Method of adjusting resistance value of thermistor

© Start of patent duration: 1982 04 15 Longest possible duration:

Issued on: 1982 12 10

Inventor:

© dependence:

© Pamphlets considered to delineate the prior art:

DE-OS 1665384 OE-OS 1690237 DE-OS 1804713 OE-OS 2437312

- 2 No. 3669186

The invention relates to a method for adjusting the resistance value of a thermistor from a plate-shaped resistor body of a thermistor semiconductor material, on which at least two electrode surfaces are applied, by partially removing one of the electrode surfaces.

A thermistor is a semiconductor, which is usually made of a base of ceramic-like material and a metal oxide. The main body is usually made of a sintered mixture of manganese oxide, nickel oxide, iron oxide, magnesium chromate, zinc chromate or the like. manufactured. In a thermistor, the resistance properties of a semiconductor are exploited. The resistance of thermistors has a high negative temperature coefficient, ie the resistance of the thermistor drops with increasing temperature. A thermistor is connected to an electrical circuit that utilizes the resistance of the thermistor. In order to connect the thermistor electrically, it is formed with contacts. These contacts can be of various kinds. They may be formed, for example, by contact surfaces or buttons on the surface of the thermistor. Further, they may be formed by non-insulated metal wires which are passed through the thermistor and abut against the ceramic material. Furthermore, they may be formed by conductors soldered to or otherwise connected to the thermistor.

The ceramic base body of thermistors can be designed in many different ways. A typical thermistor has the shape of a pearl. He can by molding or dividing a rod or the like. be made. Another typical thermistor form is a plate. Such a plate has six surfaces, of which two main surfaces face each other and four side surfaces, which also face each other. A plate-shaped thermistor can eg be cut out of a blackboard or other body of thermistor material. Furthermore, a thermistor may be made by melting. The ceramic material of the thermistor can be made in almost any size by molding or cutting. For cutting, grinding, or otherwise molding thermistor bodies to particular sizes, numerous different types of known techniques are applicable.

The resistance of a thermistor is influenced by the volume of the semiconductor material of which it consists. When the thickness of the semiconductor material between the contacts or electrodes of a given thermistor is reduced, its resistance increases. More meaningful, however, is the fact that as the thickness of the thermistor material decreases, the change in its resistance increases with any change in temperature. If a very precise setting of the thermistor is required in a particular case, it is advantageous to choose the strength of the thermistor as small as possible. This has led to the production of bead or plate thermistors with very small dimensions. For a typical die thermistor, the thickness of the semiconductor is about 0.010 mm and the larger faces have dimensions of 0.060-0.060 mm.

One way to balance the resistance of thermistors is to remove semiconductor material between the electrodes or contacts. Since the sections of material of thermistors normally constitute a mass product which is manufactured unitarily, the removal of a portion of the semiconductor material from a single thermistor requires a special technical effort. Another size that determines the resistance of a thermistor is the contact surface of the electrical contacts on the thermistor. The resistance of a thermistor at constant temperature and pressure can be expressed by the following equation:

et

R = ·. If e is the resistivity of the semiconductor material, t is the thickness of the semiconductor material at the distance of the shortest distance between its two contacts, and A is the area of ​​contact between the electrodes and the semiconductor material involved in the current flow through the thermistor. If the contacts of the thermistor consist of non-insulated conductor pieces that have passed through the thermistor, the surface of the contacts that touch the surface of the thermistor material is predetermined and can not be changed. Thus, the resistance of such a thermistor can not be changed by changing the sizes of the contact surfaces with the thermistor material.

- 3 - No.369186

In a thermistor in which the electrodes are applied to the surface of the material, the resistance of the thermistor can be changed by changing the size of the contact surfaces. It was found that in a thermistor, the only two electrodes from eg Ag or Cu deposited on opposing surfaces of the thermistor, as the contact area of ​​one or both of the electrodes decreases, the resistance of the thermistor increases by the maximum percentage reduction in the surface area of ​​one of the contacts. If, for example, the surface of one of the two electrodes is reduced by 4%, the resistance of the thermistor also increases by 4%. Is eg the thermistor designed for 5000 n, it has a resistance of 5200 n on this.

