Patent Publication Number: US-2012024593-A1

Title: Printed circuit board unit, method for manufacturing printed circuit board unit, and electric apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of Japanese Patent Application No. 2010-171797, filed on Jul. 30, 2010, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments discussed herein are related to a printed circuit board unit, a method for manufacturing a printed circuit board unit, and an electronic apparatus. 
     BACKGROUND 
       FIG. 8  is a schematic diagram illustrating a printed circuit board  27  (for example, a mother board) on which a semiconductor package, which generally includes a semiconductor chip  21  and a package substrate  24 , is mounted. The package substrate  24  and the printed circuit board  27  have a difference in coefficient of thermal expansion. For example, the package substrate  24  is made of a material with a small coefficient of thermal expansion, such as a glass ceramic, and the printed circuit board  27  is made of a material including a resin or the like that has a relatively large coefficient of thermal expansion. Owing to the difference in coefficient of thermal expansion between the package substrate  24  and the printed circuit board  27 , the amounts of changes in dimensions of the package substrate  24  and the printed circuit board  27  caused by thermal expansion largely differ from each other. The difference in the amount of change in dimension causes strain, and stress concentration occurs at solder joint  28  between the package substrate  24  and the printed circuit board  27 . In particular, the stress tends to concentrate at constricted parts of the solder joint  28 , which are barrel shaped, at the side adjacent to the printed circuit board  27 . As a result, cracks are generated and the connection reliability between the package substrate  24  and the printed circuit board  27  is reduced. 
     In this technical field, there has been a demand for increased functionality (larger number of pins). Accordingly, larger package substrates and semiconductor chips have recently been designed. When the size of the package substrate  24  is increased, the difference in the amount of change in dimension between the package substrate  24  and the printed circuit board  27 , which difference is caused by thermal expansion owing to the difference in coefficient of thermal expansion, is also increased. As a result, the problem of reduction in connection reliability between the package substrate  24  and the printed circuit board  27  due to the above-described cause has become more serious. 
     On the other hand, with increasing awareness on environmental protection, restrictions on chemicals used in electronic devices have been enforced. For example, lead-free solders are now used as solders for mounting package substrates. In general, the lead-free solders have higher melting points than that of lead solder, and do not easily creep. Therefore, when the strain in the joint between the package substrate  24  and the printed circuit board  27  is increased as described above, the lead-free solders do not creep to follow the strain as smoothly as the lead solder. As a result, when a lead-free solder is used, there is a risk that the connection reliability between the package substrate  24  and the printed circuit board  27  will be further reduced compared to the case in which the lead solder is used. 
     Accordingly, Japanese Unexamined Patent Application Publications Nos. 2001-094002 and 2006-339491 disclose structures in which spacer members are provided between the semiconductor package and the printed circuit board. 
     However, when thermally-expandable spacer members are simply arranged between the package substrate and the printed circuit board, the following problem occurs. That is, when the temperature returns to an room temperature after a reflow process and the solder joint is solidified by being cooled, the spacer members contract again. Therefore, it is difficult to accurately control the mounting height of the package substrate. 
     SUMMARY 
     According to an embodiment of the invention, a printed circuit board unit is provided with a first substrate, a second substrate, and a spacer. The second substrate is coupled to the first substrate via a solder material. The second substrate has different coefficient of thermal expansion from the first substrate. The spacer is disposed between the first substrate and the second substrate. The spacer is formed of a thermally-expandable material and a thermosetting material. The thermosetting material has a curing temperature higher than a melting point of the solder material. 
