AU710130B2 - A leakage resistant printed circuit board - Google Patents

Description

Technical Field This invention relates to reducing the susceptibility of PCB mounted circuits to faults resulting from damp heat.

Backgqround Art PCB's usually have components mounted on one side and their connection leads are inserted through corresponding holes to enable the leads to be soldered to contact pads on conductor tracks on the other side of the board. The side of the PCB on which the components are located will be referred to as the top-side, and the conductor track side as the underneath side.

A practice in some PCB manufacturing processes is to cover the underneath side with a thin layer of solder resist. The solder resist may be kept away from around the contact pads by a screen printing or etching process to enable the component leads to be soldered to the contact pads. The solder resist is usually applied by screen printing.

We have found that in some applications certain areas of PCB's may be susceptible to 15 faults due to damp heat (high humidity in warm climates).

We have found that in damp heat conditions, moisture can cause electrical faults by S" causing leakage e.g. from pad to pad, pad to track, or between tracks. The faults may be due to porosity of the solder resist layer which may result from the screen printing operation.

Specific areas of the PCB, e.g. where tracks are close together, are more susceptible to damp heat problems.

Disclosure of the Invention :We have found that damp heat leakage problems can be significantly reduced or eliminated by providing a region of thicker covering over the tracks on areas of the PCB which are susceptible to damp heat leakage.

S 25 According to the invention there is provided a printed circuit board having on at least one side thereof a plurality of solder pads and a plurality of conductor tracks Sinterconnecting groups of two or more of the solder pads, the surface of said at least one side of said printed circuit board being coated with a first coating of solder resist except for said solder pads, wherein a further non-conducting coating is provided in one or more selected areas of the solder resist coating which are susceptible to leakage under damp heat conditions.

Preferably, the thicker covering includes an additional layer over the solder resist which covers or transects the susceptible areas.

In one embodiment, the additional layer consists of solder resist.

In an alternative embodiment the additional layer consists of the labelling substrate used to provide e.g. a white background on which component identifiers can be written.

Preferably the additional layer is applied by screen printing.

Preferably, the solder resist is applied using a first screen printing mesh and the additional layer is applied using a second screen printing mesh of a different mesh size from the first mesh.

The different mesh sizes reduce the probability of a porous hole in the second layer aligning with a porous hole in the first layer, so that the two layers tend to present an intact and impervious skin to moisture. Preferably, the ratio of the meshes is not an integer.

Brief Description of the Drawings Figure 1 shows a circuit board proofed against damp heat leakage in accordance with the invention.

Figure 2 shows details of portion of a circuit board illustrating the damp heat leakage proofing in more detail.

Preferred Mode of Implementing the Invention Figure 1 is the underside view of a circuit board showing a plurality of conductor tracks having solder pads to which component leads from components mounted on the top side can be soldered. The solder pads are the generally circular .20 zones with the through-holes appearing as black dots in the centre. A number of damp heat proofed regions are shown cross-hatched, located in the top right quarter of the figure. The figure shows how damp heat proofing can be applied selectively to susceptible regions of a PCB. Substantially the whole of the underside of the board except the circular solder pads is coated with solder resist.

Figure 2 illustrated in greater detail part of the damp heat leakage proofing of Figure 1. A number of solder pads, 1 to 8, are shown together with the conductor tracks. The conductor tracks and the PCB are covered with solder resists while the solder pads are free of solder resist.

In Figure 2, pad 4 may be e.g. the solder pad for the base lead of a transistor and pad 3 may be the emitter of the transistor. Any leakage to base pad 4 may thus be amplified in the transistor so it is important to reduce the likelihood of leakage to this pad. For that reason it is completely surrounded by the additional layer 9 to reduce the chance of leakage from nearby pads and leads.

One way of determining areas of PCB's which are susceptible to damp heat leakage is the environmental testing of the boards in hot, humid conditions. Test voltages can be applied to the pads to identify where leakage occurs by identifying the leakage current paths.

A simple stripe of the additional layer transecting the shortest route between two points between which leakage may be sufficient to increase the length of the leakage path and thus to reduce the leakage current.

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The claims defining the invention are as follows: 1. A printed circuit board having on at least one side thereof a plurality of solder pads and a plurality of conductor tracks interconnecting groups of two or more of the solder pads, the surface of said at least one side of said printed circuit board being coated with a first coating of solder resist except for said solder pads, wherein a further non-conducting coating is provided in one or more selected areas of the solder resist coating which are susceptible to leakage under damp heat conditions.

2. A printed circuit board as claimed in claim 1, wherein the first coating of solder resist is applied via a first screen printing mesh having a first mesh size, and wherein the further non-conducting coating is applied via a second screen printing mesh having a :second mesh size different from the first mesh size.

3. A printed circuit board as claimed in claim 2, wherein the ratio of the first mesh size to the second mesh size is not an integer.

4. A printed circuit board as claimed in any one of the preceding claims, wherein 9* said further non-conducting coating is a second solder resist coating.

A printed circuit board as claimed in any one of claims 1 to 3, wherein said further *99* non-conducting coating is a coating of coloured background material for component S° labelling.

.o 6. A printed circuit board substantially as herein described with reference to Figures 1 -2 of the accompanying drawings.

7. A method of providing printed circuit board as claimed in claim 1, including the steps of: coating said one side of the printed circuit board with a first solder resist coating except for the said solder pads; applying a further coating of a non-conducting material in one or more selected areas of said first solder resist coating which are susceptible to leakage under damp heat conditions.

8. A method of providing a printed circuit board, substantially as herein described with reference to Figures 1 2 of the accompanying drawings.

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Claims (1)

1. 9. A method of damp heat leakage proofing a PCB substantially as herein described with reference to the accompanying drawings. DATED THIS TWENTIETH DAY OF MAY, 1996 ALCATEL AUSTRALIA LIMITED 000 005 363) *°oooo ABSTRACT Damp heat leakage on printed circuit boards is prevented by applying an additional layer, 9, over susceptible areas of the solder resist. The additional layer is applied via a screen printing mesh with a different mesh size from that used to apply the solder resist, to reduce the probability of alignment of porous holes in the two layers. Figure 2. 6 *5 ed .4O O. 4 a. a.