As mentioned above, thermistors usually have very small dimensions. Thus, the contact surfaces of the electrodes are very small. For this reason, accurate trimming of for example 1% or only a fraction of 1% is extremely difficult.

Numerous methods of trimming contacts of thermistors are known. So contacts can be filed, sanded or sanded off. Thermistors are so small and the changes in resistance that may be required can be so small that even by means of a single, light rubbing on a contact by means of a slightly roughened surface can be sufficiently removed from the contact to the resistance of the thermistor in the required To reduce the extent. Such techniques, which are done by hand to trim thermistor contacts, however, are very expensive and can make the manufacture of thermistors and in particular the setting of their resistances very expensive and expensive. For this reason, a method has already been developed using laser technology, by which alone or in combination with the fine grinding by means of a collimated laser beam, a piece of electrode contact is burned away.

All these methods of trimming a thermistor contact, such as fine grinding, laser beam trimming, etc., operate within existing tolerance limits. This means that by a certain trim method, either too little or too much can be removed from a contact. This again leads to differences between the actual values ​​and the setpoints of the thermistor. Thus, there is a need to provide a method whereby a smaller percentage of the resistance change of the thermistor is effected upon removal of a greater percentage of the surface area of ​​a contact. In such a method, an error in the change of the contacts would have a smaller effect on the resistance value of the thermistor, as is the case with currently applied trim method.

There are known thermistors, which are formed with three electrodes, wherein the third contact is larger than the other two. In a plate-shaped thermistor, the two smaller contacts lie on one major surface of the semiconductor material and the third contact extends substantially over the remaining surfaces of the semiconductor material. Such a thermistor has two different resistance values, depending on which two of the three electrodes are connected to the electrical circuit. This is due to the fact that the resistance value is influenced by the contact surfaces of the electrodes and the distance between the electrode surfaces and the thickness of the semiconductor material located therebetween.

The invention is therefore based on the object to provide an improved method for balancing the resistance of a thermistor. In particular, such a method is to be provided by which a relatively large part of the contact surface of the thermistor can be changed, thereby causing a relatively smaller change in the resistance value of the thermistor. Furthermore, the method according to the invention should be applicable to thermistors with particularly small dimensions. Finally, a very accurate comparison should be possible. These objects are achieved according to the invention in that on one of the two major major surfaces of the plate-shaped resistor body, the two first small electrode surfaces separated from each other and on the second major surface a third large electrode surface, which as Zusatzelektrodcnfläche, seen in plan view, overlaps the first two small electrode surfaces . be applied and that in order to achieve the desired resistance value, a part of the one of the three electrode surfaces is removed from the resistor body.

Preferably, the third large electrode surface is partially removed. After another

4 No. 366186 preferred method is soldered to the two small electrode surfaces in each case an electrical connector. In particular, the removal of one of the three electrode surfaces can take place during the measurement of the resistance value and its comparison with a calibration value. Finally, in particular, the partial removal of one of the three electrode surfaces may be carried out according to a relationship given and explained below.

Due to the mathematical relationship explained below, eliminating the area by an amount x of one of these three contacts, preferably the largest contact, increases the resistance of the thermistor by only a fraction of this value x. If, for example, 10% of the area of ​​a contact is removed, the value of the resistance of the thermistor increases by only 1.8%. Thus, for example, 11% of the area of ​​the contact instead of 10%, so this has a much smaller influence on the change in resistance of the thermistor, as if the same mistake would be made under a known method, since in this one

Error of 1% causes a change of 1% of the resistance value of the thermistor.

The inventive method is explained below with reference to the drawings. FIG. 1 shows a thermistor in end view, FIG. 2 shows the thermistor according to FIG. 1 being aligned, in plan view, FIG. 3 shows the thermistor in a view from below, FIG. 4 shows the thermistor in a perspective view. The thermistor is mounted on a base, and connected to the calibration to a circle, the Fig.5, 6 and 7 views of different thermistors and the

7a and 7b, the thermistor according to Figure 7 in a schematic representation.

