PATENT DOCUMENT

Publication Number: US-9593991-B2
Application Number: US-201514812958-A
Country: US
Kind Code: B2

Title: Printed circuits with embedded strain gauges

Abstract:
A printed circuit board may have embedded strain gauges. A strain gauge may be formed from a metal trace on a polymer substrate. The metal trace may form a variable strain gauge resistor that is incorporated into a bridge circuit for a strain gauge. The printed circuit may have a rigid printed circuit layer with a recess that receives the polymer substrate. Metal pads on the polymer substrate may be coupled to respective ends of the variable strain gauge resistor. The rigid printed circuit substrate with the recess may be laminated between additional rigid printed circuit layers. Vias may be formed through the additional rigid printed circuit layers to contact the metal pads. Embedded strain gauges may be used in gathering strain data when strain is imparted to a printed circuit during use of the printed circuit in an electronic device or during testing.

Claims:
What is claimed is: 
     
       1. Apparatus, comprising:
 a strain gauge formed from a strain sensor substrate containing at least one strain sensor variable resistor; and 
 a printed circuit board having a plurality of printed circuit layers, wherein the plurality of printed circuit layers comprises a dielectric layer with a recess that receives the strain sensor substrate. 
 
     
     
       2. The apparatus defined in  claim 1  wherein the strain sensor substrate has contact pads and wherein the printed circuit board has vias that respectively contact the contact pads. 
     
     
       3. The apparatus defined in  claim 2  further comprising:
 an electrical component soldered to the printed circuit using at least one solder joint, wherein the strain gauge measures strain in the printed circuit at the solder joint. 
 
     
     
       4. The apparatus defined in  claim 3  further comprising control circuitry that gathers strain data from the strain gauge. 
     
     
       5. The apparatus defined in  claim 4  wherein the control circuitry stores the strain data while the printed circuit board is subjected to strain during use of the apparatus. 
     
     
       6. The apparatus defined in  claim 5  further comprising:
 a display; and 
 a housing in which the display, the control circuitry, and the printed circuit board are mounted. 
 
     
     
       7. The apparatus defined in  claim 6  wherein the strain sensor substrate comprises a polymer layer and wherein the strain sensor variable resistor comprises a metal trace supported by the polymer layer. 
     
     
       8. The apparatus defined in  claim 7  wherein the metal trace has ends that are coupled respectively to the contact pads. 
     
     
       9. The apparatus defined in  claim 8  wherein the control circuitry stores the strain data while the printed circuit board is subjected to strain. 
     
     
       10. The apparatus defined in  claim 3  further comprising:
 test equipment that imparts strain to the printed circuit board while gathering strain data from the strain gauge. 
 
     
     
       11. The apparatus defined in  claim 10  wherein the test equipment comprises:
 an actuator that presses against the printed circuit board to impart the strain; and 
 a controller that controls the actuator while receiving the strain data. 
 
     
     
       12. An electronic device, comprising:
 an electronic device housing; 
 a printed circuit board in the housing, wherein the printed circuit board includes a dielectric layer having a recess; 
 electrical components mounted on the printed circuit board; and 
 a strain gauge formed from a strain sensor resistor on a substrate, wherein the substrate is located within the recess. 
 
     
     
       13. The apparatus defined in  claim 1  wherein the dielectric layer has first and second opposing surfaces and the recess extends from the first surface through the dielectric layer to a location between the first and second opposing surfaces. 
     
     
       14. The electronic device defined in  claim 12  wherein the printed circuit comprises a plurality of printed circuit board layers, the electronic device further comprising vias that pass through at least one of the plurality of printed circuit board layers and that contact the strain sensor resistor. 
     
     
       15. The electronic device defined in  claim 12  further comprising control circuitry that receives strain data from the strain gauge. 
     
     
       16. The electronic device defined in  claim 15  wherein the electrical components include at least one component mounted to the printed circuit board using a solder ball and wherein the strain gauge measures the strain data in the printed circuit board at the solder ball. 
     
     
       17. The electronic device defined in  claim 16  wherein the strain sensor resistor is one of a plurality of strain sensor resistors embedded within the printed circuit board each of which is oriented at a different respective angle and each of which is configured to make strain measurements at the solder ball. 
     
