Patent Publication Number: US-2003234276-A1

Title: Strengthened bonding mechanism for semiconductor package

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates to strengthened bonding mechanisms for semiconductor packages, and more particularly, to a bonding mechanism for use with a semiconductor package, so as to increase bonding strength between a solder ball or bump and the bonding mechanism where the solder ball or bump is bonded.  
       BACKGROUND OF TIE INVENTION  
       [0002] A semiconductor package, such as a BGA (ball grid array) package, is characterized by implanting a plurality of array-arranged solder balls at bond pads formed on a substrate. The solder balls serve as I/O (input/output) ports of the semiconductor package, for electrically connecting the semiconductor package to an external device e.g. a printed circuit board (PCB). The semiconductor package can be mounted on the PCB by a conventional SMT (surface mount technology) process in a manner that, the solder balls are respectively bonded to corresponding bond pads formed on the PCB. As such, a chip mounted on the substrate oppositely in position to the solder balls, can be electrically coupled to the PCB for the purposes of power supply, grounding and signal transmission in operation of the chip.  
       [0003]FIG. 4A illustrates a BGA semiconductor package mounted on a printed circuit board (PCB). As shown in the drawing, a plurality of solder balls  10  implanted on a substrate  11  are connected to a PCB  12 , allowing a chip  13  mounted on the substrate  11  to be electrically coupled to the PCB in turn through bonding wires  14  and the solder balls  10 .  
       [0004] Bond pads (not shown in FIG. 4A) formed on the substrate  11  or PCB  12  where solder balls  10  are bonded, are primarily made of copper (hereinafter referred to as “copper pad”); such a copper pad  15  is structurally illustrated in FIG. 4B. Since fabrication of copper pads  15  employs conventional technology, it is not to be further described herein.  
       [0005] Besides, bond pads are also used for accommodating solder bumps in a flip-chip structure, so as to mediate electrical connection between a flip chip and a substrate. As shown in FIG. 5A, the flip-chip structure is characterized by implanting a plurality of solder bumps  20  on an active surface  210  of a chip  21 , allowing the chip  21  to be mounted on a substrate  22  in a face-down manner. The solder bumps  20  for electrically connecting the chip  21  to the substrate  22 , are bonded to bond pads (not shown) respectively formed on the active surface  210  and the substrate  22 . Such a bond pad  23  formed on the chip  21  is stucturally illustrated in FIG. 5B, and primarily made of aluminum (hereinafter referred to as “aluminum pad”). Then, a passivation layer  24  is applied over the aluminum pad  23 , and selectively removed to expose the aluminum pad  23 . As the aluminum pad  23  is easily oxidized upon exposure to air and inhospitable to a solder bump, a more hospitable UBM (under bump metallurgy) structure  24  is consequently formed on the aluminum pad  23  for facilitating bump attachment. Since fabrication of the UBM structure  24  adopts conventional technology, it is not to be further described herein. In respect of a bond pad being formed on the substrate  22 , it is a copper pad and structured as illustrated in FIG. 4B described above, therefore not further to be here repeated.  
       [0006] However, the above conventional bonding mechanisms are inherent with significant drawbacks. Solder balls or bumps bonded to bond pads formed on a flip chip, substrate or PCB, may be easily subject to breaking or cracking upon exerted with external impact or shear force, due to unsatisfactory bonding strength between solder balls or bumps and the bond pads. Moreover, such solder balls tend to break at ball necks, and damage the underlying structure such as UBM, copper or aluminum pad; this undesirably degrades reliability of fabricated products. In addition, by virtue of unsatisfactory bonding strength and breaking of solder balls or bumps, copper pads or aluminum pads with UBM structures are hardly adapted to be densely arranged for accommodating fine-pitch arrayed solder balls or bumps. This drawback is particular unfavorable for a flip-chip structure, wherein small-sized solder bumps are preferably mounted in high density on a flip chip in compliance with the highly-integrated flip chip.  
       [0007] Therefore, it is highly desired to develop a strengthened bonding mechanism for use with a semiconductor package, so as to increase bonding strength between a solder ball or bump and the bonding mechanism, and to facilitate fine-pitch arrangement of bonding mechanisms and solder balls or bumps.  
       SUMMARY OF THE INVENTION  
       [0008] An objective of the present invention is to provide a strengthened bonding mechanism for a semiconductor package, so as to increase bonding strength between a solder ball or bump and the bonding mechanism where the solder ball or bump is bonded, thereby improving reliability of fabricated products.  
       [0009] Another objective of the invention is to provide a strengthened bonding mechanism for a semiconductor package, allowing a solder ball or bump to be strongly bonded to the bonding mechanism, such that bonding mechanisms can be densely formed for accommodating fine-pitch arrangement of solder balls or bumps.  
