Abstract:
A multi-hip semiconductor package is proposed, in which a first chip and a second chip are mounted on opposing surfaces of a lead frame in a staggered manner. This staggered arrangement assures the die bonding quality for firmly disposing the second chip in the semiconductor package without being detrimental affected by the first chip. Moreover, as both opposing surfaces of the lead frame have chips mounted thereon, a mold flow of a molding resin used in a molding process can be balanced, so that turbulence the mold flow is decreased, and void formation can be avoided. In addition, the semiconductor package can incorporate a third chip in a stacked manner with respect to the first or second chip. This therefore further improves the functionality and performance of the semiconductor package.

Description:
FIELD OF THE INVENTION 
     The present invention relates to multi-chip semiconductor packages, and more particularly, to a multi-chip semiconductor package, in which two semiconductor chips are disposed in a staggered manner. 
     BACKGROUND OF THE INVENTION 
     A conventional semiconductor package generally includes a metal lead frame having a die pad and a plurality of leads, for disposing a semiconductor chip on the die pad and electrically connecting the chip to the corresponding leads by bonding wires. The chip, the die pad and part of the leads are encapsulated in an encapsulant, in an effort to protect the semiconductor package from damp, dust or damage. 
     However, in accordance with higher demand for operational function and speed of electronic products, more semiconductor devices are employed on a motherboard, or integrated circuits with higher integration level are utilized on a semiconductor chip. Nevertheless, the motherboard needs to expand its surface area for accommodating the larger number of the semiconductor devices, and this is undesirable in response to a trend for developing low-profile electronic products. Increase in integration level for the semiconductor chip is possibly limited by present manufacturing technology and is cost-ineffective to implement. Thus, an effective solution is to incorporate two or more chips in one semiconductor package, so as to raise the density of integrated circuits, the memory capacity and the process speed. 
     U.S. Pat. No. 5,898,220 discloses a multi-chip semiconductor package. Referring to FIG. 1, the semiconductor package includes a first semiconductor chip  11 , a second semiconductor chip  12  and a lead frame  10 . The first and second chips  11 ,  12  each possesses a top surface  110 ,  120  (where electric circuits and electronic components are disposed) and an opposing bottom surface  111 ,  121 , respectively. On each of the top surfaces  110 ,  120  there are formed two rows of bond pads  112 ,  122  at two opposite sides. The lead frame  10  is composed of a plurality of leads  14 , wherein a surface of the leads  14  for mounting the first chip  11  thereon is referred to as a first surface  140 , while a surface of the leads  14  opposing the first surface  140  is referred to as a second surface  141 . 
     As shown in the drawing, a nonconductive first tape  150  is adhered to the first surface  140  of the leads  14 , and then the top surface  110  of the first chip  11  is firmly attached to the first tape  150  by using a conventional jig (not shown) under heat and pressure. Further, a plurality of gold wires  16  are used to electrically connect the bond pads  112  on the top surface  110  of the first chip  11  to the first surface  140  of the leads  14 . Similarly, the second chip  12  is attached to the second surface  141  of the leads  14  by means of a second tape  151 , and the bond pads  122  on the second chip are electrically connected to the second surface  141  of the leads  14  by a plurality of gold wires  16 . Finally, an encapsulant  17  is formed to encapsulate the first chip  11 , the second chip  12 , the gold wires  16  and a portion of the leads  14  close to the chips, so as to protect the semiconductor chips from outside moisture and pollutant. 
     In the use of the jig (as shown in FIG. 2 by a reference numeral  18 ), a heat source is provided for heating the tapes to firmly attach the semiconductor chips to the leads. However, as shown in FIG. 2, after completing the attachment of the first chip  11 , due to heat blockage by the first chip  11  and the highly heat-resistant first tape  150 , the second tape  150  receives heat in reduced transmission efficiency, and thus the second chip  12  is unable to be firmly bonded by means of the second tape  150 , resulting in increase in the reliability concern for die bonding. Moreover, in order to successively bonding the gold wires  16  to respectively the first chip II and the leads  14 , the second tape  151  needs to be dimensioned in thickness slightly larger than the height of wire loops of the gold wires  16 . In consideration of replacing the second tape  151  with a conventional nonconductive adhesive for improving the problem of poor beat transmission as described above, however, an adhesive layer formed by the adhesive with high fluidity is hardly controlled in thickness, and thus the gold wires  16  may suffer suppression from the second chip  12 , resulting in poor wire bonding quality. 