     
       18. A printed circuit board, comprising:
 a rigid dielectric printed circuit board layer having a recess; 
 a flexible polymer strain sensor substrate mounted in the recess; 
 a metal trace supported by the flexible polymer strain sensor substrate that forms a strain gauge variable resistor; and 
 a plurality of additional rigid printed circuit board layers laminated above and below the rigid dielectric printed circuit board layer that has the recess. 
 
     
     
       19. The printed circuit board defined in  claim 18  further comprising:
 vias that pass through at least one of the additional rigid printed circuit board layers and that are electrically connected to the metal trace. 
 
     
     
       20. The printed circuit defined in  claim 19  wherein the strain gauge variable resistor is a first strain gauge variable resistor in a set of three strain gauge variable resistors on the flexible polymer strain sensor substrate.

Description:
BACKGROUND 
     This relates generally to strain gauges, and, more particularly, to strain gauges for monitoring strain in printed circuits. 
     Electronic devices such as cellular telephones, computers, and other electronic devices contain integrated circuits and other electrical components. Components such as these may be mounted on printed circuits. During drop events and other situations in which an electronic device is subjected to an impact or other conditions leading to elevated stresses, solder joints may be weakened and other faults can develop in the circuitry mounted on a printed circuit. Unless care is taken, stress-induced damage to an electrical component or other circuitry in an electronic device may create reliability issues. 
     To help understand the way in which strain is distributed to the components in a printed circuit, strain tests may be performed on a test printed circuit board. External strain gauges may be attached to the upper and lower surfaces of the test board in the vicinity of integrated circuits or other components of interest. These strain gauges may be wired to test equipment that gathers strain data. Strain data may be gathered with the strain gauges while applying force to the printed circuit. By analyzing the strain data, printed circuit board designs and component layouts can be refined to enhance reliability. 
     It can be difficult to gather accurate strain data with this type of approach. Strain measurements from strain gauges mounted on the surface of the printed circuit board may be influenced by the way in which the strain gauges are adhered to the surface of the printed circuit board and other variables that are difficult to control. These strain gauges also do not gather strain data in the field to alert a user or others about the presence of excessive strain. 
     It would therefore be desirable to be able to provide improved strain gauge configurations for monitoring components on printed circuit boards. 
     SUMMARY 
     A printed circuit board may have embedded strain gauges. A strain gauge may be formed from a metal trace on a polymer substrate. The metal trace may form a variable strain gauge resistor that may be incorporated into a bridge circuit for the strain gauge. 
     The printed circuit board may have a printed circuit board layer with a recess. The polymer substrate for the strain gauge resistor may be mounted within the recess. Metal pads on the polymer substrate may be coupled to respective ends of the variable strain gauge resistor. The rigid printed circuit substrate with the recess may be laminated between additional rigid printed circuit layers. Vias may be formed through the additional rigid printed circuit layers to contact the metal pads. 
     Embedded strain gauges may be used in gathering strain data when strain is imparted to a printed circuit during use of the printed circuit in an electronic device or during testing. Strain gauges may make strain measurements on solder ball joints under electrical components mounted to the printed circuit board. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an illustrative electronic device having strain gauges for monitoring printed circuit board strain in accordance with an embodiment. 
         FIG. 2  is a cross-sectional side view of the illustrative electronic device in accordance with an embodiment. 
         FIG. 3  is a cross-sectional side view of a printed circuit board during strain measurements in accordance with an embodiment. 
         FIG. 4  is a circuit diagram of illustrative strain gauge circuitry in accordance with an embodiment. 
         FIG. 5  is a top view of an illustrative variable resistor of the type that may be used in a strain gauge in accordance with an embodiment. 
         FIG. 6  is a cross-sectional side view of a portion of a printed circuit showing illustrative printed circuit locations for strain gauges in accordance with an embodiment. 
         FIG. 7  is a top view of an illustrative pattern of strain gauges that may be used to monitor strain on a solder ball joint or other structure on a printed circuit in accordance with an embodiment. 
         FIG. 8  is a cross-sectional side view of an illustrative strain gauge structure formed from a metal trace in a polymer substrate in accordance with an embodiment. 
         FIGS. 9 and 10  are diagrams of illustrative equipment and operations involved in embedding strain gauge circuitry in a printed circuit in accordance with an embodiment. 
         FIG. 11  is a flow chart of illustrative steps involved in gathering and analyzing strain gauge data on a printed circuit and in taking suitable action in accordance with an embodiment. 
         FIG. 12  is a cross-sectional side view of illustrative printed circuit board showing how strain gauge circuitry may be mounted in layers near the top or bottom of the printed circuit board in accordance with an embodiment. 
         FIG. 13  is a cross-sectional side view of a rosette (stacked) style strain gauge structure. 
     
