Abstract:
A method of assembling a semiconductor device forming an encapsulant. The method includes providing a substrate having a plurality of semiconductor devices, respectively connecting a semiconductor chip electrically to a predetermined encapsulation area on a surface of the substrate, filling an encapsulant overlying the predetermined encapsulation area using stencil printing, sweeping excess encapsulant over the predetermined encapsulation area at a first air pressure below approximately 1 atm, sweeping encapsulant overlying the predetermined encapsulation area over the encapsulant overlying the predetermined encapsulation area using stencil printing at a second air pressure above the first air pressure, and hardening the encapsulant at a third air pressure above approximately 1 atm.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a method of assembling a semiconductor device, and more specifically to a method of assembling a semiconductor device forming an encapsulant.  
           [0003]    2. Description of the Related Art  
           [0004]    Referring to FIG. 1A, a plurality of semiconductor devices fabricated using conventional flip chip technology are shown. Substrate  100   a  has a plurality of packaging units  112   a  for assembling a plurality of the semiconductor devices. In FIG. 1A, as the name implies, flip chip technology is characterized by flipping over a semiconductor chip  10   a  for attachment to a substrate, through conductive bumps  12   a . However, as is known, significant strain is imposed on the conductive bumps  12   a  during temperature cycling, when an organic material is used as the substrate. This strain results from the significant difference between the coefficient of thermal expansion between the organic substrate (14-17 ppm/°C.) and the silicon wafer (4 ppm/°C.). Consequently, the conductive bumps  12   a  deteriorate over time at an accelerated rate.  
           [0005]    Therefore, to reduce this connection strain and enhance reliability, encapsulant  120   a  is usually filled into the space between the substrate  100   a  and the semiconductor chip  10   a . In this way, stress is dispersed to the encapsulant to alleviate stress on the conductive bumps  12   a . Thus, connection cracking is significantly reduced, and the life of the conductive bumps  12   a  is prolonged. In addition, the encapsulant  120   a  also prevents the transmission of leakage current caused by impurities between the conductive bumps  12   a . Statistical data shows that the reliability of the device can be increased five to ten times when underfill encapsulation is utilized. Therefore, underfill encapsulation has become a highly important process. However, there are problems that arise in connection with the various ways of performing the underfilling process.  
           [0006]    Conventionally, most flip chip packages are encapsulated by dispensing a liquid encapsulant with low viscosity along the periphery of the chip. Capillary action, generated from the encapsulant in the narrow space between the chip and the substrate, drives the encapsulant to fill the gap between the solder connections. Since filling is conducted by capillary action, it is very slow. For example, in a typical encapsulation operation, the filling takes several minutes to several tens of minutes depending on the filling temperature and chip size. Further, one encapsulating apparatus can only encapsulate a single chip in one run dispensing a liquid encapsulant with low viscosity along the periphery of the chip. The throughput per encapsulating apparatus is very low. Many encapsulating apparatuses are therefore necessary for mass production, negatively affecting the production cost. Furthermore, since capillary action alone is insufficient to fill all the space between the chip and the substrate, voids are easily formed in the encapsulant. Such voids require the flip chip package to be discarded from either popcorn effect, caused during subsequent thermal processes, or stress concentration, caused when the flip chip package is stressed.  
           [0007]    Referring to FIG. 1B, a plurality of semiconductor devices fabricated using conventional wire-bonding technology are shown. Substrate  100   b  has a plurality of packaging units  112   b  for assembling a plurality of the semiconductor devices. Semiconductor chips  10   b  are respectively attached to packaging units  112   b  and connect to substrate  100   b  using wires  12   b . Semiconductor chips  10   b  and wires  12   b  are covered by a molding compound  120   b . Further, another package type, tape automatic bonding (TAB), may be produced when a substrate with a plurality of pre-formed leads (not shown) is used rather than the substrate  100   b  and wires  12   b.    
           [0008]    When the semiconductor devices fabricated using conventional wire-bonding or TAB technology are encapsulated using a liquid encapsulant, a plurality of semiconductor devices can be encapsulated at the same time, but voids are still easily formed in the encapsulant.  