     In addition, in a molding process for the foregoing semiconductor package, as shown in FIG. 3, after a melted molding resin (not shown) is injected into a mold (not shown), a mold flow direction is perpendicular to gaps  142  between the leads  14 , which are interposed between the first and second chips  11 ,  12 . This therefore makes the molding resin hardly flow into the gaps  142 , and the gaps  142  not completely filled with the molding resin easily have voids formed therein. Accordingly, a popcorn effect can be generated in the semiconductor package operating in a high temperature environment due to thermal expansion of air in the voids. 
     SUMMARY OF THE INVENTION 
     A primary objective of the present invention is to provide a multi-chip semiconductor package, in which chips are disposed in a staggered manner, so as to improve heat transmission efficiency of a jig in a die bonding process and thus maintain the die bonding reliability. 
     Another objective of the present invention is to provide a multi-chip semiconductor package, in which staggered chips allow a mold flow of a molding resin to be balanced and thus avoid void formation. 
     A further objective of the present invention is to provide a multi-chip semiconductor package, in which chips are simultaneously incorporated in lead-on-chip (LOC) and non-LOC manners. 
     A further objective of the present invention is to provide a multi-chip semiconductor package, in which chips are employed with no restriction on size. 
     A further objective of the present invention is to provide a multi-chip semiconductor package, in which no special jig is needed, and thus the fabrication cost is reduced. 
     According to the foregoing and other objectives, the present invention proposes a multi-chip semiconductor package, including: a lead frame having a front surface and an opposing back surface, and consisting of a die pad and a plurality of leads surrounding the die pad, wherein the die pad is firmly held at a position deviating from the center of the lead frame by a plurality of supporting elements; a first chip having an active surface and an opposing inactive surface, and bonded to the die pad on the front surface of the lead frame by means of an adhesive; a second chip having an active surface and an opposing inactive surface, wherein the active surface is attached to the supporting elements on the back surface of the lead frame and the leads formed around the supporting elements, so as to allow the first chip and the second chip to be spatially positioned in a staggered manner, a plurality of first conductive elements for electrically connecting the first chip to the leads; a plurality of second conductive elements for electrically connecting the second chip to the leads; and an encapsulant for encapsulating the first chip, the second chip, the first conductive elements, the second conductive elements and part of the leads. 
     The semiconductor package of the invention is advantageous in that at least two semiconductor chips are incorporated in the same package simultaneously, and the first and second chips are respectively attached to the front and back surfaces of the lead frame in a staggered manner. This staggered arrangement makes die bonding processes performed for the first and second chips without interfering with each other, so that the second chip can be firmly disposed in the semiconductor package. This is more advantageous than the prior art, in which bonding quality of a second chip is detrimentally affected due to heat transmission blocked by a first chip located vertically with respect to the second chip. As a result, a special jig used in the die bonding process of the second chip for avoiding damage to the first chip is not necessary, and the cost for preparing such a jig is substantially saved. Furthermore, in a molding process, the staggered chips disposed on the opposing surfaces of the lead frame can balance a mold flow of a molding resin, so that turbulence to the mold flow is decreased and void formation can be avoided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
     FIG. 1 (PRIOR ART) is a sectional view of a conventional semiconductor package disclosed in U.S. Pat. No. 5,898,220; 
     FIG. 2 (PRIOR ART) is a schematic diagram depicting the step involved in bonding a second chip in a semiconductor package of FIG.  1 . 
     FIG. 3 PRIOR ART) is a schematic diagram depicting a mold flow curve in a molding process for a semiconductor package of FIG. 1; 
     FIG. 4 is a front view of a semiconductor package of a first preferred embodiment of the invention; 
     FIG. 5 is a sectional view of FIG. 4 cutting along a line  5 — 5 ; 
     FIG. 6 is a topical enlargement diagram depicting the completion of bonding a second chip in a semiconductor package of the invention; 
     FIG. 7 is a schematic diagram depicting a mold flow direction in a molding process for a semiconductor package of the invention; 
     FIG. 8 is a sectional view of a semiconductor package of a second preferred embodiment of the invention; 
     FIG. 9 is a sectional view of a semiconductor package of a third preferred embodiment of the invention; and 
     FIG. 10 is a sectional view of a semiconductor package of a fourth preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 4 and 5, a semiconductor package  2  of a first embodiment of the invention comprises a lead frame  20  having a plurality of leads  24  and a die pad  23  connected to a plurality of tie bars  230 ; a first chip  21  and a second chip  22  attached to front and back surfaces of the lead frame  20  respectively; a plurality of first bonding wires  260  for electrically connecting the first chip  21  to the leads  24 ; a plurality of second bonding wires  261  for electrically connecting the second chip  22  to the leads  24 ; and an encapsulant  27  for encapsulating the first chip  21 , the second chip  22 , the first bonding wires  260 , the second bonding wires  261  and part of the leads  24 . 
