Resin-sealed semiconductor device containing porous fluorocarbon resin

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.

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.

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: 
EQU 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.