Abstract:
A printed circuit board (PCB  22 ) capable of withstanding ultra high G forces and ultra high temperature as in a gas turbine ( 11 ). The PCB includes a substrate having a plurality of cavities ( 30 A,  36 A) formed therein for receiving components of a circuit, and conductors embedded in the PCB for electrically connecting the components together to complete the circuit. Each of the cavities has a wall ( 36 A′) upstream of the G-forces which supports the respective component in direct contact in order to prevent the development of tensile loads in a bonding layer ( 37 A). When the component is an integrated circuit ( 50 ), titanium conductors ( 63 ) are coupled between exposed ends of the embedded conductors and contact pads on the integrated circuit. A gold paste ( 51 ) may be inserted into interstitial gaps between the integrated circuit and the upstream wall.

Description:
This application claims benefit of the 26 Sep. 2008 filing date of U.S. provisional application No. 61/100,442. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to printed circuit boards (PCB) and in particular to a PCB structure designed to withstand the harsh environment of ultra high G-forces and ultra high temperature, such as occur on the rotating hot gas path components of a gas turbine engine. 
     BACKGROUND OF THE INVENTION 
     The temperatures inside an operating gas turbine engine are extremely high, often at levels in excess of 350° C. When it is desirable to monitor the inside temperatures of components of the turbine, such as a rotating turbine blade being exposed to thousands of G&#39;s, or to monitor stresses placed upon such components during operation, a special sensing, amplifying and transmitting circuit is required. An effective solution to this problem is the use of wireless telemetry, such as that disclosed in published U.S. Patent Application Publication No US 2005/0198967 A1 entitled SMART COMPONENT FOR USE IN AN OPERATING ENVIRONMENT. In that application, the general concept of using wireless telemetry circuitry on a moving component of a gas turbine engine is disclosed. The present patent application addresses specific problems encountered when implementing a PCB for housing and supporting the wireless telemetry circuitry, which PCB must be suitable for a harsh gas turbine environment. 
     One exemplary prior art device is disclosed in U.S. Pat. No. 5,081,562, entitled CIRCUIT BOARD WITH HIGH HEAT DISSIPATIONS CHARACTERISTIC. This patent teaches fabrication of a circuit board having a cavity for receiving an integrated circuit device and connecting leads from circuit traces on the top rim of the cavity to connection pads on the IC. This arrangement allows the connecting leads to lie flat. However, the attachment of the device to the circuit board is also stressed significantly when the PCB is exposed to centrifugal forces in the thousands of Gs. There is no suggestion or teaching of a structure that can withstand high G-forces 
     Another exemplary prior art device is disclosed in U.S. Pat. No. 7,116,557 B1, entitled IMBEDDED COMPONENT INTEGRATED CIRCUIT ASSEMBLY AND METHOD OF MAKING SAME. This patent also teaches a circuit board having a cavity for receiving an integrated circuit device and connecting leads from circuit traces on the top rim of the cavity to connection pads on the IC. However, in this case the connecting leads are arched over to make an electrical connection. An encapsulating material such as silicon gel is added so as to fill the cavity and encapsulate the connecting leads. This arrangement ensures structural integrity during vibration and G-forces in the range of 10 G&#39;s but would not work in the range of thousands of G&#39;s. Moreover, the high temperature environment of a gas turbine exceeds the temperature capability of polymeric encapsulating materials, such as silicon gel or epoxy materials. High temperature capable encapsulating materials must be developed. Ceramic cements offer the potential to encapsulate electronics for high temperature use. However, the ceramic cement musty be carefully selected so as not to be electrically conductive at high temperature, particularly at radio frequencies, which would short out the radio frequency transmitter circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in the following description in view of the drawings that show: 
         FIG. 1  is a perspective view of a turbine blade having a high temperature circuit package mounted thereon, which houses a high temperature circuit module including the PCB of the present invention. 
         FIG. 2  is an exploded view showing the elements within the high temperature circuit module, including the PCB of the present invention. 
         FIG. 3  is a plan view of the high temperature PCB including cavities formed therein for receiving components, all according to the present invention. 
         FIG. 4  is a plan view of a passive component attached in a cavity of the PCB. 
         FIG. 5  is a cross-sectional view of the passive component shown in  FIG. 4  attached in a cavity of the PCB. 
         FIG. 6  is a plan view of an active component attached in a cavity of the PCB. 
         FIG. 7  is a cross-sectional view of the active component shown in  FIG. 6  attached in a cavity of the PCB. 
         FIG. 8  is a cross section of a connection ribbon used in connecting contact pads of an active component to circuitry within the PCB. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The inventors have recognized that the prior art PCB&#39;s are inadequate for the harsh environment of a gas turbine, and in particular are inadequate for the high G-forces of a turbine blade to which the PCB is attached. Applicants also have recognized that a better geometry is needed to support circuit components when subjected to extremely high G-forces. 