     FIG. 4 illustrates a front view of a lead frame used in the semiconductor package of the invention. As shown in the drawing, the lead frame  20  has a front surface  200  and an opposing back surface  201  (as shown in FIG.  5 ), and consists of the die pad  23 , the plurality of tie bars  230  and the plurality of leads  24  surrounding the die pad  23 . The die pad  23  has a top surface  231  and an opposing bottom surface  232  (as shown in FIG.  5 ), and is held firmly at a position deviating from the center of the lead frame  20  by means of the tie bars  230  integrated with the die pad  23 . The tie bars  230  each is formed with a stamping region  233  exposed to outside of the encapsulant  27 , and a die bonding region  234  encapsulated in the encapsulant  27 , wherein surfaces thereof facing the same as the front surface  200  of the lead frame  20  are referred to as first surfaces  233   a ,  234   a , and surfaces thereof facing the same as the back surface  201  (not shown) of the lead frame  20  are referred to as second surfaces  233   b ,  234   b . Similarly, the leads  24  are also formed with outer leads  243  exposed to the outside of the encapsulant  27 , and inner leads  242  encapsulated in the encapsulant  27 , wherein surfaces thereof facing the same as the front surface  200  of the lead frame  20  are referred to as first surfaces  240 , and opposing surfaces thereof are referred to as second surfaces  241 . 
     As shown in FIG. 5, the first chip  21  such as a ASIC semiconductor chip has a top surface  210  disposed with electronic circuits and electronic components thereon, and a bottom surface  211  opposing the top surface  210 . Two rows of bond pads  212  are formed respectively at two opposite sides on the top surface  210 , and made of a conductive metal such as aluminum. Alternatively, the bond pads can be distributed at only one side or in a surrounding manner with no specific restriction. The first chip  21  has its bottom surface  211  attached to the top surface  231  of the die pad  23  by means of an epoxy resin adhesive  25  such as silver paste, or a polyimide resin tape  25 , so as to allow the first chip  21  to be. firmly bonded to the die pad  23 . 
     After completing the die bonding process for the first chip  21 , the first bonding wires  260  such as gold wires are used to electrically connect the bond pads  212  of the first chip  21  to the first surfaces  240  of the inner leads  242 . Then, a die bonding process for the second chip  22  is performed. 
     The second chip  22  such as a flash memory or DRAM semiconductor chip similarly has a top surface  220  disposed with electronic circuits and electronic components thereon, and a bottom surface  221  opposing the top surface  210 . However, the second chip  22  differs from the first chip  21  in that only one row of bond pads  222  are formed at the center or one side of the top surface  220  of the second chip  22 . As shown in FIG. 6, the second chip  22  has its top surface  220  attached to the second surfaces  241 ,  234   b  of the inner leads  242  and the die bonding regions  234  of the tie bars  230  respectively by means of an epoxy resin adhesive  25  such as silver paste, or a polyimide resin tape  25 . Therefore, the first chip  21  and second chip  22  are individually bonded to the front and back surfaces of the lead frame  20  (as shown in FIG.  5 ), and are spatially positioned in staggered manner. After completing the foregoing die bonding process for the second chip  22  by placing this semi-fabricated structure  2  into a jig (not shown), the second bonding wires  201  cross over the tie bars  230  and electrically connect the bond pads  222  on the top surface  220  of the second chip  22  to the first surfaces  240  of the inner leads  242 . 
