Abstract:
A module is placed inside a chamber that can be sealed hermetically. The module includes a substrate incorporating a plurality of electronic circuit parts, a frame connected to the substrate so as to enclose the plurality of electronic circuit parts, and a cover placed facing the substrate and connected to the frame so as to cover an opening portion of the frame. The pressure in the chamber is reduced to a first pressure P1, and then the pressure is increased from the first pressure P1 to a second pressure P2. A liquid is filled into the module through a hole formed in the cover within the chamber, and the pressure inside the chamber is increased from the second pressure P2 to a third pressure P3. Then, the hole of the cover in the chamber is closed to seal the module hermetically, thereby sealing the liquid in the module. Thereafter, the pressure in the chamber is increased from the third pressure P3 to an atmospheric pressure P0, and the module is taken out of the chamber.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a sealing method suited for sealing a liquid coolant into a module, a high-density module in particular, incorporated in a computer product such as a general-purpose computer, a server or the like. 
     A high-density module (hereinafter simply referred to as “module”) incorporated in a computer product such as a general-purpose computer, a server or the like is configured as a housing using a multilayer ceramic substrate as its base portion incorporating a plurality of electronic circuit parts such as ICs, LSIs or the like as its base portion. A heat-transferring liquid coolant is sealed in this housing. This module is mounted in a computer with the liquid coolant sealed in. The heat generated from the electronic circuit parts mounted in the multilayer ceramic substrate is radiated to the outside through the sealed liquid coolant, a comb-like microfin placed on the upper surface of the electronic circuit parts, a cooling material inserted between this microfin and the cover of the module, and an air-cooling fin placed on the top surface of the module. 
     Such a high-density module has a problem of air bubbles trapped in the liquid coolant sealed in. The module has many gaps, a gap between the electronic circuit parts and the substrate, a gap between the microfin and the cooling material, etc. It is difficult to completely fill these gaps with the liquid coolant, and air bubbles remain trapped in these gaps. The existence of these air bubbles prevents the effective cooling of heat generated from the electronic circuit parts. Reduced local cooling efficiency may cause the throughput of the electronic circuit parts to deteriorate. 
     One example of prior art known for solving this problem is disclosed in JP-A-9-213854 specification. This document discloses a method for sealing a liquid coolant with the following steps. 
     That is: 
     1. under a condition where a module is uncovered, fill the liquid coolant into the module beforehand; 
     2. place this module in a chamber that can be hermetically sealed; 
     3. reduce the pressure in the chamber to an approximate vacuum; 
     4. then, introduce an inert gas into the chamber and adjust the pressure inside the chamber to a pressure between an atmospheric pressure and the approximate vacuum; 
     5. hermetically seal the module in that state; and 
     6. increase the pressure inside the chamber to atmospheric pressure and seal the liquid coolant into the module. 
     This method allows air bubbles trapped in the liquid coolant filling the module to be eliminated when the pressure inside the chamber is reduced to the approximate vacuum. 
     SUMMARY OF THE INVENTION 
     As computer products become compact and their processing speed improved, electronic circuit parts mounted on modules are made with higher density and higher integration. This results in an increase in the number of connection terminal between LSIs and the substrate. Thus, both the diameter and pitch of the connection terminals such as solder balls, etc. are further becoming more fine. Therefore, gaps between the electronic circuit parts and the multilayer ceramic substrate are also being narrowed, and it is desired to eliminate air bubbles trapped inside these narrowed gaps. 
     Thus, it is an object of the present invention to provide a method for sealing a liquid into a module, which can seal the liquid into a high-density module housing a substrate on which high-density, highly integrated electronic circuit parts are mounted, with substantially no air bubbles trapped inside. 
     In order to attain the above object, according to the invention, a module housing a plurality of electronic circuit parts is placed in a chamber that can be hermetically sealed. The pressure inside the chamber is set to a liquid filling pressure between a vacuum pressure and atmospheric pressure, and the module is filled with a liquid. Then, the pressure inside the chamber is set to a sealing pressure between the liquid filling pressure and atmospheric pressure. In this state, the module is tightly sealed and the liquid is sealed in the module. 
