Board test apparatus and method for fast capacitance measurement

• US 5,844,412 A
• Filed: 12/19/1996
• Issued: 12/01/1998
• Est. Priority Date: 12/19/1996
• Status: Expired due to Term

• Current Assignee: Teradyne Incorporated
• Original Assignee: Teradyne Incorporated
• Inventors: Norton, Peter R.
• Primary Examiner(s): Ballato, Josie
• Assistant Examiner(s): DO, DIEP N
• Application Number:
  US08/769,556
• Time in Patent Office: 712 Days
• Field of Search: 324/519, 324/522, 324/523, 324/527, 324/537, 324/548, 324/765, 364/482, 702/64, 702/65
• US Class Current: 324/548
• CPC Class Codes: G01R 31/2806 Apparatus therefor, e.g. test stations, drivers, analysers, conveyors G01R31/2805, G01R31/281, G01R31/2818 take precedence

Abstract

A printed circuit board tester incorporating hardware for fast capacitive measurements. The circuit board tester includes a digital signal processor that can both source and measure test signals. It also includes an amplifier having an input and an output connected to probes that can contact points on the printed circuit board. In use, the board tester is configured to place a capacitor on the printed circuit board under test in the feedback path of the amplifier. The digital signal processor generates a stimulus signal to the capcitor and the output of the amplifier is passed to the digital signal processor. The digital signal processor uses an adaptive filtering approach to determine convergence of the measurement, thereby minimizing measurement time. The arrangement is flexible and can be reconfigured to measure both large and small values of capacitance.

Claims

1. A method of operating a printed circuit board tester to test a capacitance between two points, the method comprising the steps of:

a) connecting probes to the two points on the printed circuit board;

b) applying a dc stimulus signal between the two points;

c) initiating a sampling of a voltage signal between the two points;

d) using at least a portion of the samples to compute a first least squares estimate of the slope of the voltage signal at a first time;

e) using additional samples to compute a second least squares estimate of the slope of the voltage signal;

f) computing an average of at least a portion of the estimates of the slope of the voltage signal;

g) repeating steps e) and f) until the estimate of the slope at step e) differs from the average computed at step f) by less than a predetermined amount; and

h) using the slope as an indication of the capacitance.

2. The method of claim 1 wherein the step of connecting probes comprises connecting a bed of nails fixture to the circuit board.

3. The method of claim 1 wherein the step of connecting probes comprises connecting a flying prober tester to the printed circuit board.

4. The method of claim 1 wherein the step of applying a stimulus comprises applying a stimulus signal from a digital signal processor.

5. The method of claim 4 wherein the step of computing an average comprises computing an average with the same digital signal processor that supplies the stimulus signal.

6. The method of claim 1 wherein the step of connecting probes to two points on the printed circuit board comprises connecting probes to two points on the printed circuit board to place a capacitance being measured on the printed circuit board in the feedback path of an amplifier within the printed circuit board tester.

7. The method of operating a printed circuit board tester of claim 1 to determine the capacitance between two different points on a printed circuit board, additionally comprising the steps of:

a) connecting the probes to two different points on the printed circuit board;

b) applying an AC stimulus signal between the two points;

c) measuring a response signal between the two points.

8. The method of operating a printed circuit board tester using the method of testing capacitance of claim 1, comprising the steps of:

a) identifying two points on the printed circuit board where current flow therebetween exceeds a predetermined threshold when a stimulus signal is applied;

b) using the method of claim 1 to test the capacitance between the two points; and

c) indicating a short circuit between the two points when the computed capacitance is below a predetermined value.

9. A method of operating a printed circuit board tester to test a capacitance between two points, the method comprising the steps of:

a) applying a constant current to the capacitance to be tested;

b) taking initial samples of an output signal across the capacitance;

c) calculating an incremental least squares slope of the output signal;

d) computing an estimate of capacitance based on the slope computed;

e) computing a moving average of computed capacitance estimates;

f) when the computed capacitance estimate and the moving average capacitance estimate have converged to within a predetermined threshold, using the estimate as an indication of the actual capacitance; and

g) when the computed capacitance estimate and the moving average capacitance estimate have not converged to within a predetermined threshold, taking further samples of the output signal and repeating steps c) through g).