           [0009]    Referring to FIG. 1C, a plurality of semiconductor devices of multi-chip module (MCM) type fabricated using stacked-die technology are shown. Substrate  100   c  has a plurality of packaging units  112   c  for assembling a plurality of the semiconductor devices. Semiconductor chips  10   c  are respectively attached to every packaging unit  112   c  and electrically connect to substrate  100   c  through conductive bumps  12   c . Semiconductor chips  10   d  are respectively attached to and stacked overlying each semiconductor device  10   c  and electrically connect to substrate  100   c  through wires  12   d . Spaces between the substrate  100   c  and semiconductor chips  10   c  are filled by underfill  120   c . Semiconductor chips  10   d , wires  12   d , and underfill  120   c  are covered using a molding compound  120   d . Further, a substrate with both flip chip and TAB type (not shown) may be provided rather than the substrate  100   c , using a plurality of leads (not shown) electrically connecting the substrate and semiconductor chips  10   d.    
           [0010]    The semiconductor device of MCM type shown in FIG. 1C faces challenges in process complexity, process yield, and product reliability resulting from using a plurality of assembling technologies such as flip chip, wire-bonding, and TAB. The aforementioned problems in flip chip package, wire-bonding package, and TAB package will occur at the same time when fabricating the semiconductor device of MCM type. Compared to the semiconductor device fabricated using other assembling technologies, there is further an interface between the underfill  120   c  and molding compound  120   d , a negative factor affecting the process yield and product reliability of the semiconductor device of MCM type.  
           [0011]    Solutions for the void in the encapsulant of the semiconductor device have been disclosed. U.S. Pat. Nos. 5,834,339 and 6,107,123 respectively disclose a method characterized as sealing a gap between a chip and a substrate with a fluid, curable encapsulant using the application of a uniform pressure such as an isostatic or hydrostatic pressure thereto for removal of voids and gas bubbles in the encapsulant. However, the encapsulant is still provided by dispensing, so encapsulating cycle time cannot be improved. Further, an extra tooling is necessary to control the encapsulant flow when applying the uniform pressure, negatively affecting the production cost. Furthermore, the method can only be used in the underfill process. U.S. Pat. No. 6,000,924 discloses a method and device providing a special mold to surround a chip to be encapsulated in a cavity, with an encapsulant injected into the cavity at an elevated pressure, and possibly at an elevated temperature. However, the mold must be specially designed to match the size of the chip to be encapsulated, negatively affecting production cost. Further, the mold still can only encapsulate a single chip in one run, thus the throughput of the mold is not improved. U.S. Pat. No. 6,187,613 discloses a process placing a metal foil on a flip chip that has been connected to a substrate, then applying downward pressure to the metal foil so as to form space between the metal foil, flip chip, and substrate, and filling an encapsulant into the space under pressure. The metal foil is about 0.01 mm to 0.1 mm thick and can withstand an encapsulating pressure exceeding 100 psi. However, the encapsulant is still provided by dispensing, so encapsulating cycle time cannot be improved. Further, the metal foil is very expensive and may be deformed under pressure, negatively affecting the uniformity of the sizes of the encapsulated flip chips. Furthermore, the process still can only encapsulate a single chip in one run, thus the throughput is not improved.  