    
    
     DETAILED DESCRIPTION 
     An illustrative electronic device of the type that may be provided with a printed circuit board having strain gauges is shown in  FIG. 1 . As shown in  FIG. 1 , electronic device  10  may have control circuitry  16 . Control circuitry  16  may include storage and processing circuitry for supporting the operation of device  10 . The storage and processing circuitry may include storage such as hard disk drive storage, nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  16  may be used to control the operation of device  10 . The processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio chips, application specific integrated circuits, etc. 
     Input-output circuitry in device  10  such as input-output devices  22  may be used to allow data to be supplied to device  10  and to allow data to be provided from device  10  to external devices. Input-output devices  22  may include buttons, joysticks, scrolling wheels, touch pads, key pads, keyboards, microphones, speakers, tone generators, vibrators, cameras, sensors, light-emitting diodes and other status indicators, data ports, etc. A user can control the operation of device  10  by supplying commands through input-output devices  22  and may receive status information and other output from device  10  using the output resources of input-output devices  22 . 
     Input-output devices  22  may include one or more displays such as display  14 . Display  14  may be a touch screen display that includes a touch sensor for gathering touch input from a user or display  14  may be insensitive to touch. A touch sensor for display  14  may be based on an array of capacitive touch sensor electrodes, acoustic touch sensor structures, resistive touch components, force-based touch sensor structures, a light-based touch sensor, or other suitable touch sensor arrangements. 
     Input-output devices  22  may also include sensors  18 . Sensors  18  may include strain gauge sensors  20  and other sensors such as proximity sensors, ambient light sensors, touch sensors, force sensors, temperature sensors, pressure sensors, magnetic sensors, and other sensors. Strain gauge sensors  20  may include sensors mounted on the surfaces of a printed circuit board and/or embedded within a printed circuit board. 
     Control circuitry  16  may be used to run software on device  10  such as operating system code and applications. During operation of device  10 , the software running on control circuitry  16  may be used in gathering strain gauge data from embedded strain gauges and/or other strain gauges in device  10 . Strain gauge data may be analyzed during failure analysis (e.g., to help designers improve the design of a device and the printed circuits and other components within the device), may be monitored in real time to issue alerts and provide other information to a user or others, and/or may be used to take other suitable action in device  10 . 
     Device  10  may be a tablet computer, laptop computer, a desktop computer, a monitor that includes an embedded computer, a monitor that does not include an embedded computer, a display for use with a computer or other equipment that is external to the display, a cellular telephone, a media player, a wristwatch device or other wearable electronic equipment, or other suitable electronic device. 
     Display  14  may be an organic light-emitting diode display, a liquid crystal display, an electrophoretic display, an electrowetting display, a display based on an array of discrete crystalline light-emitting diode dies, or a display based on other types of display technology. 
     A cross-sectional side view of an illustrative electronic device such as device  10  of  FIG. 1  is shown in  FIG. 2 . As shown in  FIG. 2 , device  10  may have a housing such as housing  12  in which components  52  are mounted. Components  52  may include integrated circuits, connectors, sensors, input-output devices, and other circuitry. Components  52  may be mounted on one or more substrates such as illustrative substrate  48 . Substrate  48  may be a printed circuit (e.g., a rigid printed circuit board formed from fiberglass-filled epoxy or other rigid printed circuit board substrate material or a flexible printed circuit formed from a flexible layer of polyimide or a sheet of other polymer material). Configurations in which substrate  48  is a rigid printed circuit board are sometimes described herein as an example. 
     Display  14  may include display layers  42  (e.g., liquid crystal display layers, an organic light-emitting diode display, an electrophoretic display, etc.). Display layers  42  may be mounted under display cover layer  40 . Display cover layer  40  may be mounted to housing  12  and may be formed from a layer of glass, transparent plastic, sapphire or other transparent crystalline material, or other protective layer. Display layers  42  may be attached to printed circuit  48  using signal path  44 . Signal path  44  may be a flexible printed circuit that is coupled to connector  46  on printed circuit  48 . Metal traces  50  in printed circuit  48  may be used in forming signal interconnects for electrical components mounted to printed circuit  48  such as connector  46  and electrical components  52 . 