       [0010] In accordance with the above and other objectives, the present invention proposes a strengthened bonding mechanism for a semiconductor package, comprising: an aluminum pad formed on a chip; a UBM (under bump metallurgy) structure formed over the aluminum pad; and a tin layer applied over the UBM structure, for allowing a solder ball or bump to be bonded to the tin layer, wherein the UBM structure is interposed between the tin layer and the aluminum pad. The UBM structure is composed of at least two layers of metallic materials, such as copper, nickel, vanadium, gold, tungsten, titanium, chromium, aluminum or alloy thereof.  
       [0011] In another embodiment, the strengthened bonding mechanism of the invention comprises: a copper pad formed on a chip carrier; and a tin layer applied over the copper pad, for allowing a solder ball or bump to be bonded to the tin layer; wherein the chip carrier is a substrate or a printed circuit board (PCB).  
       [0012] By the above structure of bonding mechanism, with forming of a tin layer for being mounted with a solder ball or bump thereon, bonding strength between the solder ball or bump and the bonding mechanism would be significantly increased, whereby solder balls or bumps can be strongly bonded to the bonding mechanism formed on a chip, substrate or PCB, without being easily subject to breaking or cracking, making reliability of fabricated products firmly assured. As such, bonding mechanisms can be more densely arranged on the chip, substrate or PCB, for accommodating high density of solder balls or bumps thereon, which effectively facilitates electrical connection efficiency.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0013] 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:  
     [0014]FIG. 1 is a cross-sectional view of a first preferred embodiment of a bonding mechanism of the invention;  
     [0015]FIGS. 2A and 2B are cross-sectional views showing embodiments of a UBM structure in the bonding mechanism of FIG. 1;  
     [0016]FIG. 3 is a cross-sectional view of a second preferred embodiment of the bonding mechanism of the invention;  
     [0017]FIGS. 4A and 4B (PRIOR ART) are cross-sectional views respectively showing a conventional BGA semiconductor package, and a bonding mechanism for use with the semiconductor package; and  
     [0018]FIGS. 5A and 5B (PRIOR ART) are cross-sectional views respectively showing a conventional flip-chip semiconductor structure, and a bonding mechanism for use with the semiconductor package. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0019] Preferred embodiments of a strengthened bonding mechanism for a semiconductor package proposed in the present invention are described in more detail as follows with reference to FIGS.  1  to  3 .  
     First Preferred Embodiment  
     [0020]FIG. 1 illustrates a first embodiment of the bonding mechanism of the invention bonded with a solder ball or bump. As shown in the drawing, first, an aluminum pad  30  is formed on an active surface  310  of a chip  31 , and exposed by selectively removing a passivation layer  32  applied over the active surface  310  of the chip  31 . Since forming of the aluminum pad  30  employs conventional technology, it is not to be further detailed herein.  
     [0021] Then, a UBM (under bump metallurgy) structure  33  is formed over the aluminum pad  30  in a manner as to completely cover the exposed aluminum pad  30  on the chip  31 . The UBM structure is composed of at least two layers of metallic materials, such as copper (Cu), nickel (Ni), vanadium (V), gold (Au), tungsten (W), titanium (Ti), chromium (Cr), aluminum (Al), or alloy thereof  
     [0022] Finally, a tin layer  34  is applied over the UBM structure  33 , allowing the UBM structure  33  to be interposed between the tin layer  34  and the aluminum pad  30 . The tin layer  34  can be made of pure tin, and deposited on the UBM structure  33  by (but not limited to) plating technology. This therefore completes fabrication of the bonding mechanism of the invention, and an electrical connection element  35  such as a solder ball or bump can be readily bonded to the tin layer  34  of the bonding mechanism. In this embodiment, with solder balls or bumps  35  being attached to bonding mechanisms formed on the active surface  310  of the chip  31 , the chip  31  can be mounted on a chip carrier e.g. a substrate (not shown) in a face-down manner, allowing the chip  31  to be electrically connected to the substrate by the solder balls or bumps  35  that are interposed between the chip  31  and the substrate; such face-down chip arrangement is customarily referred to as a flip-chip structure.  
     [0023]FIGS. 2A and 2B illustrate embodiments of a UBM structure in the bonding mechanism of the invention. As shown in FIG. 2A, the UBM structure  33  consists of three metallic layers  330 ,  331 ,  332 , wherein the bottom metallic layer  332  is directly attached to the aluminum pad  30 , and the top metallic layer  330  is covered by the tin layer  34 . Combinations of the three metallic layers  330 ,  331 ,  332  can be, for example, Au/Ni/Cu, Au/W/Ti, Cu/Cr—Cu/Cr, etc. Alternatively, as shown in FIG. 2B, the UBM structure  33  can be made of two metallic layers  333 ,  334 , wherein the tin layer  34  is applied over the upper metallic layer  333 , and the aluminum pad  30  is in contact with the lower metallic layer  334 . Combinations of the two metallic layers  333 ,  334  are, for example, Ni/Cu, Cu/Cr, Au/Ni, etc. It should be understood that, embodiments of the UBM structure  33  exemplified herein do not set any restriction to the scope of the invention; much more other combinations for fabricating the UBM structure  33  are suitably adopted in this invention.  