           [0012]    U.S. Pat. No. 6,046.076 discloses a method of encapsulating a microelectronic assembly applying a flowable encapsulant to the assembly while maintaining the assembly at sub-atmospheric pressure, then bringing the assembly to a higher pressure after completion of the encapsulant-applying step and holding the assembly at the higher pressure, and curing the encapsulant after bring the assembly to the higher pressure. However, the encapsulant is still provided by dispensing, so encapsulating cycle time cannot be improved. Further, the dispensing volume of the encapsulant must be exactly controlled, or the encapsulant can flood the apparatus. Although a mold chest specially matching the size of the assembly can be provided for preventing encapsulant flooding, the production cost is negatively affected. U.S. Pat. No. 6,255,142 discloses a method and apparatus providing a removable seal to encapsulate a semiconductor device therein. The method further provides a pressure differential to fill an encapsulant in a gap between the semiconductor device and a substrate used therewith. The pressure differential is formed using a vacuum source. However, the encapsulant is still provided by dispensing, so encapsulating cycle time cannot be improved. Further, the gap cannot be completely filled only using the pressure differential. Furthermore, the application of the removable seal is not suitable for mass production. U.S. Pat. No. 6,255,142 discloses a method including evacuating a first chamber, placing a semiconductor chip package into a second chamber at atmosphere, evacuating the second chamber and moving the semiconductor chip package to the evacuated first chamber, dispensing an encapsulant material about the periphery of the semiconductor chip package in the evacuated first chamber, evacuating a third chamber, transporting the encapsulated semiconductor chip package from the first chamber into the evacuated third chamber, and vending the third chamber to atmosphere to force the encapsulant material into the semiconductor chip package. However, the encapsulant is still provided by dispensing, so encapsulating cycle time cannot be improved. Further, when a plurality of semiconductor chip packages is encapsulated at the same time using this method, not only is throughput low, but the encapsulating procedure is slow and complicated. Furthermore, the encapsulant cannot completely fill the gap between the semiconductor chip and substrate of the semiconductor chip package at atmosphere.  
           [0013]    As mentioned, none of the aforementioned disclosures perfectly meet the requirements of encapsulating a plurality of semiconductor devices at the same time, preventing voids in the semiconductor devices, completely covering the semiconductor chip specifically for encapsulating the semiconductor device (underfill) of flip chip type, and sustaining compatibility with all assembly technologies, such as flip chip, wire-bonding, TAB, and others.  
         SUMMARY OF THE INVENTION  
         [0014]    Thus, objects of the present invention are to provide a method of assembling a semiconductor device forming an encapsulant, the method allowing a plurality of semiconductor devices to be encapsulated at the same time, preventing voids in the semiconductor device, completely covering the chip when encapsulating the semiconductor device, specifically with flip-chip semiconductor devices, the method further supporting compatibility with all assembly technologies, such as flip chip, wire-bonding, TAB, and others, thereby improving production yield, reliability, lifetime, and throughput of the semiconductor device, and reducing the cost of encapsulating apparatus.  
           [0015]    In order to achieve the exceeding objects, the present invention provides a method of assembling a semiconductor device forming an encapsulant. First, a substrate, having a plurality of semiconductor devices, respectively having a semiconductor chip electrically connected to a predetermined encapsulation area on a surface of the substrate is provided. Next, an encapsulant is filled overlying the predetermined encapsulation area using stencil printing, sweeping excess encapsulant over the predetermined encapsulation area at a first air pressure less than approximately 1 atm. Further, encapsulant overlying the predetermined encapsulation area is swept over the predetermined encapsulation area using stencil printing at a second air pressure exceeding the first air pressure. Finally, the encapsulant is hardened at a third air pressure exceeding approximately 1 atm. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The present invention can be more fully understood by reading the subsequent detailed description in conjunction with the examples and references made to the accompanying drawings, wherein:  
         [0017]    [0017]FIGS. 1A through 1C are cross-sections illustrating conventional semiconductor devices.  
         [0018]    [0018]FIGS. 2A through 2J are cross-sections illustrating a method of assembling a semiconductor device forming an encapsulant in accordance with the first embodiment of the present invention.  
         [0019]    [0019]FIGS. 3A through 3K are cross-sections illustrating a method of assembling a semiconductor device forming an encapsulant in accordance with the second embodiment of the present invention.  
         [0020]    [0020]FIGS. 4A and 4B are photographs illustrating experimental results from example 1 of the working examples described.  
         [0021]    [0021]FIGS. 5A and 5B are photographs illustrating experimental results from example 2 of the working examples described.  
         [0022]    [0022]FIG. 6A is a photograph illustrating experimental results from example 3 of the working examples described. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    The following embodiments and examples are intended to illustrate the invention more fully without limiting the scope of the claims, since numerous modifications and variations will be apparent to those skilled in this art.  