     When a device such as device  10  is inadvertently dropped by a user or is otherwise subjected to a sharp impact, there is a potential for the circuitry mounted to a printed circuit such as printed circuit  48  to become damaged. For example, solder joints may develop cracks or integrated circuits or other components may come loose from the surface of printed circuit  48 . 
     To help warn a user or others about conditions that can lead to damage and/or to help analyze printed circuit structures and other device structures so as to improve the design of these structures in a way that avoids excessive damage when a printed circuit is stressed, printed circuit  48  may be provided with one or more strain gauges. The strain gauges may gather strain data during use of device  10  by a user and/or during testing. Strain data may be analyzed to determine when excessive strain has been experienced. A warning may be issued or a log of strain data may be maintained during use of device  10 . If desired, strain gauge data may be gathered when applying force to printed circuit  48  during testing of printed circuit  48  (e.g., a test version of printed circuit  48 ). 
     As shown in  FIG. 3 , for example, test equipment  60  may be used in gathering strain data on board  48  during stress tests. Test equipment  60  may include a controller such as controller  62  (e.g., one or more microprocessors, one or more computers, and/or other processing equipment). Controller  62  may control the operation of an actuator such as positioner  64  (e.g., a linear electromagnetic actuator or other electrically controlled device for applying force to an object). Positioner  64  may press member  66  against printed circuit  48  (e.g., by moving member  66  in direction  68  during stress testing). Strain may also be induced in board  48  using ball drop tests and other strain-inducing actions. 
     Board  48  may be provided with one or more strain gauges. Controller  62  may be coupled to the strain gauges using a signal path such as cable  70  and traces  50  in printed circuit board  48 . Using this signal path, controller  62  may gather strain data from the strain gauges of printed circuit  48  during testing. Controller  62  (and/or other equipment) may then perform failure analysis on the test data. Based on this analysis, design features can be redesigned to ensure that the printed circuit  48  will exhibit enhanced robustness and to ensure that components  52  will have a reduced likelihood of becoming damaged when board  48  is used in device  10  in the field. 
       FIG. 4  is a circuit diagram of illustrative strain gauge circuitry of the type that may be used to form strain gauges  20  in printed circuit board  48  that gather strain data. Strain gauge circuitry  72  may include a bridge circuit (e.g., a Wheatstone bridge) such as bridge circuit  74 . Bridge circuit  74  may include reference resistors R and variable resistor (strain gauge resistor RV). The resistance of resistor RV may vary as a function of applied strain and therefore serves as a strain sensing element. The illustrative bridge circuitry of  FIG. 4  includes one strain sensing variable resistor RV and three reference resistors R, but configurations with two variable resistors RV and two reference resistors R or other combinations of variable and fixed resistors may be used, if desired. 
     Power supply terminals VP and VN may respectively apply a positive power supply voltage and ground power supply voltage to bridge circuit  74 . Signal lines  76  may be used to measure voltages at measurement nodes N 1  and N 2  of circuit  74 . Differential amplifier  78  may receive the voltages on nodes N 1  and N 2  via lines  76  and may produce a corresponding analog strain gauge output signal (strain data) on output line  80 . Analog-to-digital converter  82  may convert the analog strain gauge signal on line  80  to a digital strain gauge signal on output  84 . In a testing configuration of the type shown in  FIG. 3 , strain data from output  84  may be received by controller  62 . In operation in device  10 , strain data from output  84  may be maintained in storage within control circuitry  16 . Strain data in control circuitry  16  may be used by control circuitry  16  (e.g., to generate alerts, etc.) and/or may be provided to external equipment such as controller  62  or other external computing equipment (e.g., to allow service personnel to perform diagnostics, to allow failure analysis operations to be performed, etc.). 
