Patent Publication Number: US-2010127393-A1

Title: Electronic device and semiconductor device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electronic device and a semiconductor device. 
     Priority is claimed on Japanese Patent Application No. 2008-299277, filed Nov. 25, 2008, the content of which is incorporated herein by reference. 
     2. Description of the Related Art 
     Generally, a BGA (Ball Grid Array) semiconductor device includes: a wiring board  2  having a surface  2   a  on which multiple lands  3  are provided and a surface  2   b  on which multiple connection pads  4  electrically connected to the lands  3  are provided; a semiconductor chip  5  mounted on the surface  2   b  of the wiring board  2 ; wires  7  electrically connecting electrode pads  6  on the semiconductor chip  5  and connection pads  4  on the wiring board  2 ; a seal  8  covering at least the semiconductor chip  5  and the wires  7 ; and solder balls  9  that are bumps on the lands  3 , as shown in  FIG. 5 . 
     Recently, such a BGA semiconductor device  1  and components forming the semiconductor device  1  have been further miniaturized to meet the demand for thinner and denser semiconductor devices. For example, the solder ball  9  is substantially 0.3 mm in diameter. 
     The BGA semiconductor device  1  has a problem of poor connection of bumps, such as the solder balls  9 , to be mounted on an external board for second mounting due to a decrease in the connection strength. One of the causes of the poor bump connection is a fluctuation in alloy growth between the bump and the land  3 . 
     Generally, both overheating and insufficient heating causes insufficient alloy growth, thereby degrading the connection strength. Generally, bumps are mounted on the corresponding lands  3 , and then the entire semiconductor device  1  is reflowed to implement bump connection. 
     The semiconductor device  5  is made of, for example, silicon having a specific heat capacity greater than that of the seal resin. For this reason, a difference in heating conditions occurs between the chip area where the semiconductor chip  5  is mounted and the chip-free area, thereby causing a fluctuation in alloy growth between the lands  3  under the chip area and the lands  3  under the chip-free area, resulting in a degradation of the connection strength. 
     To improve the poor bump connection, Japanese Patent Laid-Open Publication No. 2001-210749 (hereinafter, “Patent Document 1”) discloses a bump structure in which only corner bumps are made larger in size. Japanese Patent Laid-Open Publication No. H09-162531 (hereinafter, “Patent Document 2”) discloses a bump structure in which outermost bumps close to four corners are disposed along a concentric circle. 
     Further, Japanese Patent Laid-Open Publication No. 2000-243792 (hereinafter, “Patent Document 3”) discloses a BGA package including via holes in which lands outside the chip area are made larger to prevent resin from leaking through the via holes. 
     Concerning the bump structures disclosed in Patent Documents 1 and 2, the sizes and the positions of bumps are changed from a general grid arrangement, thereby requiring a design change of the lands on the external surface of the wiring board. The design change of the lands is against the demand for versatile semiconductor devices, thereby making it difficult to commercialize such a semiconductor device having the disclosed bump structure. 
     Further, Patent Document 3 does not disclose a technique of improving the connection strength in the case of a BGA semiconductor device having no via hole. 
     SUMMARY 
     In one embodiment, an electronic device may include, but is not limited to: a wiring board having first and second regions; a plurality of first lands in the first region; a plurality of second lands in the second region; and an insulator covering the wiring board. More heat is applied to the first region than the second region. The second land is smaller in volume than the first land. The insulator has a plurality of openings being adjacent to the plurality of first lands and the plurality of second lands. Each of the plurality of openings has substantially the same area. 
     In another embodiment, a semiconductor device may include, but is not limited to: a wiring board having first and second regions; a plurality of first lands in the first region; a plurality of second lands in the second region; an insulator covering the wiring board; a semiconductor chip covering the second region; and a seal covering the first region and the semiconductor chip. The second land is smaller in volume than the first land. The insulator has a plurality of openings being adjacent to the plurality of first lands and the plurality of second lands. Each of the plurality of openings has substantially the same area. The seal and the semiconductor chip are on the same side with respect to the wiring board. 
     In still another embodiment, an electronic device may include, but is not limited to: a wiring board having first and second regions; a plurality of first lands in the first region; a plurality of second lands in the second region; and an insulator covering the wiring board. The second land is smaller in volume than the first land. The insulator has a plurality of openings being adjacent to the plurality of first lands and the plurality of second lands. Each of the plurality of openings has substantially the same area. 
     Accordingly, alloy growth between a land and a bump can be uniformed for every land to prevent poor bump connection without changing the conventional design, such as the arrangement of lands in a grid, the areas of openings included in the insulating film, and the sizes of the bumps. 