       FIRST EMBODIMENT  
       [0024]    In the first embodiment of the present invention, a semiconductor device of flip chip ball grid array (FCBGA) type is used as an example to describe steps and contributions of the present invention. Note that the semiconductor device of FCBGA type is used as an example, and is not intended to limit the present invention. It will be obvious to those skilled in the art to use semiconductor devices of types such as wire-bonding, TAB, BGA, non-leaded types such as QFN, or others.  
         [0025]    Step 1  
         [0026]    Referring to FIG. 2A, a substrate  100 , such as an organic or ceramic substrate, is provided. The substrate  100  has a predetermined encapsulation area  110 , having a plurality of packaging units  112  on a surface. A top view of the surface is shown in FIG. 2B. Note that the arrangement and quantities of the packaging units  112  of the substrate  100  are used as an example, and are not intended to limit the present invention. As will be obvious to those skilled in the art, other substrates having different arrangement and quantities of packaging units will be equally suitable.  
         [0027]    In FIG. 2A, a plurality of semiconductor chips  10 , respectively having a plurality of conductive bumps  12  arranged at a predetermined pitch among each other on an active surface, is provided. The semiconductor chips  10  are respectively attached to the substrate  100 , more exactly to the packaging units  112 , after respectively aligning the semiconductor chips  10  and their conductive bumps  12  with each packaging unit  112 . As will be obvious to those skilled in the art, other substrates having different arrangement and quantities of packaging units will be equally suitable. Equally obvious should be to preform the conductive bumps  12  on the substrate  100  rather than semiconductor chip  10 .  
         [0028]    Step 2  
         [0029]    In FIG. 2C, the semiconductor chips  10  are electrically connected and fixed on the substrate  100 . The fixing procedure can be such as forming a solder paste (not shown) or flux (not shown) in predetermined positions respectively in the packaging units  112 , respectively aligning the semiconductor chips  10  and their conductive bumps  12  with each packaging unit  112 , respectively attaching the semiconductor chips  10  to the substrate, and reflowing the conductive bumps to electrically connect and fix the semiconductor chips  10  to the substrate  100 .  
         [0030]    Step 3  
         [0031]    In FIG. 2D, the substrate  100  is placed on a platform  52  in an encapsulating apparatus  50 . A stencil  70  is placed on the platform  52  and substrate  100 , with an opening  72  exposing the predetermined encapsulation area  110 . The pressure in the encapsulating apparatus  50  is set to a first air pressure, preferably between about 0.1 and 10 torr. When the pressure in the encapsulating apparatus  50  reaches the set value and stabilizes, an encapsulant  120  is formed overlying the substrate  100  by stencil printing using a wiper  60  sweeping the encapsulant  120  previously placed on the stencil  70  in direction A to form a part of the encapsulant  120  overlying the substrate  100  through the opening  72 . The stencil  70  is usually metal. The thickness of the stencil  70  and the size, quantity, arrangement of the opening  72  depend on the design of the substrate  100 , the size, quantity, and arrangement of the predetermined encapsulation area  110 , and the thickness of encapsulant  120  to be formed overlying the substrate  100 . The compositions of encapsulant  120  depend on the requirements of the semiconductor device.  
         [0032]    Step 4  
         [0033]    [0033]FIG. 2E shows wiper  60  and a part of encapsulant  120  on the other side of the stencil  70 . Some voids  122  are formed during stencil printing in the encapsulant  120  overlying the substrate  100  such as among the conductive bumps  12 . The pressure in the encapsulating apparatus  50  is then set to a second air pressure exceeding the first air pressure. When the pressure in the encapsulating apparatus  50  reaches the set value and stabilizes, voids  122  become smaller or are eliminated. Thus, some recessions  124  may appear on the surface of the encapsulant  120  overlying the substrate  100 . The surface of the encapsulant  120  overlying the substrate  100  is then leveled using the wiper  60  sweeping the encapsulant  120  on the stencil  70  through the opening  72 .  