     The object and advantages of the invention will be realized and attained at least by the elements, features, and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory, and are not restrictive of the invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram illustrating a spacer according to a first embodiment of the present invention; 
         FIG. 2  is a schematic diagram illustrating a method for manufacturing a printed circuit board unit according to a second embodiment of the present invention; 
         FIG. 3  is a schematic diagram illustrating a method for manufacturing the printed circuit board unit in reflow process according to the first embodiment of the present invention; 
         FIG. 4  is a schematic diagram illustrating a method for manufacturing the printed circuit board unit in reflow process according to the first embodiment of the present invention; 
         FIG. 5  is a schematic diagram illustrating a method for manufacturing the printed circuit board unit in reflow process according to the first embodiment of the present invention; 
         FIG. 6  is a graph of the relationship between the temperature difference (AT) and the amount of change in dimension of four types of spacers according to an embodiment of the present invention; 
         FIG. 7  is a graph of the relationship between the mounting height and the temperature cycle life of a printed circuit board unit according to an embodiment of the present invention and a printed circuit board unit of a comparative example; 
         FIG. 8  is a schematic diagram illustrating the printed circuit board unit according to the related art; and 
         FIG. 9  is a schematic diagram illustrating an electronic apparatus according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention will now be described with reference to the drawings.  FIG. 1  is a schematic diagram illustrating the structure of a spacer  10  according to a first embodiment of the present invention. In one embodiment, the spacer  10  is placed between a package substrate  24  and a printed circuit board  27  to accurately control a mounting height of the package substrate  24  with respect to the printed circuit board  27  in a reflow process. 
     The spacer  10  may include a body portion  11  and a coating portion  12  surrounding the body portion  11 . The body portion can contain a thermally-expandable material. The coating portion  12  can contain a thermosetting material. The thermosetting material forming the coating portion  12  has a curing temperature (Th) higher than a melting point (Tm) of the solder bumps which are provided on the package substrate  24 . According to an embodiment, the body portion  11  contains the thermally-expandable material, and linearly expands as the temperature increases. The coefficient of linear expansion of the spacer  10  (the thermally-expandable material) is larger than that of the solder material. 
     Therefore, when the package substrate  24  is soldered to the printed circuit board  27  through a reflow process, the body portion  11  of the spacer  10  linearly expands in a vertical direction until the temperature reaches the curing temperature (Th) of the coating portion  12 . When the reflow temperature reaches the curing temperature (Th) of the coating portion  12 , the thermosetting material contained in the coating portion  12  is cured. On the other hand, the body portion  11  linearly expands during reflow by an amount corresponding to the temperature difference between the curing temperature (Th) and room temperature. The temperature difference, the coefficient of thermal expansion of the body portion  11 , and the dimension of the body portion  11  affect the expansion amount of the body portion during reflow. Once the reflow is completed, the printed circuit board unit can be conveyed outside the reflow chamber, and thus the ambient temperature is reduced to room temperature. Nevertheless, thermal contraction of the body portion  11  is suppressed because the coating portion  12  is already cured during the reflow process. Thus, the mounting height of the semiconductor package may be accurately set by employing the above-mentioned spacer  10 . 
     In one embodiment, the curing temperature (Th) of the coating portion  12  is set to be higher than a melting point (Tm) of a solder joint material used to connect the package substrate  24  to the printed circuit board  27 . The solder joint material may include solder bumps provided on the bottom surface of the package substrate  24  and solder pastes provided on the top surface of the printed circuit board  27 . If the curing temperature of the coating portion  12  is lower than the melting point of the solder joint (Th&lt;Tm), the coating portion  12  of the spacer  10  is cured, and, thus, the thermal expansion of the body portion  11  is stopped even before the solder joint material becomes melted. As a result, the desired mounting height cannot be obtained. 
     Various materials known as thermosetting adhesives may be used for the coating portion  12 . For example, phenol resin (PF), epoxy resin (EP), melamine resin (MF), urea resin (UF), unsaturated polyester resin (UP), alkyd resin, polyurethane (PUR), and thermosetting polyimide (PI) may be used. The curing temperatures of phenol resin (PF), epoxy resin (EP), melamine resin (MF), urea resin (UF), unsaturated polyester resin (UP), and alkyd resin are about 120° C. to 200° C. The curing temperature of polyurethane (PUR) is about 60° C. to 100° C., and that of thermosetting polyimide (PI) is about 200° C. to 400° C. 
     The body portion  11  includes a material having a certain coefficient of thermal expansion. The thermally-expandable material of the body portion  11  may be selected from a material having a certain coefficient of thermal expansion such that the body portion  11  accomplishes linear expansion greater than the solder joint by a desired amount in the height direction when the reflow temperature reaches the curing temperature (Th) of the coating portion  12 . The thermally-expandable material may be preferably selected from high-heat-resistance plastics, and more preferably, from plastic materials categorized as “super engineering plastics” in accordance with the desired coefficient of thermal expansion. For example, polyphenylene sulfide (PPS), polyarylate (PAR), polyetherimide (PEI), polysulfone (PSF), polyethersulfone (PES), polyether ketone (PEK), polyether ether ketone (PEEK), polyimide (PI), polyamidoimide (PAI), and polytetrafluoroethylene (PTFE) may be used. 