     In other words, the volume of the land outside the specific region is larger than that of the land inside the specific region, less heat being applied to the specific region than the other region, thereby achieving a uniform increase in temperature of every land. Therefore, a uniform alloy growth between the land and the bump is achieved for every land, thereby preventing a fluctuation in the connection strength. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above features and advantages of the present invention will be more apparent from the following description of certain preferred embodiments taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a cross-sectional view illustrating a semiconductor device according to a first embodiment of the present invention; 
         FIG. 2  is a plane view taken along a line A-A′ shown in  FIG. 1 ; 
         FIG. 3  is a cross-sectional enlarged view illustrating bumps shown in  FIG. 1 ; 
         FIG. 4  is a plane view taken along a line B-B′ shown in  FIG. 3 ; and 
         FIG. 5  is a cross-sectional view illustrating a conventional semiconductor device. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described herein with reference to illustrative embodiments. The accompanying drawings explain a semiconductor device in the embodiments. The size, the thickness, and the like of each illustrated portion might be different from those of each portion of an actual semiconductor device. 
     Those skilled in the art will recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the present invention is not limited to the embodiments illustrated herein for explanatory purposes. 
       FIG. 1  is a cross-sectional view illustrating a semiconductor device  1 A according to a first embodiment of the present invention.  FIG. 2  is a plane view taken along a line A-A′ shown in  FIG. 1 .  FIG. 3  is a cross-sectional enlarged view illustrating bumps shown in  FIG. 1 .  FIG. 4  is a plane view taken along a line B-B′ shown in  FIG. 3 . 
     As shown in  FIG. 1 , the semiconductor device  1 A is a BGA semiconductor device and includes: a wiring board  2  that is substantially rectangular in a plane view in the direction perpendicular to surfaces  2   a  and  2   b  of the wiring board  2 ; solder balls  9  that are bumps on the surface  2   a  of the wiring board  2 ; a semiconductor chip  5  on the surface  2   b  of the wiring board  2 ; and a seal  8  covering the semiconductor chip  5  and the surface  2   b  of the wiring board  2 . 
     The wiring board  2  is a glass epoxy board having a thickness of, for example, 0.25 mm. Wires (not shown) are provided on both surfaces of the glass epoxy substrate. The wires are covered by a solder resist film  11  that is an insulating film having multiple openings  10 ,  20 . 
     Multiple lands  3  cover the wires on the surface  2   a  seen through the openings  10 . The land  3  is made of, for example, a Cu material and Ni/Au plating. As explained later, every opening  10  has substantially the same area. Multiple connection pads  4  cover the wires on the surface  2   b  seen through the openings  20 . 
     The connection pads  4  are electrically connected to the corresponding lands  3  through internal wires  12  in the wiring board  2 . The lands  3  are arranged in a grid at a given pitch, for example, the 1 mm pitch. 
     The semiconductor chip  5  is fixed on substantially the center of the surface  2   b  of the wiring board  2  through a fixing member  13 , such as an insulating adhesive or a DAF (Die Attached Film). 
     The semiconductor chip  5  is substantially rectangular in a plane view in the direction perpendicular to the surface  2   b  of the wiring board  2 . A circuit, such as a logic circuit or a memory circuit, is formed on a surface  5   a  of the semiconductor chip  5 . 
     Multiple electrode pads- 6  are provided on a periphery of the surface  5   a  of the semiconductor chip  5 . A passivation film (not shown) covers the surface  5   a  excluding regions of the electrode pads  6  to protect the circuit formation surface. 
     The electrode pads  6  on the semiconductor chip  5  are electrically connected to the corresponding connection pads  4  on the wiring board  2  using conductive wires  7  made of, for example, Au or Cu. 
     Thus, the semiconductor chip  5  and the lands  3  are electrically connected through the wires  7 , the connection pads  4 , and the internal wires  12 . 
     The seal  8  covers substantially the entire surface  2   b  so as to cover the semiconductor chip  5  and the wires  7 . The seal  8  is made of a thermosetting resin, such as an epoxy resin. The seal  8  has a thickness of, for example, substantially 400 μm. 
     The solder balls  9  that are bumps are mounted on the corresponding lands  3  on the surface  2   a  of the wiring board  2 . 
     As shown in  FIGS. 1 to 4 , the lands  3   a  are disposed outside a specific region  14  on the surface  2   a  corresponding to the chip region on the surface  2   b  on which the semiconductor chip  5  is mounted. The lands  3   b  are disposed inside the region  14 . 
