WO1990007708A2

WO1990007708A2 - Capacitive humidity sensor

Info

Publication number: WO1990007708A2
Application number: PCT/NL1989/000100
Authority: WO
Inventors: Pieter Paulus Jozef Schmitz
Original assignee: Flucon B.V.
Priority date: 1988-12-30
Filing date: 1989-12-28
Publication date: 1990-07-12
Also published as: JPH04502965A; CA2006948A1; DE68913764T2; NO912492L; ES2050428T3; FI913165A0; EP0451183A1; DE68913764D1; NO912492D0; ATE102708T1; JP2846948B2; EP0451183B1; WO1990007708A3; US5283711A; DK125891A; NO303092B1; NL8803223A; DK125891D0; FI94084C; FI94084B

Abstract

Capacitive humidity sensor comprising a substrate, a moisture-impermeable conducting bottom layer as first capacitor plate, a dielectric layer, and a moisture-permeable conducting top layer as second capacitor plate, and connecting wires associated with the first and second capacitor plates, whereby the change in the dielectric constant, and therefore in the capacitance value, due to absorption of water molecules is measured. The substrate is a flexible copper laminate in which at least the conducting bottom layer is etched, and the conducting top layer comprises a solid printed layer having a conducting grid pattern, which is printed with conducting ink.

Description

Capacitive Humidity Sensor

The invention relates to a capacitive humidity sensor comprising a substrate, a moisture-impermeable conducting bottom layer as first capacitor plate, a dielectric layer, and a moisture-permeable conducting top layer as second capacitor plate, and connecting wires associated with the first and second capacitor plates, the change in dielectric constant, and therefore in capacitance value, due to absorption of water molecules in the dielectric material being measured. Such a humidity sensor is disclosed, for example, by US patent specification 4,532,016. In such a sensor, which in f-act consists of an electrical capacitor, the humidity is determined by measuring the change in dielectric constant of the dielectric medium, and therefore the change in capacitance value, due to the absorption of water molecules by the dielectric material. If, for example, the material of a dry dielectric layer has a dielectric constant of 3.5 and water approximately has one of 80, it will be clear that the change in capacitance value due to moisture absorption may be appreciable. In order that moisture or water can be absorbed by the dielectric layer, the top iayer has to be moisture-permeable. This known sensor has a glass substrate on which a tantalum plate is deposited as first capacitor plate. Said tantalum plate is anodized, as a result of which a thin insulating layer consisting of tantalum oxide is obtained. Said insulating layer prevents resistance loss across the dielectric medium at fairly high relative humidity, or even short-circuiting between the plates, regardless of the structure of the dielectric medium. The occurrence of direct current, which would otherwise produce polarisation drift as a consequence of electrolysis, is also prevented. The dielectric layer mentioned is sufficiently porous to absorb moisture. The moisture-permeable top layer may consist, for example, of porous gold or cracked chromium. When evaporated onto the dielectric layer, chromium has in fact a high tensile force, as a result of which small cracks are also produced in the chromium and in the dielectric. A disadvantage is, however, that the cracking effect continues, especially at the beginning of the service life, as a result of which the capacitance value changes with time. To counteract this, a thermal post-treatment would be necessary.

The manufacture of said sensor also entails problems. In particular, the size of the thin glass substrate which can be processed is limited, as a result of which the rate of manufacture is restricted and the costs rise. The techniques are complicated and the finished sensors are very delicate and the output of the manufacture is relatively low. The porous gold may be affected by industrial pollutants, while the chromium cracking process is dependent on events which occur fortuitously. In regard to the finished sensor, there is also the problem that there is no separate provision for adjusting or trimming the measurement accuracy.

This group of sensors is suitable for high-cost instruments in which the relatively high costs of the sensors can be absorbed and in which the electronic circuits can be trimmed and calibrated in order to accommodate the wide tolerances in the accuracy of the sensor.

The object of the invention is to eliminate the abovementioned problems and to provide a cheap sensor which is much more robust and can be trimmed during or after manufacture in order to make interchangeability possible.

According to the invention, this is achieved in a capacitive humidity sensor of the type mentioned in the preamble in that the substrate is a flexible copper laminate in which at least the conducting bottom layer mentioned is etched, and in that the conducting top layer mentioned comprises a solid printed layer having a conducting grid pattern.

In this embodiment according to the invention, an ^* exceptionally cheap sensor for the consumer market is obtained which is also suitable for industrial applications. This sensor according to the invention can also be calibrated simply, both under dry and moist atmospheric conditions.