     The components of the circuitry supported by the PCB disclosed herein enable transmission of data via wireless telemetry circuits from regions of a gas turbine with temperatures ranging from ambient to greater than 350° C., and may include temperatures up to at least 450° C. This type of design strategy must be useful for incorporating instrumentation on a rotating hot section component, such as a gas turbine blade being subjected to G-forces in excess of 1,000 G&#39;s, because the PCB must be located on the turbine blade, and thus operate at temperatures exceeding 450° C. 
     One such instrumented moving hot section component that would enable collection of real-time temperature data is shown in  FIG. 1 , wherein a blade  10  of a turbine  11  has mounted thereon a sensor  12  and conductors  14  leading to a high-temperature electronics package  16 , which processes and transmits data derived from the sensor  12  to a receiver circuit (not shown) external the turbine blade  10 . As may be appreciated from  FIG. 1  and the discussion above, the package  16 , when mounted directly to the turbine blade  10 , is subjected to extremely high temperatures and to extremely high G-forces, often in the tens of thousands of G&#39;s, from rotation of the turbine blade. 
     Referring now to  FIG. 2 , an exploded view illustrates elements of a high temperature electronics module  18  that is secured within the high temperature electronics package  16 . Module bottom  18 A includes electrical connecting pins  20  extending from an end thereof to enable communication between the electronics inside the module  18  and external sensors, sources and antennae. In order to function at high temperatures up to at least 450° C., the package must be designed and sized to contain the electronic circuit and its substrate, hereinafter PCB  22 . A pair of gold wires  23 A and  23 B are welded to the inside walls of the bottom  18 A to secure the PCB  22  in place. A lid  18 B is next secured to the top in order to completely enclose the module structure. 
     The module  18  must be able to withstand the temperature and centrifugal loading requirements and protect the circuitry on the PCB  22 . Hence, the module  18 A and lid  18 B are made of gold-plated Kovar® alloy and the electrical connecting pins  20  are made of gold. Gold plating on the module cavity and lid prevents oxidation of the Kovar® alloy at elevated temperatures. The connectors  20  are insulated from the cavity  18 A by means of individual insulating sleeves (not shown). A pair of the pins  20  is coupled to the electrical connectors  14  which communicate with the sensor  12 , as shown in  FIG. 2 . The remaining pins may be coupled to ground potential, a source of power (two each for positive and negative ac), and to an antenna (not shown). 
     The disclosed PCB  22  is fabricated from materials capable of operation at high temperatures, for example high temperature capable materials, such as alumina, zirconia, silica, magnesia, titania, mullite, silicon carbide, silicon nitride, aluminum nitride, etc. The conductors and circuit traces in the PCB may be made of gold. The connecting pins  20  may be fabricated from platinum metal, which can withstand high temperature without melting or flexing excessively under the high G-forces. As will be discussed further hereinafter, a novel arrangement of the components within the PCB  22  provides a counter resistance to the high G-forces to which the PCB is subjected. 
     Referring now to  FIG. 3 , a plan view of the PCB  22  (unpopulated) is shown. Cavities  30 A,  30 B,  30 C and  30 D are formed for receiving capacitors (not shown in  FIG. 3 ). Gold paste  31 A,  31 B is deposited on two sides of the bottom of the cavities  30 A- 30 D for securing and making ohmic contact with each of four large capacitors. Each of the gold paste deposits makes ohmic contact with conductors (not shown) that are embedded in the PCB  22 . As will be shown hereinafter, ceramic cement may be deposited over each of the capacitors in order to secure them in place. 
     Additional cavities, such as cavity  36 A, are formed in the PCB  22  for receipt of active components, which in accordance with one embodiment are SiC JFET&#39;s. Gold paste  37 A may be deposited in cavity  36 A for securing the active component in place, and for making ohmic contact with circuitry embedded within the PCB  22 . Multiple cavities may be formed in a similar manner in the PCB  22  for receipt of the remaining components of the circuitry. 
     Referring now to  FIGS. 4  (plan view) and  5  (cross-sectional view), a typical passive component  40 , for example a resistor or a capacitor, is shown secured within the cavity  30 A. The component  40  has gold terminals  41  and  42 , which terminals make ohmic contact with gold paste pads  31 A and  31 B that in turn make ohmic contact with conductors  43  and  44  that are embedded within PCB  22 . The embedded conductors may continue through vias  45 A and  45 B to the PCB surface and connect with other circuitry (not shown). Finally, ceramic cement  46  (high temperature capable polymeric material with ceramic filler powder and binders) is placed over the component to secure it in place. The polymeric material may be a cross-linked polymer including filler powders and ceramic or metal adhesives employing binders to hold the particles together, or it may be a cross-linked epoxy including filler powders and ceramic or metal adhesives employing binders to hold the particles together. The filler powders may be selected from the group consisting of aluminum oxide, zirconium oxide, zirconium silicate, magnesium oxide, silicon dioxide, mica, graphite, silicon carbide, silicon nitride, aluminum nitride, aluminum, nickel and stainless steel. 
