Abstract:
An electronics package consisting of a molded plastic body on a ceramic substrate is provided with a layer of a ceramic surrogate material on a surface of the plastic body opposite the ceramic substrate to reduce bending stress of the package. The coefficient of thermal expansion of the ceramic surrogate layer is less than that of the ceramic substrate. Equations are developed for determining an optimum thickness for the surrogate layer.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the packaging of electronic components and, more particularly, to a package with reduced bending stress due to temperature change. 
     Some microelectronic package designs employ a thick plastic body molded on a thin substrate. A good example is a package in which a thin ceramic substrate is overmolded with an epoxy compound which encapsulates microelectronic components surface-mounted on the substrate. This substrate, since it is made of a brittle material, is prone to failure, because of the thermal contraction mismatch of the molding compound with the ceramic material as the package cools from its molding temperature to room temperature conditions. 
     The situation can be improved in many ways. For example, as disclosed in U.S. Pat. No. 5,627,407, an additional “surrogate” material (i.e., a material not needed from the standpoint of the normal operation of the package) can be applied to the outer surface of the substrate to “balance” the package bowing. Clearly, such a surrogate layer should be thin (in order not to interfere with the normal function of the electronic component, and not to make the package thicker than necessary), have a high coefficient of thermal expansion (CTE) and a high Youngs&#39;s modulus (to be effective) and, in addition, should be able to withstand high tensile stresses on both a short term and a long term basis. One could select, for example, a thermoplastic sheet that softens at the molding temperature and bonds well to the substrate, or a rigid material with an adhesive layer that bonds to the substrate during molding. This approach, based on the employment of a polymeric material molded concurrently with the “main” package, has, however, the following major shortcomings: 
     There are not too many polymer materials which have high CTE, high Young&#39;s modulus, high adhesive and cohesive strength and, at the same time, have a molding Temperature the same as the “basic” molding compound; and 
     There is a concern that this material can fail in the long-run, either adhesively or cohesively, because of acing, high tensile stresses, moisture-absorption, etc. 
     It would therefore be desirable to provide an alternative solution to the problem of reducing the package bow using a surrogate material. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a surrogate layer of a low CTE ceramic component (even in some cases a component made of negative CTE ceramic) is applied to the outer (“free”) surface of the molded plastic body. 
     In accordance with an aspect of this invention, the surrogate material (layer) is applied during manufacturing (molding) of the basic package. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing will be more readily apparent upon reading the following description in conjunction with the drawings in which like elements in different figures thereof are identified by the same reference numeral and wherein: 
     FIG. 1 is an exploded perspective view of an electronics package embodying this invention; 
     FIG. 2 is a perspective view of the package shown in FIG. 1 with its components in place; and 
     FIG. 3 is a side view of the package shown in FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     This description is divided into two parts. Part I describes the general features of a package using a surrogate layer according to the present invention to reduce bending stress, and Part II presents an analytical model useful for optimizing the surrogate layer in various applications. 
     I. The Features of the Package 
     Referring to the drawings, FIG. 1 is an exploded view showing the three main components of an electronic package in accordance with the invention. Specifically, the package comprises a body of molded plastic  10 , such as a thermosetting epoxy molding compound, a main insulating substrate  11  and a surrogate layer  12 . The molded plastic body  10  typically encapsulates one or more electronic devices such as integrated circuits (not shown). The main substrate  11  is typically a fragile material such as an insulating ceramic which needs to be protected from bending stresses. According to the present invention, the surrogate layer is a thin ceramic layer. 
     As better shown in FIG. 2, the surrogate layer  12  is disposed on the side of the molded plastic body  10  opposite the main substrate  11 . The structure is advantageously fabricated in such a fashion that both the surrogate layer  12  and the main substrate  11  bond to the plastic body  10  during the molding of the plastic body  10 . This concurrent fabrication creates a desirable stress balance in the composite structure during cooling. 
     The analysis that follows presents a calculation procedure to determine the required thickness of the surrogate layer  12  to balance the stress in the main substrate  11 . It is assumed that the plastic body  10 , the main substrate  11  and the surrogate layer  12  are molded concurrently. 
     II. Analytical Model 
     The following analysis considers an ideal tri-material structure similar to that shown in FIG.  2 . The structure consists of an epoxy molded body  10 , a ceramic substrate  11  and a ceramic surrogate layer  12 , molded together at the elevated molding temperature of the body  10  and subsequently cooled to room temperature. In the following discussion, the subscript  0  refers to the ceramic substrate  11 , the subscript  1  refers to the body  10 , and the subscript  2  refers to the ceramic surrogate layer  12 . 
     Let a tri-material elongated assembly be manufactured at an elevated temperature and subsequently cooled to a low (say, room) temperature. The induced forces in the assembly components can be determined from the equations of the compatibility of the interfacial strains 
     
       
         −α 0   Δt +λ 0 T 0 =−α 1   Δt +λ 1 T 1 =−α 2   Δt +λ 2 T 2   (1) 
       
     
     and the equilibrium equation 
     
       
         T 0 +T 1 +T 2 −0,  (2) 
       
     
     where α 0 , α 2  and α 2  are the CTE of the ceramic substrate  11 , the body  10 , and the ceramic surrogate layer  12 , respectively;                  λ   0     =     1       E   0          h   0           ,       λ   1     =     1       E   1          h   1           ,       λ   2     =     1       E   2          h   2                   (   3   )                                
     are the compliances of the corresponding components; h 0 , h 1  and h 2  are their thicknesses; E 0 , E 1  and E 2  are generalized Young&#39;s module (i.e., Young&#39;s module obtained by dividing actual Young&#39;s module by 1−ν, where ν is Poisson&#39;s ratio of the material); Δt is the change in temperature; and T 0 , T 1  and T 2  are the induced forces. 
     From equations (1) and (2) we obtain the following equations for the forces acting in the ceramic substrate  11  and the ceramic surrogate layer  12 :                        T   0     =         Δ                 t     D          [         α   0          (       λ   1     +     λ   2       )       -       α   1          λ   2       -       α   2          λ   1         ]                     T   2     =     -         Δ                 t     D          [         α   0          λ   1       +       α   1          λ   0       -       α   2          (       λ   0     +     λ   1       )         ]                 }           (   4   )                                
     where 
     
