METHOD AND APPARATUS FOR AIRTIGHTLY PACKAGING SEMICONDUCTOR PACKAGE

Abstract: An airtight packaging apparatus for covering with a cap (1) a multichip module having LSI chips (4) mounted on a ceramic substrate (2) and for sealing the cap and substrate by means of solder (12, 13) includes a receptacle (6) for ac­ commodating the multichip module, valves (8, 9) for supply­ ing gas to the receptacle and exhausting the receptacle, a retainer (14, 15) for holding the cap and ajusting the posi­ tional relation with respect to the multichip module, and a heater (16, 16′) for heating solder of the junction between the multichip module and the cap. After solder has been heated and melted, the multichip module and the cap are sealed to each other.

Description: BACKGROUND OF THE INVENTION The present invention relates to a method and an apparatus for airtightly packaging a semiconductor package, and more particular to a method and an apparatus for airtightly packaging a semiconductor package suitable to sealing with solder a multi-chip module having a large number of LSI chips mounted on a ceramic substrate. A semiconductor package is formed by attaching a cap to a substrate such as a ceramic substrate having semiconductor devices mounted thereon by means of solder and by sealing them. The semiconductor device is airtightly packaged in a space formed by the substrate and the cap. Airtight solder packaging for a multi-chip module having semiconductor devices mounted on a ceramic substrate is conducted in a hermetically sealed receptacle as described in Japanese Patent Unexamined Publication Nos. 5570/78 and 20860/78 (JP-A-53-5570 and JP-A-53-20860), for example. The latter (also published by Patent Abstract of Japan, Vol. 2, no. 56, 24 April 1978) discloses a method and an apparatus comprising the precharacterizing features of the present claims 1 and 3, respectively. As disclosed in these publications, the difference in temperature and pressure between the inside and outside of the module caused during the solidification process of solder is eliminated in order to prevent a short circuit or the like due to projection of solder in the space within the package. In methods described in Japanese Patent Unexamined Publication Nos. 148445/80 and 22493/77 (JP-A-55-148445 and JP-A-52-22493), metal for absorbing the surplus solder is disposed at a minute interval on the internal periphery of the metal portion whereto solder is to be attached. In a method disclosed in Japanese Patent Unexamined Publication No. 7172/78 (JP-A-53-7172), a groove for stopping the surplus solder is provided instead of the metal for absorbing the surplus solder. In the above-described publications, it is assumed that the weight of either the cap or the substrate (wiring substrate) is smaller than the surface tension of the solder for airtight packaging. If the weight of the cap or the wiring substrate is increased due to a large-sized package, for example, the packaging solder is crushed and scattered inside and outside the package when it is melted. As a result, the desired amount of solder may not be ensured or an electrical short circuit may be caused. It is necessary to ensure a sufficient amount of sealing solder for the purpose of absorbing the thermal strain as a buffer when there is a difference in thermal expansion between the cap and the wiring substrate and for the purpose of reliable soldering. From this point of view, a method for packaging the heavy cap and the wiring substrate by means of solder has been demanded. SUMMARY OF THE INVENTION An object of the present invention is to provide a highly reliable method for airtightly packaging a semiconductor package whereby a desired solder connection shape is obtained independently of the weight of the cap or the substrate. Another object of the present invention is to provide an airtight packaging apparatus effecting the above-described method. Said first object is achieved, according to the present invention, by a method as claimed in claim 1. Said second object is achieved, according to the present invention, by an apparatus as claimed in claim 3. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view for illustrating the structure of an apparatus according to the present invention; Fig. 2 is an operation time chart of the apparatus shown in Fig. 1; Fig. 3 is a sectional view of a sealed semiconductor package; and Fig. 4 shows the relation between the solder height and the number of repeated thermal strains eventually resulting in He leak. DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 1 shows an embodiment of the present invention. An exhaust valve 8 and a sealing gas supply valve 9 are connected to a receptacle 6 for airtight packaging. A substrate 2 of a semiconductor package having semiconductor devices 4 mounted thereon is placed on a stand contained in the receptacle 6. On the other hand, a cap 1 of the semiconductor package is retained by a retainer 14. Thermal conductive connectors 3 mounted inside the cap 1 come in contact with backs of the semiconductor devices 4 and transfer heat generated by semiconductor devices to the cap 1 to dissipate the heat. However, the thermal conductive connectors are not essential to the semiconductor package. The retainer 14 extends to the outside of the receptacle 6 and is moved upward and downward by servomechanism 15. In addition, a heater 16 for heating is disposed near seal portions of the substrate 2 and the cap 1. In the embodiment of Fig. 1, a heater 16' is disposed near the seal portion of the retainer 14. The seal portions of the cap 1 and the substrate 2 are provided with presolders 12 and 13 for sealing. A programmable controller 18 controls the valves 8 and 9, the servomechanism 15 and the heaters 16 and 16'. Fig. 2 is a time chart for illustrating the operation of the apparatus of Fig. 1 and represents the temperature T of a seal region (inclusive of the seal portions of the cap and the substrate) within the receptacle, gas pressure P within the receptacle, and gap G between the seal portions of the cap and the substrate respectively as functions of time. With reference to Fig. 1, the inside of the receptacle 6 is once evacuated to eliminate the air, and the temperature of the seal region is raised from the room temperature to approximately 150°C to effect vacuum baking. By the vacuum baking, the gas absorbed by the seal portions is eliminated. After completion of the vacuum baking, the temperature T is raised to and kept at approximately 230°C. And the sealing gas such as He gas is supplied into the receptacle 6 through the valve 9 to bring the inside of the receptacle 6 to approximately 3 atmospheric pressure and maintain that pressure. Thereby, the presolders 12 and 13 provided at the seal portions of the cap 1 and the substrate 2 are melted. At time instant t₁, the retainer 14 for retaining the cap 1 is lowered by the servomechanism 15 to joint the solders 12 and 13. In order to eliminate bubbles contained in the melted solders, the cap 1 is lowered at such a speed that the solders will not be scattered. As shown in Fig. 2, the gap G between the seal portions is preferably reduced to a value (0.2 mm, for example) smaller than a predetermined gap (0.5 mm, for example) and thereafter adjusted to have the predetermined gap. If the semiconductor package is increased in size, solder of the cap is not in contact with that of the substrate over some part of the whole periphery due to warp of the substrate, resulting in a packaging failure. This can be prevented by once pulling down the cap 1 lower than the predetermined height as described above. Further, the solder 17 takes the shape of hand-drum as shown in Fig. 3 representing the completed package by once pulling down the cap 1 lower than the predetermined height. When the semiconductor device is mounted on the substrate by means of solder balls, it is said that solder shaped like a hand-drum provides a higher connection reliability. In the packaging according to the present invention as well, a similar effect can be anticipated by making solder shaped like a hand-drum. From time instant t₂ whereat the gap between the cap and the substrate has reached the predetermined value, the heater temperature is lowered to start the cooling process. Since the temperature within the package generally drops faster than that within the receptacle, the gas pressure P₁ within the package becomes lower than the gas pressure P₂ within the receptacle. Therefore, it is necessary to maintain the relation P₁ = P₂ until the temperature whereat solder is solidified is reached, by controlling the valves to control the gas pressure of the receptacle concurrently with the drop in temperature. If the gas pressure P₁ within the package becomes lower than the gas pressure outside the package, the melted solder flows into the space within the package, resulting in solder projection. In order to attain the relation P₁ = P₂ in the present apparatus, the gas pressure P₁ within the package is experimentally derived with respect to various cooling instants. The resultant characteristic curve representing the gas pressure P₁ within the package as a function of cooling time t is stored in the controller 18. A curve P of Fig. 2 after time instant t₂ is that characteristic curve. After cooling has been