WO2000074452A1

WO2000074452A1 - Electronic circuitry sealing system

Info

Publication number: WO2000074452A1
Application number: PCT/AU2000/000600
Authority: WO
Inventors: Ian Robert Cocks; Braham Martin Basser
Original assignee: Innovag Pty Ltd
Priority date: 1999-05-28
Filing date: 2000-05-26
Publication date: 2000-12-07
Also published as: AUPQ064899A0

Abstract

Apparatus (10) is disclosed in which a substance of low electrical conductivity and low coefficient of thermal expansion (dimethylpolysiloxane) is used to fill a volume (4) within an enclosure (1) to protect an electronic circuit (2) from environmental damage.

Description

ELECTRONIC CIRCUITRY SEALING SYSTEM

Technical Field of the Invention

The present invention relates generally to hermetically sealed apparatus and, in particular, to hermetically sealed apparatus that are exposed to wet environments and extremes of temperature. The present invention in a first aspect relates to a hermetically sealed apparatus and in a second aspect relates to a temperature sensor.

Background

Electronic systems are often used in locations where environmental conditions are, practically speaking, uncontrolled. In dairies for example, electronic sensing apparatus may be subject to regular hosing and immersion in water and cleaning solutions. In many cases the apparatus may be subjected to rapid temperature change while immersed, such as during washing of the dairy plant. Modern washing systems can expose the dairy to a cold rinse at ambient temperature, followed by a hot wash. The ambient temperature is sometimes only marginally above freezing point while the temperature of the hot wash can be close to boiling point.

It is therefore not surprising to find that a major cause of failure of electronic apparatus in such environments is by way of water ingress into an enclosure housing the sensing electronics. One solution to this problem involves removing the electronics apparatus to a controlled environment and leaving only remote sensors arranged in the uncontrolled environment. An example of this is the use of thermocouples as sealed temperature sensors in dairy washing equipment. However, such remote sensing presents its own difficulties in the communication of the sensed value to the processing apparatus located in the controlled environment. A desire therefore remains for electronic sensing.

In an attempt to overcome the problem of locating electronics sensors in uncontrolled environments, manufacturers attempt to protect the electronics circuitry by means of a hermetically sealed enclosure. Some manufacturers also dip or coat the circuitry in protective lacquer or epoxy resins.

It has been found that even in the most tightly sealed enclosures, water can, over an extended period of time, be found to enter in sufficient quantities to cause failure. If the manufacturer has also taken the precaution of coating the circuitry in a protective lacquer or epoxy resin, once the outer enclosure has been breached, water which may also contain dissolved corrosive substances can then be in continuous contact with the protective layer. This water often finds small entries or imperfections in the protective coating where it can corrode the highly reactive metals used in the electronic circuit. In some cases the corrosive substances can break down the protective coating directly.

In order to understand how water ingress occurs in even the most tightly sealed enclosures, one has to look at the thermal physics of the environment.

Traditional sealing of electronics is achieved by placing the circuitry in an enclosure and providing an external seal, where entries for connecting cables and the like, are sealed against the enclosure wall. The seal can take the form of solder, sealing glands and compounds, to name but a few. As the temperature of the enclosure rises, air within the enclosure is heated, thus increasing the pressure within the enclosure. Observations by the present inventors indicate that even tight seals allow a few molecules of air to escape when pressurised. Escape routes can be around seals, for example from scratches in cable walls, or through the cable itself, between the inner conductors and outer insulation.

When the enclosure is rapidly cooled the internal pressure falls. Often at this time the outer seals (e.g. cable glands, etc) are soaked with fluid and surface tension has formed a meniscus over the interface between seal and enclosure. The negatively pressurised enclosure then draws small amounts of water through microscopic imperfections in the seal interface. Over time, quantities of water can accumulate within the enclosure sufficient to cause failure of the electronics by either direct contact, or by increasing the ambient humidity within the enclosure.

In the dairy environment, the present inventors have found that electronic sensing apparatus including even the most thorough and well-formed traditional seals fail after approximately 6 months. Subsequent "surgical" examination of the enclosure and seals has revealed no apparent wear or deterioration, thus supporting the findings noted above.

It is therefore desirable for alternative means for protecting electronics apparatus be devised. Advantageously, such means will provide for longevity of use, whilst remaining cost-effective in comparison with existing arrangements.

Summary of the Invention

The inventors have realised that the hub of the above problem lies not with the individual seals of the enclosure, but with the air within the enclosure. According to the present invention, it is proposed that, prior to sealing the enclosure, the air be displaced by a material that is substantially electrically non-conductive, and has a low coefficient of thermal expansion over the temperature range in which the apparatus is to be used.

According to a first aspect of the invention, there is provided an apparatus comprising: an enclosure constructed of a non-porous material, said enclosure having an internal cavity defining a first volume; electronic circuitry within said enclosure and occupying at least part of said first volume; sealing means for sealing said enclosure; and a fluid occupying a remainder of said first volume, said fluid having a low coefficient of thermal expansion and being substantially electrically non-conductive. According to a second aspect of the invention, there is provided a temperature sensor comprising: a hermetically sealed container; an electronic temperature sensor circuit connected to an interface cable, said circuit being within said container with said interface cable protruding from at least one seal of said container; and a fluid within said container, said fluid having a low coefficient of thermal expansion and being substantially electrically non-conductive.