     An illustrative strain gauge resistor such as resistor RV of  FIG. 4  is shown in  FIG. 5 . As shown in  FIG. 5 , resistor RV may have an elongated metal trace  86  with a meandering path. Trace  86  may be formed from a metal such as nichrome, constantan, or other metals (elemental metals or metal alloys). Trace  86  may have a series of interconnected segments that run parallel to axis  92  and may be supported by a substrate such as substrate  88 . Substrate  88  may be plastic or other dielectric. For example, substrate  88  may be formed from a sheet of polyimide or other flexible polymer. The resistance of trace  86  (and therefore the resistance of resistor RV) may be measured across terminals such as contact pads  90  at opposing ends of trace  86 . When resistor RV is bent about axis  94 , trace  86  will become thinner and elongated, increasing the resistance of resistor RV. In this way, resistor RV can serve as a strain sensing element for strain sensor circuitry  72  of  FIG. 4 . If desired, multiple strain sensing resistors (e.g., two, three, more than three, etc.) may be mounted on a single substrate such as substrate  88 . 
       FIG. 6  is a cross-sectional side view of printed circuit  48  showing illustrative locations where strain gauges can be incorporated into printed circuit  48 . As shown in  FIG. 8 , one or more components such as component  52  may be soldered to printed circuit  48 . Printed circuit  48  may have traces that form contacts such as solder pads  114 . Component  52  may have mating contacts such as contacts  116 . Solder  110  may be used to solder component  52  to printed circuit  48  (i.e., solder balls formed from solder  110  may be used to solder contacts  116  to contacts  114 ). This type of arrangement may be used to form an array of solder balls (e.g., a ball grid array when component  52  has a ball grid array package) or other suitable solder joint pattern. In general, any suitable components may be mounted on printed circuit  48  (e.g., components mounted in land grid array (LGA) packages, other types of surface mount technology (SMT) packaging, integrated circuits in chip-scale packages (CSP), etc.). Configuration in which solder balls form joints for ball grid array packages are merely illustrative. 
     When subjected to strain, there is a risk that solder joints such as solder joints formed from solder  110  of  FIG. 6  may become damaged. Accordingly, it may be desirable to locate strain gauges on printed circuit  48  at locations where the strain experienced by the solder balls can be measured (e.g., at certain selected solder balls). As an example, external strain gauges can be mounted on the top of printed circuit  48  in locations such as locations  102  or on the bottom of printed circuit  48  in locations such as locations  100  (i.e., on the side of printed circuit  48  opposing the side of printed circuit  48  on which component  52  is mounted). Strain gauges may also be embedded within printed circuit  48  in locations such as locations  98  and  96 . Locations such as locations  98  are closer to the surface of printed circuit  48  on which component  52  are mounted and therefore may provide strain data of enhanced accuracy. The depth at which the strain gauges may be buried below the surface of printed circuit  48  may be 10-100 microns, more than 5 microns, less than 200 microns, less than 40 microns, or other suitable depth. The strain sensor circuitry that is embedded within printed circuit  48  may include variable resistors such as variable resistor RV and, if desired, circuitry of the type shown in  FIG. 4  (e.g., a Wheatstone bridge). Configurations in which the strain sensor embedded within printed circuit  48  includes variable resistor RV are sometimes be described herein as an example. 
       FIG. 7  is a top view of component  52  showing how solder balls  110  may be patterned in an array (as an example). In this type of situation, corner ball  110 ′ may be more susceptible to damage than other balls because corner ball  110 ′ receives less support from surrounding solder balls than solder balls that are surrounded by solder balls on all sides. Using an array of three strain sensing resistors RV (e.g., resistors RV- 1 , RV- 2 , and RV- 3 , which are each angularly offset by 45° with respect to the next) that are pointed at corner solder ball  110 ′, the strain experienced by corner solder ball  110 ′ may be accurately monitored. Resistors RV- 1 , RV- 2 , and RV- 3  may be formed on individual polymer substrates or may be formed on a common polymer substrate. Other strain resistor deployment patterns may be used when monitoring solder ball strain for component  52 , if desired. The example of  FIG. 7  is merely illustrative. The package of component  52  may overlap some or all of strain gauge resistors RV- 1 , RV- 2 , RV- 3 , as illustrated by dashed component outline  52 ′. Strain gauge resistors RV- 1 , RV- 2 , and RV- 3  may be embedded within the layers that make up printed circuit  48 . 
