Chip resistor and manufacturing method thereof

A chip resistor (A1) includes a chip-like resistor element (1), two electrodes (31) spaced from each other on the bottom surface (1a) of the resistor element, and an insulation film (21) between the two electrodes. Each electrode (31) has an overlapping portion (31c) which overlaps the insulation film (21) as viewed in the vertical direction.

FIELD OF THE INVENTION

The present invention relates to a chip resistor and a method of making the same.

BACKGROUND ART

FIG. 15of the present application shows a chip resistor disclosed in Patent Document 1 below. The disclosed chip resistor B includes a metal resistor element90and a pair of electrodes91fixed to the bottom surface90aof the resistor element. The electrodes91are spaced from each other by a predetermined distance s5. Each of the electrodes91has its lower surface formed with a solder layer92.

When the size of the resistor element90is unchanged, the resistance of the chip resistor B is in proportion to the distance s5between the electrodes91. Thus, the resistance of the chip resistor B is changed by varying the distance s5. As understood fromFIG. 15, to increase the distance s5decreases the width s6of each electrode91, and to decrease the distance s5increases the width s6.

As described above, in the conventional chip resistor B, the change of the distance s5affects the width s6, which gives rise to the following problem.

In use, the chip resistor B is soldered to a circuit board, for example. At this stage, each electrode91of the resistor B should be properly bonded, electrically and mechanically, to the relevant connection terminal formed on the circuit board. To achieve this, the size of the connection terminal matches the size of the electrode91. With the conventional design described above, however, the size of the connection terminal needs to be changed every time the resistance of the chip resistor B is changed. Unfavorably, this lowers the productivity of circuit boards and increases the production costs.

DISCLOSURE OF THE INVENTION

The present invention has been proposed under the circumstances described above. It is an object of the present invention to provide a chip resistor whose electrode size remain unchanged even when its resistance is varied. Another object of the present invention is to provide a method of making such a chip resistor efficiently and appropriately.

A chip resistor provided by a first aspect of the present invention includes: a chip-like resistor element which has a bottom surface, an upper surface opposite to the bottom surface, two end surfaces and two side surfaces; two electrodes spaced from each other on the bottom surface of the resistor element; and an insulator between the two electrodes. At least one of the two electrodes overlaps the insulator as viewed in a direction in which the bottom surface and the upper surface are spaced from each other.

Preferably, the insulator is provided by a resin film which is flat as a whole, and the above-mentioned at least one of the electrodes includes an overlapping portion extending onto the resin film. Alternatively, the insulator includes a first portion between the two electrodes, and a second portion formed integral with the first portion, and the second portion extends on the above-mentioned at least one of the electrodes.

Preferably, the chip resistor further includes a soldering-facilitation layer which covers the end surfaces of the resistor element and the electrodes.

Preferably, the chip resistor further includes an additional insulation film formed on the upper surface of the resistor element, and two auxiliary electrodes spaced from each other via the additional insulation film.

A method of making a chip resistor provided by a second aspect of the present invention includes the steps of: patterning an insulation film on a surface of a metal resistor element; forming a conductive layer on the surface of the resistor element to extend on both the insulation film and a region at which the insulation film is not present; and dividing the resistor element into a plurality of chips so that part of the conductive layer is formed into a pair of electrodes spaced from each other via part of the insulation film.

Preferably, the resistor element is either a metal plate or a metal bar.

Preferably, the step of forming a conductive layer includes: a printing process of forming a first conductive layer extending on both the insulation film and the region at which the insulation film is not present; and a plating process of forming a second conductive layer on the first conductive layer.

Preferably, the patterning of the insulation film is performed by thick-film printing.

A method of making a chip resistor according to a third aspect of the present invention includes the steps of: patterning a first insulation film on a surface of a metal resistor element; forming a conductive layer on a region of the surface of the resistor element in which the insulation film is not present; patterning a second insulation film on the surface of the resistor element so that the second film extends on both the first insulation film and the conductive layer; and dividing the resistor element into a plurality of chips so that part of the conductive layer is formed into a pair of electrodes spaced from each other via part of the first insulation film.

