AU718894B2 - Suppression of silver migration in electrical circuits - Google Patents

Description

P/00/011 28/5/91 Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: "SUPPRESSION OF SILVER MIGRATION IN ELECTRICAL CIRCUITS" The following statement is a full description of this invention, including the best method of performing it known to us:- 2 Technical Field This invention relates to the reduction of the migration of metal in electrical circuits. This phenomenon is a particular problem in relation to printed circuit boards which include silver conductor tracks. Silver is particularly prone to the problem of silver migration in which minuscule branches of silver grow out from the conductor tracks. In some instances this can cause short circuits between adjacent conductor tracks when the branches bridge the intervening insulating space or partially bridge the space so it is no longer adequate to maintain electrical isolation between the tracks.

Background Art Silver migration may be reduced by covering the silver tracks with a passivating layer. One reason for the reduction of silver migration by this method is that it reduces the amount of atmospheric moisture which reaches the circuit board so that there is insufficient "eledrolyte" to facilitate the migration.

1: 5 In some applications it is desired that the metal should be used as a contacting surface e.g. in electrical connectors or as the printed matrix of a PCB keypad in which a pair of conductor tracks are bridged by a contacting element which is resiliently mounted above the conductor tracks. Silver is particularly suitable for these applications because of its low contact impedance.

Where such mechanical contacting arrangements are involved, an insulating .passivating layer cannot be used.

In such circumstances, we have used a conductive carbon loaded layer to coat the silver tracks.

S"This process requires an additional processing step and thus adds to the manufacturing cost.

Disclosure of the Invention This specification discloses an arrangement for reducing the effect of metal migration the arrangement including a source of polarity alternating voltage applied to metal parts susceptible to metal migration.

In a preferred embodiment the source of polarity alternating voltage includes a DC voltage source connected to a change-over switch arrangement controlled by control means.

The control means may be a multivibrator or it may employ a more complex algorithm which suppresses the change-over while specific conditions apply. For example, where the metal is used as the "matrix" paths for a PCB push button keypad array, change-over may be suppressed while a button is pushed and the bridging contact bridges a pair of the metal conductor tracks.

Brief Description of the Drawing Figure 1 is a schematic representation of an embodiment of the invention.

Best Mode of Carrying Out the Invention The invention will be described with reference to Figure 1 which is a schematic representation of two rows of the conductive tracks of a PCB push-button keypad to which the invention has been applied.

Each push button is defined by a pair of conductive pads 8/9 18/19, which are bridged by a corresponding bridging contact (not shown) when a push button located above and normally spaced from the pair of pads is depressed.

~In Figure 1, pads 8, 10 and 12 are connected in series with resistor 7 in a first row, while their corresponding pads 9, 11 and 13 are each located in different columns and are connected to respective resistors 4, 5 and 6.

Similarly, pads 14, 16 and 18 are in series with resistor 22 and pads 15, 17 and 19 are connected to respective resistors 4, 5 and 6, i.e. 15, 17 and 19 are in the same columns as 9, 11 and 13 respectively.

:OO When no button is pushed, all the row pads 8; 10; 12; 14; 16; 18 are at the same potential because no current flows through resistors 7 or 22 e.g. these pads are at OV.

Similarly, all the column pads 9, 11, 13, 15, 17, 19 are at the same potential, e.g. +5V, button is operated.

However, if the button which bridges pads 10 and 11 is pushed current flows through resistors 5 and 7. Thus by detecting voltage drops across the resistors the operated button can be identified.

Normally the pads and connecting wires are silver tracks printed on a PCB.

The tracks and pads are often closely placed and we have found that silver migration I can cause faults.

r To reduce the effects of silver migration, the polarity of the columns and rows is reversed periodically. This is represented schematically in Figure 1 by a change-over switch inserted between the battery 1 and the keypad. Control means controls the operation of the change-over switch.

The control means switches the polarity of the voltage applied to the keypad tracks according to a pre-determined algorithm or on a regular basis e.g. at 71 Hz.

In applying this principle to a keypad in a telephone subset having a microprocessor to manage its functions, the rows and columns are regularly scanned by applying a test voltage thereto. Scanning may be carried out at a rate of e.g.

200Hz.

By modifying the scanning routine so that the polarity applied between the rows and columns is regularly reversed, the average voltage can be made approximately equal to zero so that the "galvanic" effect is neutralized and silver migration is largely o, suppressed.

Figure 2 shows an embodiment of the invention which is a keypad having rows and columns adapted to be scanned to detect an operated button.

Figure 2 shows an array having a plurality of rows, 2 of which 21 and 22 are shown for simplicity, and columns, 3 of which 23, 24, and 25 are shown. At the intersection of each row and column the contact arrangement is shown schematically as a switch 31, 32, 33, 34, 35, 36.

Each row and column is connected to the voltage supply VDD via a corresponding resistor, 41 to 45, and to the other side of the voltage supply by a corresponding transistor switch, 51 to The condition of each row and column is available to be monitored at the oooeo S•output of its corresponding buffer amplifier 61 to In operation, the transistor switches 51 and 52 of the rows are switched ON for half the time and the transistor switches 53, 54 and 55 of the columns are switched OFF while the row switches are ON. In the second half of each switching period, the switching is reversed.

When one of the switches 51 to 55 is OFF, the corresponding row or column is pulled up to V,,DD via the corresponding pull up resistor 41 to 45, and when a switch is ON the corresponding row or column is set LOW by the open drain of the transistor.

By switching the row and column switches in antiphase the average potential between the rows and columns is Os,, thereby inhibiting silver migration.

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Claims (9)

1. A method of reducing metal migration from a metallic conductor means in an electrical circuits, said conductor means being subject to electrical voltage, the method including the step of alternating the polarity of the electrical voltage such that the net voltage/time product is approximately zero.

2. A method as claimed in claim 1, wherein the frequency of the polarity reversals is sufficiently high to suppress substantially the formation of metal branches.

3. A method as claimed in claim 1 or 2, wherein the metal is silver.

4. A method as claimed in any one of claims 1 to 3 wherein the metallic conductor means includes a metal track on a Printed Circuit Board. A method of reducing metal migration substantially as herein described with reference to the accompanying drawings.

6. An arrangement for reducing metal migration from a metallic conductor means in an electrical circuit, said conductor means being subject to electrical voltage, the arrangement including voltage polarity reversing means connected to the metallic conductor means, the polarity reversing means reversing the polarity of the voltage applied to the metallic conductor means in a predetermined manner.

7. An arrangement as claimed in claim 6 wherein the polarity reversing means reverses the polarity so that the net voltage/time product is approximately zero. *l An arrangement as claimed in claim 6 or claim 7 wherein the polarity reversing means reverses the polarity with a frequency sufficient to suppress substantially the formation of metal branches.

9. An arrangement as claimed in any one of claims 6 to 8 wherein the metallic conductor means is a silver track on a printed circuit board. An arrangement for reducing silver migration substantially as herein described with reference to the accompanying drawings.

11. A telephone subset including an arrangement as claimed in claim 8 or claim 9 for suppressing silver migration on a Printed Circuit Board keypad associated with said telephone subset.

12. A telephone subset as claimed in claim 11 wherein the polarity reversing means includes a processor controlled scanning arrangement which scans the rows and columns of the keypad and which regularly reverses the polarity of the scanning voltage. DATED THIS TWENTY THIRD DAY OF FEBRUARY 2000 ALCATEL AUSTRALIA LIMITED 000 005 363) o