Brief Description of the Drawing A preferred embodiment of the present invention will now be described with reference to the drawing, in which:

Fig. 1 schematically depicts a temperature sensor of the preferred embodiment. Detailed Description including Best Mode

Fig. 1 shows a temperature sensor 10 formed by a length of stainless steel tubing 1 enclosing a temperature sensing electronics module 2 connected to a number of conductors 8 of an interface cable 3. The tubing 1 is hermetically sealed at one end 5 by coating the end 5 with RTV silicone sealant and pinching and folding the tubing 1 to form a crimped seal. The other end 6 of the tubing 1 is sealed with glue-filled environmental heat shrink tubing 7. The sealing of each of the ends 5 and 6 is achieved in this fashion using high quality traditional sealing techniques.

Prior to final sealing, a volume 4 within the tubing 1 and surrounding the electronics module 2 is filled with Dow Corning ® 200 Fluid. This particular fluid is a silicone liquid (Dimethylpolysiloxane).

Dow Corning ® 200 Fluid is a relatively-inert silicone fluid available in either food or medical grades in a range of viscosities. It is known for its ability to maintain viscosity over a wide temperature range and has a low toxicity when consumed. Dow

Corning ® 200 Fluid has previously found application as a lubricant for food processing machinery, heat transfer fluid for industrial cooling systems, high-viscosity breast implant filling fluid and in cosmetic creams.

The present inventors have ascertained that the fluid also has the following desirable characteristics: (i). a minimal coefficient of thermal expansion (m³/°C), thus being able to prevent pressure changes occurring within the tubing 1 ;

(ii). a high dielectric breakdown voltage and insulation resistance, to avoid influence on the operation of the electronics module 2;

(iii). it has low toxicity (Food Grade), which is advantageous should a leak occur and traces of fluid enter food-carrying vessels or pipes; and

(iv). a sufficiently low viscosity, so the fluid is readily flowable to all areas of the container during production, and can be dispensed easily.

In the preferred embodiment the Food Grade Dow Corning ® 200 Fluid is used, and although the product is available in a range of viscosities (0.65 centistokes to 100 x 10³ centistokes), fluid having a viscosity of 350 centistokes is preferred for its medium flowability and ease of dispensing using standard production equipment. This compares

with a viscosity of approximately 0.9 centistokes for water at 25°C.

A comparison of Dow Corning 200 ® Fluid against petroleum oil and dry air

reveals that Dow Corning 200 ® Fluid has a coefficient of volumetric thermal expansion

of 0.096 %/°C which is lower than petroleum oils (0.11 %/°C) and dry air (0.326 %/°C).

This also indicates that Dow Corning ® 200 Fluid has a low coefficient of thermal

expansion in comparison to these other fluids since the coefficient of thermal expansion is proportional to the coefficient of volumetric thermal expansion.

Although the coefficient of volumetric thermal expansion of dry air is

approximately three times that of Dow Corning ® 200 Fluid, this is still not large enough to account for the amount of water ingress into traditional sealed enclosures. However, it must be kept in mind that in a normal operating environment, the air will always have some water content. Since water has an extremely large coefficient of volumetric thermal expansion, if a small amount is admitted into an enclosure, the pressure inside the enclosure for a given temperature will be much higher than the pressure prior to the water entering. It can be appreciated that the cyclic pressure change inside the enclosure caused by hot and cold washing cycles, for example, will increase exponentially with time as more water enters the enclosure.

It is noted that the fluid sealing technique can be applied to any electronic product that is subjected to temperature change, moisture and is production-sealed. Also, depending on the environment in which the apparatus operates, it may not be necessary for the fluid to have a low toxicity.

Industrial Applicability

It is apparent from the above that the embodiment(s) of the invention are applicable to industries, such as the dairy industry, where electronic equipment is exposed to a wet environment which experiences extremes of temperature.

The foregoing describes only one embodiment of the present invention, and modifications and/or changes can be made thereto without departing from the scope and spirit of the present invention.

Claims

CLAIMS:

1. Apparatus comprising: an enclosure constructed of a non-porous material, said enclosure having an internal cavity defining a first volume; electronic circuitry within said enclosure and occupying at least part of said first volume; sealing means for sealing said enclosure; and a fluid occupying a remainder of said first volume, said fluid having a low coefficient of thermal expansion and being substantially electrically non-conductive.

2. Apparatus according to claim 1, wherein said fluid has a viscosity sufficiently low to enable said fluid to flow into all unoccupied areas of said first volume.

3. Apparatus according to claim 1 or 2, wherein said fluid has low toxicity.

4. Apparatus according to claim 3, wherein said fluid is Dow Corning ® 200 Fluid

(Food Grade).

5. Apparatus according to claim 1, wherein said fluid is a dimethylpolysiloxane.

6. A temperature sensor comprising: a hermetically sealed container; an electronic temperature sensor circuit connected to an interface cable, said circuit being within said container with said interface cable protruding from at least one seal of said container; and a fluid within said container, said fluid having a low coefficient of thermal expansion and being substantially electrically non-conductive.

7. A temperature sensor according to claim 6, wherein said fluid has a viscosity sufficiently low to enable said fluid to flow into all unoccupied areas of said first volume.

8. A temperature sensor according to claim 6 or 7, wherein said fluid has low toxicity.

9. A temperature sensor according to claim 8, wherein said fluid is Dow Corning ® 200 Fluid (Food Grade).

10. A temperature sensor of according to claim 6, wherein said fluid is a dimethylpolysiloxane.

11. An apparatus substantially as herein described with reference to any one of the embodiments as that embodiment is illustrated in the drawings.

12. A temperature sensor substantially as herein described with reference to any one of the embodiments as that embodiment is illustrated in the drawings.