Preferably, the patterning of the first insulation film and the second insulation film is performed by thick-film printing.

Preferably, the conductive layer is formed by plating.

Other characteristics and advantages of the present invention will become clearer from the following detailed description to be made with reference to the attached drawings.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1throughFIG. 4show a chip resistor according to a first embodiment of the present invention. The chip resistor A1includes a resistor element1, insulation films21-23, a pair of lower electrodes31, a pair of upper electrodes (auxiliary electrodes)33, and a pair of plated layers4(not illustrated inFIG. 4) to facilitate soldering. The chip resistor A1has a low resistance of 0.5 mΩ˜100 mΩ for example. It should be noted, however, that this range of resistance is nothing more than an example, and the scope of the present invention is not limited to resistors which have such a low resistance.

The resistor element1is a chip which has a uniform thickness and a rectangular plan view, and as shown inFIG. 2orFIG. 3, has a bottom surface1a, an upper surface1b, two end surfaces1c(spaced from each other in the direction X) and two side surfaces1d(longitudinal in the direction X). The resistor element1is made of a Ni—Cu alloy or a Cu—Mn alloy for example. It should be noted that the present invention is not limited by these examples. The resistor element1may be made of other materials which have an appropriate resistivity for a target resistance.

Each of the insulation films21-23is made of an epoxy resin for example. The insulation film21covers a region between the two lower electrodes31on the bottom surface1aof the resistor element1. The insulation film22covers a region between the two auxiliary electrodes33on the upper surface1bof the resistor element1. The insulation film23covers all of the side surfaces1dof the resistor element1.

The lower electrodes31are formed on the bottom surface1aof the resistor element1, spaced from each other in the direction X. As shown inFIG. 2, each of the electrodes31has a two-layer structure consisting of a first conductive layer31A and a second conductive layer31B formed on the first layer. As understood fromFIG. 2andFIG. 4, each electrode31covers part of the bottom surface1aof the resistor element1(the region not covered by the insulation film21) and part of the insulation film21. A portion of each electrode31which overlaps the insulation film21will hereinafter be called “overlapping portion (indicated by a sign31c)”. InFIG. 4, hatched areas are the overlapping portions31c.

The auxiliary electrodes33are spaced from each other on the upper surface1bof the resistor element1, with the insulation film22in between. The auxiliary electrodes33are made of the same material as that of the second conductive layer31B of the lower electrode31, and are formed by e.g. copper plating.

As shown inFIG. 2, the plated layers4cover the lower electrodes31, the auxiliary electrodes33and the end surfaces1cof the resistor element1, as an integrally formed layer. The plated layers4are made of e.g. Sn, and may be made of other materials.

The resistor element1has a thickness of e.g. 0.1 mm through 1 mm. The lower electrodes31and the auxiliary electrodes33have a thickness of e.g. 30 through 100 μm. Each of the insulation films21-23has a thickness of e.g. 20 μm, and the plated layers4have a thickness of e.g. 5 μm. The resistor element1has a length and a width of e.g. 2 through 7 mm. Obviously, the sizes of the resistor element1are not limited to the dimensions exemplified above, and may be selected as appropriately in light of the desired resistance.

Next, a method of manufacturing the chip resistor A1will be described with reference toFIG. 5throughFIG. 8.

First, a frame from which resistor elements1are to be made is prepared.FIG. 5Ashows such a frame F prepared by e.g. punching a metal sheet of a uniform thickness. The frame F includes a plurality of bars11which extend in parallel to each other, and a rectangular support12which supports these bars11. Mutually adjacent bars11are spaced from each other by a slit13. Each bar11has two connection tabs14, each of which is formed at a longitudinal end of the bar, and connects the bar with the support12. As shown inFIG. 5B, each connection tab14has a width W1which is smaller than a width W2of the bar11. Therefore, the connection tabs14can easily be twisted to rotate the bar11about its longitudinal axis.FIG. 5Ashows an instance in which one of the bars11is rotated by 90 degrees in the direction indicated by Arrow N1. Rotating the bar11in such a way makes it easy to perform the step of forming the insulation film23(to be described later) on the side surfaces11dof the bar11.

