Abstract:
A packaging substrate and a package structure are provided. The packaging substrate includes a plurality of dielectric layers, two of which have a difference in thickness; and a plurality of circuit layers alternately stacked with the dielectric layers. Therefore, the package warpage encountered in the prior art is avoided.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention The present invention relates to packaging substrates and package structures, and, more particularly, to a packaging substrate and a package structure having a plurality of stacked dielectric layers and circuit layers. 
         [0002]    2. Description of Related Art 
         [0003]    Flip-chip technologies facilitate to reduce chip package area, signal path and so on, and therefore have been widely used for chip packaging, such as chip scale package (CSP) and multi-chip module (MCM). 
         [0004]      FIG. 1  is a cross-sectional view of a conventional flip-chip  1  substrate. Referring to  FIG. 1 , the substrate  1  has a plurality of dielectric layers  11  and a plurality circuit layers  12  alternately stacked with each dielectric layers  11 . If each dielectric layer  11  in a packaging substrate  1  that has four dielectric layers  11  is 40 μm thick, a thickness sum of the four dielectric layers  11  is 160 μm. 
         [0005]    However, during a temperature cycle test of a flip-chip packaging process, warpage easily occurs to the package substrate due to a big CTE (Coefficient of Thermal Expansion) mismatch between the chip and the package substrate, a surface area of the packaging substrate being too large, or asymmetrical heating. Consequently, it becomes difficult to form good joints between conductive bumps around an outer periphery of the chip and contacts of the package substrate, thereby reducing the product yield. 
         [0006]    The warpage problem may be alleviated by increasing the thickness of the dielectric layer, which, however, also increases the overall thickness of the packaging substrate and unable to meet the low-profiled, compact-sized requirements. 
         [0007]    Accordingly, there is an urgent need to solve the drawbacks encountered in the prior art. 
       SUMMARY OF THE INVENTION 
       [0008]    In view of the foregoing problems, the present invention provides a packaging substrate, comprising: a plurality of dielectric layers, at least two of which having a difference in thickness; and a plurality of circuit layers alternately stacked with the dielectric layers. 
         [0009]    The present invention further provides a package structure, comprising: a packaging substrate having a plurality of dielectric layers and a plurality of circuits layers alternately stacked with the dielectric layers, wherein at least two of the dielectric layers have a difference in thickness.; and a chip mounted on a first surface of the packaging substrate. 
         [0010]    In an embodiment, the packaging substrate further comprises a first surface and a second surface opposing the first surface, wherein one of the dielectric layers that is disposed on or close to the first surface is thicker than another one of the dielectric layers that is disposed on or close to the second surface; the first surface is a chip mounting side, and the second surface is a non-chip mounting side; one of the dielectric layers that is closest to the first surface has a thickness greater than each of thicknesses of the others of the dielectric layers, and one of the dielectric layers that is closest to the second surface has a thickness less than each of thicknesses of the others of the dielectric layers; and the dielectric layers, except the one that is closest to the first surface and the one that is closest to the second surface, have the same thickness. 
         [0011]    In an embodiment, the thicknesses of the dielectric layers gradually decrease from the from the first surface to the second surface; and the sum of the thickness of half of the dielectric layers that are close to the first surface is greater than the sum of the thickness of the other half of the dielectric layers that are close to the second surface. 
         [0012]    In an embodiment, a thickness ratio between a thickest one of the dielectric layers and a thinnest one of the dielectric layers is 1.2:1 to 2:1, preferably 1.5:1. 
         [0013]    In an embodiment, a thickest dielectric layer and the thinnest dielectric layers have a thickness difference of 50 μm, preferably 10 μm to 25 μm. 
         [0014]    In an embodiment, the number of the dielectric layers is even or odd; a core layer is disposed therebetween, and the dielectric layers are symmetrically distributed on two opposing surfaces of the core layer with the core layer as a center. 