     Furthermore, the above described sealing pressure is set so that the pressure inside the module becomes lower than atmospheric pressure even while the electronic circuit parts operate and the temperature of the sealing liquid in the module increases. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross-sectional view of a module used in an embodiment of the invention and in a state before sealing; 
     FIG. 2 is a schematic view showing a state of the module placed in a chamber according to the embodiment of the invention; 
     FIG. 3 is a flow chart showing the procedure according to the embodiment of the invention; and 
     FIG. 4 is a view showing a pressure in the chamber at each step according to the embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the invention will be explained below with reference to the attached drawings. The invention, however, is not limited to this embodiment. 
     FIG. 1 illustrates a module  1  used in this embodiment. 
     As shown in the figure, the module  1  includes a ceramic wiring board  2  forming a base portion of the module  1 , a frame  3  soldered to the ceramic wiring board  2  in such a way as to surround LSIs (electronic circuit parts)  20 , which are mounted on this ceramic wiring board  2 , and a cover  4  that covers an opening portion  30  of this frame  3 . 
     The LSI  20  is fixed to the ceramic wiring board  2  via a plurality of solder balls  21  having a diameter on the order of tens of microns. Moreover, a cooling fin  22  is arranged on the upper surface of the LSI  20 . Springs (not shown) are provided between the cooling fin  22  and a cooling material  49 , and the cooling fin  22  is kept in close contact with the LSI  20  by the forces of the springs. 
     At the top end portion of the frame  3 , a flange section  31  extending horizontally is provided. This flange section  31  is formed with a groove  33 , in which a heat-resistant sealing material  32  is disposed to assure airtightness with the cover  4 , and screw holes  34  for fastening screws  40  to secure the cover  4  to the frame. 
     A groove  43  is formed in the periphery of the cover  4 , which corresponds to the groove  33  in the flange section  31  of the frame  3 . Screw holes  44  are also provided in the periphery of the cover  4 , which correspond to the screw holes  34 . Furthermore, two sealing holes  45  are formed in the periphery further inside than the groove  43 , which serve as inlets to charge a liquid coolant. These sealing holes  45  are  10  each provided with screw grooves and flanges. After the module is filled with the liquid coolant, the module  1  can be tightly sealed by pressing O-rings  47  attached to the flange with the sealing screws  46 . Furthermore, an air-cooling heat sink  48  is bonded to the top surface of the cover  4 . A cooling material  49  having fins complementary to the shape of the cooling fin  22  is bonded to the lower surface of the cover  4 . 
     A chamber used to fill the module  1  with the liquid coolant will be explained below. 
     FIG. 2 illustrates a state of the module  1  placed in the chamber  5  that can be hermetically sealed. 
     As shown in the figure, the chamber  5  includes an xy table  6  that positions and fixes the module  1 , a fastening unit  7  that closes the sealing holes  45  of the cover  4  of the module  1  by means of the sealing screws  46 , a filling unit  8  that fills the module  1  with the liquid coolant through the sealing holes  45  of the cover  4 , a gas supply unit  9  that supplies an inert gas and an oil-sealed rotary vacuum pump  10  that deaerates the inside of the chamber  5 . 
     The xy table  6  is a well-known movable table provided movably in the two orthogonal directions on a horizontal plane (x, y directions). The xy table  6  has feed screws provided in the x, y directions, a motor that rotates these feed screws, an encoder and a table that moves according to these feed screws and guides (both not shown). A set jig  60  is provided on the xy table  6  to position and fix the module  1 . A chuck  61  is provided in the chamber  5  to grip the sealing screw  46  described above. 
     The fastening unit  7  includes a motor-driven driver  72  that has a driver bit  71  corresponding to the sealing screw  46 , and a lifting device  74  that moves this driver  72  up and down along a guide  73 . The driver  72  is provided with a torque controller  75  that adjusts a torque for fastening. 