     The shape of the body portion  11  is not particularly limited. For example, the shape of the body portion  11  may be selected as appropriate from various shapes such as a pillar shape (a circular columnar shape, a rectangular parallelepiped shape, a cubic shape, a polygonal columnar shape, etc.), a plate shape, and a spherical shape in accordance with the structures of the package substrate  24  and the printed circuit board  27  to be connected, and limitations regarding processing thereof, etc. In the present embodiment, the surface of the body portion  11  may be covered (surrounded) by the coating portion  12  having a thermosetting property. Alternatively, the spacer may be formed by dispersing thermosetting resin powders into a matrix of thermally-expandable material. Also, the spacer  10  may be formed of a material having both thermal-expansion and thermosetting property. 
     The vertical length (height) of the body portion  11  can be determined based on the amount of thermal expansion to be accomplished and the coefficient of thermal expansion of the material of the body portion  11 . For example, in the case of a body portion  11  whose initial height at room temperature is greater than the distance (stand-off height) between the package substrate  24  and the printed circuit board  27 , the spacer  10  may be partially affixed within a recess provided on the printed circuit board  27  as illustrated in the example of  FIG. 2 . Alternatively, the spacer  10  can be partially affixed within a through hole provided in the printed circuit board  27 , and the bottom end of the spacer  10  may be secured at an appropriate location in the through hole. Also, the through hole may be formed in the package substrate  24  to secure the top end of the spacer  10  therein. 
     On the other hand, in the case of a body portion  11  whose initial height at room temperature is smaller than the distance (stand-off height) between the package substrate  24  and the printed circuit board  27 , an auxiliary member may be placed between the spacer  10  and the printed circuit board  27  and/or the package substrate  24  so as to absorb the distance gap. 
     Hereinafter, a method for manufacturing a printed circuit board unit according to a second embodiment of the present invention will be described with reference to  FIGS. 2 to 5 . 
       FIG. 2  illustrates a method for manufacturing a printed circuit board unit provided with the spacer  10 , the package substrate  24 , and the printed circuit board (e.g., a mother board)  27 , according to one embodiment. A semiconductor chip  21  is electrically flip-chip mounted on the top surface of the package substrate  24  with conductive bumps  22  provided therebetween. Ball grid array (BGA) solder bumps  25  may be placed on electrodes provided on the bottom surface of the package substrate  24 . Solder pastes  26  can be placed on electrodes provided on the top surface of the printed circuit board  27 . In addition, the spacers  10  according to the first embodiment of the present invention are arranged on the top surface of the printed circuit board  27 . The package substrate  24  and the printed circuit board  27  may be arranged such that the solder bumps  25  are opposed to the corresponding solder pastes  26 . Here, the printed circuit board  27  may be regarded as a first substrate according to the present invention, and the package substrate  24  may be regarded as a second substrate according to the present invention. 
     In the second embodiment, when the package substrate  24  has a rectangular parallelepiped shape, the spacers  10  may be disposed on the printed circuit board  27  at positions corresponding to the four corners of the package substrate  24 . However, the positions at which the spacers  10  are arranged may be set as appropriate in accordance with a shape of a package substrate, arrangements of electronic components, and circuit patterns, etc., taking into consideration processing. In the second embodiment, the spacers  10  are placed on the top surface of the printed circuit board  27 . Alternatively, the spacers  10  may be provided on the bottom surface of the package substrate  24 . 
     In the second embodiment, the semiconductor chip  21  may be electrically flip-chip mounted on the package substrate  24  with the conductive bumps  22  provided therebetween, and an underfill layer  23  can be provided so as to fill the gap between the package substrate  24  and the semiconductor chip  21 . However, this structure is merely an example, and the present embodiment is not limited in any way to this example. 
