[0001] The present invention relates to a resistor and a method of manufacturing the resistor.
In particular it relates to a surface mounted chip resistor which does not have a
lead frame.
[0002] A typical surface mounted chip resistor is shown in Fig. 1. It comprises a ceramic
substrate 10, having a thick film resistor 20 on a top surface thereof. Thick film
printed conductor portions 30 are provided on either side of the thick film resistor
20 on the top surface of the substrate 10. Corresponding portions of thick film printed
conductor 30 are also provided on the bottom surface 40 of the substrate 10. Conductor
plates 50 are wrapped around each end of the substrate 10 and have portions resting
on and in conductive contact with the thick film printed conductor sections 30. The
conductor plates 50, have flat bottom surfaces 55 under the bottom 40 of the substrate.
An over glaze layer 60 is provided over the thick film resistor 20 and acts as a protective
coating. A further protective coating, in the form of an insulating epoxy layer 70,
is provided over the over glaze layer 60. Ideally the epoxy layer 70 is flush with
the upper surfaces 80 of the conductor plates 50.
[0003] In use the conductor chip is surface mounted onto a printed circuit board 200 such
that the lower faces 55 of the conductive plates 50 rest over and are in electrically
conductive contact with conductive lines of the circuit board 200. Usually, the resistor
chip is attached by solder. Current can then be passed through the thick film resistor
20 of the resistor chip, via the conductive plates 50 which contact the circuit board
200 and thick film printed conductor sections 30 which are in conductive contact with
the thick film resistor 20.
[0004] The surface mounted chip resistor described above is very different to the lead frame
mounted resistors shown in Figs. 2a, and 2b. A through hole lead frame resistor 100,
shown in Fig. 2a, has a resistor component 110, which is attached to a circuit board
200 by a pair of lead frame pins 120 which are inserted through holes 210 in the circuit
board 200. The pins or wires 210 are then generally secured in place by solder. As
can be seen in Fig. 2a, the resistor 110 is suspended above the circuit board by the
wires or pins 120. The resistor component 110, in this example, comprises a substrate
110a in which is mounted a resistor element 110b. In Fig. 2a the resistor element
110b is in a serpentine pattern. Fig. 2b shows a surface mounted lead frame resistor
in which the resistor component 110 has a lead frame with two wire legs 120 which
are surface mounted onto the circuit board 200. In contrast a surface mounted resistor
chip does not have a lead frame, instead its conductive terminal plates form an integral
part of the chip which is mounted on the circuit board. Furthermore, the lead mounted
resistor of the type shown in Fig. 2a and 2b are conventionally attached manually
to the circuit board, whereas surface mounted chip resistors are suitable for automatic
attachment to the circuit board. The flat conductor plates of the surface mounted
resistor chip make it easy to attach by high volume assembly processes by means of
industry standard, automatic, high speed pick-and-place equipment.
[0005] Another difference is that lead frame mounted resistors are typically larger than
10mm x 6mm (dictated by the size of the lead frame), whereas the dimensions of a surface
mounted resistor is typically, 6.5mm x 3.2mm or smaller (dimensions being measured
as length x width, rather than depth, of a chip). In summary surface mounted chip
resistors and lead frame resistors are quite different from each other and not considered
by persons skilled in the art to be similar.
[0006] It would be desirable to provide a surface mounted chip resistor which has a higher
pulse energy rating than a conventional surface mounted chip resistors without increasing
its size. The pulse energy rating of a resistor is the safe limit to the amount of
energy generated when a short pulse of given power and duration is passed through
the resistor (above the safe limit the resistor may be destroyed).
[0007] Current attempts to achieve the above rely on minimising or omitting the laser trimming,
which is conventionally used to cut the thick film resistor to achieve the desired
ohmic value or size during the manufacturing process. That is current methods rely
on avoiding or minimising removal of the resistive film.
[0008] Accordingly, at its most general, the present invention proposes providing a resistor
element on both sides of the substrate. As the pulse energy rating for short pulses
is proportional to the active resistive film area, providing two resistor elements
effectively doubles the pulse energy rating for a chip of a given size.
[0009] A first aspect of the present invention may provide a leadless chip resistor for
surface mounting on a circuit board, having:
a substrate;
a pair of electrically conductive plates mounted to at least a first face of the substrate,
said conductive plates being configured for surface mounting on a circuit board;
a first resistor layer on said first face of the substrate between said pair of conductive
plates and in (electrically) conductive contact with said plates; and
a second resistor layer on a second face of the substrate, opposite the first face,
said resistor layer being in conductive contact with said pair of conductive plates.