     The coolant filling unit  8  includes a syringe  82  having a supply nozzle  81 , which can be inserted into the sealing hole  45 , and a lifting device  84  that moves this syringe  82  up and down along a guide  83 . The syringe  82  is provided with a piston  85  and a valve  86  so as to adjust the volume of the liquid coolant supplied from the supply nozzle  81 . Opening this valve  86  allows the liquid coolant to be supplied from the supply nozzle  81  by the self weight of the piston  85  and a pressure difference between atmospheric pressure and the inner pressure of the decompressed chamber  5 , as will be described later. 
     The gas supply unit  9  includes a gas bomb  91  filled with compressed nitrogen gas for pressure adjustment and a pipe  92  that leads the nitrogen gas in this gas bomb  91  into the chamber  5 . The pipe  92  is provided with a flow rate control valve  93  that controls the flow rate of the nitrogen gas supplied. 
     Next, the procedure for sealing the liquid coolant into the above described module  1  will be explained with reference to the flow chart in FIG. 3, and FIGS. 2 and 4. Sealing the liquid coolant into the module  1  is carried out at ordinary temperatures. 
     In Step  1 , as shown in FIG. 2, the module  1  before sealing (before filling it with the liquid coolant) is positioned and fixed by the set jig  60  on the xy table  6  disposed in the chamber  5 . At this time, the frame  3  and cover  5  are fastened together with the fastening screws  40 . As shown in FIG. 4, the pressure inside the chamber  5  and the module  1  at this time is atmospheric pressure P 0 . 
     Moreover, the syringe  82  of the coolant filling unit  8  has beforehand been filled with the liquid coolant C. As the liquid coolant C, it is preferable to use thermal conductive, insulating and heat resistant oil. In this embodiment, poly α olefin is used. It is also preferable to use the liquid coolant C well deaerated using a deaerator (not shown) such as a deaerating chamber and stirrer beforehand. 
     The amount of the liquid coolant C to be filled, since the module  1  is normally used in an upright position, is set to take a level at which a LSI at the top of this module in the upright position and the cooling fin attached thereto are almost immersed in the liquid coolant. Accordingly, the nitrogen gas is also contained in the sealed module  1 . This nitrogen gas serves to prevent the oxidation of the liquid coolant C, absorb the volume expansion of the liquid coolant C when the volume of the liquid coolant C expands and prevent the deformation of the frame  3  or defective coupling between the frame  3  and the ceramic wiring board  2 . The volume ratio of the liquid coolant C to the nitrogen gas after filling and sealing of the liquid coolant C in the inner space of the module  1  is set, for example, to 8:2. 
     Then, in Step  2 , the vacuum pump  10  of the chamber  5  is operated and the pressure P 1  inside the chamber  5  is reduced from atmospheric pressure P 0  to an approximate vacuum pressure P 1  (see FIG.  4 ). The pressure P 1  inside the chamber  5 , though it depends on the capacity of the vacuum pump, is from 0.1 to 0.2 Torr in this embodiment, and the chamber  5  and module  1  are deaerated to an approximate vacuum condition. 
     Then, in Step  3 , the vacuum pump  10  is stopped, the flow rate control valve  93  is opened and the nitrogen gas is introduced from the gas bomb  91  into the chamber  5  so that the pressure inside the chamber  5  becomes a predetermined coolant filling pressure P 2 . Due to a pressure increase accompanying the introduction of the nitrogen gas, the pressure inside the chamber  5  and module  1  is increased to the coolant filling pressure P 2 . The coolant filling pressure P 2  is from 50 to 150 Torr in this embodiment. 
     In Step  4 , under this coolant filling pressure P 2 , the supply nozzle  81  is lowered using the lifting device  84  and the end of the supply nozzle  81  is inserted into the sealing hole  45  of the module  1 . Then, the valve  86  is opened and the liquid coolant C stored in the syringe  82  is filled into the module  1  through the sealing hole  45 . During this filling, since the pressure inside the module is lower than atmospheric pressure, the liquid coolant C is filled into the module by this pressure difference and the self weight of the piston. Moreover, since the pressure inside the module  1  is higher than the vacuum pressure, this prevents the entry of air bubbles from the joints of the syringe  82  and nozzle  81  or the reverse spouting of the liquid coolant through the sealing hole  45  of the module  1 , enabling smooth filling while suppressing the mixture of air bubbles during filling to a minimum. 