     In the state in which the package substrate  24  and the printed circuit board  27  are positioned as described above, the package substrate  24  and the printed circuit board  27  are heated to reflow the solder bumps  25  and the solder pastes  26 . When the temperature of the solder bumps  25  and the solder pastes  26  reaches the melting point thereof, the solder bumps  25  and the solder pastes  26  melt and are combined with each other due to surface tension, as illustrated in the example of  FIG. 3 . Thus, the package substrate  24  and the printed circuit board  27  are electrically connected to each other. Then, as illustrated in  FIG. 4 , the body portions  11  having a certain coefficient of thermal expansion, thermally expand in the height (longitudinal) direction and push up (uplift) the package substrate  24  as the temperature increases. As a result, the distance (stand-off height) between the package substrate  24  and the printed circuit board  27  is increased by the spacers  10  while solder joint  28  are in a molten state. Until the temperature reaches the curing temperature of the coating portions  12  of the spacers  10 , the body portions  11  continue to thermally expand in the height direction. 
     When the temperature is further increased and reaches the curing temperature (Th) of the thermosetting material contained in the coating portions  12 , the coating portions  12  are cured with the thermal expansion amount of the body portions  11  corresponding to the curing temperature (Th), as illustrated in  FIG. 5 . Once the coating portions  12  are cured, further thermal-expansion of the body portions  11 , which are surrounded by the cured coating portions  12 , is restrained. This allows the printed circuit board unit to maintain the uplifted stand-off height. After the reflow process, the temperature returns to the room temperature. However, since the coating portions  12  have been already cured, the spacers  10  do not thermally contract and the uplifted stand-off height is still maintained. As a result, the desired mounting height is reliably obtained. Accordingly, in one example, a shape of the solder joint  28  is formed similar to a pillar in accordance with the increase in the stand-off height. Thus, even when thermal strain is propagated to the solder joint  28  due to difference in the coefficient of thermal expansion between the package substrate  24  and the printed circuit board  27 , the thermal stress may be dispersed over the entire bodies of the pillar-shaped solder joint  28  as compared to the barrel-shaped solder joint in the related art. As a result, defects such as cracks in a solder joint may be effectively reduced. 
     With the spacer according to the first embodiment of the present invention, the amount of thermal expansion of the body portion in the vertical (longitudinal) direction may be accurately controlled by selecting a thermosetting material having a certain curing temperature (Th) as the coating portion, or by selecting a thermally-expandable material having a certain coefficient of thermal expansion as the body portion. By employing the spacer according to the first embodiment of the present invention, thermally-expanded spacer may be cured in the reflow process while the package substrate  24  is uplifted with the molten solder joint. Even when the temperature is reduced to the room temperature after the reflow process, the uplifted height of the spacer may be maintained. As a result, the mounting height of the package substrate with respect to the printed circuit board may be accurately controlled to a desired height. 
     According to embodiments of the present invention, despite a difference of coefficient of thermal expansion between the package substrate and the printed circuit board in the soldering process, thermal stresses may be suppressed from concentrating on the constricted parts of the solder joint, and be dispersed over the entire bodies of the pillar-shaped solder joint  28 . This allows for an improvement in the connection reliability of the solder joint in the printed circuit board unit. 
     As an acceleration test for evaluating the connection reliability of an electronic component, a heat cycle test under the conditions described below may be performed. Specifically, each solder joint to be tested is electrically connected in series with a daisy chain configuration to form a circuitry. A given direct current (DC) is applied to the circuitry. In this state, the temperature is set to −65° C. for 15 minutes, to the room temperature for 2 minutes, and to 125° C. for 15 minutes. The heat cycle is repeated. Since each solder joint is electrically connected with the daisy chain configuration, the direct current stops flowing when the electrical connection is terminated at any of the solder joint. The number of the heat cycles which have been repeated until the direct current is interrupted, is recorded. This may be used as an index of the connection reliability. The larger the recorded heat cycles, the higher the connection reliability in solder joints. 
       FIG. 9  is a schematic diagram illustrating a server computer  110  as an example of an electronic apparatus according to the present invention. The server computer  110  includes an enclosure  120 . A housing space is provided in the enclosure  120 . A board unit including a mother board, which is an example of a printed circuit board  27 , and a semiconductor package mounted on the mother board is placed in the housing space. Another example of the electronic apparatus according to the present invention is a supercomputer. 
