DE9419603U1 - Flat wide-angle mounting structure for infrared heat sensor elements - Google Patents

Description

K 30 465

Flat wide-angle mounting structure for infrared heat sensor elements

The present invention relates to planar mounting devices and structures for pyroelectric infrared thermal sensors and more particularly to such a device and structure having a larger detection angle.

Heat sensitive security monitoring systems have been widely used for some time. Such systems are based on the fact that a pyroelectric infrared heat sensor element has been developed. The principle of operation of pyroelectric infrared heat sensor elements is that the elements generate a corresponding electrical signal in response to infrared heat radiation emitted by an approaching person or heat source. The electrical signal is then amplified by an amplifier circuit and generates a preventive alarm or trigger function and serves for security purposes. Such pyroelectric infrared heat sensor elements have the following advantages:

1. The pyroelectric effect itself is a differential function of time, so it can provide automatic suppression of the background (static background heat sources), thereby creating a dynamic sensing effect and simplifying signal processing;

2. Pyroelectricity is generated spontaneously without the need for a bias voltage, which causes the

noise generated by such bias voltage is avoided; and

3. the elements are manufactured using the same simple technique as for the manufacture of typical ceramic capacitors. They can be manufactured by only depositing coatings forming the electrodes and a blackbody film by vapor deposition, thus reducing costs.

Due to the above advantages, inexpensive non-contact radiant heat sensor elements that can operate at room temperature are feasible. In recent years, such heat sensor elements have been widely used in security systems for burglar alarm and fire detection, as well as in sensor elements for light control for energy saving. As a result, more and more research is currently being carried out on the pyroelectric infrared heat sensor elements.

In general, it is necessary to make the thermal sensor elements as thin as possible to reduce their thermal capacity, while at the same time providing them with a suspension structure to reduce their heat loss, i.e. to increase their thermal resistance. Therefore, when a certain amount of thermal energy is absorbed by the elements, the elements produce a higher temperature rise and therefore have a better thermal response, which in turn results in a better pyroelectric response. The requirements of a thin and suspended structure mean that the adhesive technology required for assembling any sensor elements becomes complicated, which is also the case for the manufacture of pyroelectric infrared thermal sensors.

This means that the complicated structure of radiant heat sensor elements currently in production leaves a lot of room for improvement.

In Fig. 7 (in the dashed box) there is shown the equivalent circuit of a conventional two-part pyroelectric sensor element, in which the pyroelectric sensor elements Al and A2 can be made of thin layers of a pyroelectric material such as LiTaO ₃ , PZT, PVF ₂ or TGS. The pyroelectric sensor elements Al and A2 each have electrodes deposited on both sides, which in turn are coated with a blackbody film on their upper surface, thereby increasing the heat absorption effect. The two sensor elements have opposite polarity, as shown by the arrows, with the polarity of one element pointing upwards and that of the other element pointing downwards in Fig. 7. The static charge or voltage resulting from the heat induction of the elements Al and A2 is transferred to a high input impedance junction field effect transistor (JFET) Q, which serves as an impedance buffer for an external circuit. The junction field effect transistor Q outputs a signal at a terminal L2. A load resistor RL with a high specific resistance is provided in parallel with the terminals of the layer sensor elements Al and A2, which facilitates the compensation of the leakage current of the pyroelectrically induced charge, the transmission of the low-frequency noise and the selection of the bandwidth of the circuit. The use of two elements eliminates the microphone noise generated by vibration and the common-mode signal that arises when the background thermal radiation changes. The assembly formed from the above elements must be placed in a sealed metal container in order to avoid a reduction in the signal-to-noise ratio.

due to interference from electromagnetic noise. When an element S is used in a heat-inducing system as shown in Fig. 5, a multiple infrared collecting lens array S is arranged in front of the element S, dividing the observation field of the system into several small observation fields (shown by oblique lines) so that any heat wave from a moving heat-radiating body, e.g. the heat wave from a person, can be "imaged" onto and absorbed by one or the other of the two pyroelectric sensor elements. This causes an alternating rise and fall in temperature in the two elements, producing and outputting a differential signal with a frequency of approximately 1 to 10 Hz.