     Then, in Step  5 , after the filling of the liquid coolant C is completed, the flow rate control valve  93  is opened again and the nitrogen gas is introduced from the gas bomb  91  into the chamber  5  again so that the pressure inside the chamber  5  becomes a sealing pressure P 3  between the liquid coolant filling pressure P 2  and atmospheric pressure P 0 . Due to a pressure increase accompanying the introduction of the nitrogen gas, the pressure inside the chamber  5  and module  1  is increased to the sealing pressure P 3 . This sealing pressure P 3  is set to such a value that the pressure inside the module  1  is lower than the atmospheric pressure P 0  even when the LSI  20  operates and its temperature increases to the allowable upper limit temperature, for example, 70° C. and the temperature of the liquid coolant C increases. In this embodiment, it is set from 400 to 500 Torr. 
     Then, in Step  6 , the module  1  is moved using the xy table  6  and positioned so that the sealing hole  45  comes right under the fastening unit  7 . Subsequently, the driver bit  71  is lowered using the lifting device  74 . By running the motor  72 , the sealing screw  46  held by the chuck  61  is fastened into the sealing hole  45  using the driver bit  71 . At this time, the torque is adjusted by the torque controller  75  so that the sealing screw is fastened with a torque enough to deform the O-ring  47 . 
     This fastening operation is repeated the number of times corresponding in number with the sealing holes  45  provided (two in this embodiment) by moving the module  1  using the xy table  6 . In this way, the module  1  is sealed hermetically and the sealing of the liquid coolant C into the module  1  is completed. 
     Then, in Step  7 , the flow rate control valve  93  is closed to stop the supply of the nitrogen gas from the gas bomb  91 . A leak valve (not shown) of the vacuum pump  10  is opened to introduce external air into the chamber  5 . After ensuring that the pressure inside the chamber  5  has returned to the atmospheric pressure P 0 , the module  1  is taken out of the chamber  5 , and this completes the procedure. 
     According to this embodiment, when filling the liquid coolant C into the module  1 , the pressure inside the chamber  5  is set to the filling pressure P 2 , which is a negative pressure, and therefore it is possible to minimize the generation of air bubbles during filling as compared with the conventional way of filling the liquid coolant C under atmospheric pressure. Thus, this embodiment can improve the cooling performance of the module  1  and thereby the throughput of a computer incorporating this module, as comparison with the conventional way. 
     Moreover, the pressure inside the module  1  is set to be lower than the atmospheric pressure even when the LSI  20  operates and the temperature of the liquid coolant C sealed in the module  1  increases. Accordingly, even if the volume of the liquid coolant C increases as the temperature increases, this will not cause the frame  3  to be deformed or result in defective coupling between the ceramic wiring board  2  and the frame  3 , making it possible to suppress the leakage of the liquid coolant C while the module  1  is operating. 
     In the above embodiment, explained as a module in which a liquid coolant is to be sealed is the air-cooled module having the air-cooling heat sink  48  on the cover  4 . However, the method of sealing a liquid coolant into a module according to the invention is also applicable to a water-cooled module having a cooling water path provided in a module cover if it has a sealing hole that serves as the inlet. 
     As the pressure adjustment gas, it is preferable to use nitrogen gas, which is an inert gas, as is in the embodiment. However, other inert gases such as helium, argon, neon or other gases, if they at least do not deteriorate the effects of the invention, may also be used. 
     In the above described embodiment, two sealing holes are formed as the inlets, but the number of sealing holes may also be one or two or more. 
     Furthermore, the above described embodiment describes the case where sealing operation is carried out at ordinary temperatures, but it is possible to change the temperature for operation as far as it does not deteriorate the effects of the invention.