       FIG. 8  is a cross-sectional side view of an illustrative strain gauge resistor such as variable resistor RV of  FIG. 5 . As shown in  FIG. 8 , variable resistor RV may have a metal resistor trace  86  supported by substrate  88 . Substrate  88  may be a polymer such as polyimide or other dielectric. The thickness T of substrate  88  may be less than 50 microns, less than 20 microns, less than 5 microns, or less than 2 microns. Metal trace  86  may have a thickness of less than 1 micron, less than 0.5 microns, less than 0.2 microns, or other suitable thickness. Metal trace  86  may have a series of parallel segments coupled in series to form a resistor with opposing ends coupled to pads  90 . Pads  90  may have metal structures such as nickel structures  90 - 2 , copper structures  90 - 1 , or structures formed from other metals. Metal trace  86  may be formed on the surface of substrate  88  or within substrate  88 , as shown in  FIG. 8 . With one illustrative arrangement, pads  90  are configured to form terminals for variable resistor RV and serve as a laser drilling stopping layer. This allows laser drilling operations to be used to form printed circuit via holes that terminate on pads  90  after resistor RV has been embedded within the layers of printed circuit  48 . 
     Illustrative equipment and operations in forming printed circuits with embedded strain sensors are shown in  FIGS. 9 and 10 . As shown in  FIG. 9 , milling tool  112  or other equipment may form recess  115  in printed circuit layer  48 - 1 . Printed circuit layer  48 - 1  may be a layer of rigid printed circuit board material (e.g., fiberglass-filled epoxy material) such as a layer of FR4 printed circuit board material. 
     After forming recess  115  in printed circuit layer  48 - 1 , pick and place tool  117  or other robotic assembly equipment may be used to place variable resistor RV on substrate  88  in recess  115 . Lamination tool  118  may then use heat and pressure to attach additional printed circuit layers such as layers  48 - 2 ,  48 - 3 ,  48 - 4 , and  48 - 5  to the upper and lower surfaces of printed circuit layer  48 - 1 . Layers  48 - 2  may be rigid printed circuit board layers that include patterned metal traces (e.g., copper traces) such as metal traces  120  on rigid printed circuit board material  122  (e.g., FR4). Adhesive may be used between adjacent layers during printed circuit lamination operations. Any suitable number of printed circuit layers may be laminated together, if desired. The operations of  FIG. 9  in which four printed circuit layers have been laminated to layer  48 - 1  (two layers above layer  48 - 1  and two layer below layer  48 - 1 ) is merely illustrative. 
     As shown in  FIG. 10 , after laminating the printed circuit layers together, laser drilling equipment  124  or other suitable via formation equipment may be used to form via holes  126 . Equipment  124  may be a pulsed or continuous wave laser operating in the visible spectrum, infrared spectrum, or ultraviolet light spectrum. For example, laser drilling equipment  124  may be a Nd:YAG laser. The laser light from equipment  124  may penetrate through the metal and dielectric of layers  48 - 2  and  48 - 3  to reach pads  90  and thereby expose pads  90  without penetrating through pads  90 . 
     After forming via holes  126 , plating equipment  126  or other metal deposition equipment may be used to form metal  128  in via holes  126 , thereby forming vias  130 . Contact pads may be formed from metal layer  120 . These contact pads may be coupled to vias  130 , thereby allowing terminals  90  of sensor resistor RV to be electrically accessed at the surface of printed circuit  48 . Sensor signal processing circuitry such as amplifier  78  and analog-to-digital converter circuitry  82  may be interconnected with vias  130  and/or other sensor circuitry  72  using traces  50  in printed circuit  48 . Circuitry such as reference resistors R (e.g., bridge circuit  74 ) may be formed on the same substrate (e.g., substrate  88 ) as variable resistor(s) RV or reference resistors R may be formed elsewhere on printed circuit  48  or external to circuit  48 . After forming printed circuit  48  of  FIG. 10 , components  52  may be mounted on printed circuit  48  (e.g., using pick and place equipment, a solder reflow oven, and/or other component mounting techniques). Printed circuit  48  may then be used as a test board in a test setup of the type shown in  FIG. 3  or may be installed into device  10  and used in the field. 
     Illustrative steps involved in using strain gauge circuitry  72  (e.g., embedded strain sensors  20 , etc.) are shown in  FIG. 11 . 
     At step  200 , strain gauge measurements may be initiated. For example, in a test system of the type shown in  FIG. 3 , controller  62  may direct one or more strain gauges  20  to use circuitry such as strain gauge circuitry  72  of  FIG. 4  to collect strain data. The strain gauges may be mounted on the exterior surfaces of printed circuit board  48  and/or may be embedded within printed circuit board  48 . Controller  62  may be coupled to board  70  (and thereby the strain sensor circuitry) using a cable such as cable  70  of  FIG. 3  or using test probes (as examples). If desired, control circuitry  16  that is mounted in an electronic device housing with printed circuit  48  may direct strain gauge(s)  20  to initiate strain data measurements (e.g., based on user input, based on sensor data such as data from an accelerometer indicating that device  10  has been dropped, based on a predetermined schedule, or based on satisfaction of other suitable criteria). 
