Abstract:
A method of evaluating configuration of solder external terminals of a BGA-type tape-based semiconductor device mounted on a board such that the external terminals are joined to lands provided on the mounting board is provided. The method includes the step of obtaining geometric data related to opening of a tape substrate of the semiconductor device, solder balls to be placed at positions corresponding to the openings, and the lands of the mounting board and the step of deribing configuration of the solder external terminal based on the geometric date. The method further includes the step of calculating the volume of voids to be produced in the external terminals, so as to compensate for the geometric data related to the tape substrate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally related to a method of evaluating configuration of solder external terminals of semiconductor device, and particularly relates to evaluation of failures of configuration of solder external terminals of a BGA (Ball Grid Array) semiconductor device after secondary mounting process. 
     2. Description of the Related Art 
     Recently, a semiconductor device is required to have a package structure that can achieve higher density, higher speed and higher power at a low cost. An FBGA (Fine pitch Ball Grid Array) type semiconductor device having a fine-patched structure has been developed to meet such requirements. The FBGA type semiconductor device are now used in various types of electronic equipment. There are some FBGA type semiconductor devices provided with tape substrates to achieve a further fine-pitched structure. Such fine-pitched semiconductor devices must also be mounted on mounting boards with high reliability. 
     One method of evaluating the reliability is to estimate the configuration of ball-shaped solder external terminals. In the estimation, the configuration of the solder external terminal is estimated for a state after the semiconductor device has been mounted on the mounting board. When the solder external terminals are arranged in a fine pitched structure, it becomes difficult to estimate the configuration of the solder external terminals. Accordingly, there is a need for a simple and accurate method of evaluating the configuration of the solder external terminals. 
     FIG. 1 is a schematic diagram showing a semiconductor device  1  having a package structure of a FBGA type. The semiconductor device  1  generally comprises a tape  2 , a semiconductor chip  4 , ball-shape solder external terminals  6  and a sealing resin  8 . 
     The tape  2  is made of a material such as polyimide resin. Electrode patterns  10  and bonding pads  12  are provided on a first (upper) surface  2   a  of the tape  2 . Also, the semiconductor chip  4  is mounted on the first surface  2   a  of the tape  2 . In order to provide the electrode patterns  10  and the bonding pads  12 , a copper layer is formed on tape  2  and then an etching process is implemented on the copper layer to form predetermined patterns. The electrode patterns  10  and the bonding pads  12  are electrically connected by wiring patterns(not shown). 
     Also, wires  14  are provided between the semiconductor chop  4  and the bonding pads  12 . Accordingly, the semiconductor chip  4  and the electrode patterns  10  are electrically connected via the wires  14 , the bonding pads  12  and the wiring patterns. Further tape opening  16  are formed through the tape  2  at positions corresponding to the electrode patterns  10  of the tape  2 . 
     The ball-shaped solder external terminals  6  are provided such that a solder ball parts are on a second (lower) surface  2   b  of the tape  2 . The ball-shaped solder external terminals  6  are provided at positions corresponding to tape openings  16 . The ball-shaped solder external terminals  6  are joined to the electrode patterns  10  via the tape openings  16 . That is to say, the ball-shaped solder external terminals  6  are joined to the electrode patterns  10  and thus the ball-shaped solder external terminals  6  are attached to the tape  2 . 
     The ball-shaped solder external terminals  6  are joined the electrode patterns  10  in the following manner. First, the tape  2  provided with the electrode patterns  10  and the tape openings  16  is reversed. Then, solder paste  13  is filled in the tape openings  16  (see FIG.  5 ). Solder balls  6 A are placed on the solder paste  13  provided in the tape opening  16 . A heat treatment is carried out to fuse the solder balls  6 A and the solder paste  13 . Then, the fused solder is subjected to a cooling treatment, so that the solder is cured and thus the solder balls  6 A are attached to the electrode patterns  10 . It is to be noted that the openings  16  are filled with solder. 
     Thus, with the processes described above, the solder balls  6 A are joined to the electrode patterns  10  to for ball-shaped solder external terminal  6 . Also, the ball-shaped solder external terminal  6  is shaped such that a portion protruding from the tape opening  16  becomes spherical due to surface tension during the fusing step. 
     FIGS. 2A to  2 C show various steps of mounting the semiconductor device  1  of the above structure onto the mounting board  3 . As shown in FIG. 2A, solder paste  19  is provided on lands  17  formed in the mounting board  3 , for example, by printing. After registering the ball-shaped solder external terminals  6  and the lands  17 , the semiconductor device  1  is placed on the mounting board  3 . The solder paste  19  is a mixture of solder particles and a viscous organic agent. The organic agent serves as an adhesive agent to temporarily fix the semiconductor device  1  on the mounting board  3 . 
     Subsequently, the mounting board  3  and the semiconductor device  1  temporarily fixed thereon are subjected to a heat treatment in a reflow oven. Accordingly, the solder particles included in the solder balls  6  and the solder paste  19  are fused, and the organic agent in the solder paste  19  is vaporized. Thus, the ball-shaped solder external terminals  6  and the lands  17  are soldered and the semiconductor device  1  is mounted on the mounting board  3 . 
     In the process of mounting the semiconductor device  1  onto the mounting board  3 , there may be a case in which the ball-shaped solder external terminals  6  are not properly joined to the lands  17 . This will be described with reference to FIGS. 2A to  2 C. 
     The process of mounting the semiconductor device  1  on to the mounting board  3  has been described above with reference to FIG.  2 A. In the process described above, the solder paste  19  is provided on the lands  17 . Then, the solder balls  6  and the lands  17  are temporarily fixed by means of the solder paste  19 . Thereafter, a reflow process is carried out, so that the solder particles in the solder balls  6  and the solder paste  19  for mounting are fused. 
     The above described process may be unsuccessful if un appropriate selections are made for the size of the tape openings  16  formed in the tape  2 , total amount of the solder, and the size of the lands  17 . As shown in FIG. 2B, there may be a case in which most of the solder may flow towards the lands  17  and the ball-shaped solder external terminal  6  is necked within the tape opening  16  (hereinafter referred to as a necking failure). Further, as shown in FIG. 2C, the ball-shaped solder external terminal  6  may be detached from the electrode pattern  10  (hereinafter referred to as an open failure). 
     If necking failures and/or open failures occur during mounting process of the semiconductor device  1 , the mounting reliability will be considerably decreased. Accordingly, in order to detect such failures, it is known to carry out an estimation of the configuration of the ball-shaped solder external terminals for a state after mounting the semiconductor device  1  onto the mounting board  3 . 
