Patent Publication Number: US-2019178904-A1

Title: Device, system and method for stress-sensitive component isolation in severe environments

Description:
BACKGROUND 
     Inertial Measurement Units (IMUs) are devices that can sense the rotation and acceleration of an object. For example, IMUs can be utilized to detect the rate of acceleration and the change in rotational attributes of objects about three axes for a given period of time. In space applications, IMUs are utilized in navigational and guidance systems for launch vehicles, spacecraft, satellites and the like. In other applications, IMUs are utilized to guide (e.g., gun-launched) large caliber projectiles. Notably, during the launch of a spacecraft, satellite or large caliber projectile, the electronic components in the onboard IMUs are subjected to the high temperatures, high shock loads and high vibration levels (e.g., referred to herein as “severe environments”) caused by the high acceleration and g-force levels involved. 
     Cavity potting is one process utilized to encapsulate and support electronic components (e.g., components mounted on a printed board assembly or PBA) by placing a component in a container, filling a cavity between the container and the component with a suitable potting material (e.g., a resin), and curing the material to form an integral potted component. However, when cavity potting is utilized to encapsulate electronic components in order to meet the stringent system requirements imposed for severe environments encountered, for example, during high g-force gun launches, the potting materials utilized to support the electronic components can induce high levels of stress on the potted components during the life of the device. 
     Designers typically attempt to minimize this stress by selecting more flexible (e.g., lower elastic modulus) potting materials and/or utilizing potting materials having matching coefficients of thermal expansion (CTEs). However, as the severity of an environment is increased (e.g., higher g-forces, temperatures, shock loads, vibrations, etc.), the selection of suitable potting materials becomes more limited because these materials are required to support the higher loads. Consequently, the potting materials needed to support these higher loads must be less flexible (e.g., higher elastic modulus), which can induce additional stress into the encapsulated components involved. 
     In summary, fewer potting materials have become available for electronic component encapsulation as the severities of the operating environments have increased. Consequently, the conventional practice of attempting to match the CTEs of such a limited number of potting materials has become more challenging, and has resulted in non-optimal design conditions in which additional stress can be induced into the electronic components involved. 
     For the reasons stated above, and for other reasons stated below, which will become apparent to those skilled in the art upon reading and understanding the specification, there is a need in the art for a way to isolate stress-sensitive electronic components from stresses induced by high modulus potting materials utilized in severe environments. 
     SUMMARY 
     The embodiments of the present invention provide ways to isolate stress-sensitive components from higher modulus potting materials utilized in severe environments, and will be understood by reading and studying the following specification. 
     A device, system and method for stress-sensitive component isolation in severe environments are provided. For example, a device for stress-sensitive component isolation is disclosed. The device includes a circuit board assembly, a plurality of electronic components mounted onto a surface of the circuit board assembly, and a protective cap disposed over at least one electronic component of the plurality of electronic components and mounted onto the surface of the circuit board assembly. As such, the protective cap isolates the stress-sensitive electronic component from stresses induced by higher modulus potting materials utilized to encapsulate and support the electronic components throughout the life of the device. 
    
    
     
       DRAWINGS 
       Embodiments of the present invention can be more easily understood and further advantages and uses thereof more readily apparent, when considered in view of the description of the preferred embodiments and the following figures in which: 
         FIGS. 1A-1D  are related structural diagrams illustrating perspective views of a device that can be utilized to implement one example embodiment of the present invention. 
         FIGS. 2A-2B  are related, structural diagrams illustrating cross-sectional, side views of the device depicted in  FIG. 1C . 
         FIGS. 3A-3B  are related, structural diagrams illustrating cross-sectional, side views of the device depicted in  FIG. 1D . 
         FIG. 4  illustrates a method that can be utilized to implement one example embodiment of the present invention. 
         FIG. 5  illustrates a system that can be utilized to implement one example embodiment of the present invention. 
     
    
    
     In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize specific features relevant to the exemplary embodiments. Reference characters denote like elements throughout the figures and text. 
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of specific illustrative embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense. 