After preparing the frame F, plural pieces of a rectangular insulation film are formed on a first surface11a(e.g. an upper surface as inFIG. 5) in each bar11and on the surface away therefrom, i.e. a second surface11b(a lower surface as inFIG. 5). Specifically, as shown inFIG. 6A, plural pieces of an insulation film21are formed on all of the first surfaces11aof the bars11so that the film pieces are spaced from each other in the longitudinal direction of the bar. Likewise, as shown inFIG. 6B, plural pieces of an insulation film22are formed on all of the second surfaces11bof the bars11so that the film pieces are spaced from each other in the longitudinal direction of the bar. Each of the insulation films21,22is formed of the same material (an epoxy resin for example) by thick-film printing. Thick-film printing methods serve to form the pieces of insulation films21,22precisely to the desired dimensions. Surfaces of the insulation films22may have printed marks and symbols indicating characteristics of the resistor.

Next, as shown inFIG. 7, plural pieces of a rectangular conductive layer31A are formed on all of the first surfaces11aof the bars11so that the film pieces are spaced from each other in the longitudinal direction of the bar. Each piece of the conductive layer31A is formed to overlap a region where there is no insulation film21formed and a region formed with an insulation film21. The region not formed with the insulation film21includes a region where the conductive layer31A is not formed yet. In this particular region which is not formed with the conductive layer, the original surface of the bar11is exposed. A plating process to be described later causes the conductive layer31B to form directly upon this particular region where there is no conductive layer, establishing the reliable bond of the conductive layer31B to the bar11. The formation process of the conductive layer31A includes a step of printing using a paste which contains a metal powder provided primarily by e.g. silver. According to such a printing technique, it is easy to form the conductive layer31A accurately to the desired dimensions.

Next, an insulation film21is formed on each of the side surfaces11dof all the bars11(SeeFIG. 8A). The formation of the insulation film23is made with the same material as used in the formation of the insulation films21,22. To form the insulation film23on the side surfaces11d,each bar11is first rotated to an attitude drawn in the phantom lines inFIG. 5A. Then, side surfaces11dare dipped in the coating liquid to apply the coating material on the side surfaces and finally, the coating material is dried on the surfaces.

Next, as shown inFIG. 8A,8B, copper-plating is performed to make a conductive layer31B′ and a conductive layer33′ on the first surface11aand the second surface11brespectively of each bar11. More specifically, the conductive layer31B′ is formed as shown inFIG. 8A, on the first surface11ato cover the above-described region where no conductive layer is formed and also to cover the conductive layer31A (SeeFIG. 7). Each region covered with the conductive layer31B′ will serve as part of an electrode31. Similarly, as shown inFIG. 8B, the conductive layer33′ is formed on the second surface11b, to cover the region where no insulation film22is formed. Each region covered with the conductive layer33′ will serve as an auxiliary electrode33.

As described above, the conductive layer31A is also formed on the insulation film21. Therefore, it is easy to form the conductive layer31B′ on the insulation film21by a plating process. By plating, the conductive layers31B′,33′ are formed simultaneously, with an improved production efficiency compared to the instance where two conductive layers31B′,33′ are formed in separate steps.

After the plating process, each bar11is cut along phantom lines C1as shown inFIGS. 8A,8B into individual chip resistors A1′. The phantom lines C1are perpendicular to the longitudinal direction of the bar11. Further, each phantom line C1divides pieces covered with the conductive layer33′ equally into two halves. Therefore, each resistor A1′ thus obtained includes a pair of lower electrodes31and a pair of auxiliary electrodes33. Since a single frame F produces a plurality of chip resistors A1′, the method is highly productive.