started, the controller 18 derives the gas pressure within the receptacle on the basis of the characteristic curve and controls the valve 8. Such control for attaining the relation P₁ = P₂ is conducted until solder is solidified. In Pb-Sn eutectic solder, for example, control for attaining the relation P₁ = P₂ is effected until the solid phase temperature of 183°C is reached. Fig. 4 shows experimental results concerning the number of repeated thermal strains causing a leak failure of He contained in a semiconductor package sealed by means of solder in accordance with the method of Fig. 1. The number is plotted as a function of the height of sealing solder. As the solder height is increased, the number of thermal strains until the He leak is caused is also increased, resulting in a longer life. By increasing the solder height, therefore, it is possible to absorb the difference in thermal expansion between the cap and the substrate. Accordingly, is becomes possible to select a material such as copper or aluminum having a property such as high thermal conductivity desired as the property of the cap without being subject to restriction by the thermal expansion difference from the substrate made of alumina ceramic or mullite ceramic, for example. It is thus possible to realize high seal reliability and cooling capability. Therefore, the controller 18 adjusts the solder height so as to produce flexibility enough to absorb the thermal expansion difference between the substrate and the cap. As described above, the present invention provides an accurate solder connection shape irrespective of the weight of the cap or wiring substrate. As a result, a highly reliable structure for airtightly packaging a semiconductor package can be obtained.

Claims: A method for airtightly packaging a semiconductor package by sealing with solder a cap (1) with respect to a substrate (2) having semiconductor devices (4) mounted thereon, comprising the steps of: placing said substrate (2) and said cap (1) within a receptacle (6) having a controlled gas pressure; melting said solder and adjusting a gap between a seal portion of said substrate (2) and a seal portion of said cap (1) by a relative movement between said substrate (2) and said cap (1) with at least one of said substrate (2) and said cap (1) being held, so that the seal portions of said substrate (2) and said cap (1) come in contact with each other through the melted solder; and cooling said seal portions, characterized in that the adjustment of said gap includes narrowing said gap to a value lower than a predetermined height and thereafter keeping said gap at said predetermined height, and that the relation P₁ = P₂ of the gas pressure P₁ within the package and the gas pressure P₂ within the receptacle (6) is maintained, until the temperature at which said solder is solidified, is reached. A method according to claim 1, wherein the gas pressure P₂ in said receptacle (6) is lowered during said cooling step. An apparatus for airtightly packaging a semiconductor package by sticking with solder a cap (1) to a substrate (2) having semiconductor devices (4) mounted thereon and sealing them, comprising: a receptacle (6) for accommodating said semiconductor package therein; means (8, 9) for supplying gas to said receptacle (6) and exhausting said receptacle; holding means (14, 15) for holding at least one of said substrate (2) and said cap (1) in said receptacle (6) and adjusting a positional relation between said substrate and said cap; heating means (16, 16') for heating solder at a seal region between said substrate (2) and said cap (1) to melt the solder; and control means (18) for controlling said gas supplying and exhausting means (8, 9) to control the gas pressure in said receptacle (6), for controlling the movement of said holding means (14, 15), and for controlling said heating means (16, 16'), characterized in that after said solder has been melted, said control means (18) controls said holding means (14, 15) so that a gap between said substrate (2) and said cap (1) is changed to a height lower than a predetermined height and thereafter to said predetermined height, said control means (18) controls said heating means (16, 16') to start cooling upon change of said gap to said predetermined height, and said control means (18) includes means for controlling said gas supplying and exhausting means (8, 9) to make the gas pressure P₂ in said receptacle (6) equal to the gas pressure P₁ in said semiconductor package.

Inventors: KOBAYASHI, FUMIYUKI, TAKENAKA, TAKAJI, WATANABE, HIDEKI
Applicants: HITACHI, LTD.
Application year: 1986
Main classification: H01L21
IPC codes: H01L21, H01L23