The thermistor -10- according to FIGS. 1 to 3 consists of a sintered, metal-oxide, ceramic semiconductor body -12-, which has a square shape with an upper major surface -14- and a lower major surface -16-. On the entire upper main surface -14- a metallic electrode -20- is applied. This electrode -20- consists of a mixture of silver and glass grains. This mixture was applied to the surface of the ceramic

Semiconductor material melted on.

On the lower surface -16- of the ceramic body -12-, two electrodes -22 and 24- are arranged, which consist of the same material as the electrode -20-. The electrodes -22 and 24- were applied in the same way as the electrode -20- as a single layer covering the entire area -16-. However, to form separate electrodes 30-22 and 24-, this surface was divided by cutting, grinding or filing to form a gap -26- which is free of contact material. To achieve exact dimensions, this gap -26- can be produced by means of laser technology. An accurate gap width is therefore necessary for the resistance of the thermistor to be constant over the entire temperature range to which the thermistor is exposed. The gap 26 is formed so that the electrodes 22 and 24 have approximately equal contact surfaces with the ceramic body 12. An area equality is not critical, as can be seen from the formula for the thermistor resistance, which is given below. The thermistor -10- is turned on by means of metal conductors -30, 32- which are connected to the electrodes -22, 24- in an electrical circuit. Unless the resistance of the thermistor 12 is measured 40 and found to be too small, a portion of its electrode surfaces, preferably a portion of the third electrode surface -20- is removed, whereby its resistance is increased. As shown in Figs. 1 and 2, a corner portion -36- of the electrode surface -20- is removed. According to a modified method, the electrode surface -20- does not have to extend over the entire surface -14- and the electrode surfaces -22 and 24-can differ

Show sizes.

Such a thermistor can be used in a thermometer as a temperature-dependent component.

FIG. 4 shows a method for dimensioning a thermistor and a device used to dimension the thermistor. The thermistor -10- is adjusted to a desired resistance by removing 50 of a portion of the surface of the electrode area -20-. The resistance value has thereby been increased. Since it is not possible to reduce the resistance value of the thermistor, it is necessary for the electrode area -20- to be slightly larger than required for a certain resistance value.

Nr.369186

The thermistor -10- should have a certain resistance at given values ​​of temperature, humidity and other environmental conditions. The resistance is measured using a standard resistor. Thereafter, the electrode surface -20- is changed so that the resistance value of the thermistor -10- to the standard resistance assumes a predetermined ratio. Thereafter, the thermistor -10-, as shown in Fig.l is placed on the guide -30 and 32-. The conductors -30, 32- are foil strips made of metal, which are attached, for example glued, to a non-conductive, elongate support -40-. The pad and conductors -30 and 32- terminate together at end surface -42- of pad -40-. The end areas -44 and 46- of the ladder are provided with push-in terminals. The upper ones

Surfaces of the film conductors are formed with a thin solder layer for attachment to contacts -22 and 24-.

The pad -40- is then cut to form a strip -47- between the conductors -30 and 32-. The strip is raised to form a gap between it and the rest of the backing -40-. In this gap, the thermistor -10- is inserted, the contacts -22 and 24- come to the respective conductors -30 and 32- come. Then the strip -47- is released. The base consists of a flexible plastic material, which has a so-called memory, such as the material Mylar. Since the stripe -47- tends to return to its initial state, it thus holds the thermistor.

The thermistor is so much heat supplied that the solder layer is melted, so that between the electrode surfaces -22 and 24- and the conductors -30 and 32- a mechanical and electrical connection is made. The solder layer has a sufficiently low melting point to preclude the thermistor from being damaged by the heat. It may be convenient to cover the thermistor, pad and conductors to protect them.

The gap -26- between contacts -22 and 24- can be made before thermistor -10- is applied to conductors -30 and 32-. The entire pad is a simple means of holding the thermistor and working with the thermistor. Since a thermistor is very small, care must be taken to have an effective holding device available. It is also possible to form the gap only when the thermistor has been mounted on the substrate, eg by guiding a laser beam longitudinally along the center line of the substrate 40 at the level of the metal layer from which the electrode surfaces are formed. 22 and 24- are formed.