Next, a plated layer4is formed on each end surface1cof the resistor element1in the chip resistor A1′, as well as surfaces of each electrode31and surfaces of each auxiliary electrode33. Formation of the plated layers4are performed by barrel plating for example. In the barrel plating, a plurality of chip resistors A1′ are placed in a single barrel. Each chip resistor A1′ has exposed metal surfaces in each end surface1cof the resistor element1, the surface of each electrode31and the surface of each auxiliary electrode33, while all of the other portions are covered with the insulation films21through23. Therefore, it is possible to form the plated layers4efficiently and appropriately only on the metal surfaces described above. Before the formation of plated layers4, formation of a protective film provided by e.g. Ni may be performed on the metal surfaces, as an under coating for the plated layers4. Formation of such protection films is preferred since it provides anti-oxidation barriers for the electrodes31and the auxiliary electrodes33. The formation of protective films can also be made by barrel plating. The sequence of steps so far described above enables efficient manufacture of the chip resistors A1inFIG. 1throughFIG. 4.

In use, chip resistors A1are surface-mounted onto a circuit board by a solder re-flow process for example. In the solder reflowing, the chip resistors A1are placed in alignment with the electrically conductive terminals31which are formed on the circuit board, and then the substrate and the resistors A1are heated together in a reflow furnace.

The functions of the chip resistor A1will be described below.

As shown inFIG. 2, in the above-described chip resistor A1, the overlapping portion31cof each lower electrode31rides on the insulation film21. More specifically, when viewed in a manner such that the line of sight extends in parallel to the vertical direction (in which the bottom surface1aand the upper surface1bare spaced from each other) (or simply “when viewed in the vertical direction”), each lower electrode31and the insulation film21at least partially overlap with each other. For the left-hand-side electrode31, the overlapping portion31cextends to the right, from a region (“left-hand-side contact region”) where the left-hand-side electrode31makes direct contact with the resistor element1. Likewise, for the right-hand-side electrode31, the overlapping portion31cextends to the left, from a region (“right-hand-side contact region”) where the right-hand-side electrode31makes direct contact with the resistor element1.

According to the above arrangement, the resistance of the chip resistor A1is determined, not by the shortest distance between the two lower electrodes31(i.e. the distance between the two overlapping portions31c), but by the shortest distance between the left-hand-side contact region and the right-hand-side contact region (“resistance determining distance”). On the other hand, according to the manufacturing method which has been described with reference toFIG. 5throughFIG. 8, the resistance determining distance is equal to a dimension s1of the insulation film21. This means that by varying the dimension s1of the insulation film21, it is possible to vary the resistance determining distance, thereby varying the resistance of the chip resistor A1, without changing the dimension s2of each lower electrode31.

As described above, there is no need in the chip resistor A1to change the dimension s2of the lower electrode31for changing the resistance. Therefore, the size of connection terminals on the circuit board does not need to be changed even when there is a change, for example, in the electric circuit specifications which requires a change in the resistance of the chip resistor A1to be mounted on the circuit board. Further, when a plurality of chip resistors A1of different resistances are to be mounted on a single circuit board, all the connection terminals for the resistors A1can be of the same size.

According to the chip resistor A1, the dimension s1of the insulation film21can be varied over a wider range if a greater initial value is given to the dimension s2of each lower electrode31, resulting in a wider adjustment range of the resistance of resistor A1. Also, the greater the dimension s2of the electrode31, the more efficient heat radiation will be achieved from the electrically heated resistor element1through the electrode31. Further, the greater the dimension s2of the electrode31, the greater the area of solder bonding in the electrode31, leading to increased bonding strength to the circuit board.