[0010] By "leadless" it is meant that the resistor does not have a lead frame.
[0011] The conductive plates are configured for surface mounting on a circuit board. The
surfaces for mounting on the circuit board may be substantially flat and are preferably
parallel to each other so that the resistor can easily be laid down on the circuit
board. The conductive plates need not be directly mounted on the substrate and there
may be one or more intermediate layers between the conductive plates and the substrate.
[0012] As the plates are on either side of the first resistor layer and in electrically
conductive contact therewith, a current can be passed between the two plates through
the first resistor layer. The first resistor layer need not be in direct contact with
the conductive plates, but may be in conductive contact via an intermediate conductive
element or layer. For example, an intermediate conductor mounted on the surface of
the substrate between each conductive plate and the resistive layer. The second resistor
layer is also in conductive contact (direct or indirect) with the conductive plates,
so that current can be passed through both first and second resistor layers simultaneously.
[0013] By the above configuration, the resistor chip can have a higher pulse energy rating
than a conventional surface mounted resistor of the same size (double if the resistive
layers are of the same size). Furthermore, the above configuration allows for a lower
minimum resistance than has previously been achievable with the same components. For
example, if the second resistor layer is the same size as the first resistor layer,
then the resistance will be approximately half that of single sided resistor only
having the first resistor layer on a first face of the substrate. This is important
in the area of low value current sense resistors, where minimum resistance is desirable.
Furthermore, as two resistor layers are used, it may be possible to achieve higher
manufacturing yields and therefore lower costs than for conventional surface mounted
resistors.
[0014] Preferably the conductive plates wrap around the substrate, such that each conductive
plate has a first portion on the first face of the substrate, a second portion on
the second face of the substrate and an intermediate portion joining the first and
second portions and wrapping round an edge of the substrate. Each conductive plate
may take the form of an approximately U-shaped layer of material wrapping around an
edge of the substrate. This makes it easy to connect the first and second resistor
layers to the conductive plates. The conductive plates may be formed from an electroplated
conductive epoxy resin.
[0015] Usually the dimensions of the, resistor chip will be 6.5mm x 3.2mm or less usually
the dimensions will be at least 0.5mm x 0.25mm. Dimensions means the length x width
of the chip.
[0016] The second resistor layer is usually the same size as the first resistor layer.
[0017] The substrate is not electrically conductive (i.e. it is an insulator). Preferably
the substrate is made of a ceramic material.
[0018] The resistor layers may be made of any suitable material. Usually they will take
the form of thick film resistors. Ruthenium components, e.g. ruthenium oxide, are
particularly suitable for the resistor layers, but alternatives will be apparent to
a person skilled in the art.
[0019] In one embodiment intermediate conductors are provided on the substrate at both ends
of and in contact with each resistor layer on both faces of the substrate. The conductive
plates wrap over and connect intermediate conductor sections on opposite faces of
the substrate.
[0020] Preferably one or more protective and/or insulating layers are provided over the
resistor layers and/or the intermediate conductors. Preferably the outermost protective
or insulating layer is flush with the surface of the conductive plates. This provides
a smooth surface for the resistor enabling it to be easily mounted on a circuit board.
However it is possible for the protective or insulating layer to be recessed relative
to the surface of the conductive plates. Protective layers help to prevent corrosion
of the resistor layers and intermediate conductors, while insulating layers help to
prevent shorting of the resistor.
[0021] Preferably an over glaze is provided on one or both faces of the resistor, covering
the surface of the resistor layer(s). The over glaze helps to protect the internal
parts of the resistor from corrosion.
[0022] Preferably an epoxy layer is provided over the resistor layer and over the over glaze
(if present). The epoxy layers help to insulate the resistor layer(s) and help prevent
shorting between the conductive plates.
[0023] A second aspect of the present invention may provide a method of making a leadless
chip resistor comprising placing or forming a first resistor layer on a first face
of a substrate and forming or placing a second resistor layer on a second face of
the substrate, opposite the first face, forming or placing electrically conductive
plates on both sides of said first resistor layer on said first face of said substrate,
such that said conductive plates are in electrical contact with said resistor layers
and adapted for surface mounting on a circuit board.
[0024] The resistor layers may be formed by printing and firing resistive materials (preferably
thick film) onto the substrate (which is preferably ceramic). The conductive plates
may be electroplated.
[0025] Preferably a plurality of resistor layers are formed on both faces of a substrate
and the substrate is subsequently cut, snapped or otherwise separated into separate
sections (typically strips comprising a row of one or more resistor chips). The conductive
plates are preferably placed or formed after this separation.