         [0034]    Further, the thickness of the encapsulant  120  overlying the substrate  100  may substantially equal the sum of the thickness of the conductive bumps  12  and the semiconductor chip  10  to expose another surface thereof. The encapsulant  120  overlying the substrate  100  may completely cover the semiconductor chips  10 .  
         [0035]    Step 5  
         [0036]    In FIG. 2F, the wiper  60  and stencil  70  are removed. At this point, most voids  122 , leaving only a possible few.  
         [0037]    Step 6  
         [0038]    In FIG. 2G, at least one magazine (not shown) is provided to transport a plurality of substrates  100  of the same lot as needed. Next, the magazine transporting the substrates  100  is put in a pressure oven (not shown) or pressure chamber (not shown). The pressure surrounding the substrates  100  is set at a third air pressure of about 1 atm to 30 kgf/cm 2 , preferably about 3 kgf/cm 2  to 15 kgf/cm 2 , to eliminate any remaining voids  122 . The atmosphere surrounding the substrates  100  is preferably dry air, nitrogen, or inert gases.  
         [0039]    It is possible to perform an encapsulant hardening procedure when the substrates  100  are pressurized. The hardening parameters are set according to the characteristics of the encapsulant  120  so as to transfer the encapsulant  120  to a fully-cured molding compound  126  with good elasticity, air-tightness, high chemical stability, and high insulating properties.  
         [0040]    Further, since voids  122  previously formed in the molding compound  126  are completely eliminated after application of the third air pressure, there is no reliability concern from voids in the subsequent processes.  
         [0041]    Step 7  
         [0042]    In FIG. 2H, the substrate  100  may be turned upside down as required to form a plurality of ball-like contacts  130  in predetermined positions on its other surface. The ball-like contacts  130  may be formed by solder paste (not shown) or flux (not shown) in the predetermined positions on the other surface of the substrate  100 , respectively placing a solder ball (not shown) with a predetermined diameter on the solder paste or flux, or reflowing the solder balls to fasten to the substrate  100  so as to form the ball-like contacts  130 .  
         [0043]    Step 8  
         [0044]    In FIG. 2I, the substrate  100  is divided according to the packaging units  112  shown in FIG. 2A using a rotating blade  80  so as to form the semiconductor devices  150  of FCBGA type shown in FIG. 2J.  
         [0045]    Description  
         [0046]    The step 7 in accordance with the first embodiment of the present invention will not be performed when substrate  100  is a lead-frame of QFN type.  
         [0047]    Further, it is possible to perform an extra hardening procedure after step 6 to ensure that the molding compound  126  is completely hardened, under the same conditions as step 6 or at 1 atm, as required.  
         [0048]    According to this embodiment, the inventive method can encapsulate a plurality of semiconductor devices at the same time, prevent voids therein, and completely cover the chip thereof during encapsulation, specifically for flip-chip types, thereby improving the throughput, production yield, reliability, and lifetime of the semiconductor device and reducing the cost of encapsulating apparatus.  
       SECOND EMBODIMENT  
       [0049]    In the second embodiment of the present invention, a semiconductor device of MCM type is used as an example to describe steps and contributions of the inventive method.  
         [0050]    Step 1  
         [0051]    Referring to FIG. 3A, a substrate  200 , such as an organic or ceramic substrate, is provided. The substrate  200  has a predetermined encapsulation area  210 , having a plurality of packaging units  212  on a surface. Top view of the surface, similar to that of the substrate  100  shown in FIG. 2B, is not shown. Note that the arrangement and quantities of the packaging units  212  of the substrate  200  is used as an example, and is not intended to limit the present invention. Equally obvious should be to provide other substrates having different arrangement and quantities of the packaging units from those of the substrate  100 .  
