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| United States Patent |
5,281,853
|
|
Hazaki
, et al.
|
January 25, 1994
|
Resin-sealed semiconductor device containing porous fluorocarbon resin
Abstract
A resin-sealed semiconductor device, including a chip mounting die pad,
porous fluorocarbon material located just beneath the die pad, beneath a
die-pad supporting layer, gold lead wires, or in a sealing resin
surrounding the other components, wherein any water vapor generated by the
heat of soldering will be held within the porous fluorocarbon rather than
crack the sealant under internal pressure.
| Inventors:
|
Hazaki; Yoshito (Okayama, JP);
Hatakeyama; Minoru (Okayama, JP);
Fukutake; Sunao (Okayama, JP);
Urakami; Akira (Okayama, JP)
|
| Assignee:
|
Japan Gore-Tex, Inc. (Tokyo, JP)
|
| Appl. No.:
|
841685 |
| Filed:
|
February 26, 1992 |
Foreign Application Priority Data
| Mar 08, 1991[JP] | 3-67530 |
| Mar 12, 1991[JP] | 3-70378 |
| Intern'l Class: |
H01L 023/36; H01L 023/02; H01L 023/12; H01L 039/02 |
| Field of Search: |
357/72
257/682,683,687,702
|
References Cited
U.S. Patent Documents
| 3083320 | Mar., 1963 | Godfrey et al. | 257/682.
|
| 3670091 | Jun., 1972 | Frantz et al. | 257/786.
|
| 4888634 | Dec., 1989 | Lai et al. | 357/72.
|
| 4933744 | Jun., 1990 | Seqawa et al. | 357/72.
|
| 4985751 | Jan., 1991 | Shiobara et al. | 357/72.
|
| 5015675 | May., 1991 | Walles et al. | 523/443.
|
| 5034801 | Jul., 1991 | Fischer | 357/72.
|
| 5097317 | Mar., 1992 | Fujimoto et al. | 357/72.
|
| 5117272 | May., 1992 | Nomura et al. | 357/72.
|
| 5122858 | Jun., 1992 | Mahulikar et al. | 357/70.
|
| Foreign Patent Documents |
| 0015448 | Jan., 1988 | JP | 357/72.
|
| 0010858 | Jan., 1990 | JP | 357/72.
|
| 0027557 | May., 1991 | JP | 357/72.
|
Primary Examiner: James; Andrew J.
Assistant Examiner: Wallace; Valencia M.
Attorney, Agent or Firm: Samuels; Gary A.
Claims
We claim:
1. A sealed semiconductor device consisting of
(a) a chip-mounting element
(b) a semiconductor chip mounted on the chip-mounting element;
(c) a layer of porous fluorocarbon material located on at least a portion
of the lower surface of the chip-mounting element;
(d) a multiplicity of leads located on said chip-mounting element and
connected to said chip by gold wire leads; and
(e) a sealing resin surrounding as a unit said chip, said chip-mounting
element, said layer of porous fluorocarbon material, the interior ends of
said leads and said wire leads.
2. A device of claim 1 wherein said porous fluorocarbon material is
selected from a group consisting of polytetrafluoroethylene,
tetrafluoroethylene-hexafluoropropylene copolymers, poly
chloro-trifluoroethylene, perfluoroethylene-perfluoroalkyl vinyl ether
copolymers, and ethylene-tetrafluoroethylene copolymer.
3. A device of claim 2 wherein said wire leads comprise gold.
4. A device of claim 3 wherein said sealing resin comprises an epoxy-based
resin.
5. A device of claims 1 or 3 wherein said porous fluorocarbon material is
filled with particulate inorganic material.
6. A resin-sealed semiconductor device consisting of:
(a) a semiconductor chip mounted on a chip-mounting element;
(b) a multiplicity of leads located on said chip-mounting element and
connected to said chip by gold wire leads; and
(c) a sealing resin containing small pieces of porous fluororesin
surrounding as a unit said chip, said chip-mounting element, the interior
ends of said leads, and said wire leads.