     The first chip  21  and second chip  22  are not particularly limited in surface area relative to each other, that is, the first chip  21  can be dimensionally larger than, equal to or smaller than the second chip  22 . Thus, the first chip  21  and second chip  22  can be more flexibly selected and combined for use in type and size (for example, a microprocessor chip or other types of chips can all be utilized), as long as they are sufficiently dimensioned to accommodate the die pad  23  and tie bars  230 . Moreover, due to the two chips spatially staggered in the semiconductor package of the invention, in the die bonding process for the second chip  22 , heat transmission from the jib to the second chip  22  is not affected by the staggered first chip  21 , so that the second chip  22  can be firmly disposed in the semiconductor package. Further due to no concern for damaging the first chip  21  and the first bonding wires  260 , no special jig is needed, and thus the cost for the jig used in the die bonding process can be efficiently reduced. 
     After completing the die bonding and wire bonding processes, the semi-fabricated structure  2  is placed into a molding cave of a mold (not shown) to carry out a molding process. As shown in FIG. 7, a fluid melted molding resin (not shown) is injected from an injection inlet  29  into the molding cavity for forming the encapsulant  27  (not shown) encapsulating the semiconductor chips  21 ,  22 . Since the first chip  21  and second chip  22  are spatially staggered, and the front and back surfaces  200 ,  201  of the lead frame  20  are each disposed with one of the chips, thus a mold flow of the molding resin can be balanced (as indicated by arrows in FIG.  7 ), so that turbulence to the mold flow is decreased, and void formation can be avoided. 
     After curing the encapsulant at a high temperature, the outer leads  243  of the semi-fabricated structure  2  can be optionally trimmed or formed into a shape according to practical application. The outer leads can be bent into a gull-wing shape, a J-like shape, a L-like shape, etc. This therefore completes the fabrication of the semiconductor package of the invention. 
     FIG. 8 illustrates a second embodiment of the semiconductor package of the invention. The semiconductor package of the second embodiment is structurally identical to that of the first embodiment as described above, with the only difference in that, in this embodiment, besides a first chip  31  disposed on a top surface  331  of a die pad  33  of a lead frame, the die pad  33  can further accommodate a third chip  38  on a bottom surface  332  thereof The third chip  38  is not particularly limited in size and type, as long as it can fit the sizes of the die pad  33  and tie bars  330 . Therefore, the semi conductor package of the invention can further raise the integration of the semiconductor device, in a manner that more semiconductor chips can be incorporated into the same package for improving functionality and processing speed of the package. 
     FIG. 9 illustrates a third embodiment of the semiconductor package of the invention. The semiconductor package of the second embodiment is structurally identical to that of the first embodiment as described above, with the only difference in that, chips in this embodiment are disposed in a stacked manner, wherein, after attaching a second chip  32  to a lead frame  30 , a third chip  38  is additionally bonded to a bottom surface  321  of the second chip  32  in a back-to-back manner by means of an adhesive  35 . The third chip  38  can have bond pads formed thereon at one single side, at central positions or in a surrounding manner with no particular limitation. 
     FIG. 10 illustrates a fourth embodiment of the semiconductor package of the invention. The semiconductor package of the second embodiment is structurally identical to that of the first embodiment as described above, with the only difference in that, chips in this embodiment are disposed in a stacked manner, wherein a third chip  38  is bonded to a top surface  310  of a first chip  31  by means of an adhesive layer  35  made of an epoxy resin adhesive such as silver paste, or a polyimide resin tape. Since the adhesive layer  35  is made of a rubber material with elasticity, after bonding the third chip  38  to the first chip  31 , the elastic adhesive layer  35  can buffer and absorb a downward pressure to the first chip  31 , and thus provide protection for the top surface  310  of the first chip  31  from being damaged by the pressure. The third chip  38  needs to be dimensionally slightly smaller than the first chip  31 , and the adhesive layer  35  is necessarily applied to area excluding bond pads  312  on the top surface  310  of the first chip  31 , so as not to interfere with a wire bonding process of first bonding wires  360 . The third chip  38  can have bond pads formed thereon at one single side, at central positions or in a surrounding manner with no particular limitation. 
     The semiconductor package with staggered double chips of the invention incorporating a further chip in a stacked manner can significantly improve the integration of a semiconductor device, and thus more semiconductor chips can be incorporated into the same package without increasing the overall thickness of the package. This therefore significantly raise the functionality and processing speed of integrated circuits in response to the increase in complexity of electronic signal transmission in a package. 
     The invention has been described using exemplary preferred embodiments. However, it is to be understood that the scope of the 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.