         [0052]    In FIG. 3A, a plurality of semiconductor chips  20 , respectively having a plurality of conductive bumps  22  arranged at a predetermined pitch among each other on an active surface, is provided. The semiconductor chips  20  are respectively attached to the substrate  200 , more exactly to the packaging units  212 , after respectively aligning the semiconductor chips  20  and their conductive bumps  22  with each packaging unit  212 . It will be obvious to those skilled in the art to provide other semiconductor chips having different arrangement and quantities of conductive bumps from those of the conductive bumps  22 . Equally obvious should be to previously form the conductive bumps  22  on the substrate  200  rather than semiconductor chip  20 .  
         [0053]    Step 2  
         [0054]    In FIG. 3B, the semiconductor chips  20  are electrically connected and fixed on the substrate  200 . The fixing procedure can be such as forming a solder paste (not shown) or flux (not shown) in predetermined positions respectively in the packaging units  212 , respectively aligning the semiconductor chips  20  and their conductive bumps  22  with each packaging unit  212 , respectively attaching the semiconductor chips  20  to the substrate, and reflowing the conductive bumps to electrically connect and fix the semiconductor chips  20  to the substrate  200 .  
         [0055]    Step 3  
         [0056]    In FIG. 3C, a plurality of semiconductor chips  30  are provided. Semiconductor chips  30  are respectively attached to each semiconductor chip  20  with an active surface as the top surface. Semiconductor chips  30  are respectively electrically connected to the substrate  200  using wires  32 . Wires  32  are usually Au-based alloy made by adding some microelements to adjust the physical properties of the wire  32 . In some cases, wires  32  may be Al-based alloy.  
         [0057]    It will be obvious to those skilled in the art to change to stacked-die numbers in the semiconductor device. Equally obvious should be to provide a TAB or other method rather than the wire-bonding method.  
         [0058]    Step 4  
         [0059]    In FIG. 3D, the substrate  200  is placed on a platform  54  in an encapsulating apparatus  50 . A stencil  90  is placed on the platform  54  and substrate  200 , with an opening  92  exposing the predetermined encapsulation area  210 . The pressure in the encapsulating apparatus  50  is set to a first air pressure, preferably between about 0.1 and 10 torr. When the pressure in the encapsulating apparatus  50  reaches the set value and stabilizes, an encapsulant  220  is formed overlying the substrate  200  by stencil printing using a wiper  60  sweeping the encapsulant  220  previously placed on the stencil  90  in direction A to form a part of the encapsulant  220  overlying the substrate  200  through the opening  92 . The stencil  90  is usually metal. The thickness of the stencil  90  and the size, quantity, arrangement of the opening  92  depend on the design of the substrate  200 , the size, quantity, and arrangement of the predetermined encapsulation area  210 , and the thickness of encapsulant  220  to be formed overlying the substrate  200 . The compositions of encapsulant  220  depend on the requirements of the semiconductor device.  
         [0060]    Step 5  
         [0061]    In FIG. 3E, wiper  60  and a part of encapsulant  220  are positioned on the other side of the stencil  90 . Some voids  222  are formed during stencil printing in the encapsulant  220  overlying the substrate  200  such as among the conductive bumps  22 . The pressure in the encapsulating apparatus  50  is then set to a second air pressure exceeding the first air pressure. When the pressure in the encapsulating apparatus  50  reaches the set value and stabilizes, voids  222  are reduced in size or eliminated. Thus, some recessions  224  appear on the surface of the encapsulant  220  overlying the substrate  200 . The surface of the encapsulant  220  overlying the substrate  200  is then leveled using the wiper  60  sweeping the encapsulant  220  on the stencil  90  through the opening  92 .  
         [0062]    Further, the encapsulant  220  overlying the substrate  200  must completely cover the semiconductor chips  30  and wires  32 .  
         [0063]    Step 6  
         [0064]    In FIG. 3F, the wiper  60  and stencil  90  are removed. Most of the voids  222  in the encapsulant  220  overlying the substrate  200  have been eliminated, leaving only a possible few.  