2 and 3 each show the structure of one type of pyroelectric sensor elements currently commercially available. In the conventional structure shown in Fig. 2, two strip-like insulating bumps 102, 103 are bonded in advance to a metallic base 101, and then, at a position where the optical system is focused, two layer pyroelectric elements Al and A2 (as shown in Fig. 1) are mounted so as to bridge the insulating bumps 102, 103. Furthermore, other elements such as the junction field effect transistor JFET and the load resistor RL as shown in Fig. 1 must be mounted on an insulating bump 104 which is in turn fixed to the base 101 to prevent a short circuit with this metallic base 101. Once this complicated assembly is completed, lead wires must be made in ultrasonic welding steps according to conventional semiconductor assembly techniques to produce the necessary circuit wiring. The structure is then sealed by applying a

Cover 106 is welded. A multilayer silicon filter 105 for long-wave infrared light (LWIR) is glued to the cover 106, which filter transmits heat waves with a wavelength of 6 to 14 µm, but suppresses background light rays of other wavelengths.

In the structure of another conventional pyroelectric sensor element shown in Fig. 3, a plurality of curved welding wire rings 108 are provided on a ceramic insulating bump 107, and then pyroelectric layer members (denoted by reference character A) are bonded to the upper ends of the respective welding wire rings 108, thereby obtaining a suspended heat insulation structure.

In the above two conventional structures for pyroelectric elements, a stacking arrangement is used for suspension and heat insulation, which makes the assembly of the elements and the structure of the products relatively complicated and therefore not suitable for automated assembly, resulting in high labor costs. In addition, the metal base is quite expensive because it accounts for a relatively high cost share (about 25%) of the materials, and the electrical conductivity of the base requires the provision of the insulating bumps, which significantly increases the manufacturing and processing costs.

In order to eliminate these adverse influences, the applicant of the present invention has proposed in ROC Utility Model Application No. 67164 a special planar mounting structure as shown in Fig. 4A, in which a conductive copper foil 204 having a special circuit design and wiring is formed on a double-sided circuit board 201,

such that the elements can be arranged as shown in Fig. 1 using surface mount technology (SMT). A plurality of through holes 202 are provided in the circuit board 201 through which pins 203 pass and are secured by means of an automatic riveting technique so that they can serve for positioning so that the pyroelectric elements can be precisely arranged at desired positions during assembly. The structure of the above-mentioned ROC Utility Model Application No. 67164 is characterized in that an undercut slot 205 is formed in the area of the circuit board where the layer of pyroelectric material Al and A2 is provided so that the layer of pyroelectric material Al and A2 bridges this undercut slot 205. The undercut slot 205 is also lined with copper foil 204. In Fig. 4B, a side sectional view of the structure is shown. When the pyroelectric elements Al and A2 bridge the area of the well 205, a suspended frame is formed, which provides an excellent heat insulating effect. On the other hand, with such a suspended structure, both the thin layer pyroelectric elements Al and A2 and the other elements added can be arranged on the same plane of the circuit board, so that an automated assembly operation can be carried out by using the surface mounting technique and equipment. Furthermore, because of the electrical insulation of the circuit board itself, there is no need for additional insulating bumps, and the required number of wires welded by ultrasonic welding can be reduced since the circuit board can be designed for many different applications. Finally, the double-sided circuit board can effectively prevent interference from electromagnetic noise.

After the structure shown in Figs. 4A and 4B has been assembled and tested, it can be installed in the cover housing. As shown in Fig. 4C, the top of the cover 206 has an infrared window 207, which has a multilayer thermal infrared filtering plate 208 that allows the thermal radiation energy to pass through. The cover 206 also has flanges on its inner edge (not shown) so that when the circuit board 201 is mounted in the cover 206, this circuit board fits snugly against the flanges and there is a fixed relative distance between the pyroelectric elements Al and A2 on the one hand and the infrared window 207 on the other hand, which enables precise optical detection. Finally, on the side of the circuit board 201 opposite to the side fitted into the cover 206, an epoxy resin is injected and thermally cured to encapsulate the circuit board 201 in an airtight manner. The cover 206 may be made of metal molded by pressing technology or plastic molded by die casting and then coated with metal to provide electrical insulation and noise insulation as well as water resistance properties.

In addition, pins are no longer required for the through holes 202 of the circuit board 201. After the structure shown in Figures 4A and 4B has been assembled and tested, it can be welded to the pin ends (elements 210 shown in Figures 6A and 6B) of a conventional TO metal base so that it "floats" on that base surface. A metal cover is then welded to a filter window using standard TO metal encapsulation equipment, thus obtaining the finished product.