[0026] Separate intermediate conductor layers or sections for electrically connecting the
resistor layers with the conductive plates can be formed on the substrate as well.
If a plurality of resistor layers are formed on the substrate, before the substrate
is broken into separate sections, then the intermediate conductors are preferably
formed before this separation step. The intermediate conductors may be formed by printing
and firing a conductive material onto the substrate.
[0027] Preferably an over glaze layer is added to cover the resistive layers. Preferably
a laser is used to adjust the ohmic value (by shape or size trimming) of the resistor
layers, by trimming off unnecessary portions. An epoxy layer may also be added, for
example over the resistor layer or the over glazed layer (where present). Generally,
these steps will be carried out before the substrate is separated into separate sections.
[0028] Preferably the conductive plates are such that they wrap around ends of the substrate
to extend over both first and second faces of the substrate. Preferably the conductive
plates are formed by dipping the ends of the resistor chip into a conductive resin.
Preferably the conductor plates are electroplated.
[0029] The method of the second aspect of the present invention may be adapted to use components
or produce resistor chips having any of the features described in the first aspect
of the present invention.
[0030] An embodiment of the present invention will now be described with reference to the
accompanying drawings in which;
Fig. 1 is a cross-sectional view of a single sided surface mount resistor chip and
has already been described above;
Figs. 2a and 2b shows two types of lead frame mounted resistor and has already been
described;
Fig. 3 is a cross-sectional view of a resistor chip according to the present invention;
Fig. 4 shows steps in the first stage in the manufacturing process of the chip of
Fig. 3; and
Fig. 5 shows manufacturing steps in the second stage of the manufacturing process.
[0031] Fig. 3 shows a resistor chip 300 according to an embodiment of the present invention.
It comprises a ceramic substrate 10 (which is an insulating material), a first thick
film resistor 20 mounted on a first (upper) face of the substrate 10 and a second
thick film resistor 320 mounted on a second face of the substrate 10 opposite to the
first face. In this embodiment the thick film resistors are parallel to each other
and extend approximately ¾ of the length of the substrate. On each face of the substrate
10, intermediate or internal conductors 30 in the form of thick film printed conductor
sections, are mounted on either side of the thick film resistors 20, 320 and cover
the remainder of each face of the substrate 10. Electroplated conductive plates 50
are wrapped around the ends of the substrate 10 and over a portion of the intermediate
conductors 30. The conductor plates 50 thus have a cross-section corresponding approximately
to a square U-shape. The legs of the U form substantially flat sections 55 and 80
on either side and parallel to the upper and lower sides of the substrate 10. These
flat surfaces 55 act as conductive terminals when the resistor 300 is mounted on a
circuit board 200 (shown in dotted lines in Fig. 3). Alternatively the resistor could
be mounted the other way up with the flat surfaces 80 acting as conductive terminals.
[0032] Each electrically conductive plate 50 wraps around the ends of the substrate 10,
as described above, and thus puts the intermediate conductors 30 which it contacts
on opposite faces of the substrate 10 in electrically conductive contact with each
other. In this way current can be passed through both the first 20 and second 320
resistor layers when the device is in use.
[0033] As there is a resistor layer (in this embodiment a thick film resistor) on either
face of the substrate 10, the pulse energy rating of the resistor chip is increased
and the resistance is smaller than if there was only a single resistor on one face
of the substrate.
[0034] An over glaze layer 60 is provided over each face of the resistor chip 300 covering
the respective resistor layer 20, 320 on each face of the substrate 10. The overglaze
layer 60 is itself covered with an epoxy layer 70 which has an outer surface, preferably
flush with the outer surface of the conductor plates 50 to form a smooth consistent
surface for mounting on the printed circuit board 200.
[0035] The resistor chip 300 can be attached to the printed circuit board 200 by solder
and this may be carried automatically with the aid of a machine. Suitable machines
are well known and standard in the industry.
[0036] The method of manufacturing the resistor chip will now be described. This is similar
to the conventional method for manufacturing a single sided resistor chip (i.e. a
resistor chip with only a single resistive layer on one face thereof), but certain
steps have to be carried out twice so that components are applied to both faces of
the substrate and special care has to be taken to make sure that the components are
not too thick on either face. In contrast to the conventional single sided resistor
chip shown in Fig. 1, the resistor chip of the present embodiment can be surface mounted
on either face, both of which have components, so it is necessary to make sure that
neither the resistor layer, over glaze layer or epoxy layer are too thick. Preferably
they are flush with the outer surfaces of the conductive plates.