     Conventionally, the evaluation of solder external terminals has been carried out by visual inspection or by X-ray photography after actually mounting the semiconductor device  1  onto the mounting board  3 . Thus, with the solder shape evaluation method of the related art, the evaluation of the ball-shaped solder external terminals cannot be implemented before the mounting step. If any failure is produced, it is necessary to replace the solder balls  6  by solder balls of different size and then carrying out the mounting and estimation steps again until the size of the solder balls becomes appropriate. 
     Accordingly, the evaluation method of the related art has a drawback that it is time-consuming to carry out. Also, since the evaluation method is carried out by visual inspection or by X-ray photography, there is a further drawback of low evaluation accuracy. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a general object of the present invention to provide a method of evaluating the configuration of solder external terminals which can obviate the drawbacks described above. 
     It is another and more specific object of the present invention to provide a method of evaluating the configuration of solder external terminals which can be implemented before a mounting step with improved accuracy and with reduce time. 
     In order to achieve the above objects according to the present invention, a method is provided for evaluating configuration of solder external terminals of a BGA-type tape-based semiconductor device mounted on a mounting board such that the external terminals are joined to lands provided on the mounting board, the method including the steps of: 
     a) obtaining geometric data related to openings of a tape substrate of the semiconductor device, solder balls to be placed at positions corresponding to the openings, and the lands of the mounting board; and 
     b) deriving configuration of the solder external terminal based on the geometric data. 
     With the invention described above, since the estimation is based on known geometric data of the tape openings, solder balls and the lands, the configuration of the solder external terminals can be evaluated without actually mounting the solder balls on the mounting board. Accordingly, the solder balls can be evaluated accurately and with reduced time. 
     It is yet another object of the present invention to provide a method of evaluating the configuration of solder external terminals which may be applied to a case where voids are produced in the solder external terminals. 
     In order to achieve the above object, the method further includes the step of calculating the volume of voids to be produced in the external terminals. When deriving configuration of the solder external terminal based on the geometric data, the volume of voids is taken into account to compensate for the geometric data related to the tape substrate. 
     With the invention described above, the solder external terminals can be accurately evaluated even in a case were voids are produced in the solder external terminals. 
     The present invention also relates to an apparatus for implementing the method described above. The present invention further relates to a computer readable storage medium with program for implementing the method described above. 
     Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional diagram showing a part of a semiconductor device which can be used in the method of the present invention. 
     FIGS. 2A to  2 C are diagrams showing how connection failure of the solder is caused. 
     FIG. 3 is a block diagram showing an apparatus of the present invention. 
     FIG. 4 is a flowchart showing various steps of a method of the present invention. 
     FIG. 5 is a schematic diagram illustrating step  11  of the flowchart shown in FIG.  4 . 
     FIG. 6 is a schematic diagram illustrating step  12  of the flowchart shown in FIG.  4 . 
     FIG. 7 is a schematic diagram illustrating step  13  of the flowchart shown in FIG.  4 . 
     FIG. 8 is a graph of ball fall-off occurrence rate against tape thickness ratio (T 1 /T 2 ). 
     FIG. 9 is a diagram showing a map used in step  15  of the flowchart shown in FIG.  4 . 
     FIGS. 10A to  10 D are diagrams showing how connection failure of the solder is caused when there are voids in the solder. 
     FIG. 11 is a schematic diagram showing how the method of the present invention is carried out when there are voids in the solder. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, principles and embodiments of the present invention will be described with reference to the accompanying drawings. In the following description, like reference numerals are used to indicate like elements through out the Figures. 
     FIG. 3 is a block diagram showing a solder shape evaluation apparatus  20  which may be used during BGA mounting. The solder shape evaluation apparatus  20  includes a main control part  21 , an input/output control part  22 , a display unit  24 , an output unit  25  and a storage unit  26 . 
     The main control part  21  is embodied as a microcomputer and implements a solder shape evaluation process in accordance with a solder shape evaluation program stored in the storage unit  26 . The input/output unit  22  controls information communication between the units  23  to  26  (to be described later) and the main control part  21 . The input unit  23  may be, for example, a keyboard via which various parameters required for solder shape evaluation process are input. 
     The display unit  24  may be, for example, a CRT on which various information required for input/output process of the various parameters and for solder shape evaluation process are displayed. The output unit  25  may be, for example, a printer with which the result of solder shape evaluation process is printed out. Further, the storage unit  26  stores the solder shape evaluation program shown in FIG. 4 and a two-dimensional map showing a relationship between solder shape determining value and tape thickness (see FIG.  9 ), etc. 
     It is to be noted that the solder shape evaluation program (FIG. 4) is stored in the storage medium according to the present invention and the storage unit  26  stores the solder shape evaluation program read out from the storage medium. 
     Referring to FIGS. 4 to  9 , the solder shape evaluation process will be described which is implemented by the main control part  21  and in accordance with the solder shape evaluation program. 
     When the solder shape evaluation process of FIG. 4 is initiated, an operator of the solder shape evaluation apparatus  20  inputs various parameters through the input unit  23  (step  10  (S 10 )). The parameters inputted at step  10  are thickness of the tape  2  (tape thickness) T 2 , diameter of the tape opening  16  (tape opening diameter) TD, radius of the land  17  formed on the mounting board  3  (land radius) b, radius of the solder ball  6 A (solder ball radius) BD and thickness of the solder paste  19  (paste thickness) MT. 
     The input parameters T 2 , TD, b, BD and MT are known before actually mounting the solder balls  6  on the mounting board  3 . The input parameters T 2 , TD, b, BD and MT are transmitted to the main control part  21  via the input/output control part  22 . 
     After step  10 , the main control part  21  calculates total solder volume V based on the input parameters T 2 , TD, b, BD and MT (step  11 ). As shown in FIG. 5, the total solder volume V is given as a sum of three sub-volumes V 1 , V 2  and V 3 . First sub-volume V 1  is a volume of solder included in the solder paste  13  that fills the tape opening  16 . Second sub-volume V 2  is a volume of solder of the solder ball  6 A. Third sub-volume V 3  is a volume of solder included in the solder paste  19  provided on the land  17 . Thus, the total volume V may be expressed as (V 1 +V 2 +V 3 ). 
     Each of the sub-volumes V 1 , V 2  and V 3  may be derived by the following equations: 
     
