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Method for a capacitive probe on a PCB trace for signal sourcing and sensing

IP.com Disclosure Number: IPCOM000021732D
Publication Date: 2004-Feb-04
Document File: 5 page(s) / 21K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for a capacitive probe on a printed circuit board (PCB) trace for signal sourcing and sensing. Benefits include improved functionality and improved performance.

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Method for a capacitive probe on a PCB trace for signal sourcing and sensing

Disclosed is a method for a capacitive probe on a printed circuit board (PCB) trace for signal sourcing and sensing. Benefits include improved functionality and improved performance.

Background

In signal impedance sensitive or limited-access board designs, such as small form-factor boards and high-speed boards, adding test points that require additional routes out from the signal net is difficult or impossible. Adding additional test points can create problems, such as requiring board surface space, creating an antenna effect, and/or altering the electrical impedance characteristics.

Structural test solutions, such as automated optical inspection and automated x-ray inspection, are conventionally used to detect manufacturing faults. These solutions utilize imaging technology and are only able to detect the manufacturing defect spectrum of the device under test (DUT) solder profile. No electrical test is conventionally performed. As a result, the testing capability is limited. Furthermore, test equipment is very expensive and is not suitable for low profit margin board manufacturing.

Capacitance is calculation using the following formula:

         The capacitance equation contains the following values:

•         e0 is dielectric constant of the free space (8.854 x 10-12 F/m)

•         A is the area of the electrical conductor

•         t is the thickness of the insulator layer

•         er is the dielectric constant of the insulator layer

For example, based on conventional PCB technology, capacitance can be calculated as follows:

t = Thickness of PCB’s solder mask layer = ( 0.4 to 2mils )

A = Square width of the PCB trace (5 mils)

e0 = 8.854 x 10-12 F/m

Note: 1 mils = 2.540 x 10-5 meter

Assuming the worst-case scenario, where t = 0.4 mils, and A = (5 mils)2:

C = ( 8.854 x 10-12 F/m ) x er x ( 5 x 2.540 x 10-5 m x 5 x 2.540 x 10-5 m )

0.4 x 2.540 x 10-5 m

= 1.41 x 10-14 x er (Farad)

Assume the following values:

er = 5000, C = 1.41 x 10-14 x 5000

= 70.5 x 10-12 F

= 70.5 pF

The capacitance value of 70.5 pF is detectable by most test equipment.

To obtain high capacitance, a material with a high dielectric constant is required.

Traces can not be probed directly as damage can result from excessive probe force. Furthermore, trace widths are decreasing (5 mils), increasing the challenge to probe accuracy.

General description

The disclosed method is a capacitive probe on a PCB trace for signal sourcing and sensing.

         The key elements of the method include:

•         Embedded high-dielectric constant material seated on PCB trace

•         PCB trace that...