The invention will be explained in more detail on the basis of an exemplary embodiment with reference to the drawing, in which: Figure 1 shows a perspective view of the layered structure of the exemplary embodiment of a sensor according to the invention; and Figure 2 shows a plan view of the top layer, which acts as second capacitor plate, of the exemplary embodiment of Figure 1.

As stated above, the object of the invention is to provide a cheap sensor for consumer applications.

According to the invention, the glass substrate material, which is per se an excellent material, is now replaced in practice by copper laminate which is frequently used, for example, in printed circuit boards (PCBs). In a further embodiment, said copper laminate may be constructed with polysiloxane-glass textile or fabric. The moisture-impermeable bottom layer can be etched on the copper surface of such a laminate as first capacitor plate, if necessary with a second thinner contact plate next to it which acts as contact plate for the moisture-permeable top layer. Connecting wires can be attached to said bottom layer and contact layer in a later phase of the manufacture. The use of such a thin flexible PCB laminate has the advantage that use can be made of PCB manufacturing techniques, the finished result also yielding a more robust sensor than the one manufactured from thin-film glass substrates. It is then possible to start with larger substrates, for example, resulting in a quantity of 50 x 50 = 2500 sensors. Under certain conditions, in which an insulating polymer is deposited as insulating layer on the moisture-impermeable bottom layer, it is possible in practice to process two substrates simultaneously back to back, as a result of which a quantity of 5000 sensors can be produced in one processing phase.

Figure 1 shows diagrammatically the structure of a sensor according to the invention.

1 indicates the substrate consisting of a thin PCB copper laminate in which two copper layers 2 and 3 are etched. The layer 2 indicated at the left-hand side is the moisture-impermeable layer which acts as first capacitor plate and the layer 3 is the contact plate which makes contact with the moisture-permeable top layer 4, which acts as second capacitor plate, in a later phase of the manufacture. When the substrate is cut up in a final phase and the individual sensors are separated, the connecting wires 5 are attached to the layers 2 and 3. 6 indicates a polymer coating which functions as insulating layer and which, although extremely thin, protects the surface of the active copper capacitor plate 2 against attack and contamination. Since the layer 6 is itself moisture-impermeable, any short circuiting between the capacitor plates mentioned is consequently prevented, regardless of the structure of the dielectric material, and direct current, which would otherwise produce polarisation drift as a consequence of electrolysis, is prevented from flowing. The layer will prevent breakdown occurring as a consequence of the occurrence of so-called "pinholing" in the active dielectric 7. The polymer 6 does not extend over the thinner copper contact layer 3. In the latter case, an inert, non-reactive pinhole-free barrier polymer is preferably used.

The dielectric layer 7 is composed of an active polymer which can be spun, can be deposited by means of screen printing or in another manner, so that an extremely thin layer is produced which nevertheless contains a sufficient quantity of pores required for the operation of the humidity sensor. In this manner, a dielectric "sponge" is obtained which ensures a rapid change in the capacitance value under the influence of moisture. A large "inhalation" and "exhalation" surface is consequently achieved. The figure indicates that the layer 7 does not extend far enough to lie on top of the contact layer 3. But other methods of manufacture are conceivable.

4 indicates the moisture-permeable top layer as second capacitor plate which extends over the whole. The part of said top layer extending over the dielectric layer 7 has the form of a grid printed on the dielectric layer with conducting ink.

The plan view shown in Figure 2 shows the plate 7 mostly as a rectangular grid pattern, although other patterns are also possible. The grid openings ensure moisture transmission. The geommetry of the grid 10 is important. A maximum (conducting) ink surface area will produce a maximum capacitance value, but then the time necessary for absorption and desorption will be large, unless the moisture-transmitting surface area is sufficiently large. A good result is produced with a relatively large sensor surface area, such as, for example, 10 mm², in which case the response time is adversely affected to a certain extent.

11 indicates the part of the top plate which is in solid printed form, ie. does not have any openings, and which is intended to make contact with the contact layer 3 on the substrate 1.

12 indicates a row of small conducting printed areas in an insulated arrangement. In constructing the capacitor, it is intended that said row of small areas 12 is situated only above the barrier polymer and not above the active polymer. The capacitances thereby formed have a minimum or zero value.

In manufacture, the design is such that the sensor lies somewhat below its normal tolerance of capacitance values, both under dry and moist conditions (delta = 0% to 100% relative humidity range) . For automatic calibration, several of the small areas 12 are connected in the case of a dry reference environment to the main grid by means of small pads of conducting ink in order to obtain the required fixed (dry) capacitance value.