     At step  202 , printed circuit  48  (either a test printed circuit in the test system of  FIG. 3  or a printed circuit board that has been assembled with other structures to form device  10 ) may be used to gather strain data from strain gauges  20  (e.g., strain sensors mounted on the surface of board  48  and/or strain sensors mounted within the layers of printed circuit  48  such as a strain sensor based on variable resistor RV on substrate  88 ) while printed circuit board  48  is subjected to strain (e.g., strain induced by a test system, strain from normal wear and tear on device  10 , strain imparted to board  48  due to a drop event in which device  10  is dropped on the ground or is otherwise subjected to a sharp impact, etc.). Strain data may be stored in controller  62  and/or control circuitry  16  of device  10  or other electronic equipment. 
     At step  204 , the strain data that has been gathered at step  202  may be analyzed and appropriate action taken in response. As an example, test data gathered using controller  62  may be analyzed as part of a failure analysis operation that seeks to understand how to improve the design of printed circuits such as printed circuit  48 , how to improve the mounting of components  52 , and/or how to make other design enhancements. As another example, strain data may be analyzed by control circuitry  16 . Control circuitry  16  may, as an example, compare measured strain data to predetermined thresholds to determine whether to issue an alert. If device  10  is subjected to more than a given amount of strain, control circuitry  16  may use display  14  or another output device to issue a warning to a user of device  10  (e.g., a message may be presented to a user suggesting that device  10  be serviced by a technician). As another example, a user may be presented with a warning message that serves as a reminder to treat device  10  with care (e.g., “caution, you may risk damage to your device by dropping your device repeatedly”). Strain data may be logged and retrieved at a later time by computing equipment in a service facility. For example, strain data may be analyzed to determine whether certain components should be repaired or replaced, to determine how device  10  has been handled, etc. 
     To make sensitive strain gauge measurements, it may be desirable to locate strain gauges near the upper and/or lower layers of board  48 , where stresses tend to be maximized.  FIG. 12  is a cross-sectional side view of an illustrative printed circuit board with strain gauge substrates  88 T and  88 B that are respectively located in the second from the top and second from the bottom layers of printed circuit  48 . Printed circuit  48  has multiple metal layers  120  (e.g, copper layers) and dielectric layers  122 . Solder mask layers  121  may be formed on the surface of the outermost metal layers  120 . 
     If desired, measurements of strain in printed circuit  48  may be made using a rosette (stacked) style gauge. This type of arrangement is shown in  FIG. 13 , in which a stacked strain gauges  220 - 1 ,  220 - 2 , and  220 - 3  are embedded within dielectric  224  on substrate  222 . Vias  130  may allow signals to be routed to and from gauges  220 - 1 ,  220 - 2 , and  220 - 3 . The structures of  FIG. 13  may form all or part of a printed circuit. For example, the structures of  FIG. 13  may form a stacked strain gauge that is embedded within one or more layers of printed circuit  48  of  FIG. 6  or printed circuit  48  of  FIG. 12  (as examples). 
     The foregoing is merely illustrative and various modifications can be made by those skilled in the art without departing from the scope and spirit of the described embodiments. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20150729
Publication Date: 20170314
Grant Date: 20170314
Priority Date: 20150729
Inventors: Mason Anne M.
MCDONALD BRYAN
ARNOLD SHAWN X.
CASEBOLT MATTHEW
PYPER DENNIS R.
Assignee: APPLE INC
CPC Classifications: [{"code": "G01L1/2287", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01L5/0047", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01M5/0083", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01L3/1457", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01L1/22", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01L1/2287", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01M5/0033", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01L1/2262", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01L1/2262", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01L3/1457", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01L1/22", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01L1/2287", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 57882377