7. A device of claim 6 wherein said porous fluororesin is selected from the
group consisting of tetrafluoroethylene, tetrafluoroethylene copolymers of
ethylene and hexafluoropropylene, polychlorotrifluoroethylene and
perfluoroethylene-perfluoroalkyl vinyl ether copolymers.
8. A device of claim 7 wherein said sealing resin comprises an epoxy-based
resin.
9. A device of claims 6 or 7 wherein said quantity of said fluororesin
pieces comprises from 5 to about 90% by volume of said sealing resin.
10. A device of claims 6, 7, or 9 wherein said fluororesin has a porosity
of about 20% to about 90%.
11. A device of claims 6, 7, 8, 9, or 10 wherein said porous fluororesin is
filled with particulate inorganic material.
Description
FIELD OF THE INVENTION
The present invention relates to resin-sealed semiconductor apparatus and
more specifically to preventing both the formation of cracks owing to the
thermal stress inside the resin and the formation of cracks in the resin
owing to water vapor pressure generated by rapid heating of the resin
during solder mounting of the device.
BACKGROUND OF THE INVENTION
Owing to the gradual miniaturization of electron instruments and to their
constant improvement in recent years, resin-sealed semiconductor devices
wherein semiconductor chips are sealed with a resin, such as an epoxy
resin, have become widely used as thin and compact semiconductor packages
and other devices of the surface-mounted type.
Resin-sealed conventional semiconductor devices generally have a
semiconductor chip mounted on a chip-mounting component commonly referred
to as a die pad, the electrodes of the semiconductor chip and the inner
leads connected by thin gold wires, the components sealed with an
epoxy-based or other resin and subsequently connected to conductive leads
and coated with solder.
Japanese provisionally published Patent Applications 61-23348 and 63-54757
show techniques for manufacturing such resin-sealed semiconductor devices,
including machining the undersurfaces of the tabs to relieve stress.
However, a serious disadvantage of conventional resin-sealed semiconductor
devices similar to that described above is their poor thermal resistance
during solder mounting of the package. Specifically, since vapor phase
reflow soldering or infrared heating are generally used as the method for
solder mounting of these semiconductor packages on printed circuit boards
(PCB) and the like, not only the components to be joined by soldering but
also the packages themselves are rapidly heated during such solder
mounting and the water which is absorbed in the bulk of the resin and
which has penetrated the semiconductor device during storage rapidly
evaporates owing to said heating. The vapor generated diffuses along the
interface either between the resin and the chip-mounting component or
between the resin and the semiconductor chip. The components become
detached from each other, and the vapor penetrates between the detached
portions, increases its internal pressure, and causes cracks in the resin
to form. Such crack formation becomes especially noticeable in compact and
thin semiconductor devices. A resin, such as an epoxy resin, absorbs
moisture during the period of storage which follows manufacture but
precedes soldering. With formation of cracks, the resin-sealing effect is
considerably reduced and the performance and life span of semiconductor
devices are severely impaired.
Although the techniques proposed in the above Japanese provisionally
published applications 61-23348 and 63-54757 are effective in preventing
cracks from being formed on the lower surfaces of chip-mounting elements,
these techniques do not prevent the formation of cracks on the upper and
lateral surfaces, which cause wire breakage and other failures fatal to
semiconductor devices. Additional disadvantages include a larger number of
operations and higher production costs caused by the machining of the
reverse surfaces of the elements.
SUMMARY OF THE INVENTION
The present invention provides a means for securely preventing resin cracks
from being caused both by abrupt thermal stress during solder mounting and
by the accompanying generation of water vapor even in resin-sealed layers
made into compact and thin layers.
The invention comprises a resin-sealed semiconductor device which comprises
a semiconductor element mounted on a chip-mounting component, a porous
fluorocarbon body located either on at least a portion of the lower
surface of the chip-mounting component or small pieces of porous
fluororesin incorporated into a sealant surrounding the components as a
unit.