         [0065]    Step 7  
         [0066]    In FIG. 3G, at least one magazine (not shown) can be provided for transporting a plurality of substrates  200  of the same lot as needed. Next, the magazine transporting the substrates  200  is put in a pressure oven (not shown) or pressure chamber (not shown). The pressure surrounding the substrates  200  is set at a third air pressure about 1 atm to 30 kgf/cm 2 , preferably about 3 kgf/cm 2  to 15 kgf/cm 2  to eliminate any possibly remaining voids  222 . The atmosphere surrounding the substrates  200  is preferably dry air, nitrogen, or inert gases.  
         [0067]    It is possible to perform an encapsulant hardening procedure when the substrates  200  are pressurized. The hardening parameters are set according to the characteristics of the encapsulant  220  so as to transfer the encapsulant  220  to a molding compound  226  with good elasticity, air-tightness, high chemical stability, and high insulating properties.  
         [0068]    Further, since any voids  222  in the molding compound  226  are completely eliminated after application of the third air pressure, there is no reliability concern in the subsequent processes resulting therefrom.  
         [0069]    Step 8  
         [0070]    In FIG. 3H, the substrate  200  can-be turned upside down as required to form a plurality of ball-like contacts  230  in predetermined positions on the other surface of the substrate  200 . The ball-like contacts  230  may be formed by a solder paste (not shown) or flux (not shown) in the predetermined positions on the other surface of the substrate  200 , respectively placing a solder ball (not shown) with a predetermined diameter on the solder paste or flux, or reflowing the solder balls to fasten the solder balls to the substrate  200  so as to form the ball-like contacts  230 .  
         [0071]    Step 9  
         [0072]    In FIG. 3I, the substrate  200  is divided according to the packaging units  112  shown in FIG. 3A using a rotating blade  80  so as to form the semiconductor devices  250  of MCM type shown in FIG. 3J.  
         [0073]    Description  
         [0074]    Step 8 of this embodiment will not be performed when substrate  200  is a lead-frame of QFN type.  
         [0075]    Further, it is possible to perform an extra hardening procedure after step 7 to ensure that the molding compound  226  is completely hardened. The extra hardening procedure may be performed under the same conditions as step 7 or at 1 atm as required.  
         [0076]    The semiconductor device of MCM type may not only be semiconductor devices  250  formed using stacked-die technology, but as the semiconductor device  260  without stacked die shown in FIG. 3K. It will be obvious to those skilled in the art to design a semiconductor chip with both stacked and unstacked dies.  
         [0077]    According to this embodiment, the inventive method can encapsulate a plurality of semiconductor devices at the same time, prevent voids therein, and completely cover the chip thereof during encapsulation, specifically for flip-chip types, thereby improving the throughput, production yield, reliability, and lifetime of the semiconductor device and reducing the cost of encapsulating apparatus.  
         [0078]    According to the first and second embodiments of the present invention, the method of assembling a semiconductor device forming an encapsulant according to the present invention achieves all desired objects of the present invention.  
       Working Examples  
       [0079]    In the subsequent working examples, three sets of semiconductor devices of FCBGA type are fabricated to demonstrate that the inventive method effectively prevents voids in the molding compound of semiconductor devices, including devices of flip chip type and wire-bonding, TAB, or other designs.  
         [0080]    The fabrication steps of the subsequent working example all follow the steps of the first embodiment because semiconductor devices of FCBGA type are fabricated therein. Examples 1 and 2 are experimental groups, following the first embodiment. Example 3 is a reference group, not completely following the first embodiment. The parameters used in Examples 1 and 2 represent all possible parameters, rather than acting as a limitation. Those skilled in the art will be able to make modifications as required.  
       Example 1  
       [0081]    A first substrate, having a predetermined encapsulation area of approximately 40 mm×40 mm, having nine packaging units arranged in a 3×3 array, was provided. Steps were performed from the first embodiment. In step 3, the stencil opening was approximately 40 mm×40 mm, and the thickness of the stencil approximately 0.8 mm. The provided encapsulant was CV-5400A from Matsushita Electric Works Co. (MEW), with viscosity of approximately 120 Pa.S. The first substrate underwent stencil printing at an air pressure of approximately 0.7 torr, using a Teflon wiper. Before performing step 7, void location and distribution in the encapsulant overlying the first substrate were recorded. In step 7, the first substrate was put in a pressure oven at an air pressure approximately 5 kgf/cm 2  in an atmosphere of dry air. The first substrate was further heated to approximately 100° C. for approximately 1 hour to perform the first hardening procedure during pressurization. After the first hardening procedure, the first substrate was moved to a general oven at 1 atm to be heated to approximately 150° C. for approximately 3 hours to perform the second hardening procedure. Void location and distribution were again checked.  