13 indicates another row of small conducting printed areas in an insulated arrangement. In the construction of the capacitor, however, these are situated above the active polymer. These small areas can also be connected to the main grid in order to adjust the required delta capacitance value in a humid or moist reference en¬ vironment.

In the above, it has been assumed that it is easier to add than to subtract a capacitance value. Such a subtraction may, however, also be achieved by laser trimming. After trimming, the conducting ink tracks can be hardened.

The advantages of this "printing" method are that the exact printing geommetry can be defined and there is no dependence on fortuitous phenomena. Trimming can be carried out both for the fixed and the delta values. If process faults are made during printing, these can be "erased", the more expensive part of the substrate clad with the barrier polymer being left intact.

The small trimming areas are shown as small squares. It will be clear that they can also be provided in a smaller number and, for example, in a binary surface area progression.

During manufacture, sensors can be manufactured in accordance with the required application having an improved response speed by making one dimension greater, as a result of which a longer sensor is obtained. Since the contact surface occupies an appreciable part of the total surface, a more slender design will make it possible for the contact surface area to decrease in favour of a larger active surface area. Since the barrier polymer is expensive, the copper laminate is preferably clad only on one side. This can best be achieved by processing the substrate plates two at a time, back to back. To do this, it is necessary to mask the contact layer or even to grind off the barrier layer obtained. Since the laminates can be obtained processed on both sides, it is possible to provide both contact layers on the other side, unclad by the barrier layer and consequently readily solderable and to connect them by means of through-hole plating or by means of specially designed "knife-edge" contact terminals. All the inactive surfaces which are not covered by the barrier polymer can be covered with solder resist in the normal manner of protecting PCB laminates.

The cheap humidity sensor specified above, which can be manufactured in a reproducible manner, may be used for many types of applications, such as for air treatment systems, in the motor vehicle industry for window demisting systems, brake (shoe) systems and for internal combustion engines in which the combustion process is strongly- dependent on the relative humidity.

Claims

1. Capacitive humidity sensor comprising a substrate, a moisture-impermeable conducting bottom layer as first capacitor plate, a dielectric layer, and a moisture-permeable conducting top layer as second capacitor plate, and connecting wires associated with the first and second capacitor plates, the change in the dielectric constant, and therefore in the capacitance value, due to absorption of water molecules being measured, characterized in that the substrate is a flexible copper laminate in which at least the conducting bottom layer mentioned is etched, and in that the conducting top layer mentioned comprises a solid printed layer having a conducting grid pattern.

2. Capacitive humidity sensor according to Claim 1 , characterized in that the grid pattern of the top layer mentioned is printed with conducting ink.

3. Capacitive humidity sensor according to Claim 1 , in which the bottom layer mentioned and a contact layer making contact with the moisture-permeable top layer are deposited parallel to each other in the longitudinal direction on the substrate, a moisture- impermeable -insulating layer being present at least between the bottom layer mentioned and the dielectric layer, and the top layer mentioned extending both over the dielectric layer and over the contact layer, characterized in that the insulating layer is an inert, pore-free insulating polymer, in that the dielectric layer is a dielectric polymer which is deposited such that an extremely thin layer with adequate porosity is produced, and in that the top layer mentioned has a grid geometry such that the conducting ink surface area and the moisture-permeable surface area are matched so as to obtain an adequate capacitance value and a short absorption and desorption time.

4. Capactive sensor according to Claim 3, characterized in that the contact layer is etched in the copper laminate.

5. Capacitive humidity sensor according to Claim 3, characterized in that the copper laminate is constructed with polysiloxane-glass textile material.

6. Capacitive humidity sensor according to Claim 3, characterized in that the insulating polymer is^"a barrier polymer.

7. Capactive humidity sensor according to Claim 3, characterized in that the top layer has a row of small ink areas at at least one of the sides of the grid pattern.

8. Capacitive humidity sensor according to Claim 7, characterized in that the row of small ink areas (12) in the finished sensor is situated such that the dielectric polymer is not situated under the row but the insulating polymer is, so that, during precalibration of the capacitance value in a dry atmosphere, small ink areas can be connected, if necessary, to the grid pattern.

9. Capacitive humidity sensor according to Claim 7, characterized in that the row of small ink areas (13) in the finished sensor is situated such that the dielectric polymer is situated under the row and the insulating polymer is not, so that, during precalibration of the capacitance value in a moist atmosphere, small ink areas can be connected, if necessary, to the grid pattern.

10. Capacitive humidity sensor according to Claim 3, characterized in that the grid pattern is trimmed during precalibration of the capacitance value by means of laser irradiation.