     [0024] The above bonding mechanism of the invention can provide significant benefits. It is a characteristic feature of forming a tin layer on a UBM structure, whereby a solder ball or bump would be strongly bonded to the tin layer of the bonding mechanism without being easily subject to breaking or cracking. A table below illustrates experimental data for testing bonding strength between a solder ball and a bonding mechanism. As shown in the table, a solder ball bonded and reflowed conventionally to a UBM structure formed on an aluminum pad, can only sustain a shear force up to 650 g before it breaks from the UBM structure. In another case, a solder ball reflowed to a tin (Sn) layer formed on a UBM structure would be able to sustain a much greater shear force of 930 g, which indicates around 50% increase in bonding strength by provision of the tin layer in the bonding mechanism. Moreover, if a solder ball bonded to the Sn/UBM structure is exerted with an even greater shear force than 930 g, it would break at Sn/UBM interface, instead of ball neck as for a solder ball directly bonded to a UBM structure. Breakage at Sn/UBM interface would not result in craters formed on the breaking surface, thereby reducing damage to the underlying structure such as UBM, aluminum pad.  
                                                   Reflowed to UBM structure   Shear force                          Solder ball   650 g           Solder ball + tin layer   930 g                      
 
     [0025] Therefore, by using the bonding mechanism with a tin layer of the invention, bonding strength between the solder ball or bump and the bonding mechanism would be significantly enhanced, making the solder ball or bump firmly bonded to the bonding mechanism, such that reliability of fabricated products can be greatly improved.  
     [0026] Furthermore, as a solder ball or bump can be strongly bonded to the bonding mechanism of the invention, thereby bonding mechanisms can be densely arranged in a manner as to reduce pitch spacing between adjacent bonding mechanisms. This advantage is important for a flip-chip structure, in which aluminum pads formed on a flip chip are preferably high-density arrayed in compliance with the small sized and highly integrated chip, such that fine-pitch arrangement of solder balls or bumps can be accommodated for achieving desirable electrical connection efficiency.  
     Second Preferred Embodiment  
     [0027]FIG. 3 illustrates a second embodiment of the bonding mechanism of the invention bonded with a solder ball or bump. As shown in the drawing, first, a copper pad  40  is formed on a chip carrier  41  such as a substrate or printed circuit board (PCB); a chip carrier serves for accommodating a chip (not shown) thereon. And, a solder mask layer  42  applied over the chip carrier  41  is selectively removed to expose the copper pad  40 . Since fabrication of the copper pad  40  adopts conventional technology, it is not to be further detailed herein.  
     [0028] Then, a tin layer  43  is applied over the copper- pad  40  in a manner as to completely cover the exposed copper pad  40  formed on the chip carrier  41 . The tin layer  42  can be made of pure tin, and deposited on the copper pad  40  by (but not limited to) plating technology. This then completes fabrication of the bonding mechanism of the invention, and thereby, an electrical connection element  44  such as a solder ball or bump can be readily bonded to the tin layer  43  of the bonding mechanism.  
     [0029] In the case of the chip carrier  41  being a substrate for use in a semiconductor package such as a BGA package, bonding mechanisms formed on the substrate can be bonded with solder balls or bumps, which serve as I/O ports for electrically connecting a chip (not shown) mounted on the substrate to an external device such as a PCB. Alternatively, bonding mechanisms formed on the substrate are used for accommodating solder balls or bumps bonded to an active surface of a chip (not shown), whereby the chip can be mounted and electrically connected to the substrate in a face-down manner; this forms a flip-chip structure as described above.  
     [0030] In the case of the chip carrier  41  being a PCB, when a BGA semiconductor package is mounted on the PCB by conventional SMT technology, solder balls or bumps as I/O ports of the semiconductor package are adapted to be bonded to bonding mechanisms formed on the PCB.  
     [0031] The above structured bonding mechanism can also provide improvements rendered in the first embodiment. With forming of a tin layer on a copper pad, bonding strength between a solder ball or bump and the bonding mechanism for a substrate or PCB, would be significantly increased, making solder balls or bumps strongly bonded to the substrate or PCB without being easily subject to breaking or cracking, whereby reliability of fabricated products can be firmly assured. As such, bonding mechanisms can be more densely arranged on the substrate or PCB, for accommodating high density of solder balls or bumps thereon, which facilitates electrical connection efficiency. In a flip-chip structure, such a substrate would allow highly effective electrical connection between the substrate and a flip chip. For a BGA semiconductor package mounted on a PCB, solder balls or bumps as I/O ports of the semiconductor package would be firmly bonded to a substrate and the PCB, thereby making electrical connection quality effectively improved.  
     [0032] 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.