Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With housing or contact structure
Reexamination Certificate
2002-04-08
2004-06-08
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With housing or contact structure
C257S098000, C257S100000
Reexamination Certificate
active
06747293
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a light emitting device, in particular, having an excellent emission property and a high reliability.
Light emitting devices combining LEDs (light emitting diodes) or other semiconductor light emitting elements and fluorescent elements have been remarked as inexpensive, long-lived light emitting devices, and are widely used as various kinds of indicators, light sources, flat-type display devices, backlight of liquid crystal displays, and so forth.
As typical light emitting devices, there are those mounting semiconductors light emitting elements in resin stems.
FIGS. 37A and 37B
show such a typical conventional light emitting device.
FIG. 37A
is a plan view showing a configuration of the substantially part thereof, and
FIG. 37B
is a cross-sectional view thereof.
The light emitting device shown here is of a so-called “surface mounting” type, including a package (resin stem)
800
, semiconductor light emitting element
802
and sealing element
804
of a resin.
The resin stem
800
has a structure molding a pair of leads
805
,
806
shaped from lead frames with a resin portion
803
of a thermoplastic resin. The resin portion
803
has an opening
801
, and the semiconductor light emitting element
802
is place therein. Then the semiconductor light emitting element
802
is sealed with an epoxy resin
804
.
The semiconductor light emitting element
802
is mounted on the lead
806
. An electrode (not shown) of the semiconductor light emitting element
802
and the lead
805
are connected to each other by a wire
809
. When en electric power is supplied to the semiconductor light emitting element
802
through those two leads
805
,
806
, the semiconductor light emitting element
802
emits light, and the light is extracted from an emission surface
812
via the epoxy resin
804
.
The Inventor, however, made researches and has found that conventional light emitting devices of this type have still room for improvement from the viewpoint of reliability and long-time stability.
That is, through temperature cycle tests of 700 cycles under temperatures in the range from −40° C. to 110° C., various undesirable phenomena were observed, such as cracks C in the epoxy resin
804
as shown in
FIG. 38
, or exfoliation of the epoxy resin
804
at the interface I with the resin stem
800
. In some cases, the semiconductor light emitting element
802
broke, or exfoliated from the mount surface, and the wire
809
cut down.
The light emitting device shown in
FIGS. 37A and 37B
certainly meets the requirements currently in force, i.e., 100 cycles as the current level of temperature cycle tests requested for ordinary civilian uses, and 300 cycles for car-borne uses. However, for further improvement of the reliability toward the future uses, essential review is required.
The same circumstances commonly exist in all structures sealing semiconductor elements with epoxy resin, without being limited to that shown in
FIGS. 37A and 37B
.
As a result of a careful review of mechanisms of malfunctions, the Inventor has realized that the epoxy resins
804
is physically hard and fragile and produces a large stress upon hardening and that there still exists room for improvement in quality of close contact with the resin portion
803
of a thermoplastic resin that surrounds it.
Apart from this, there are semiconductor devices of a type as shown in
FIGS. 37A and 37B
but including two or more chips mounted in the opening
801
.
Those having two or more semiconductor elements common in emission wavelength, for example, are enhanced in output.
Those having two or more semiconductor elements different in emission wavelength can provide mixed color, thereby to diversify the color representation. In this case, two complementary colors can produce white light.
It is sometimes desirable to mount an element for protecting the light emitting element in a common package. Incase of a light emitting element of a nitride semiconductor, it is often desirable to connect a Zener diode in a parallel opposite directions for the purpose of protecting the light emitting element from static electricity.
However, the light emitting device shown in
FIGS. 37A and 37B
cannot provide a sufficient space for mounting the chip and for bonding the wire as well. If two chips are packed in the narrow opening by force, the optical axis of the light emitting element will largely offset from the center of the opening, and the intensity profile of the emitted light, i.e., luminous intensity property, will become asymmetrical. Then, the light emitting device cannot provide a uniform emission pattern required in applications such as the back light of a liquid crystal display.
FIG. 39
is a schematic diagram showing a plan-viewed configuration of a light emitting device prepared by the Inventor for trial toward the present invention.
The light emitting device shown here has an approximately rectangular opening
901
formed in a resin portion
903
, and chips
902
A,
902
B mounted on opposed leads
905
,
906
, respectively, at the bottom of the opening
901
. Wires
909
A,
909
B extending from the chips
902
A,
902
B are connected to the opposed leads
906
,
905
, respectively.
As a result of evaluation of this light emitting device, the following problems were found.
The fist problem is that a part of an adhesive extruding out upon mounting the chips
902
A,
902
B causes insufficient bonding of the wires
909
A,
909
B. For mounting the chips
902
A,
902
B to the leads, pastes such as silver paste or solders such as gold-tin (AuSn) or gold-germanium (AuGe) solder is usually used.
However, such an adhesive often extrudes on the leads
905
,
906
upon mounting. If the extruded adhesive reaches the wire bonding region, it makes it difficult to bond wires
909
A,
909
B by thermo compression bonding or ultrasonic welding. For example, when a silver paste exists, so-called “breeding” occurs, and it makes wire bonding difficult. Even if they are once bonded, their bonding force will soon degrade significantly.
An attempt of locating the wire bonding site remote from the chip for the purpose of preventing that problem will need a larger opening
901
against the restriction on size.
The second problem lies in that the illustrated rectangular shape of the opening
901
causes side walls of the resin portion
903
to be uniformly thin, and makes the mechanical strength insufficient. This problem becomes serious especially when a soft resin is used as the sealing element buried in the opening. For example, a silicone resin used as the sealing element is advantageous for reducing the residual stress and thereby reducing cracks of the sealing element and breakage of the wire. However, in case the side wall of the resin portion
903
is thin, the relatively soft silicone resin often fails to prevent an external lateral force to act on the chip and the wire. For example, upon picking up the light emitting device by grasping from its side surfaces for assembly and a test, the force actually acted upon the chip and the wire, and often deformed the wire.
The third problem is that the illustrated rectangular shape of the opening
901
need a larger quantity of resin buried therein, and sometimes increases the resin stress. The resin filled in the opening
901
produces a stress upon curing, or thereafter upon an increase of decrease of the temperature.
The degree of the stress depends on the buried quantity of the resin, and tends to increase as the buried quantity increases. Moreover, as already explained with reference to
FIG. 38
, epoxy resins exhibit a large stress.
Therefore, the sealing resin filled in the illustrated rectangular opening
901
produced a large stress, and is liable to cause exfoliation of the chips
902
A,
902
B, and deformation or breakage of the wires
909
A,
909
B.
That is, the attempt of mounting two or more chips in the light emitting device invites various problems contravening the requirements about the external dimensions.
As reviewed above, conventio
Komatsu Takeshi
Nitta Koichi
Oshio Hiroaki
Shimomura Kenji
Flynn Nathan J.
Hogan & Hartson LLP
Kabushiki Kaisha Toshiba
Mondt Johannes P.
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