The chip resistor A1also has the following technical advantages. Specifically, when solder reflowing is used to mount the resistor A1on a circuit board, the plated layers4will melt. As described above, the plated layer4is formed on the end surfaces1cof the resistor element1and on the auxiliary electrodes33. Thus, the solder reflowing will form solder fillets Hf as shown in phantom lines inFIG. 1. Therefore, simple visual inspection to the shape of solder fillets Hf will tell whether the chip resistor A1is appropriately mounted or not. In addition, formation of the solder fillets Hf helps increase bonding strength of the chip resistor A1to the circuit board.

The pair of auxiliary electrodes33serve to release the heat generated by the electricity which passes through the resistor element1, increasing heat radiation effect. In addition, the auxiliary electrodes33may be used as follows. The pair of electrodes31is used for supplying electric current whereas the pair of auxiliary electrodes33is used for voltage measurement. When detecting an electric current in the circuit, a resistor A1(whose resistance is given) is connected in series to the circuit via a pair of current supplying electrodes (electrodes31), whereas a pair of voltage measurement electrodes (auxiliary electrodes33) are connected with a voltmeter. Under such a configuration, voltage drop in the resistor element1of the chip resistor A1is measured with the voltmeter. From the measured voltage value and the known resistance of the resistor A1, the value of electric current which passes through the resistor element1can be obtained by using the Ohm's Law.

Since the insulation film21is formed by thick-film printing, highly accurate formation to predetermined target sizes is possible. This enables to decrease errors in setting the resistance which is dependent on the accuracy of the dimension s1of the insulation film21.

FIG. 9andFIG. 10show a chip resistor A2according to a second embodiment of the present invention. It should be noted that in the following embodiments, elements which are identical or similar to those in the first embodiment will be indicated by the same reference signs.

The chip resistor A2includes a resistor element1, insulation films21-23, a pair of lower electrodes32, a pair of auxiliary electrodes33and a pair of plated layers4. The lower electrodes32are spaced from each other by a predetermined distance (“resistance determining distance”). Each electrode32covers a region not formed with the insulation film21in a bottom surface1of the resistor element1, so as not to ride on the insulation film21. The insulation film21consists of a first insulation layer21A and a second insulation layer21B which is formed on the first insulation layer. The first and the second insulation layers21A,21B are formed of the same resin material as will be described later, so the insulation film21can be considered as a single element. As shown inFIG. 9, the first insulation layer21A is formed between the lower electrodes32. The second insulation layer21B has overlapping portions21cpartially masking both the electrodes32. Thus, when viewed in the vertical direction, the insulation film21at least partially overlaps with each of the electrodes32.

A method of manufacturing the chip resistor A2will be described with reference toFIG. 11throughFIG. 13.

First, a frame F which is like the one as used in the first embodiment is prepared. Next, as shown inFIGS. 11A and 11B, a plurality of rectangular pieces of an insulation layer21A (FIG. 11A) and a plurality of rectangular pieces of an insulation film22(FIG. 11B) are formed on a first surface11aand on a second surface11bin each bar11. The insulation layer21A and the insulation film22is made of the same material such as epoxy resin applied by a thick-film printing method. Advantageously, thick-film printing makes it possible to form the insulation layer21A and the insulation film22precisely to the desired width and thickness.

Then, an insulation film23is formed on all the side surfaces11dof each bar11. The insulation film23is made of the same material as that used for making the insulation layer21A and the insulation film22. The insulation film23may be formed by the same method as used in the formation of the insulation film23in the embodiment 1.

Next, as shown inFIGS. 12A and 12B, plural pieces of a conductive layer31B′ and a plural pieces of a conductive layer33′ are formed (each indicated by cross-hatching) on the first surface11aand the second surface11bof each bar11where the insulation layer21A and the insulation film22are not present. Each region on the first surface11acovered by the conductive layer32′ will provide a lower electrode32and each region on the second surface11bcovered by the conductive layer33′ will provide an auxiliary electrode33. The conductive layers32′,33′ may be formed by copper plating for example.

