Method for detecting power plane-to-power plane shorts and I/O net-to power plane shorts in modules and printed circuit boards

• US 6,242,923 B1
• Filed: 07/16/1998
• Issued: 06/05/2001
• Est. Priority Date: 02/27/1997
• Status: Expired due to Fees

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: Scaman, Michael E., Yarmchuk, Edward J., Halperin, Arnold
• Primary Examiner(s): Metjahic, Safet
• Assistant Examiner(s): KERVEROS, DEMETRIOS C
• Application Number:
  US09/116,396
• Time in Patent Office: 1,055 Days
• Field of Search: 324/529, 324/530, 324/528, 324/524, 324/526, 324/531, 324/537, 324/95, 438/17
• US Class Current: 324/529
• CPC Class Codes: G01R 1/07314 the body of the probe being perpendicular to test object, e.g. bed of nails or probe with bump contacts on a rigid support on an elastic support, e.g. a film, G01R1/0735
• CPC Class Codes: G01R 31/281 Specific types of tests or tests for a specific type of fault, e.g. thermal mapping, shorts testing G01R31/2818 takes precedence
• CPC Class Codes: G01R 31/52 Testing for short-circuits, leakage current or ground faults

Abstract

A method of locating in a non-destructive and non-invasive manner power plane-to-power plane shorts or I/O net-to-power plane shorts found in a printed circuit board or a multi-chip-module by way of a magnetic field generating probe is described. Thousands of nets can be simultaneously tested to detect not only the presence of a short but also to accurately pinpoint its position. For high resistance shorts, the probe is provided with a pot core housed inductor located at its tip, and is used at low frequencies to minimize the effect of the capacitive impedance between the power planes. For low resistance shorts, the probe is used at high frequencies, delivering equal but opposite current to each of two matched inductors at the tip of the probe to maximize mutual inductive coupling while minimizing electrostatic capacitive coupling with the board or module. In both cases, the highest current stress is on the probe rather than on the expensive and fragile package under inspection. This allows the test to be both more sensitive to high resistance shorts at low frequencies and is less destructive, thereby being less likely to blow filamentary shorts due to high current stresses through the board or module.

Claims

1. A method of inspection for pinpointing the position on a board or a module of a short between power planes of between an I/O net and a power plane, said short being known to exist somewhere on said board or said module but its exact location being unknown, the method comprising the steps of:

movably scanning said board or said module with a magnetic field generation probe searching for a minimum or a maximum induced voltage; and

pinpointing the exact position of said short by sensing said induced voltage between said shorted power planes or said shorted I/O net to power plane when said magnetic field generating probe detects said induced voltage at said minimum or said maximum induced voltage.

2. The method as recited in claim 1, further comprising the step of placing said board or said module on a test fixture.

3. The method as recited in claim 2, wherein said test fixture includes a conductive block such that at least two of said I/O nets are simultaneously contacted.

4. The method as recited in claim 3, wherein a resistive test provides detection of a short between said power planes and any one of said I/O nets contacted by said conductive block.

5. The method as recited in claim 1, further comprising the step of coupling said magnetic field generating probe to a reference terminal of a lock-in amplifier to provide means for generating a magnetic field and coupling said power planes to input terminals of said lock-in amplifier for sensing an induced voltage.

6. The method as recited in claim 1, wherein detecting said power plane-to-power plane short or said I/O net-to-power plane short is achieved by first subjecting said board or said module to a resistive test.

7. The method as recited in claim 1, wherein said magnetic field generating probe is an inductor.

8. The method as recited in claim 1, wherein said magnetic field generating probe is a transformer.

9. The method as recited in claim 1, wherein said magnetic field generating device is mounted at the tip of a mechanical probe.

10. The method as recited in claim 1, wherein said magnetic field generating device is mounted to the tip of a microscope objective.

11. The method as recited in claim 1 wherein a layer of conductive foil is placed at the tip of said magnetic field generating probe to minimize electrostatic coupling between a substrate under inspection and said magnetic field generating probe.

12. The method as recited in claim 11 wherein said layer of conductive foil is grounded and positioned between said substrate under inspection and said magnetic field generating probe.

16. The method as recited in claim 5 wherein said layer of conductive foil is grounded and positioned between said substrate under inspection and said pot core.