From the foregoing description, such a conventional structure (according to ROC Utility Model Application No. 67164) has the following advantages:

1. All elements are bonded to the same plane of the circuit board. Therefore, the process for automated assembly using currently available surface mount equipment is simplified, so that the advantages of mass production can be achieved at the lowest cost;

2. the pre-manufactured undercut shaft 205 enables the pyroelectric elements mounted thereon to be implemented in a suspension structure without the need for the stools, thereby reducing the number of processing steps and saving materials, thus achieving cost savings;

3. the circuit board is used as the base, so that the expensive metal base can be omitted ;

4. Insulation bumps are no longer required for the sensor elements, which can further reduce material costs and reduce the number of processing steps;

5. The layout wiring of the circuit board allows reducing the number of wires to be welded.

However, all of these structures, including those of the above-mentioned ROC Utility Model Application No. 67164, have the disadvantage that the observation angle

of such a thermal sensor element is not more than 110°. If the observation angle is increased, the material cost and the size of the structure are increased. The reasons for this will be described with reference to Figs. 6A and 6B.

Fig. 6A shows a schematic cross-section of the mounting structure according to ROC Utility Model Application No. 67164. From the light beams r _ml and r^ it can be seen that the sensor devices according to this arrangement have a maximum observation angle of 9 _m , which is generally about 110° for currently available products. In use, such an observation angle can hardly cover the semicircular angle, ie 180°, since the angle is limited by the wall (see the diagram shown in Fig. 5). To increase the observation angle, two solutions are possible:

The first approach is to increase the dimensions of the observation window 207 for the filtering plate 208, designated by reference B in Fig. 6A. However, this increases both the material cost and the package dimensions because the filtering plate 208, which is transparent to thermal infrared with a wavelength of 8 to 14 µm, is made of expensive silicon chips coated with several tens of layers of germanium and zinc sulfide films, which is a time-consuming and very expensive process. In addition, with increased dimensions, the metal footprint used must be increased, so that the volume and material cost are further increased.

The second solution is to reduce the distance A as shown in Fig. 6A between the sensor elements Al and A2 on the one hand and the filtering plate 208 on the other hand. This attempted solution does not have the disadvantage

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part of the first attempt at a solution. However, a wire 209 must be welded to the upper surface of the elements Al and A2 for connection to external circuits, so that a gap must be provided for this wire 209 to be welded. Thus, a larger volume is required for such a structure, so that the observation angle is not actually improved. This means that the increase in the observation angle has an adverse effect on the cost and dimensions of the elements.

It is therefore the primary object of the present invention to provide a planar mounting device and structure for pyroelectric infrared heat sensor elements which is improved over that previously disclosed by the applicant of the present invention in ROC Utility Model Application No. 67174.

It is another object of the present invention to provide a planar mounting device and structure for pyroelectric infrared thermal sensor elements having a larger observation angle without increasing the cost and/or size.

These objects are achieved according to the invention by a planar mounting structure for pyroelectric infrared heat sensor elements, which has the features specified in claim 1.

The dependent claims are directed to preferred embodiments of the present invention.

Further objects, features and advantages of the invention will become apparent upon reading the following description of preferred embodiments, which refers to the drawings, in which:

Fig. 1 is an exploded view of a mounting structure of a pyroelectric heat sensor element of the present invention;

Fig. 2 shows the previously mentioned structural view of a conventional pyroelectric sensor element;

Fig. 3, 3A the aforementioned structural view of another conventional pyroelectric sensor element;

Fig. 4A is the aforementioned structural view of a conventional pyroelectric sensor element disclosed by the applicant of the present invention, in which the heat sensor elements A1 and A2 are suspended on a circuit board;

Fig. 4B is the aforementioned sectional view of the suspended structure shown in Fig. 4A;

Fig. 4C is the previously mentioned exploded view of a mounting structure of the pyroelectric sensor element of Fig. 4A;

Fig. 5 the above-mentioned viewing angle subdivision diagram of a matrix of infrared lenses;

Fig. 6A, B the already mentioned cross-sectional view and longitudinal sectional view of the mounting structure of Fig. 4C;

Fig. 7 the already mentioned equivalent circuit of a conventional two-part pyroelectric sensor element;

Fig. 8A is a schematic cross-sectional view of the mounting structure of the pyroelectric sensor element of Fig. 1;

Fig. 8B is a plan view of the wiring diagram of the pyroelectric sensor elements and the base of the structure shown in Fig. 8A;

Fig. 8C is a cross-sectional view of the pyroelectric sensor element according to the present invention along the line C-C in Fig. 8A, from which it can be seen that the thermal sensor elements Al and A2 are suspended on the circuit board at an inclination;

Fig. 9 Signal response distribution curves of the pyroelectric sensor elements according to the present invention for different inclination angles;

Fig. 10 the far-field profile of the measured signal intensity with a thin pyroelectric plate tilted at an angle of 25°; and

Fig. 11 is a plan view showing the structure of the pyroelectric thermal sensor element according to the present invention.