       
         D=λ 0 λ 1 +λ 1 λ 2 +λ 0 λ 2   (5) 
       
     
     is the determinant of the system of equations (1) and (2). Using equation (3), one can write the equations (4) as follows:                        T   0     =       -       Δ                 t       D   1              E   0            h   0          [       -       α   0          (         E   1          h   1       +       E   2          h   2         )         +       α   1          E   1          h   1       +       α   2          E   2          h   2         ]                       T   2     =       -       Δ                 t       D   1              E   2            h   2          [         α   0          E   0          h   0       +       α   1          E   1          h   1       -       α   2          (         E   0          h   0       +       E   1          h   1         )         ]                 }           (   6   )                                
     where the following notation is used: 
     
       
         D 1 =E 0 h 0 +E 1 h 1 +E 2 h 2.   (7) 
       
     
     Obviously, no bow can occur if the moment          M   0     -       T   0              h   0     +     h   1       2                              
     applied to the molded body  10  by the ceramic substrate  11  is in equilibrium with (i.e., equal and opposite to) the moment          M   2     -       T   2              h   1     +     h   2       2                              
     applied to the molded body  10  by the ceramic surrogate layer  12 . This yields: 
     
       
         T 0 (h 0 +h 1 )−T 2 (h 1 +h 2 )=0  (8) 
       
     
     Introducing equation (6) into equation (8), we obtain the condition of zero bow in the form: 
      E 2 [(α 0 −α 2 )E 0   h   0 +(α 1 −α 2 )E 1   h   1   ]h   2   2 +E 2 [(α 0 −α 2 )( h   0 +2 h   1 )E 0   h   0 +(α 1 −α 2 )E 1   h   1   2   ]h   2   
     
       
         −E 0   h   0 E 1   h   1 ( h   0   +h   1 )(α 1 −α 0 )=0 
       
     
     This expression results in the following quadratic equation for the dimensionless thickness (η 2 =h 2 /h 0 ) of the ceramic surrogate layer  12 : 
     
       
         η 2   2 +2γη 2 −δ−0  (9) 
       
     
     where the following notation is used:                      γ   =           (     1   -       α   _     2       )                     (     1   +     2        η   1         )       +       (         α   _     1     -       α   _     2       )          e   1          η   1   2           2        [     1   -       α   _     2     +       (         α   _     1     -       α   _     2       )          e   1          η   1         ]                     δ   =         (         α   _     1     -   1     )          e   1            η   1          (     1   +     η   1       )             e   2          [     1   -       α   _     2     +       (         α   _     1     -       α   _     2       )          e   1          η   1         ]                 }           (   10   )                                
     and                  η   1     =       h   1       h   0         ,       η   2     =       h   2       h   0         ,       e   1     =       E   1       E   0         ,       e   2     =       E   2       h   0         ,         α   _     1     =       α   1       α   2         ,         α   _     2     =       α   2       α   0                 (   11   )                                
     From equation (9) we find:                η   2     =     γ        (         1   +     δ     γ   2           -   1     )               (   12   )                                
     In the simplest case, when the ceramic surrogate material has the same properties as the ceramic substrate material (e 2 =1,{overscore (α)} 2 =1), this equation yields: η 2 =1. The equation (12) enables one to compute the thickness of the surrogate layer for the given materials, and the given thicknesses of the molding compound and the ceramic substrate. The effect of the application of a low expansion ceramic as a surrogate layer can be enhanced, of course, if a ceramic with a negative CTE is employed. 
     Let, for example, the generalized Young&#39;s module of the materials be E 0 =14000 kgf/mm 2 , E 1 =1400 kgf/mm 2 , E 2 =14000 kgf/mm 2 , their CTE&#39;s be α 0 =6×10 −6 /° C., α 1 =18×10 −6 /° C., α 2 =10 −6 /° C., and the thicknesses of the material layers in the package be h 0 =1.0 mm and h 1 =4.0 mm. Then the equations (11) result in the following dimensionless parameters: η 1 =4; e 1 =0.1; e 2 =1.0; {overscore (α)} 1 =3.000; {overscore (α)} 2 =0.167. From equation ( 10) we find: γ=3.0593 and δ=2.039, and the equation (12) yields: η 2 =0.3161. Hence, h 2 =η 2 h 0 =0.316 mm. The surrogate layer can be made even thinner, if a ceramic material with negative CTE is used. Let, for instance, α 2 =−1.5×10 −6 /° C. Then we obtain: {overscore (α)} 2 =−1.0; γ=3.3889; δ=1.1111; η 2   =0.1601; h   2 =0.160 mm. 
     Accordingly, there has been disclosed an improved electronics package with reduced bending stress. It is to be understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments which can represent applications of the principles of the invention. Numerous and varied other arrangements can be made by those skilled in the art without departing from the spirit and scope of the invention, as defined by the appended claims.