13. A method of inspection for pinpointing the position on a board or a module of a short between power planes or between and I/O net and a power plane, said short having a resistance exceeding 1 KΩ

being known to exist by prior testing but its exact location being unknown, the method comprising the steps of;

movably scanning said board or said module with a magnetic field generating probe searching for a minimum or a maximum induced voltage, said magnetic field generating probe generating electrical signals having a frequency ranging from 100 Hz to 100 KHz at the tip of said magnetic field generating probe;

coupling said magnetic field generating probe to a reference terminal of a lock-in amplifier to provide means for generating a magnetic field and coupling said power planes to input terminals of said lock-in amplifier for sensing said induced voltage; and

pinpointing the exact location of said short by sensing said induced voltage between said shorted power planes or between said shorted I/O net and said power plane, and correlating said induced voltage to said current passing through said tip when said magnetic field generating probe detects said induced voltage at said minimum or said maximum induced voltage.

14. The method as recited in claim 13, wherein said magnetic field generating probe is an inductor housed in a pot core open at one end.

15. The method as recited in claim 13 wherein a layer of conductive foil is placed at the tip of said magnetic field generating probe, said probe including a pot core to minimize electrostatic coupling between a substrate under inspection and said magnetic field generating probe.

17. A method of inspection for pinpointing the position on a board or a module of a short having a resistance below 1 KΩ

between power planes or between an I/O net and a power plane, said short being known to exist by prior testing but its exact location being unknown, the method comprising the step of;

movably scanning said board or said module with a magnetic field generating probe ending in a tip, said probe searching for a minimum or a maximum induced voltage, said magnetic field generating probe generating electrical signals having a frequency ranging from 100 KHz to 3 MHz; and

pinpointing the exact position of said short between said shorted power planes or between said I/O net and said power plane by sensing said induced voltage at said minimum or said maximum induced voltage.

18. The method as recited in claim 17, wherein said magnetic field generating probe is a dual coil transformer with a center tapped grounded secondary coil.

19. The method as recited in claim 18, wherein said dual coil transformer drives matched dual wound inductors.

20. The method as recited in claim 18, wherein said dual coil transformer drives a simple inductive coil.

21. The method as recited in claims 18, wherein said dual coil transformer minimizes a capacitive coupling between said magnetic field generating probe and said board or said module.

22. The method as recited in claim 17 wherein a layer of conductive foil is placed at the tip of said magnetic field generating probe that includes an inductor to minimize electrostatic coupling between a substrate under inspection and said magnetic field generating probe.

23. The method as recited in claim 22 wherein said layer of conductive foil is grounded and positioned between said substrate under inspection and said inductor.

24. A method of inspection for pinpointing the position on a board or a module of a short having a resistance below 1 KΩ

between power planes or between an I/O net and a power plane, said short being known to exist by prior testing but its exact location being unknown, the method comprising the steps of;

movably scanning said board or said module with a magnetic field generating probe ending in a tip, said magnetic field generating probe generating electrical signals having a frequency ranging from 100 kHz to 3 MHz, searching for a minimum or a maximum induced voltage;

coupling said magnetic field generating probe to a reference terminal of a lock-in amplifier to provide means for generating a magnetic field and coupling said power planes to input terminals of said lock-in amplifier for sensing an induced voltage; and

pinpointing the exact position of said short by sensing said induced voltage between said shorted power planes or between said shorted I/O net and said power plane, and correlating said induced voltage to current passing through said tip when said magnetic field generating probe detects said induced voltage at said minimum or said maximum induced voltage.

25. The method as recited in claim 24, wherein said dual coil transformer drives matched dual wound inductors.

26. The method as recited in claim 24, wherein said dual coil transformer drives an inductive coil.

27. The method as recited in claims 24, wherein said dual coil transformer minimizes a capacitive coupling between said magnetic field generating probe and said board or said module.

28. The method as recited in claim 24, wherein a layer of conductive foil is placed at the tip of said magnetic field generating probe to minimize electrostatic coupling between a substrate under inspection and said magnetic field generating probe.

29. The method as recited in claim 28 wherein said layer of conductive foil is grounded and positioned between said substrate under inspection and said magnetic field generating probe.