The present invention improves the observation angle of the pyroelectric infrared heat sensor based on the earlier ROC Utility Model Application No. 67174.

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As shown in Figs. 1 and 8A to 8C, the planar mounting structure of the pyroelectric infrared heat sensor device according to the present invention includes a base plate 301, a cover 306 having an infrared window 307, and an infrared filtering plate 308. The base plate 301 is provided with a recess 305 having a specific configuration. The recess 305 is formed in the base plate 301 by a pressing technique. The heat sensor elements Al and A2 are arranged across the width of the recess 305, thus forming a suspended structure.

The recess 305 is designed to have a rectangular shape, but two pairs of opposite extended recess portions 305' are provided on its sides contacting the thermal sensor elements Al and A2, whereby the width of the recess 305 at these extended portions is greater than the length of the thermal sensor elements Al and A2. The width of the opposite extended recess portions 305' is smaller than that of the thermal sensor elements Al and A2, respectively. Therefore, when the thermal sensor elements Al and A2 are welded to the base plate 301 (at welding points 310), the elements Al and A2 partially tilt into the recess 305, whereby they are inclined with respect to this recess 305 and face each other, but still suspended in the recess 305, as shown in Fig. 8A.

As can be seen from Fig. 8A, the inclined yet suspended structure of the present invention has the following advantages:

1. The oblique arrangement causes the spaced outer edges of the heat sensor elements Al and A2 to be

Filtering plate 308 have a distance H ¹ which is smaller than the distance H in Fig. 6, whereby, as explained above, the observation angle of the device can be increased; and

In the conventional horizontally arranged structure shown in Fig. 6, the "projection exposure" of the light incident on the thermal sensor elements is calculated by multiplying the incident light quantity by the projection of the angle of incidence, i.e., cosa. Thus, the amount of light actually received varies with the angle of incidence. As the angle of incidence approaches 90°, cosa approaches 0, so that the sensitivity of the thermal sensor device is lowered and the increase in the actual observation angle is made difficult. However, in the proposed structure of the present invention as shown in Fig. 8A, since the thermal sensor elements are positioned obliquely, the angle of incidence of the light on the sensor elements (α' in Fig. 8) is reduced to be smaller than the angle α in Fig. 6, resulting in the amount of light actually received being much larger than in the conventional structure. The far field profile of the signal intensity has been theoretically calculated for various tilt angles and shown in Fig. 9. From this figure it can be seen that the tilt of the thermal sensor elements allows for a large angle of observation. Although certain signals are suppressed to some extent for smaller angles of observation, the tilted structure provides significantly better detection results since the instrument is generally positioned such that a moving object being detected typically enters the field of observation from the side at large angles of observation.

angle (as shown by the direction of motion P of the moving object in Fig. 5).

Fig. 10 shows the measurement results on the samples we examined. The curve with white dots represents the measurements of a given conventional (Japanese) product available on the market. The data indicated by black dots represent the measurements on the structure according to the present invention, with the heat sensor elements inclined by about 25°. It is obvious that at half the maximum signal, the observation angle for the conventional product is only about 125°, while for the structure of the present invention it is almost 150°; at 1/10 of the maximum signal, the conventional product gives 140°, while the structure according to the invention gives an observation angle of 170°, which is approximately a semicircle (limit value).

The following describes an exemplary process for manufacturing the structure of the pyroelectric infrared heat sensor device according to the present invention:

1. A base plate (having holes 302 and pins 303 as shown in Fig. 8C) is formed by circuit wiring and pressing as shown in Fig. 11, where all of points a, b, c and d include both discontinuous layers and upper and lower copper foil layers (the shaded portion) .

2. On the points b and c, where the pyroelectric heat sensor elements Al and A2 are arranged, as well as on the points e, f and g, where the junction field effect transistor JFET and the load resistor

stand RL, a silver adhesive is applied, whereby this silver adhesive is applied to the interrupted circuit board layers at points b and c.

3. Each of the pyroelectric thermal sensor elements Al and A2, as well as the junction field effect transistor JFET and the load resistor RL are bonded at positions shown in the drawings and then hardened by heating in an oven.