[0037] In broad terms the manufacturing method has two stages. In the first stage, shown
in Fig. 4, various components are added to a ceramic substrate 10 in a printing and
firing process. In the second stage, shown in Fig. 5, the coated substrate is snapped
into a plurality of separate sections and the conductive plates are then formed.
[0038] In Fig. 4, at step 410, a plurality of conductive layers 30 are formed by printing
and firing conducting material onto a first (rear) face of the substrate 10. These
conductive layers 30 later form the intermediate conductors 30 described above.
[0039] In step 420 the same printing and firing process is carried out to form corresponding
conductive layers 30 on the second (front) face of the substrate 10.
[0040] In step 430 resistor layers 20 are then formed by printing and firing resistive thick
film materials onto the first face of the substrate 10. These resistor layers 20 are
formed between and contact the conductor layers 30 which were formed previously. This
step 430 is then repeated for the second face of the substrate 10.
[0041] At step 440 an overglaze layer is formed by printing and firing an over glaze material
on to the first face of the substrate 10, so that the resistive layers 20 are covered.
This step is then repeated for the second face of the substrate 10.
[0042] At step 450 a laser is used to obtain the desired ohmic value for then resistor layer
20 by trimming off any unwanted portions of the resistor layers 20. This step is then
repeated for the resistors on the second face of the substrate 10.
[0043] In step 460 printing and curing is carried out on the first and second faces to add
a protective layer.
[0044] In step 470 each resistor is marked.
[0045] Thus far, a plurality of separate resistors have effectively been formed as a grid
on the same substrate. This is then snapped into strips, each comprising a plurality
of resistors in a single row, in step 480 which is shown in Fig. 5.
[0046] In step 490 each strip is dipped into a conductive epoxy resin, so as to provide
a conductive path around the ends of each resistor chip on the strip. The conductive
epoxy resin thus forms the conductive plates 50 shown in Fig.3
[0047] In step 500 each strip is separated into separate sections by snapping, cutting or
otherwise. Then in step 510 the conductive plates formed by the conductive resin are
electroplated to enhance their solderability.
[0048] In step 520 each chip is officially inspected for flaws. Then in step 530 chips which
pass this step are measured to check their ohmic value.
1. A leadless chip resistor for surface mounting on a circuit board, having:
a substrate;
a pair of electrically conductive plates mounted to at least a first face of the substrate,
said conductive plates being configured for surface mounting on a circuit board;
a first resistor layer on said first face of the substrate between said pair of conductive
plates and in conductive contact with said plates; and
a second resistor layer on a second face of the substrate, opposite the first face,
said second resistor layer being in conductive contact with said pair of conductive
plates.
2. A resistor according to claim 1 wherein the conductive plates wrap around the substrate,
such that each conductive plate has a first portion on the first face of the substrate,
a second portion on the second face of the substrate and an intermediate portion joining
the first and second portions and wrapping round an edge of the substrate.
3. A resistor according to claim 1 or claim 2 wherein the dimensions of the chip resistor
are 6.5mm x 3.2mm.
4. A resistor according to any one of the above claims wherein the conductive plates
are formed from an electroplated conductive epoxy resin.
5. A resistor according to any one of the above claims wherein the resistor layers are
in conductive contact with the conductive plates via intermediate conductors provided
on the substrate between the resistor layers and said conductive plates.
6. A resistor according to any one of the above claims wherein one or more protective
and/or insulating layers are provided over the resistor layers.
7. A method of making a leadless chip resistor comprising placing or forming a first
resistor layer on a first face of a substrate and forming or placing a second resistor
layer on a second face of the substrate, opposite the first face, forming or placing
electrically conductive plates on both sides of said first resistor layer on said
first face of said substrate, such that said conductive plates are in electrical contact
with said resistor layers and adapted for surface mounting on a circuit board.
8. A method according to claim 7 wherein the dimensions of the chip resistor are 6.5mm
x 3.2mm.
9. A method according to claim 7 or 8 wherein the conductive plates are such that they
wrap around ends of the substrate to extend over both first and second faces of the
substrate.
10. A method according to claim 9 wherein the conductive plates are formed by dipping
the ends of the resistor chip into an electrically conductive resin.
11. A method according to any one of claims 7 to 10 wherein a plurality of resistor layers
are formed on both faces of a substrate and the substrate is subsequently cut, snapped
or otherwise separated into separate sections.
12. A method according to claim 11 wherein the conductive plates are placed or formed
after separation of the substrate into separate sections.