     Embodiments of the present invention improve on the prior art by utilizing a cavity potting process that enables any electronic component that would be adversely affected by the potting material to be isolated from that material. In one example embodiment, a protective cap (or cover) is placed over an electronic component to isolate the component from a high modulus potting material to be utilized. In a second example embodiment, a second potting material having a lower elastic modulus than that of the first potting material (e.g., in order to match the two materials&#39; CTEs) is utilized to fill the cavity between the protective cap and the electronic component involved. As such, in accordance with the teachings of the present disclosure, higher modulus potting materials can be utilized to support electronic components that are less sensitive to stress, and electronic components that are more sensitive to stress can be isolated from the higher modulus potting material by the protective cap utilized. Furthermore, if such an isolated stress-sensitive component requires additional support, for example, in order to meet more stringent mission requirements, the cavity between the protective cap and the component can be filled with a lower modulus, less stress-inducing potting material to provide the additional support required. Thus, in accordance with the teachings of the present disclosure, stress-sensitive components can be isolated from stress-inducing potting materials and thereby enabled to withstand the severe environments encountered during a launch. 
       FIGS. 1A-1D  are related structural diagrams illustrating perspective views of a device  100   a - 100   d , which can be utilized to implement one example embodiment of the present invention. Referring to  FIG. 1A , the device  100   a  includes a PBA  102   a . Notably, although the PBA  102   a  is shown in the embodiment illustrated in  FIG. 1A , any suitable device (e.g., printed circuit, printed wire assembly or PWA, and the like) that can provide a base for mounting and encapsulating one or more electronic components can be utilized. The device  100   a  also includes an electronic component  104   a  mounted onto the upper surface of the PBA  102   a  (e.g., utilizing a known component mounting and soldering process). Notably, in one embodiment, in an IMU and/or guidance system, the electronic component  104   a  can be a Micro-Electro-Mechanical Systems (MEMS) component mounted on the upper surface of the PBA  102   a . For example, the electronic component  104   a  can be one sensor of a plurality of MEMS sensors mounted on the PBA  102   a . As such, in one embodiment, the electronic component  104   a  can be, for example, a rate sensor, accelerometer, or oscillator mounted on the PBA  102   a  in an enclosure within an IMU. However, although a sensor is contemplated to implement the electronic component  104   a  in one embodiment, in a different embodiment, any electronic component having a suitable functional capability can be utilized to implement the electronic component  104   a.    
     Referring to  FIG. 1B , the device  100   b  further includes a protective cap (or cover)  106   b  disposed over the electronic component (e.g.,  104   a  in  FIG. 1A ) and mounted onto the upper surface of the PBA  102   b . For example, in one embodiment, the protective cap  106   b  can be made of a suitable material (e.g., an epoxy, an encapsulating material and the like) that is capable of withstanding the extreme conditions (e.g., heat, shock, vibration, stress) encountered in the severe environment created during a launch. In a second embodiment, the protective cap  106   b  can be, for example, a container that can form a suitable cavity for use in a cavity potting process. 
     Referring to  FIG. 1C , a cross-sectional, side view of the device  100   b  illustrated in  FIG. 1B  is depicted. As such, the example embodiment illustrated in  FIGS. 1B and 1C  includes a cavity  108   c , which is formed by the open space between the protective cap  106   c  and the electronic component  104   c , which are both mounted onto the upper surface of the PBA  102   c . Accordingly, the protective cap  106   c  and the cavity  108   c  function to isolate and thereby protect the (e.g., stress-sensitive) electronic component  104   c  from the launch-induced stresses created by the high modulus potting material utilized to encapsulate the PBA  102   c.    
     Referring to  FIG. 1D , a second, cross-sectional, side view of the device  100   b  illustrated in  FIG. 1B  is depicted. For this example embodiment, the device  100   d  includes an electronic component  104   d  and a protective cap  106   d , which are both mounted onto the upper surface of the PBA  102   d . However, for this embodiment, additional support is desired to strengthen the electronic component  104   c  (e.g., in order to meet more stringent mission requirements). Consequently, in order to provide the additional support desired, the cavity  108   c  depicted in  FIG. 1C  is filled with a suitable potting material  110   d  (as depicted in  FIG. 1D ), or, for example, in a second embodiment, the filler material utilized can be an expandable polystyrene bead foam. As such, for this embodiment, the cavity  108   c  depicted in  FIG. 1C  is filled with a lower modulus, less stress-inducing material  110   d  to provide the additional support required without substantially increasing the stress on the electronic component  104   d  throughout the life of the device. 