By installing a porous fluorocarbon body on the lower surface of a
chip-mounting component and installing a semiconductor element on the
mounting component, and by sealing these two components with a resin
sealant, the vapor that is generated by rapid heating during solder
mounting is caused to diffuse into the porous body, an internal pressure
rise is avoided, and the formation of cracks is prevented.
Where small pieces of porous fluororesin are incorporated into the molding
resin, the stress caused by the pressure of the water vapor generated
inside said resin designed for molding is absorbed by the small pieces of
porous fluororesin and the formation of cracks is prevented. Even when
cracks have been formed, the spreading of the cracks can be deterred by
the small pieces of porous fluororesin. By using small pieces of porous
fluororesin, it becomes possible to suitably absorb the thermal stress and
to lower the internal stress caused by rapid heating during solder
mounting because the porous resin pieces display low elasticity and excel
in stress relaxation.
The small pieces of porous fluororesin measure 1 to 500 microns and
preferably 10 to 100 microns, and have a porosity of 20 to 90%, and
preferably 60 to 80%. The pieces are used in an amount (in terms of
volume) of 5 to 90% with respect to the sealing resin layer. Further, the
small pieces of porous fluororesin are positioned evenly throughout the
sealing resin, ensuring that the semiconductor device is rendered more
compact and flat.
In addition, the chip-mounting component can have supporting extensions.
Where a porous fluorocarbon body is installed on the lower surface of the
supporting portion, the vapor diffused in by the heat of soldering is
allowed to reach the supporting portion, is allowed to pass through the
interface between the supporting portion and the sealing resin, and is
discharged to the outside of the sealed component, thereby dispersing an
increase in the pressure inside the package and preventing the formation
of cracks.
By using a porous fluorocarbon body which displays low elasticity and is
excellent in stress relaxation, it becomes possible to suitably absorb the
thermal stress and to lower the internal stress, which are caused by rapid
heating during solder mounting.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a semiconductor chip mounted on a die
pad, connected by wire leads to electrodes, and the components surrounded
by a resin seal in the conventional manner.
FIG. 2 is a cross sectional view of a device of the invention wherein the
semiconductor chip thereof rests on a chip support layer, which in turn
rests on a layer of porous fluorocarbon.
FIG. 3 is a top view of a device of the invention.
FIG. 4 is a cross-sectional view of a device of the invention wherein small
pieces of fluororesin are incorporated into the sealant.
DETAILED DESCRIPTION OF THE INVENTION
The invention is now described in terms of the drawings to more clearly
delineate the scope and important details of the invention.
The porous fluorocarbon body has a pore diameter of about 0.1 to 2 microns,
preferably 0.5 to 1.2 microns and a porosity of 30 to 90%, preferably 50
to 80%. The porous fluorocarbon resin body can usually be shaped as a thin
layer, so that even when the porous body is secured to the bottom surface
of the chip-mounting component and is stably positioned inside a resin
sealing component, it is still possible to make a semiconductor device
more compact and flat by suitably shaping the resin coating on said
components in the form of a thin layer.
The porosity and the pore diameter of the porous fluorocarbon body can be
calculated from the ethanol bubble point and the density by methods such
as ASTM-F316.
Ethanol Bubble Point (EBP)
Ethanol was spread over the surface of the material (film) sample, the
sample placed horizontally on a fixing apparatus, and the EBP measured.
Here, air was blown from below the sample. The EBP is the initial pressure
(kg/cm.sup.2) at the point air bubbles are continuously exiting from the
surface on the reaction side. The average pore diameter can be calculated
from the EBP by a method, such as ASTM-F316.
