Method and apparatus for strain monitoring of printed circuit board assemblies

• US 20070205791A1
• Filed: 02/21/2006
• Published: 09/06/2007
• Est. Priority Date: 02/21/2006
• Status: Active Grant

• Current Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Original Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Inventors: Teng, Sue, Ahmad, Mudasir
• Granted Patent: US 7,621,190 B2
• Time in Patent Office: Days
• US Class Current: 73/862.474
• CPC Class Codes: G01R 31/2817 Environmental-, stress-, or burn-in tests of IC's G01R31/2855; of individual semiconductors G01R31/2642; of other circuits G01R31/2849
• CPC Class Codes: H05K 1/0268 for electrical inspection or testing
• CPC Class Codes: H05K 1/0271 Arrangements for reducing stress or warp in rigid printed circuit boards, e.g. caused by loads, vibrations or differences in thermal expansion
• CPC Class Codes: H05K 2201/10151 Sensor
• CPC Class Codes: H05K 2203/163 Monitoring a manufacturing process

Abstract

A technique of monitoring strain on a printed circuit board assembly involves the use of a strain detector mounted on a printed circuit board. The strain detector is formed of a non-ductile material. The strain detector has a narrowed portion forming a weak link that has a characteristic of breaking when a critical strain limit is exceeded. The method of monitoring can include visual or electrical inspection. The electrical inspection can include having a strain monitoring device. The strain monitoring device has a timer connected to at least one strain detector and a memory for storing results connected to the timer. The capacitance of at least one of the strain detectors is sampled and any change in capacitance is recorded to the memory. In one embodiment, a time stamp occurs in the memory based on when an electrical property changes across at least one of the strain detectors.

Claims

1. A printed circuit board assembly comprising:

a circuit board substrate;

a plurality of mounting points for receiving electrical components carried on the circuit board substrate;

a plurality of electrically conductive pathways carried on the circuit board substrate between mounting points; and

at least one strain detector mounted on the flat board, the strain detector formed of a non-ductile material.

2. A printed circuit board assembly of claim 1, wherein the strain detector has a portion composed of alumina to provide desired failure of the alumina at an appropriate strain level.

3. A printed circuit board assembly of claim 1, wherein the strain detector is planar and has a narrowed portion forming a weak link in the detector to allow failure in a pre-defined location.

4. A printed circuit board assembly of claim 3, wherein the strain detector has a dog-bone shape defining the narrowed portion.

5. A printed circuit board assembly of claim 3, wherein the strain detector has a rectangular shape with an opening to define at least one narrow portion forming a weak link in the strain detector to allow failure in a pre-defined location at an appropriate strain level.

6. A printed circuit board assembly of claim 1 further comprising:

a monitor system carried by the board substrate, the monitor system having a timer and a memory for recording the change in electrical capacitance of at least one strain detector.

7. A printed circuit board assembly of claim 6 wherein one of the electrical components is an integrated circuit package having a plurality of corners, and further comprising:

at least two strain detectors, each strain detector extending along a longitudinal axis, each of the strain detectors located in proximity to a corner of the integrated circuit package, the longitudinal axis of the strain detector bisecting the adjacent corner.

8. A strain detector comprising a planar body having a narrowed portion forming a weak link in the body to allow failure in a pre-defined location, the planar body composed of a non-ductile material.

9. A strain detector of claim 8, wherein the strain detector has a portion composed of alumina 99% to provide desired failure of the strain detector at an appropriate strain level.

10. A strain detector of claim 8, wherein the strain detector has a dog-bone shape defining the narrow portion.

11. A strain detector of claim 8, wherein the strain detector has a rectangular shape with an opening to define at least one narrow portion forming a weak link in the strain detector to allow failure in a pre-defined location at an appropriate strain level.

12. A method of testing a printed circuit board assembly for failure comprising the following steps:

providing a printed circuit board;

mounting at least one strain detector having a sensing portion formed of a non-ductile material on the printed circuit board; and

monitoring the sensing portion of the strain detector on the printed circuit board to determine if strain related failure has occurred.

13. A method of testing of claim 12 wherein monitoring includes visually inspecting the sensing portion.

14. A method of testing of claim 13 wherein visually inspecting includes applying a surface dye penetrant to the sensing portion to determine whether the sensing portion includes a crack.

15. A method of testing of claim 13 wherein visually inspecting includes inspecting the sensing portion with a non-invasive crack detection technique of ultrasound to determine whether the sensing portion includes a crack.

16. A method of testing of claim 12 wherein monitoring includes electrically inspecting the sensing portion to determine if an electrical property of the strain detector has changed.

17. A method of testing of claim 16 further comprising the steps:

mounting at least one integrated circuit package to the printed circuit board, the integrated circuit package having a plurality of corners;

wherein the step of mounting at least one strain detector comprises mounting at least three strain detectors, each of the strain detectors located in proximity to a corner of the integrated circuit package, the longitudinal axis of the strain detector bisecting the adjacent corner; and

wherein the step of observation is done by electronic method comprising the step of providing a strain monitoring device having a timer connected to at least one strain detector and a memory for storing results connected to the timer;

sampling the capacitance of at least one of the strain detectors; and

recording when the capacitance changes.

18. A method of testing of claim 17 further comprising the steps of providing a time stamp in the memory based on when the capacitance across at least one of the strain detectors changes.

19. A method of testing of claim 18 wherein the strain detectors are wired together in parallel to allow testing of the strain detectors concurrently.

20. A method of testing of claim 18 further comprising the step of providing a battery on the printed circuit board to power the strain monitoring device.

21. A method of testing of claim 12 wherein the step of mounting the strain detector occurs on production printed circuit board assemblies.

22. A method of testing of claim 12 wherein the step of mounting the strain detector occurs on printed circuit boards not designed to receive a strain detector.