       FIGS. 2A-2B  are related, structural diagrams illustrating cross-sectional, side views of the device  100   c  depicted in  FIG. 1C . However, the example embodiment illustrated in  FIGS. 2A-2B  depicts the entire device  200   a ,  200   b  encapsulated with a suitable (e.g., high modulus of elasticity) potting material  210   a ,  210   b . Referring to  FIG. 2A , the device  200   a  includes a PBA  202   a . An electronic component  204   a  is shown mounted onto the upper surface of the PBA  202   a , and a protective cap  206   a  is disposed over the electronic component  204   a  and also mounted onto the upper surface of the PBA  202   a  (e.g., utilizing a known component mounting and soldering process). Thus, a cavity  208   a  is formed by the space between the electronic component  204   a  and the protective cap  206   a . Referring to the expanded view depicted in  FIG. 2B , for this example embodiment, the electronic component  204   b  is a sensor (e.g., a MEMS sensor, rate sensor, accelerometer or oscillator), and the protective cap  206   b , cavity  208   b , and sensor  204   b  are encapsulated with the (e.g., high modulus) potting material  210   b.    
       FIGS. 3A-3B  are related, structural diagrams illustrating cross-sectional, side views of the device  100   d  depicted in  FIG. 1D . However, the example embodiment depicted in  FIGS. 3A-3B  depicts the cavity ( 208   a  and  208   b  in  FIGS. 2A, 2B ) filled with a suitable (e.g., low modulus of elasticity) potting material  310   a ,  310   b . Also, the entire device  300   a  is encapsulated with a suitable (e.g., high modulus) potting material  312   a ,  312   b . Referring to  FIG. 3A , the device  300   a  includes a PBA  302   a . An electronic component  304   a  is shown mounted onto the upper surface of the PBA  302   a , and a protective cap  306   a  is disposed over the electronic component  304   a  and also mounted onto the upper surface of the PBA  302   a . Referring to the expanded view depicted in  FIG. 3B , for this example embodiment, the electronic component  304   b  is a sensor (e.g., a MEMS sensor, rate sensor, accelerometer or oscillator), and the protective cap  306   b , filled cavity  310   b , and sensor  304   b  are encapsulated with the (e.g., high modulus) potting material  312   b.    
       FIG. 4  illustrates a method  400 , which can be utilized to implement one example embodiment of the present invention. For example, the method  400  can be utilized to implement the exemplary devices  100   a - 100   d  illustrated in  FIGS. 1A-1D . As such, referring to  FIGS. 1A-1D and 4 , the method  400  begins by providing or forming an electronic component on a PBA ( 402 ). For this example embodiment, the electronic component thus provided or formed is the electronic component  104   a  and the PBA is the PBA  102   a . Next, a protective cap (or cover) is formed over the electronic component ( 404 ). For example, the protective cap is the protective cap  106   b  in  FIG. 1B . A determination is then made about whether or not additional structural support is needed for the electronic component ( 406 ). If additional structural support is needed for the electronic component, the cavity (e.g.,  108   c  in  FIG. 1C ) between the protective cap  106   c  and the electronic component  104   c  is filled with a (e.g., low elastic modulus) potting material  110   d  in  FIG. 1D  ( 408 ). The method is then terminated. However, if ( 406 ) additional structural support is not needed for the electronic component, the method is terminated. 
       FIG. 5  illustrates a simplified, schematic block diagram of a system  500 , which can be utilized to implement one example embodiment of the present invention. For this embodiment, the system  500  illustrated in  FIG. 5  is a spacecraft, such as, for example, a satellite. In a second embodiment, the system is a guided projectile, such as, for example, a large caliber projectile. Referring to  FIG. 5 , the exemplary system  500  includes (among other components) a sensor system  502 , which is a component of a navigational and guidance subsystem for guiding a spacecraft or large caliber projectile. The sensor system  502  includes (among other components) a plurality of inertial sensors  504 , and a plurality of other sensors  506  that also function to guide the spacecraft or large caliber projectile. The plurality of inertial sensors  504  includes at least one printed board assembly (PBA)  508 . The PBA  508  includes (e.g., among other components) a plurality of MEMS sensors (e.g., sensors  204   a ,  204   b ,  304   a ,  304   b  in  FIGS. 2A-3B ) mounted onto the PBA (e.g., PBA  102   a  in  FIG. 1A ), and the PBA  508  includes an electronic component and a protective cap (e.g.,  104   c  and  106   c  in  FIG. 1C ) that form an isolated device  510  (e.g., isolated stress-sensitive component). If warranted, the cavity formed between the electronic component and the protective cap can be filled with a (e.g., low modulus) potting material. As such, for this exemplary embodiment, the system  500  provides stress-sensitive component isolation for operations in severe environments. 
