Surface-mount coil

Details Surface-mount coil Surface-mount coil

1999-12-16

2001-09-18

Mai, Anh (Department: 2832)

Inductor devices

Winding with terminals, taps, or coil conductor end...

C336S209000, C336S210000

Reexamination Certificate

active

06292083

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates in general to a surface-mount coil, such as a choke coil, and more particularly, to a structure of lead frames of a surface-mount coil in which a plurality of external connection terminals are directly disposed on a bottom face side of a lower flange portion of a drum-shaped core body.
BACKGROUND OF THE INVENTION
Conventionally, a structure in which a core comprising a wind frame portion provided with winding, and an upper and lower flange portions formed to extend at opposite ends of the wind frame portion is fixed by bonding to a pedestal made of synthetic resin mounted with external connection terminals. Further, end portions of the winding are tied to and soldered to roots of the external connection terminals as a main stream of the surface-mount coil such as the choke coil.
Recently, however, a structure wherein lead frames having external connection terminals are directly fixed through an adhesive or the like to the bottom face of the lower flange portion of the core made of ferrite, ceramic, or the like, for example, has been frequently employed in order to decrease the height. Such external connection terminals are separated from the core, and the base of the core is bonded only to the core mount surfaces of the lead frames.
FIG. 6
is a perspective view of a conventional surface-mount coil seen from a bottom face side (mount board side) and which is of a type as disclosed in Japanese Patent Application Laid-open No. 9-213539, wherein an electrode is directly fixed by bonding to the lower flange portion of the drum-shaped core.
In
FIG. 6
, a surface-mount coil
20
comprises a coil body including a core
5
, and a wind frame portion
3
of which winding (not shown) is applied. The core
5
having the wind frame portion
3
and upper and lower flange portions
1
and
2
is formed to extend at opposite ends of the wind frame portion
3
. Lead frames
10
and
11
, include external connection terminals
6
and
7
to be surfaces of which solderable layers (such as tinning layers, copper plating layers, gold plating layers, solder plating layers, and the like which have a solder wetting property equal to or better than that of lead frame material) are bonded. Core placing portions
8
and
9
extend from the external connection terminals and have widths larger than widths W of the external connection terminals. A bottom face of the lower flange portion
2
of the coil body is fixed by bonding with an adhesive or the like to upper faces
8
a
and
9
a
(lower invisible faces in
FIG. 6
) of the core placing portions
8
and
9
of the lead frames, and end portions of the winding are conductively connected by soldering to packing terminals
15
and
16
of lead frames
10
and
11
.
Height differences H are formed at boundary portions between the external connection terminals
6
and
7
and the core placing portions
8
and
9
. The external connection terminals are separated from the core, and the base of the core is bonded only to the core mount surfaces of the lead frames. Reference numerals
13
and
14
designate engaging projections formed on the lead frames
10
and
11
for positioning lead frames
10
and
11
on the coil body.
In the above-described conventional surface-mount coil
20
, by disposing the wide core placing portions
8
and
9
higher (by height differences H) than external connection terminals
6
and
7
on the lead frames
10
and
11
(by a bending process), insulation between a wiring pattern in the mount board face right under the lead frames and the lead frames can be maintained.
PROBLEMS TO BE SOLVED BY THE INVENTION
However, as the size and a height of this conventional surface-mount coil
20
are further decreased, it is necessary to decrease the height differences H. However, when the surface-mount coil
20
is mounted on electrode lands of a circuit board coated with solder paste and the solder paste is melted and hardened in a reflow soldering furnace to solder the electrode lands to the external connection terminals
6
and
7
of the surface-mount coil, molten solder moves and diffuses from surfaces of the external connection terminals
6
and
7
of the lead frames
10
and
11
toward lower faces of the core placing portions
8
and
9
. As a result, if a solder adhesion preventing layer is not provided on the lower surface of the core placing portions
8
and
9
, heat from the diffused molten solders is conducted to the upper surface of the core placing portions
8
and
9
, thereby melting the solder bondable layer.
On the other hand, in response to recent rising tide of environmental protection, a movement for removing lead included in conventional solder has been promoted. Use of lead-free solder (including approximately 96 wt % of tin, and 4 wt % of silver, bismuth, copper, or a mixture of them as the remainder) is considered to increase in the future.
However, because the melting point of the lead-free solder is higher than that of low-temperature solder (tin-lead eutectic solder including about 10% of lead and having a melting point of about 220° C.) which is used conventionally, wetting cannot be obtained unless the temperature of the external connection terminals
6
and
7
of the lead frames
10
and
11
is sufficiently increased during soldering. On the other hand, since the lead frames
10
and
11
are inevitably exposed to high temperatures for a long period in this case, the molten solder can easily enter to reach the core placing portions
8
and
9
during heating at high temperatures. As a result, the above problem of the defect in soldering is further highlighted.
For example, if the surface-mount coil
20
is soldered to the electrode land of the circuit board, molten solders are diffused through the lower surfaces of the core mount portions
8
and
9
and the heat of the diffused molten solders are conducted to upper surfaces
8
a
and
9
a
of the core mount portion
8
. The solder bondable layer
31
(solder plating or the like) is molten by the heat. As a result, the fixing strength between the core
5
, which is only bonded onto the upper surfaces
8
a
and
9
a
of the core mount portions
8
and
9
, and the core mount portions
8
and
9
of the lead frame
10
disadvantageously decreases.
OBJECTS OF THE INVENTION
The object of the present invention, with the above circumstances in view, is to provide a surface-mount coil having a structure which prevents decrease in the fixing strength between the core and the lead frames, as encountered with prior art devices.
SUMMARY OF THE INVENTION
To achieve the above object, according to the present invention, there is provided in one embodiment a surface-mount coil comprising a coil body including a core to a wind frame portion of which winding is applied, the core having the wind frame portion and upper and lower flange portions formed to extend at opposite ends of the wind frame portion. Lead frames include external connection terminals to surfaces of which solderable layers are bonded, and core placing portions extend from the external connection terminals and have widths larger than widths of the external connection terminals. The coil body is fixed by bonding to upper faces of the core placing portions of the lead frames, and end portions of the winding are conductively connected to the lead frames, wherein solder adhesion preventing layers are formed at lower faces of the core placing portions of the lead frames.
To achieve the above object, according to the present invention, there is provided yet another embodiment where a surface-mount coil according to the above embodiments have solder adhesion preventing layers formed before forming of the solderable layers.


REFERENCES:
patent: 5153549 (1992-10-01), Morinaga
patent: 5457872 (1995-10-01), Sakata et al.
patent: 5530416 (1996-06-01), Wakamatsu et al.
patent: 5831510 (1998-11-01), Zhang et al.
patent: 5864281 (1999-01-01), Zhang et al.
patent: 9-213539A (1997-08-01), None

Affiliated with

Also associated with

Rating

Surface-mount coil does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Surface-mount coil, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Surface-mount coil will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2449793