       
           V   1 =( TD   2   ×T   2 ×π)/4  (1)  
       
     
     
       
           V   2 =( BD   3 ×π)/6  (2)  
       
     
     
       
           V   3 =( b   2   ×MT ×π)/4  (3)  
       
     
     Therefore, the total solder volume V may be expressed as: 
     
       
           V ×{( TD   2   ×T   2 ×π)/4}+{( BD   3 ×π)/6}+{( b   2   ×MT ×π)/4}  (4)  
       
     
     In the following description, it is assumed that the radius of the solder paste  19  and the radius of the land b are equal. 
     At step  12 , the height and the contact angle of a spherical solder drop are calculated at the main control part  21 . In the present application, the spherical solder drop refers to a drop of solder that is formed when the total solder volume V is fused and joined to the land  17 . 
     FIG. 6 is a diagram showing the spherical solder drop. As shown in FIG. 6, the spherical solder drop conforms to the shape of the configuration of the land  17  at a portion joined to the land  17 . That is to say, when viewed along line A—A, the spherical solder drop is circular at the joining portion. Above the joining portion, the spherical solder drop has a configuration that is substantially spherical due to surface tension. In the following description, the solder having the shape of a spherical solder drop is referred to as a drop-shaped solder  6 A. 
     The volume V of the spherical can also be defined by the height of the spherical solder drop “h” and the contact angle “θ” between the spherical solder drop and the land  17  at the peripheral part of the contact portion. The volume of the spherical solder drop is defined by the following equation:              V   =       1   6        π                   h        (       h   2     +     3        b   2         )                 (   5   )                                
     Then, the height h can be derived by the following equation:              h   =         (         3      V     π     +           (       3      V     π     )     2     +     b   6           )       1   /   3       +       (         3      V     π     -           (       3      V     π     )     2     +     b   6           )       1   /   3                 (   6   )                                
     The contact angle θ can be derived by the following equation:          tan                 θ     =       2        b        (     j   +   k     )             3        b   2       -     j   2     -     k   2                   where                 h     =     j   +     k   .                              
     At step  13 , the height T 1  of the spherical solder drop within the tape opening is calculated based on the height h and the contact angle θ. Firstly, a position is derived where the diameter of a circle on a horizontal plane traversing the spherical solder drop is equal to the tape opening diameter TD. This position is referred to as an equal diameter level. Then, the height T 1 , which is a distance between the equal diameter level and the highest point of the spherical solder drop, is determined. 
     Referring to FIGS. 6 and 7, the process implemented in step  13  will be described. The equal diameter level corresponds to a position where the periphery of the tape opening  16  comes in contact (shown by arrows B) with the drop shaped solder  6 A when the tape  2  having the tape opening  16  of diameter Td is covered on the drop shaped solder  6 A (see FIG.  7 ). Therefore, step  13  corresponds to a process of deriving the distance T 1  between the contact position and the highest point of the drop shaped solder  6 A in a vertical direction. 
     Since the equations defining the spherical shape of the drop shaped solder  6 A can be easily derived and the diameter TD of the tape opening  16  is known, coordinates of the position shown by the arrows B in FIG. 7 can be derived easily. Accordingly, the distance T 1  between the position B and the highest point of the drop shaped solder  6 A can also be derived easily. 
     In step  14 , a ratio between the distance T 1  and the tape thickness T 2  is derived. The ratio between the distance T 1  and the tape thickness T 2  is hereinafter referred to as a tape thickness ratio (T 1 /T 2 ). 
     The inventors have carried out an experiment to derive a relationship between the tape thickness ratio (T 1 /T 2 ) and a rate of occurrence of open failure (rate of occurrence of ball disconnection). The experiment was carried out using the semiconductor device  1  of a mass-produced type. Various semiconductor devices  1  with different tape thickness ratio (T 1 /T 2 ) were mounted on the mounting board and the rate of occurrence of ball-dropping was derived. FIG. 8 is a graph showing the result of the experiment. In FIG. 8, the vertical axis represents the rate of occurrence of ball-dropping and the horizontal axis represents the tape thickness ratio. 