4. Then, the silver adhesive is applied to points a and d and cured by heating in an oven so that the copper layers Pl and P2 on the discontinuous layer are electrically connected to the pyroelectric heat sensing elements Al and A2. This step avoids the difficulty of welding wires on the inclined surface.

5. Wires are then welded to complete the required connection circuit.

6. The structure to which infrared filter lenses are attached after the above step is inserted into a metal case, after which an adhesive is applied to the back side and cured by heating in an oven. Then the manufacturing process of the sensor device is completed.

Another example of the manufacturing process of the structure of the pyroelectric infrared heat sensor device according to the present invention is described below:

1. A base plate (without pins 303, as shown in Fig. 8C) is formed by circuit wiring and pressing as shown in Fig. 11, where all of points a, b, c and d include both discontinuous layers and upper and lower copper foil layers.

2. A silver adhesive is applied to points b and c where the pyroelectric thermal sensor elements Al and A2 are arranged, and to points e, f and g where the junction field effect transistor JFET and the load resistor RL are arranged, with the silver adhesive being applied to the interrupted PCB layers at points b and c.

3. Both the pyroelectric thermal sensor elements Al and A2 and the junction field effect transistor JFET and then the load resistor RL are bonded at positions shown in the drawings and then cured by heating in an oven.

4. Then, the silver adhesive is applied to points a and d and cured by heating in an oven so that the copper layers Pl and P2 on the discontinuous layer are electrically connected to the pyroelectric heat sensing elements Al and A2. This step avoids the difficulty of welding wires on the inclined surface.

5. Then the wires are welded to complete the required wiring circuit.

6. The base plate 301 is held high above a pin of a TO metal base, aligned with the through holes 302 and then welded in place.

7. The 306 metal cover and the metal base are sealed and welded, and the product is completed.

From the above description, it is apparent that by adhering the thermal sensor elements Al and A2 to the recess 305 in the base plate 301 in an inclined and suspended manner, the planar mounting structure can increase the observation angle without requiring more material or increasing the dimensions.

The structure of the present invention has been described with reference to a preferred embodiment of a pyroelectric infrared heat sensor device. It will be understood that many different changes and modifications may be made by those skilled in the art without departing from the scope defined in the appended claims. For example, the pyroelectric infrared heat sensor elements A1 and A2 may be replaced by any heat-sensitive resistive layer elements (bolometers) of similar size or other similar elements having similar heat-inducing functions.

Claims (7)

K 30 465 Protection claims 1. Level mounting device for pyroelectric infrared heat sensor elements (Al, A2), with a circuit board (301) on which two pyroelectric heat sensor elements (Al, A2) and associated electronic components (JFET, RL) are mounted, and a cover (306) containing a window (307) and an infrared filtering plate (308), wherein the circuit board (301) has been subjected to press molding in advance to form a recess (305) at a portion to which the pyroelectric heat sensor elements (Al, A2) are attached, so that the pyroelectric heat sensor elements (Al, A2) bridge the recess (305),
characterized in that the recess (305) is shaped such that the pyroelectric heat sensor elements (Al, A2) can be arranged obliquely in the recess (305) and formed as a suspended structure and the angle of inclination of the pyroelectric heat sensor elements (Al, A2) can be adjusted as desired. 2. Planar mounting device for pyroelectric infrared heat sensor elements according to claim 1, characterized in that the angle of inclination is in the range of 5° to 45° and preferably in the range of 10° to 30°. 3. Planar mounting device for pyroelectric infrared heat sensor elements according to claim 1, characterized in that several through holes (302) are provided on the circuit board (301) through which externally connected pins (303) can be guided and secured by riveting. 4. Planar mounting device for pyroelectric infrared heat sensor elements according to claim 1, characterized in that the circuit board (301) is a ceramic plate, wherein the recess (305) with a special shape is formed in advance by sintering. 5. Planar mounting device for pyroelectric infrared heat sensor elements according to claim 1, characterized in that the cover (306) is manufactured by stamping a metal housing or by die casting from plastic and then coating the surface with metal so that it provides insulation against electromagnetic noise. 6. Planar mounting device for pyroelectric infrared heat sensor elements according to claim 1, characterized in that the circuit board (301) is suspended from a conventional TO metal base in such a way that external leads of the board (301) and protruding pins on the metal base can be aligned and welded together, and the metal cover (306) and the metal base are sealed and welded to form the final product. 7. Planar mounting device for pyroelectric infrared heat sensor elements according to claim 1, characterized in that the pyroelectric infrared heat sensor elements {Al, A2) can be replaced by infrared heat sensor layer elements made of heat-sensitive resistance materials.