     Example Embodiments 
     Example 1 includes a device for stress-sensitive component isolation, comprising: a circuit board assembly; a plurality of electronic components mounted onto a surface of the circuit board assembly; and a protective cap disposed over at least one electronic component of the plurality of electronic components and mounted onto the surface of the circuit board assembly. 
     Example 2 includes the device of Example 1, further comprising: a first potting material disposed within a cavity formed between the protective cap and the at least one electronic component of the plurality of electronic components. 
     Example 3 includes the device of any of Examples 1-2, further comprising: a first potting material disposed within a cavity formed between the protective cap and the at least one electronic component of the plurality of electronic components; and a second potting material disposed on the protective cap, wherein the first potting material has a first modulus of elasticity and the second potting material has a second modulus of elasticity. 
     Example 4 includes the device of Example 3, wherein a modulus value for the first modulus of elasticity is lower than a modulus value for the second modulus of elasticity. 
     Example 5 includes the device of any of Examples 3-4, wherein the protective cap is configured to isolate the at least one electronic component from the second potting material. 
     Example 6 includes the device of any of Examples 3-5, wherein the protective cap is configured to mitigate stress induced into the at least one electronic component by the second potting material during the life of the device. 
     Example 7 includes the device of any of Examples 1-6, wherein the protective cap comprises at least one of a molded plastic material or a formed metal material. 
     Example 8 includes the device of any of Examples 2-7, wherein the first potting material comprises a plurality of polystyrene beads. 
     Example 9 includes the device of any of Examples 3-8, wherein the second potting material comprises a material having a high modulus of elasticity. 
     Example 10 includes the device of any of Examples 3-9, wherein a coefficient of thermal expansion associated with the first potting material is substantially equal to a coefficient of thermal expansion associated with the second potting material. 
     Example 11 includes a method for stress-sensitive component isolation, comprising: providing an electronic component on a surface of a board assembly; forming a protective cap over the electronic component; determining if the electronic component requires structural support; and filling a cavity between the electronic component and the protective cap with a first potting material if the electronic component requires structural support. 
     Example 12 includes the method of Example 11, further comprising: forming a layer of a second potting material on the surface of the board assembly and the protective cap. 
     Example 13 includes the method of Example 12, wherein the filling comprises filling the cavity with the first potting material having a first modulus of elasticity; and the forming the layer comprises forming the layer of the second potting material with the second potting material having a second modulus of elasticity, wherein a value of the first modulus of elasticity is lower than a value of the second modulus of elasticity. 
     Example 14 includes the method of any of Examples 11-13, wherein the filling the cavity comprises filling the cavity with an expandable polystyrene bead foam. 
     Example 15 includes the method of any of Examples 12-14, wherein the forming the layer comprises forming the layer with a layer of a high modulus material. 
     Example 16 includes a system, comprising: a sensor system; a plurality of inertial sensors in the sensor system; a circuit board assembly in an inertial sensor of the plurality of inertial sensors; a plurality of electronic components mounted onto a surface of the circuit board assembly; and a protective cap disposed over at least one electronic component of the plurality of electronic components and mounted onto the surface of the circuit board assembly. 
     Example 17 includes the system of Example 16, wherein the sensor system is a subsystem of a navigational and guidance system configured to guide a vehicle during or after a launch of the vehicle. 
     Example 18 includes the system of any of Examples 16-17, wherein the system comprises a spacecraft. 
     Example 19 includes the system of any of Examples 16-18, wherein the plurality of inertial sensors comprises a plurality of MEMS inertial sensors. 
     Example 20 includes the system of any of Examples 16-19, wherein the sensor system is a subsystem of a guided projectile. 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiments shown. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.