     The direction of G-force load in  FIGS. 4 and 5 , when viewed in a conventional manner, is from right to left. Hence, wall  30 A′, which is on the right-hand side of the cavity  30 A in the figures, is the wall that will support or brace the component  40  (when enshrouded in the ceramic cement  46 ) in resistance to the applied G-force load. Wall  30 A′ is sometimes referred to herein as the wall upstream of the G-forces. It is also pointed out that the drawings are not drawn to scale and that the thickness of the ceramic cement adjacent the wall  30 A′ is much thinner than may appear. The thickness of the ceramic cement support layer may be less than 2 mm. 
       FIGS. 6  (plan view) and  7  (cross-sectional view) illustrate an active component  50  secured to the bottom of the cavity  36 A by means of the gold paste  37 A. It is noted that the direction of the G-force load in  FIGS. 6 and 7 , when viewed in a conventional manner, is from right to left. Hence, wall  36 A′, which is on the right-hand side of the cavity  36 A in the figures, is the wall that will support or brace the component  50  in resistance to the applied G-force load. Wall  36 A′ is sometimes referred to herein as the wall upstream of the G-forces. Wall  36 A″, which is adjacent to wall  36 A′, also helps support the component  50  by resisting G-loads that are not strictly perpendicular to wall  36 A′, and thus wall  36 A″ may also be considered to be a wall upstream of G-forces. Accordingly, it may be appreciated that the component  50  is secured into a corner of the cavity  36 A. 
     In this manner, extremely high G-loads may be resisted and carried into the base material of the PCB  22  in direct compression, without relying upon the strength of the bond between the gold paste  37 A and the component  50 . Prior art components lacking such direct contact support will develop a bending moment (compression on the upstream side and tensile on the downstream side) in the underlying bonding layer due to the vertical displacement of the center of gravity of the component above the plane of the underlying bonding surface. Even prior art components that were potted into place would develop such bending moments under very high G-loads (such as are experienced by turbine blades) due to the inherent flexibility of the posting material. The arrangement of the present invention avoids such bending moments/tensile loads in the underlying gold paste  37 A by directly resisting all G-loads as compressive force along the side of the component  50  bearing on cavity wall  36 A′ (and optionally  36 A″). 
     Gold paste  51  may be coated on the walls of the cavity  36 A and the component  50  pushed into a corner defined by walls  36 A′ and  36 A″. Gold paste  51  may also be placed into the space between the component  50  and the wall opposite the wall  36 A″. The gold paste  51  on the upstream wall  36 A′ ( 36 A″) of the cavity  36 A is kept to a minimum thickness due to the fact that the upstream wall(s) directly support the component  50  against the extremely high G-load forces. The space between the component  50  and the side opposite the upstream side  36 A′ may be left open for allowance of any expansion/contraction of the component  50 . Since the walls of the cavity  36 A or the edges of the component  50  may not be perfectly planar or may not align precisely with the cavity, the gold paste  51  is used to fill in any interstitial gaps or small crevices between the device and the wall surfaces  36 A′ and  36 A″. It is noted again that the PCB  22  (and the components mounted therein) are subjected to extremely high G forces and any gap between the cavity  36 A and the component  50  could dislodge the component as a result of twisting and resulting tensile forces. Accordingly, the gold paste serves to fill in any gaps that may occur and to firmly secure the component in place. It is also pointed out that the drawings are not to scale and that the thickness of the gold paste adjacent the wall  36 A′ or  36 A″ is much thinner than it appears in the figures. The thickness of the gold paste may be less than 2 mm. 
     As may be seen in the cross-sectional view of  FIG. 7 , the gold paste (die attach)  37 A makes ohmic contact with an embedded conductor  53 . The conductor  53  may continue up to the surface of the PCB by means of via  54 . Contact pads  55 ,  56 , and  57  on the top surface of the component  50  are coupled to other surface conductors  59 ,  60 , and  61 , respectively, by means of bonding conductors or ribbons  63 ,  64  and  65 . The ribbons  63 ,  64 , and  65  may be affixed to the contact pads  55 ,  56 , and  57 , respectively, by means of thermosonic welding. Likewise, the other end of the ribbons  63 ,  64 , and  65  are affixed to the conductors  59 ,  60 , and  61 , respectively, by thermosonic welding. It is noted that the component shown in  FIGS. 6 and 7  is illustrative only, and in most cases active components include many more contact pads and bonding conductors than is shown herein. 
     According to an embodiment, the bonding conductors or ribbons  63 ,  64  and  65  are made of platinum. It is pointed out that alignment of the ribbons  63 ,  64  and  65  is parallel to the G-load forces. This arrangement minimizes any warping of the ribbons due to the heavy G-loading, which warping could be more severe if the ribbons were aligned perpendicular to the G-loading. A cross-sectional view of a typical ribbon is shown in  FIG. 8 . The ribbons  63 ,  64 , and  65  are typically made with an aspect ratio (W/T) of something greater than 1:1, such as an aspect ratio of 5:1 or between 1:1 and 5:1. This is preferred in order for the ribbons to withstand the extremely high G-forces exerted on the PCB  22  and the component  50  buckling or warping. Other metals suitable for the ribbons  63 ,  64  and  65  include Ni, Pt, Pd, Ti, Ta, W, etc. 
     While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.