     It can be seen from FIG. 8 that a slight ball-dropping occurs at a tape thickness ratio of 0.7, and the rate of occurrence of ball-dropping increases as the tape thickness ratio decreases. That is to say, the occurrence of ball-dropping (or, the shape of solder) can be estimated by the tape thickness ratio. 
     FIG. 9 shows a two-dimensional map of a solder shape evaluation with respect to the tape thickness ratio. The map shown in FIG. 9 is created from the results of experiment shown in FIG.  8 . In the present embodiment, a grade AA is given for a case where (T 1 /T 2 ) 1.0, since no ball-dropping occurs due to the defectiveness of the solder shape as shown in FIG. 8. A grade D is given for a case where (T 1 /T 2 )&lt;0.3, since a considerable number of ball-dropping occurs due to frequent occurrence of the defectiveness of the solder shape as shown in FIG.  8 . Between grade AA and grade D, three grades A to C are provided. The solder shape evaluation indicated by grades AA to D can be obtained from the two-dimensional map shown in FIG. 9 by taking the tape thickness parameter (T 1 /T 2 ) as a parameter. 
     In step  15 , the main control part  21  accesses the two-dimensional map (see FIG. 9) stored in the storage unit  26 , so as to select the grade corresponding the tape thickness ratio (t 1 /T 2 ) that is in interest. The result of the solder shape evaluation of step  15  is displayed on the output unit  25  and is also outputted from the output unit  25 . Accordingly, the solder shape evaluation process of FIG. 4 terminates. 
     As has been described above, in the solder shape evaluation process of the present invention, the solder shape is estimated by predetermined operations based on parameters such as the total solder volume V, the land radius b, and the opening diameter TD, all of which being measurable in advance. Accordingly, the solder shape can be estimated before the mounting process accurately within a reduced time. Also, it is now possible in a designing step to estimate the occurrence of failures, such has open failures and necking failures, that may occur during the mounting process. Therefore, the semiconductor device  1  having a high reliability and without any necking failures can be provided in the designing step. 
     Referring now to FIGS. 10A to  10 D, another aspect of the present invention will be described. It is known that voids (air bubbles) may be produced in the solder external terminals  6 . When voids are produced in the solder external terminal  6 , the probability of occurrence of open failures and necking failures increases. 
     FIGS. 10A to  10 D show states where the solder external terminals  6  with voids  15  is mounted on the mounting board  3 . As shown in FIG. 10A, the voids  15  may exist in the solder external terminal  6 . Then, when a heat treatment applied by a reflow process, the solder external terminals  6  and the solder paste  19  will fuse. Then, as shown in FIG. 10B, the voids  15  or the bubbles starts moving upwards within the fused solder. 
     Then, the voids  15  are expelled from the solder external terminal  6  and the air remains within the tape opening  16 . Therefore, as shown in FIG. 10C, the gap in the tape opening  16  increases and the solder external terminal  6  will be pressed downwards. That is to say, since the voids  15  are expelled and the air remains in the tape opening  16 , the solder external terminal  6  will be in a more necked state inside the tape opening  16 . Accordingly, when the voids  15  exist in the solder external terminal  6 , the rate of occurrence of the open failures and the necking failures will increase. 
     Therefore, in order to implement an accurate solder shape evaluation, the effect of the voids  15  should be taken into account. 
     In order to reflect the effect of the voids, the volume of the voids (v 4 ) that may occur in the solder external terminal  6  is derived in advance. The number of voids to occur and the volume of the voids are known experimentally and thus are known values. Also, for increased accuracy, the volume V 4  of the voids  15  may be directly obtained by implementing an X-ray imaging of the solder external terminal  6 . 
     Thus obtained volume V 4  of the voids  15  is used in step  14  of FIG. 4 to compensate for the thickness (T 2 ) of the substrate. That is to say, when the voids  15  are expelled inside the tape opening  16 , the gap within the tape opening  16  increases as compared to a case where there is no void  15 . 
     Therefore, as shown in FIG. 11, it can be regarded that the height of the tape opening (or the tape thickness T 1 ) is increased (ΔT 2 ) by an amount corresponding to the volume V 4  of the voids  15 . Then, even if voids  15  exist in the solder external terminal  6 , the solder shape evaluation may be implemented using the two-dimensional map of FIG.  9 . ΔT 2  may be shown by the following expression: 
     