         [0015]    Therefore, by providing a thickness difference between the dielectric layers, the present invention increases the rigidity of the package substrate so as to reduce warpage of the package substrate. Further, the sum of the thicknesses of the dielectric layers s can be kept unchanged so as not to increase the overall thickness of the package substrate and structure. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0016]    The present invention can be more fully understood by reading the following detailed description of the preferred embodiments, with reference made to the accompanying drawings, wherein: 
           [0017]      FIG. 1  is a cross-sectional view of a conventional flip-chip packaging substrate; 
           [0018]      FIG. 2  is a cross-sectional view of a packaging substrate in accordance with an embodiment of the present invention; 
           [0019]      FIG. 3  is a cross-sectional view of a packaging substrate in accordance with another embodiment of the present invention; and 
           [0020]      FIG. 4  is a cross-sectional view of a package structure of an embodiment in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0021]    The present invention is described in the following with specific embodiments, so that one skilled in the pertinent art can easily understand other advantages and effects of the present invention from the disclosure of the present invention. 
         [0022]    Referring to  FIG. 2 , a cross-sectional view of a packaging substrate  2  in accordance with an embodiment of the present invention is shown. The packaging substrate  2  comprises: dielectric layers  21   a   n , dielectric layers  21   b   m  and a plurality of circuit layers  22  respectively alternately stacked with the dielectric layers  21   a   n  and the dielectric layers  21   b   m . There is a difference in thickness between one of the dielectric layers  21   a   n  and one of the dielectric layers  21   b   m . 
         [0023]    The packaging substrate  2  has a first surface  2   a  (referred as to an upper surface) and a second surface  2   b  (referred as to a lower surface). The first surface  2   a  is opposite to the second surface  2   b.  The subscripts “n” and “m” in the dielectric layers  21   a   n  and  21   b   m  can be the same or different integers, and can be odd or even numbers. If n=1, the dielectric layers  21   a   n  have only one dielectric layer. If n=2, the dielectric layers  21   a   n  have two dielectric layers. Same rule is applied to the number of layer of the dielectric layers  21   b   m , and therefore will not be described further therein. One of the dielectric layers  21   a   n  disposed on or close to the first surface  2   a  of the packaging substrate  2  is thicker than another one of the dielectric layers  21   b   m  disposed on or close to the second surface  2   b  of the packaging substrate  2 . The first surface  2   a  of the packaging substrate  2  is a chip mounting side, and the second surface  2   b  is a non-chip mounting side, or vice versa. 
         [0024]    Under a condition that the total thickness of the dielectric layers  21   a   n  and dielectric layers  21   b   m  is equal to the total thickness of the dielectric layers  11  of the conventional packaging substrate  1 , the ratio between the thickest one of the dielectric layers  21   a   n  and the thinnest one of the dielectric layers  21   b   m  is 1.2:1 to 2:1, preferably 1.5:1. The thickest one of the dielectric layers  21   a   n  is thicker than the thinnest one of the dielectric layers  21   b   m  by 10 to 50 μm, preferably 10 to 25 μm. Through increasing the thicknesses of the dielectric layers  21   a   n  while reducing the thicknesses of the dielectric layers  21   b   m , the occurrence of deformation is reduced to 5% to 25%, as compared to the prior art. 
         [0025]    In an embodiment, if the total thickness of the dielectric layers  21   a   n  and dielectric layers  21   b   m  equals to the total thickness (e.g., 160 μm) of the dielectric layers  11  of the conventional packaging substrate  1 , and the packaging substrate  2  has four dielectric layers (i.e., n=2 and m=2), each of the dielectric layers  21   a   n  is 50 μm thick, and each of the dielectric layers  21   b   m  is 30 μm thick. In other words, the dielectric layers  21   a   n  are thickened, while the dielectric layers  21   b   m  are thinned. 