In the measuring arrangement, a first potentiometer -50- is provided, through which the resistance of an electrically connected to this object is measurable, which is readable from a digital display -52-. The conductors -54 and 56 coming from the potentiometer -50- are connected to connectors -58 and 60- inside a hollow clamping bush -62-. The opening leading into the clamping bush 62 is adapted to securely receive both the base 40 and the conductor ends 44 and 46 and electrically connect them to the associated plugs 58 and 60. A spring provided in the plug can additionally hold the terminals together. In this way, the thermistor -10- is connected via its electrode surfaces -22 and 24- to the potentiometer -50-. When the potentiometer -50- is put into operation, it indicates the resistance of the thermistor -10-.

As can also be seen from Figure 4, the measuring arrangement includes a further plate-shaped thermistor -70-, whose resistance has the value to which the thermistor -10- to be set. This calibration thermistor should exactly match the thermistor -10- to be set, since changes in the measurement conditions could affect different thermistors differently. The conductors -72 and 74- on their support -75- are connected to the same contacts of the thermistor -70-. They are also connected to a second, conventional potentiometer -80- whose digital display -82- indicates the resistance of the thermistor -70-.

The thermistor -10- and its associated thermistor -70- are inserted into a chamber -84-. The most important feature of the Chamber is that

No.369186 are the same in terms of temperature, pressure, humidity, air condition, etc. for both thermistors -10 and 70-.

In the measuring arrangement shown in Figure 4, the thermistor -10- is brought to its value to a value of 4910 ß, whereas the thermistor -70- contains a value of 5000 ß. The resistance of the thermistor -10- is thus 1.8% less than the resistance of the thermistor -70-.

Thereafter, the electrode surface -20- of the thermistor -10- is changed by a part of the contact surface is removed, for example by forming a cut -36. To increase the resistance of the thermistor -10- by about 1.8% to 5000 ß, 10% of the contact area -20- must be removed. For this purpose, a laser tube -90- is inserted into the chamber and aligned so that its collimated light beam impinges on a corner of the electrode surface -20-. Thereafter, the laser tube is moved and kept in operation until the laser beam has burned away that amount of contact material as required for the desired resistance value. In practice, a precise measurement of the area of ​​the electrode surface -20- and the portion to be removed from this is not necessary, since the resistance values ​​of the thermistors -10 and 70-, while the surface of the contact surface -20- processed, are continuously measured, and the processing is terminated when the resistance values ​​of the two thermistors -10 and 70- match.

By machining the contact surfaces by abrasion or by laser technology, the temperature of the thermistor -10- may increase slightly. As soon as the processing is finished, the temperature of the thermistor immediately returns to the value prevailing in the chamber -84-.

The following is an explanation for the change in the resistance of a thermistor by removing a part of the electrode surface:

In Fig.5 a conventional two-contact thermistor -100- is shown, which has a length L and a width W and which is provided on its underside and on its upper side with contacts -101 and 102- with the same dimensions. This thermistor has the structure and operation of a capacitor. The resistance of thermistor -100- is determined by the following formula:

r = -21A

The symbols used have the following meanings at normal temperature of 25 ° C. and normal pressure of 1 bar:

R .... resistance;

e. resistivity of the semiconductor material (a feature of the particular material and the prevailing temperature and pressure);

t .... thickness of the thermistor, ie the distance between the contacts -101 and 102-;

A .... surface of the overlapping areas of the contact surfaces -101 and 102-.

The overlapping area is the area in which they overlap in plan view of the two contacts. In Figure 5, the two contact surfaces -101 and 102- have the same size and are also superimposed, where A = LW. For example, if -102-10% of the contact area is removed from the pad, contacts -101 and 102- would overlap only 90% of the area. The basic form shows that the resistance of the thermistor -100- would decrease by 10%. The same change would also occur if both contact surfaces -101 and 102- were reduced by 10% of their surface area.

