Thermistor elements

Details Thermistor elements Thermistor elements

1998-03-02

2002-08-13

Enad, Elvin (Department: 2832)

Electrical resistors

Resistance value responsive to a condition

Current and/or voltage

C338S308000, C338S025000

Reexamination Certificate

active

06433666

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to thermistor elements which are used for detection of temperature and temperature compensation of circuits. In particular, this invention relates to thermistor elements having an external electrode structure which is suited for surface mounting.
Since high-density mounting of electronic components is desired, thermistor elements are required to be surface-mountable, say, to a printed circuit board.
FIGS. 13 and 14
(or
FIGS. 13A
,
13
B,
14
A and
14
B) show examples of prior art surface-mountable thermistor elements.
FIGS. 13A and 13B
show a thermistor element
61
having electrodes
63
and
64
formed so as to cover the two end surfaces of a thermistor body
62
made of a material with resistance having a negative temperature coefficient (NTC). These electrodes
63
and
64
are each formed not only on one of the end surfaces of the thermistor body
62
in the shape of a rectangular parallelepiped but also so as to reach the remaining four surfaces adjoining that end surface, that is, the upper, lower and two side surfaces. Thus, such a thermistor element
61
could easily be surface-mounted by attaching its lower surface
62
a
to an electrode land formed on a printed circuit board, for example, by soldering.
FIGS. 14A and 14B
show a thermistor element
65
of the type disclosed in Japanese Patent Publication Tokkai 7-29704, characterized as having a first electrode
67
and a second electrode
68
formed on the lower surface of a thermistor body
66
in the shape of a rectangular parallelepiped so as to be mutually opposite to each other with a specified distance therebetween. If such a thermistor element
65
is desired to be miniaturized and the distance between its electrodes
67
and
68
is excessively reduced, however, there arises the danger of a short-circuiting.
In order to prevent the occurrence of short-circuiting, the thermistor element
65
is provided with an insulating layer
71
of an inorganic material, as shown in
FIGS. 14A and 14B
, so as to cover the lower surface of the thermistor body
66
between two external electrodes
69
and
70
which are formed respectively on the first and second electrodes
67
and
68
, separated from each other by a distance larger than the gap between the first electrode
67
and the second electrode
68
. Since the first and second electrodes
67
and
68
, as well as these external electrodes
69
and
70
, are all formed only on the lower surface of the thermistor body
66
without reaching any other surfaces, the thermistor element
65
, too, can be easily surface-mounted by attaching its lower surface
66
a
to a printed circuit board, for example, by using a solder for reflow mounting or flow mounting.
With the thermistor element
61
shown in
FIGS. 13A and 13B
, each of the electrodes
63
and
64
is formed so as to reach five of the surfaces of the thermistor body
62
. Thus, although it can be surface-mounted, say, onto a printed circuit board by soldering, the solder tends to form swollen parts referred to as “fillets” which make high-density mounting difficult. This may be explained as follows. Suppose that the thermistor element
62
is surface-mounted onto a printed circuit board by applying a solder on the lower surface
62
a
of the thermistor body
62
. If this is done, the parts of the electrodes
63
and
64
situated on the lower surface of the thermistor body
62
may be joined by the solder but the molten solder will swell along the three surfaces perpendicular to the lower surface of the thermistor body
62
and fillets are thereby formed. Thus, the area required for the mounting becomes far greater than the flat area of the thermistor element
61
. This is a serious problem in the attempt to achieve high-density mounting.
As for the thermistor element
65
shown in
FIGS. 14A and 14B
, on the other hand, the external electrodes
69
and
70
for making connections are provided only on the lower surface of the thermistor body
66
. Thus, there is no problem of fillets and hence the area for mounting can be made smaller and higher-density mounting can be accomplished than in the case of the thermistor element
61
of
FIGS. 13A and 13B
. The thermistor element
65
of
FIGS. 14A and 14B
, however, was originally for using a reflow mounting method with a solder paste or a flow mounting method with molten solder. Thus, higher mounting densities are very difficult to achieve with such mounting methods, for example, for the following reasons:
(1) High density mounting is not possible unless solder lands to be formed (say, on a printed circuit board) by a printing process is done with a high degree of accuracy but there have been limits to the accuracy in the printing of solder lands;
(2) When a solder material is melted, the thermistor element tends to be displaced from the solder land onto the base board; and
(3) It is difficult to control the thickness of a solder layer and hence it was difficult to control the mounting displacement of the thermistor element in the direction of the height.
By the reflow and flow methods, furthermore, the mechanical strength of joint becomes weaker due to the embrittlement of the solder and the electrical connections of the chip parts are sometimes deteriorated. Since thermistors which are used for the detection of temperature are required to be accurate to the level of about 1%, such a deterioration of electrical contacts could be a fatal defect.
Recently, a new mounting method referred to as the bump mounting is becoming popular as an improved method of mounting by which higher density mounting becomes possible than by the reflow or flow mounting method. The bump mounting method is a technology whereby a cylindrical or square pillared protrusion called a bump, usually comprising Au or Sn—Pb, is inserted between a chip component and a base board and the bump is joined together with the board and the chip component by thermocompression bonding or by eutectic alloy formation.
By this method, a bump can be formed on a chip component or a base board with very high accuracy and, as long as a bump can be formed accurately, the chip component can be accurately attached to the base board. Another advantage of this method is that there is no problem of fillets.
Among the bump joints, Au bump joints are particularly favorable because they have a high mechanical strength and hence there is no embrittlement problem of the kind encountered with solder materials. Thus, reliable joints can be thereby realized.
The prior art thermistor elements
61
and
65
described above, however, are not suited for bump mounting because they were basically intended to be mounted by using a solder material, having the base layers for their electrodes comprised of a conductive paste. In other words, the electrodes
63
and
64
are formed by applying a conductive paste on a thermistor body
62
and baking it in order to obtain base layers and then forming a layer of Sn or a Sn—Pb alloy in order to improve the solder wettability. As for the thermistor element
65
, its first and second electrodes
67
and
68
are formed by applying a conductive paste such as of Ag on the lower surface
66
a
of the thermistor body
66
and then subjecting them to a baking process.
Thus, if external electrode layers for external connections are formed by plating Ni or Sn—Pb on the electrodes formed by applying a conductive paste and subjecting it to a baking process as described above, the base layers are thick and uneven. As a result, the surfaces of the external electrodes thereabove were necessarily also uneven.
If a thermistor element is to be mounted onto a base board by a bump mounting method, the bumps and the electrodes of the thermistor element must be firmly in contact with each other. Thus, if the thermistor has external electrodes with very uneven surfaces with large indentations and protrusions, a dependably firm contact cannot be expected by a bump joint method.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide the

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