for fabricating the chip resistor  200 , although in other implementations other suitable materials may be used. The substrate  210  may be made of alumina 96%, 15 mil thick. The contacts  230  may be made of conductor ink Ferro C4270 made of platinum/gold. The resistor element  240  may be made of DuPont resistor ink at 50 ohms/square for a 131 ohm resistor, or DuPont resistor ink at 100 ohms/square for a 262 ohm resistor. The protective coating  245  may be made of Ferro blue protective coating. 
     The previous description of the disclosed examples is provided to enable any person of ordinary skill in the art to make or use the disclosed methods and apparatus. Various modifications to these examples will be readily apparent to those skilled in the art, and the principles defined herein may be applied to other examples without departing from the spirit or scope of the disclosed method and apparatus. The described implementations are to be considered in all respects only as illustrative and not restrictive and the scope of the application is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.