Porosity
The porosity of the polymer film prior to impregnation was obtained by
measuring the density of the material. The density of the material
(polytetrafluoroethylene) was 2.2 g/cm.sup.3. The porosity was calculated
using the equation:
Porosity=(2.2-sample density):2.2.times.100
In a conventional mounting of a semiconductor device as shown in FIG. 1, a
semiconductor element or chip 10 is mounted on a chip-mounting component 1
and the electrodes of the semiconductor chip connected to leads 2 by means
of thin gold wires 4, which have excellent electrical characteristics. In
the present invention, however, a porous fluorocarbon body 5 is
additionally installed either on the bottom surface of a supporting
portion 1' or on the bottom surface of the chip-mounting portion of a
chip-mounting component 1, as shown in FIG. 2, ensuring that lead
formation and solder coating are conducted in a suitable manner because
the resin 3 encloses entirely and seals such a chip-mounting portion and
porous fluorocarbon body 5.
A fluorocarbon body that is rendered porous by subjecting a
poly(tetrafluoroethylene) film to an expansion (drawing) treatment is
preferable as the porous fluorocarbon body 5. The porosity obtained should
be 30 to 90%, and should preferably be about 50 to 80%.
Fine pores in the body do not transmit some liquids such as water, but
transmit most organic liquids such as the usual organic solvents.
In case of epoxy resin, it is precluded from penetrating inside body 5
because of its high viscosity and the air enclosed into the fine pores in
the body. In order to penetrate such viscous liquid into such fine pores,
impregnation of any liquid having permeability into the pores and
miscibility with the viscous liquid is first done, then the first liquid
is replaced with the viscous liquid. In the present invention, the above
mentioned process is not conducted for the fine pores to be able to hold
the air through which vapor can pass.
However, the porous fluorocarbon body 5 is not limited to only expanded
polytetrafluoroethylene (PTFE) and may also be a continuous foamed body of
fluorocarbon. Further, nonwoven or woven fabric manufactured from a
fluorocarbon fiber may be used. The body 5 may be a product obtained by
including an inorganic filler in a porous body. Especially preferable
fillers are glass, quartz, titanium dioxide, barium titanate, and calcium
titanate because of their excellent thermal conductivity.
In addition to PTFE, tetrafluoroethylene-hexafluoropropylene copolymers
(FEP), polychlorotrifluoroethylene (PCTFE),
perfluoroethylene-perfluoroalkyl vinyl ether copolymers (PFA), and
ethylene-tetrafluoroethylene copolymers (ETFE) can be used as the
fluorocarbon, for example.
Since the porous fluorocarbon 5 in FIG. 2 always has low elasticity and
high porosity, the water vapor that is generated by the rapid heating
during solder mounting is dispersed throughout the structure. The low
elasticity plays an important part in absorbing thermal stress. The
internal stress caused by heat is thus alleviated, thereby preventing
cracks from forming in sealing resin 3. Therefore, it is possible to
provide semiconductor chips with characteristics that are stable over a
long period of time.
As an example, eight samples of each of two types of device were prepared.
In the first type of device that pertains to the present invention, an
epoxy adhesive was applied to the outer surface of porous fluorocarbon 5
which was an expanded porous PTFE film that had a thickness of 50 microns,
a porosity of 80%, and a maximum pore diameter of 1 microns. In the second
type of device, conventional devices without the porous fluorocarbon body
5 were used. The test pieces were first caused to absorb moisture for 72
hours in an atmosphere at a temperature of 85.degree. C. and a humidity of
85% and were then immersed in a solder bath for ten seconds. The presence
of cracks formed in the packages was investigated. The results showed
that, whereas no cracks whatsoever were formed in the examples pertaining
to the present invention, the formation of cracks was detected in all the
conventional examples under the rigorous testing conditions similar to
those described above.
Specific embodiments of the invention are now described wherein small
pieces of porous fluororesin are incorporated into the molding resin
surrounding the electional components. As shown in FIG. 4, a semiconductor
chip 10 is mounted on a chip-mounting element 11 and the electrodes of
chip 10 connected to leads 12 by means of thin gold wires 14 in the same
manner as conventional devices. In the present invention, the
circumference of the chip-mounting element 11 is enclosed and sealed by a
molding resin 13 which contains small pieces 15 of a porous fluororesin.