         [0026]    In another embodiment, if the total thickness of the dielectric layers  21   a   n  and dielectric layer  21   b   m  equals to the total thickness (i.e., 160 μm) of the dielectric layers  11  of the conventional packaging substrate  1 , and the packaging substrate  2  has four dielectric layers (i.e., n=2 and m=2), the four dielectric layer are 50 μm, 40 μm, 40 μm, and 30 μm thick, respectively. In other words, only the dielectric layer  21   a   2  that is disposed on and closest to the first surface  2   a  is thickened, and only the dielectric layer  21   b   2  that is disposed on and closest to the second surface  2   b  is thinned 
         [0027]    In yet another embodiment, if the total thickness of the dielectric layers  21   a   n  and dielectric layers  21   b   m  equals to the total thickness (i.e., 160 μm) of the dielectric layers  11  of the conventional packaging substrate  1 , and the packaging substrate  2  has four dielectric layers (i.e., n=2 and m=2), the dielectric layers  21   a   1 , the dielectric layers  21   a   2 , the dielectric layers  21   b   1 , and the dielectric layers  21   b   2  are 50 μm, 45 μm, 35 μm, and 30 μm thick, respectively. In other words, the thicknesses of the dielectric layers  21   a   n  and dielectric layers  21   b   m  are progressively reduced. In yet another embodiment, if the total thickness of the dielectric layers  21   a   n  and dielectric layers  21   b   m  equals to the total thickness (i.e., 160 μm) of the dielectric layers  11  of the conventional packaging substrate  1 , and the packaging substrate  2  has four dielectric layers (i.e., n=2 and m=2), the dielectric layers  21   a   1  and the dielectric layers  21   a   2  can have their thicknesses irregularly increased, and the dielectric layers  21   b   1  and the dielectric layers  21   b   2  can have their thicknesses irregularly decreased, while the total thickness of the dielectric layers  21   a   1  and the dielectric layers  21   a   2  is still equal to the total thickness of the dielectric layers  21   b   1  and the dielectric layers  21   b   2 . For example, the dielectric layers  21   a   1 , the dielectric layers  21   a   2 , the dielectric layers  21   b   1 , and the dielectric layers  21   b   2  are 50 μm, 45 μm, 25 μm, and 40 μm thick, respectively. 
         [0028]      FIG. 3  is a cross-sectional view of a packaging substrate  2  in accordance with another embodiment of the present invention. The packaging substrate  2  shown in  FIG. 3  differs from the packaging substrate  2  shown in  FIG. 2  in that the packaging substrate  2  shown in  FIG. 3  has a core board  21 ′ disposed therebetween, and the dielectric layers  21   a   n  and  21   b   m  are symmetrically distributed between two opposing surfaces of the core layer  21 ′, i.e., n=m. The thickness of each of the dielectric layers  21   a   n  and  21   b   m  are modifiable according to the descriptions in  FIG. 2 . The materials of the core board  21 ′ and the dielectric layers are the same. In another embodiment, the core board  21 ′ is made of glass fiber, epoxy resin, polyimide tape, FR4 resin or BT resin. 
         [0029]      FIG. 4  is a cross-sectional view of a package structure in accordance with an embodiment of the present invention. The package structure has a chip  31  mounted on the first surface  2   a  of the packaging substrate  2 , and an encapsulant  32  encapsulating the chip  31 . The remaining technical features are similar to that described before, and therefore will not be described in details herein. 
         [0030]    In summary, in comparison with the prior art, the present invention provides a solution through adjusting the thickness of parts of the dielectric layers in the package substrate, to increase the overall strength of the substrate in a condition where the total thickness of the dielectric layers of the substrate equals to that of a conventional substrate, thereby preventing the occurrence of substrate warpage (5-25% reduction), therefore increasing the yield of the products. Besides, it should be noted that since not all the dielectric layers are increased in thickness, the total thickness summed up from all the dielectric layers could remain the same, such that the overall thickness is not increased, thus the present invention is able to meet the low-profile requirement nowadays. 
         [0031]    The present invention has been described using exemplary preferred embodiments. However, it is to be understood that the scope of the present invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements. The scope of the claims, therefore, should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.