       Example 2  
       [0082]    A second substrate, having a predetermined encapsulation area of approximately 40 mm×40 mm, having nine packaging units arranged in a 3×3 array, was provided. Steps were performed from the first embodiment. In step 3, the stencil opening was approximately 40 mm×40 mm, and thickness of the stencil approximately 0.8 mm. The provided encapsulant was CV-5351A from MEW, with viscosity of approximately 30 Pa.S. The second substrate underwent stencil printing at air pressure of approximately 0.7 torr, using a Teflon wiper. Before performing step 7, void location and distribution in the encapsulant overlying the second substrate were recorded. In step 7, the second substrate was put in a pressure oven at an air pressure of approximately 5 kgf/cm 2  in an atmosphere of dry air. The second substrate was further heated to approximately 100° C. for approximately 1 hour to perform the first hardening procedure during pressurization. After the first hardening procedure, the second substrate was moved to a general oven at 1 atm to be heated to approximately 150° C. for approximately 3 hours to perform the second hardening procedure. Void location and distribution were again checked.  
       Example 3  
       [0083]    A third substrate, having a predetermined encapsulation area of approximately 40 mm×40 mm, having nine packaging units arranged in a 3×3 array, was provided. Steps were performed from the first embodiment. In step 3, the stencil opening was approximately 40 mm×40 mm, and the thickness of the stencil approximately 0.8 mm. The provided encapsulant was CV-5351A from MEW, with viscosity of approximately 30 Pa.S. The third substrate underwent stencil printing at an air pressure of approximately 0.7 torr, using a Teflon wiper. Before performing step 7, void location and distribution in the encapsulant overlying the third substrate were recorded. In step 7, the third substrate was put in a general oven at 1 atm to be heated to approximately 100° C. for approximately 1 hour to perform the first hardening procedure, and then to approximately 150° C. for approximately 3 hours to perform the second hardening procedure. Void location and distribution were again checked.  
         [0084]    Description  
         [0085]    [0085]FIGS. 4A and 4B are photographs illustrating experimental results from example 1 of the working examples described. FIG. 4A shows a void before performing step 7, and FIG. 4B shows the same void eliminated thereafter, proving that the inventive method effectively prevents voids in the molding compound of semiconductor devices.  
         [0086]    [0086]FIGS. 5A and 5B are photographs illustrating experimental results from example 2 of the working examples described. The difference between examples 1 and 2 is the encapsulant used therein. That used in example 1 has viscosity of 120 Pa.S, exceeding that of example 2 with a viscosity of 30 Pa.S. FIG. 5A shows a void before performing step 7, smaller than that recorded in FIG. 4A, and FIG. 5B shows the same void eliminated thereafter, successfully proving that the inventive method effectively prevents voids in the molding compound of semiconductor devices.  
         [0087]    [0087]FIG. 6A is a photograph illustrating experimental results from example 3 of the working examples described. The difference between examples 3 and 1 is that the encapsulant overlying the third substrate of example 3 is not pressurized as in the first embodiment. The voids recorded before step 7 are not substantially improved after step 7, remaining in the hardened molding compound overlying the third substrate.  
         [0088]    As mentioned, the results from the working examples prove efficacy of the inventive method at effectively preventing voids in molding compound of semiconductor devices, thereby achieving the desired objects of the present invention.  
         [0089]    Although the present invention has been particularly shown and described with reference to the preferred specific embodiments and examples, it is anticipated that alterations and modifications thereof will no doubt become apparent to those skilled in the art. It is therefore intended that the following claims be interpreted as covering all such alteration and modifications as fall within the true spirit and scope of the present invention.