Small pieces of fluororesin 15 that have been rendered porous by subjecting
the resin, such as polytetrafluoroethylene, for example, to a drawing
treatment or to a treatment with a foaming agent in the case of a
thermoplastic fluororesin, are preferred. The size of the pieces are 1 to
500 microns (preferably 10 to 100 microns) of a porosity of 20 to 90%
(preferably 60 to 80%) (calculated from the density of the resin). Even in
cases when pieces 15 are incorporated into the molding resin 13 in an
amount of 5 to 90% by volume, the fine pores, although transmitting vapor
and other gases, do not transmit liquids, thereby precluding the resin
from penetrating inside the porous structure of the particles. The fine
pores are retained unchanged. Therefore, the vapor that is generated by
rapid heating during solder mounting is effectively absorbed in these fine
pores.
The small pieces 15 of porous fluororesin are not limited to porous
polytetrafluoroethylene but may also be fluororesin foams or filled porous
resins containing an organic filler, for example. Especially preferable
fillers are glass, quartz, titanium oxide, barium titanate, and calcium
titanate because of their excellent thermal conductivity.
In addition to porous polytetrafluoroethylene (PTFE),
tetrafluoroethylene-hexafluoropropylene copolymers (FEP),
poly(chlorotrifluoroethylene) (PCTFE), perfluoroethylene-perfluoroalkyl
vinyl ether copolymers (PFA) and ethylene-tetrafluoroethylene copolymers
(ETFE) can be used as the fluororesins.
Epoxy resins, polyimide resins, bismaleimide triazine resins (BT resins)
and other resins compatible with fluororesins may be used as the molding
resins. However, the use of epoxy resins and polyimide resins is
preferable.
Since the small pieces of a porous fluororesin that are incorporated into
the molding resin as described above always have low elasticity, internal
stress is alleviated and absorbed even during the application of thermal
stress after rapid temperature changes and moisture absorption in the
course of solder mounting, making it possible to prevent, in a suitable
and secure manner, the formation of cracks in the molding resin.
Therefore, it is possible to provide semiconductor chips with
characteristics that are stable over a long period of time.
As an example, twenty samples of each of two types of device were prepared.
In the first type of device that pertains to the present invention, 30
weight percent of the small pieces of porous PTFE of the size of 20
microns, a porosity of 70%, and a maximum pore diameter of 1 micron, were
added to the molding resin. In the second type, conventional devices
without the small pieces of porous fluororesin. The test pieces were first
caused to absorb moisture for 72 hours in an atmosphere at a temperature
of 85.degree. C. and humidity of 85% and were then rapidly heated at
260.degree. C. at 30 seconds. The presence of cracks formed in the resin
was determined. The results showed that no cracks whatsoever formed in the
samples pertaining to the present invention. The formation of cracks was
detected in 70% of the conventional samples under similar testing
conditions.
When an epoxy resin was used as molding resin 13, the dielectric constant
of the devices was 3.6 in conventional samples, but reached only 2.6 in
the samples pertaining to the present invention, wherein 30 weight percent
of the small pieces of porous fluororesin was added to the epoxy resin.
The dielectric constant of the packages was then observed to be lowered.
Through the present invention it is possible to securely prevent crack
formation by alleviating and absorbing in small pieces of porous
fluororesin the stress that is created in a resin layer when vapor is
generated or the temperature is changed as a result of rapid heating
during solder mounting of a resin-sealed semiconductor device. It is also
possible to suppress to a minimum the formation of cracks and to lower the
dielectric constant, thereby ensuring, among other effects, an increase in
the speed with which signals are transmitted in semiconductor devices.
Additionally, the devices have relatively thin-layer films and manufacture
is not complex or expensive.
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