       
           ΔT   2 =(4 ×V   4 )/( TD   2 ×π)  (8)  
       
     
     Accordingly, since the voids  15  existing in the solder external terminal  6  are taken into account by compensating for the tape thickness T 2 , the solder shape evaluation can be accurately implemented even if the voids  15  exist in the solder external terminal  6 . 
     It is to be noted that, in the embodiment described above, the solder shape evaluation is based on the tape thickness ratio (T 1 /T 2 ), but the solder shape can also be estimated based on a volume ratio. The volume ratio is defined as a ratio of the volume V 1  of a part of drop-shaped solder  6 A that is above a level shown by line B—B in FIG. 7 against the volume V 2  of the tape opening (V 1 /V 2 ). The volume V 1  of the drop-shaped solder  6 A and the volume V 2  of the tape opening  16  may be derived by the following equations, respectively:                      V   1     =       ∫     H   -     TT        (   I   )         H            X   2             y                     =       ∫     H   -     TT        (   I   )         H            {       R   2     -       (     Y   -   b     )     2       }             y                       (   9   )                 V   2     =         TAPE                     (     2   ,   I     )     2     ×   TAPE                   (     1   ,   I     )       4        π             (   10   )                                
     Further, the present invention is not limited to these embodiments, but variations and modifications may be made without departing from the scope of the present invention. 
     The present application is based on Japanese priority application No. 2000-350221 filed on Nov. 16, 2000, the entire contents of which are hereby incorporated by reference.