     In the above description, the solder joint between the semiconductor package and the printed circuit board, such as a mother board, is explained. However, the above-described embodiments may also be applied to a solder joint between other types of substrates, such as a semiconductor chip and a package substrate. Also in this case, the connection reliability may be enhanced. 
     Hereinafter, experimental examples according to embodiments of the invention are explained. As a body portion of a spacer, four types of high-heat-resistance plastics each of which has different coefficients of thermal expansion (50 ppm/° C., 60 ppm/° C., 70 ppm/° C., and 80 ppm/° C.) were prepared as seen in the example of  FIG. 6 . The height of these components was set to 3.43 mm. The above components were coated with thermosetting epoxy resin having a curing temperature of about 230° C. by dip coating. The coating portion was formed, and the spacer was completed. Assuming that the room temperature is 25° C., the temperature difference (AT) between the room temperature and the curing temperature of the spacer is 230° C.−25° C.=205° C. 
     In the experiments, it was aimed that 430 μm of the initial stand-off height of the package substrate is to be uplifted to 600 μm by using the above-mentioned spacer. Therefore, 170 μm of the stand-off height is to be increased by thermal expansion of the spacer in the height (vertical) direction.  FIG. 6  illustrates a graph of the relationship between the temperature difference (ΔT) and the amount of thermal expansion according to the four types of spacers. Referring to the example of  FIG. 6 , it may be concluded that the spacer having a coefficient of thermal expansion of 70 ppm/° C. is to be used for achieving the aimed stand-off height of 170 μm at the above-mentioned temperature difference (ΔT=205° C.) (see the box-shaped area in  FIG. 6 ). Thus, the structure of spacers (combination of materials of the body portion and the coating portion) may be determined by the above-described procedure in accordance with the desired mounting height of the package substrate. 
     With the spacer having the coefficient of thermal expansion of 70 ppm/° C. and the curing temperature of 230° C., a mounting experiment of a BGA semiconductor package was performed on a printed circuit board in accordance with embodiments of the invention with reference to  FIGS. 2 to 5 . 
     As described above, in this example, the mounting height of the semiconductor package that is 430 μm without spacers is to be increased to 600 μm by using the spacers having a curing temperature of 230° C. and a coefficient of thermal expansion of 70 ppm/° C. Therefore, it is desirable that the amount of projection of the spacers from the printed circuit board is 430 μm at the room temperature (25° C.) before the reflow heating process, and is 600 μm at the curing temperature (Th=230° C.) of the coating portions of the spacers. Accordingly, the spacers of this example, which have a height of 3.43 mm, were placed between the printed circuit board and the package substrate such that the spacers were embedded by 3 mm in recesses formed in the printed circuit board. In this example, the reflow temperature was about 250° C. 
     A temperature acceleration test was performed under the above-described conditions by using a semiconductor package according an embodiment of the present invention that was mounted as described above and a semiconductor package according to a comparative example without using spacers. Thus, the connection reliability of the solder joint was evaluated for each semiconductor package. 
       FIG. 7  illustrates a graph of the relationship between the mounting height and the number of times the heat cycle was repeated in the temperature accelerated test for the semiconductor package of the example and the semiconductor package of the comparative example. As is clear from  FIG. 7 , in the semiconductor package manufactured by using the spacers according to the present example, the mounting height was increased from 430 μm to 600 μm, compared the semiconductor package of the comparative example. According to the increase in the mounting height, the number of times the temperature cycle was repeated in the temperature accelerated test was largely increased from 720 to 1,000. 
     As is clear from this result, the mounting height of the semiconductor package with respect to the printed circuit board may be accurately controlled by using the spacers according to embodiments of the present invention which are coated with a coating material having a curing temperature higher than or equal to a melting point of the solder. As a result, the connection reliability of the solder joint may be largely increased. 
     All examples and conditional language recited herein are intended for pedagogical objects to aid the reader in understanding the invention and the concepts contributed by the inventors to further the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority or inferiority of embodiments of the invention. Although the embodiments of the invention have been described in detail, it will be understood by those of ordinary skill in the relevant art that various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention as set forth in the claims.