As shown inFIG. 13A, plural pieces of a second insulation layers21B which are rectangular are formed on the first surface of each bar11. Each piece of the second insulation layer21B covers a piece of the first insulation layer21A, while also overlapping the two abutting conductive layers32′ on both sides. The formation of the second insulation layer21B is made by thick-film printing using the same material as that used for the first insulation layer21A and the insulation films22,23. After the formation of the second insulation layer21B, each bar11is cut as shown inFIGS. 13A and 13Binto individual chip resistors A2′. In this cutting process, each bar11is cut at phantom lines C2so that each resulting piece contains the first and the second insulation layers21A,21B abutted by parts of the conductive layer32′ from both sides. Each phantom line C2divides a set of the conductive layers32′,33′ into two equal halves in a direction perpendicular to the longitudinal direction of the bars11. In this process therefore, the chip resistor A2′ is formed with a pair of lower electrodes32and a pair of auxiliary electrodes33. Then, a plated layer4is formed by barrel plating process, on each end surface1cof the chip resistor A2′, surfaces of each lower electrode32and surfaces of each auxiliary electrode33. According to the above-described steps, efficient production of the chip resistor A2shown inFIGS. 9 and 10is possible.

Next, functions of the chip resistor A2will be described.

As shown inFIG. 9, the resistance of the chip resistor A2is determined by a dimension s3of the first insulation layer21A. By varying the dimension s3, the resistance of the chip resistor A2can be varied. Further, according to the chip resistor A2, the second insulation layer21B has its overlapping portions21cwhich overlap the lower electrodes32. Therefore, even when the dimension s3of the insulation layer21A is changed in order to change the resistance, it is possible to maintain the dimension s4, i.e. the dimension of the exposed portion of the electrode32. Therefore, the same technical advantages as achieved by the first embodiment are enjoyed.

FIGS. 14A and 14Bshow a chip resistor A3according to a third embodiment of the present invention. As shown inFIG. 14B, the chip resistor A3is provided with four electrodes32B on a bottom surface1aof a resistor element1. These electrodes32B are formed by first forming a cross-shaped insulation layer21A on the bottom surface1aof the resistor element1and then plating the bottom surface1a.Thereafter, by forming a second insulation layer21B, the chip resistor A3is obtained. It should be appreciated that the figure does not show plated layers which is formed to facilitate soldering, for convenience of description.

The chip resistor A3has four electrodes32B, and can be utilized in the following way. Supposing that the resistance of the chip resistor A3is given, two of the four electrodes32B are used for supplying electric current, and the other two electrodes32B are used for voltage measurement. The pair of current application electrodes are connected to the circuit so as to allow the electric current to pass, and the pair of voltage measurement electrodes are connected to a voltmeter to measure a voltage drop between the two voltage detection terminals. From the measured voltage value and the known resistance, the value of electric current which passes through the resistor element1can be known by using the Ohm's Law.

The present invention is not limited to the embodiments described above. The design of a chip resistor according to the present invention may be varied in many ways. For example, the lower electrodes31in the first embodiment may have a single-layer structure formed by printing a metal paste and then baking the paste.

In the first embodiment, both of the lower electrodes31overlap the insulation film21. However, only one of the paired electrodes31may overlap the insulation film21. Likewise, in the second embodiment, the second insulation layer21B is formed to overlap both of the lower electrodes32. Alternatively, the layer may overlap only one of the electrodes.

In each of the chip resistor manufacturing methods described above, use of the frame may be replaced by use of a plate-like member. In this instance, the insulation films (21,22) are formed on one of the surfaces and on the other of the surfaces of the plate-like member respectively, and then the plate-like member is divided into a plurality of bars. After the division, the remaining steps such as formation of the insulation film (23) on the side surfaces of each bar may be performed to produce desired chip resistors. Instead of dividing a large plate-like member, a chip resistor may be produced by starting with preparing a small bar-like member, followed by an appropriate process.