     The diameter Xa of the land  3   a  is greater than the diameter Xb of the land  3   b . In other words, the land  3   a  has a larger area than the land  3   b.  Specifically, the area of the land  3   a  is 1.1 to 2.0 times larger than that of the land  3   b.    
     The land  3   a  and the land  3   b  have the same thickness. Consequently, the land  3   a  has a larger volume than the land  3   b.  Specifically, the volume of the land  3   a  is  1 . 1  to 2.0 times larger than that of the land  3   b.    
     The openings  10  inside and outside of the region  14  have the same diameter Y, and therefore have the same area. The solder balls  9  inside and outside of the region  14  are of the same size. 
     Since the semiconductor device  1 A has the above structure, a specific heat capacity of the land  3   a  is greater than that of the land  3   b.  Consequently, the temperature of the lands  3   a  is prevented from increasing at the time of heating in a reflow process. For this reason, a fluctuation of alloy growth can be reduced, thereby achieving better bump connection. 
     The area ratio of the land  3   a  to the land  3   b  may be adequately adjusted in a range from 1.1 to 2.0 so that the degree of alloy growth and the degree of melting and solidification of the solder balls  9  are equalized between inside and outside of the region  14 , thereby achieving better bump connection. 
     Specifically, the relationship between an increase in temperature and an amount of heat can be expressed as ΔT=Q/CM where ΔT denotes a variation in temperature, Q denotes heat, C denotes specific heat capacity, and M denotes mass. Since mass=volume×density, the above expression can be expressed as ΔT=Q/CVD, where V denotes volume, and D denotes density. 
     Assuming that the heat Q applied to the semiconductor device  1 A (and respective lands  3 ) is constant, the lands  3  are made of the same material, and the specific heat capacity C and the density D are constant, the above expression can be expressed as ΔT=α/V, where α denotes a constant value. 
     Further, assuming that the volume V=area×thickness, and the lands  3  have the same thickness, the above expression can be expressed as ΔT=β/S , where β denotes a constant value, and S denotes the area. 
     As understood from the expression, an increase in temperature of the land  3  is inversely proportional to the area of the land  3 . Melting of bump and alloy growth between the land  3  and the bump are correlated to the temperature of the land  3 . A variation in the temperature of each land  3  can be controlled by changing the area of each land  3 . For example, if the area of the land  3  doubles, the increase in the temperature of the land  3  is reduced by half (the effects of the wires on the substrate and peripheral atmosphere are ignored). 
     The semiconductor chip  5  has a specific heat capacity of approximately 700 J/kg√K, which is greater than that of the seal resin. Consequently, an increase in the temperature of the lands  3   b  is smaller than that of the lands  3   a  upon heating in a reflow process for bump connection or second mounting. 
     For this reason, the area of the land  3   a  is made larger so that the thermal behavior of the land  3   a  is the same as that of the land  3   b.  Thus, an increase in the temperature of the lands  3   a  is prevented, thereby achieving a uniform temperature of the lands  3   a  and  3   b.    
     Consequently, alloy growth between every land  3  and the corresponding bump is equalized, thereby preventing a fluctuation of the connection strength, and therefore enhancing the connection reliability of the semiconductor device  1 A. 
     Every opening  10  of the solder resist  10  has the same area with respect to any land  3 . For this reason, every bump has the same shape, and therefore design change of lands on the surface  2   b  of the wiring board  2  is not necessary. Further, poor mounting and a decrease in the mounting reliability due to the change of the bump shape do not occur. 
     It is apparent that the present invention is not limited to the above embodiments, but may be modified and changed without departing from the scope and spirit of the invention. 
     For example, the size, the type, the number, and the mounting direction of the semiconductor chip, and the type of bumps are not limited to those of the embodiment. An element to be mounted in a package is not limited to the semiconductor chip, and another semiconductor package, such as a PiP (Package in Package), or passive and active elements may be used. 
     To further control the volumes of lands, lands in a specific area may be thicker, or via holes filled with, for example, Cu may be formed by etching the substrate. Moreover, the shape of the land is not limited to a circle, and a rectangular land may be used. 
     As used herein, the following directional terms “forward, rearward, above, downward, vertical, horizontal, below, and transverse” as well as any other similar directional terms refer to those directions of a device equipped with the present invention. Accordingly, these terms, as utilized to describe the present invention should be interpreted relative to a device equipped with the present invention. 
     The terms of degree such as “substantially,” “about,” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. For example, these terms can be construed as including a deviation of at least ±5 percents of the modified term if this deviation would not negate the meaning of the word it modifies.