Fig. 6 shows another type of plate thermistor -103-. In this case, the two electrode surfaces -104 and 105- are on the same surface -106- of the body -107- of semiconductor material. In the case of a thin wafer -107- made of semiconductor material, the same basic formula is applicable again: R = e. However, as also shown in Fig. 6, with a thin plate -A-, the range of the thickness dimension of the body -107- is along the side -109- with a contact -105- extending along its edge,

No.369186 and t is the width of the distance -110- between contacts -104 and 105-. A depends on the length L of contacts -104 and 105- along -109-. At A, only the length L over which the contacts extend is considered. If one contact -104 or 105- has a smaller length L than the other, the shorter length is that which enters the calculation A. It should be noted here that the respective widths of the contacts -104 and 105- have no influence on R, the arrangement of the gap -110- being insignificant, but the width of which is of great importance. In order to change the resistance of the thermistor -103-, the length L of one or both contacts -104 and 105- is correspondingly reduced. If the length L is reduced by 10%, the value R increases by 10% according to the formula.

In Fig.7 a thermistor -120-- is shown. It consists of a body -122--

Semiconductor material, a contact -124-, which extends over the entire surface and on the opposite side of two separated by a gap contacts -126 and 128-.

The values ​​shown in FIG. 7 show the dimensions of an exemplary embodiment of such a thermistor.

From Figure 7a it can be seen that the thermistor -120- has three different values ​​for R and t between the three different pairs of the contact combination. From Fig. 7b it can be seen that the values ​​R of the thermistor -120- series resistors -Rj and R₂ - are to the resistance -R₃- lie in parallel. The resistance of the thermistor -120- can be calculated in the following way:

^R! = θ (1000 0.01) (0.028, 0.06)

5950 n

Herein A means₁overlap.

the smallest surface L χ W over which the contact surfaces -124, 126- _L = (looo o.oi)

A, (0.028-0.06) = 5950β

Herein A means₂ the smallest area L χ W, over which the contact surfaces -124, 128- overlap.

R = e. -l³- = ⁽l-000- · 0.004), .. _{= 6670 a}

A₃ (0.01, 0.06)

Herein A means₃ the surface -129-, which has been explained with reference to Figure 6.

The resistance of the circuit diagram shown in Fig. 7b results therefrom as follows:

(R + RJR 11900, 6670

R. = -: --- = - = 4270 °^ta R₁+ R₂+ R₃ 18570

If the contact surface -124- of the thermistor removes -120- 10%, then Rj is changed. This trimming of the contact surface -124- may be accomplished by the laser technique, wherein a portion of the contact -124- or the entire side edge of the thermistor including the body of semiconductor material, for example, by grinding a wedge-shaped portion, 35 of the conductor -124- comprises , or by grinding a rectangular section, which detects the two conductors -124 and 126-, can be removed. In any case, the area Aj becomes • '· · t₂

10% decreases and becomes the resistance -Rj- according to the formula Rj = e. - increased by 10%. Thus, 110% of R_J = 6545 ß.

Nr.369186

R, total (6545 + 5950). 6670

- 4349 a

6545 + 5950 + 6670

The change of R_total on ^Rtota /^New^ ^concerningis 79 n.

a is 1.85% of the initial value of 4270 a of the thermistor -120-, with a change of 10% of the surface of a contact of the thermistor -120- causes only 1.85% change in its resistance.

It is to be understood that the above formulas are based on the assumption that a thin wafer of semiconductor material is used, neglecting any fraying. Fraying means losses due to the thickness of the semiconductor material and due to the fact that the electromagnetic flux from the direct path spills between the two contacts -126, 128-.

In a study carried out on a thermistor adjusted according to the invention, an increase in the resistance of 2% was found with a reduction of 10% of the contact area -124-. This discrepancy of 0.015% from the theoretical change in resistance may have been due to the change in normal environmental conditions and so on. However, this difference is insignificant in the context of determining or sizing the thermister value, especially when the method explained with reference to Figure 4 is used, in which the resistance of the thermistor is constantly monitored.