Abstract:
A solder joint structure comprises a solder of a Sn alloy especially having Cu element contained therein, a contact region having a Ni layer been composed therein. In which, by means of controlling the Cu concentration to select an interface reaction product for reducing the consumption rate of the Ni layer of the contact region so as to provide an durable strength therefore.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a structure of a solder joint of an electronic device, more particularity, to a structure of a solder joint that the consumption rate of a Ni layer of a contact region is reduced by controlling the concentration of copper element in the solder joint such that a more protective reaction product forms at the interface between the solder joint and the Ni layer of the contact region.  
           [0003]    2. Description of the Related Art  
           [0004]    In the electronic age, electronic products are wildly used in the daily life of everyone. The central part of any electronic product is the integrated circuit chip. The chip has to be connected electrically to a substrate by a packaging process, and then the substrate has to be connected electrically to a motherboard. Solder joints of various types are used to connect the chip to the substrate, and to connect the substrate to the motherboard. The solder joints take responsibility of electrical connecting as well as physical supporting, therefore a solder joint should has good physical strength to prevent the electronic device form damages. In fact, as many as 80% of the failures in electronic devices are due to failures of poor solder joints, therefore improving the quality of solder joints is an important problem to be solved.  
           [0005]    [0005]FIG. 1 is a cross-sectional view of a conventional solder joint showing a solder joint  120  and a contact region  110  seated on an electronic device  100 . The contacting region  110  can be called a “soldering pad” or an “under bump metallurgy” according to the field of application. The contacting region can be composed of a Cu layer  112  over the electronic device  100 , a Ni layer  114  over the Cu layer  112 , and a Au layer  116  over the Ni layer  114 . The solder  120  is composed of an alloy of Sn, such as Pb—Sn or Ag—Sn.  
           [0006]    At the beginning of soldering, the gold in the Au layer  116  will enter the solder  120  rapidly to form the compound (Au, Ni)Sn 4 . After the Au layer  116  is consumed completely, the Ni layer  114  then comes into contact with the solder  120  and starts to react with Sn in solder  120  to form the compound Ni 3 Sn 4 . When the Ni layer is also fully consumed, the Cu layer  112  will rapidly reacted with the solder  120 . Since the reacting rate of the Cu layer  112  with the solder is at least 10 times faster than the reacting rate of the Ni layer  114  with the solder, the Cu layer  112  will be consumed rapidly after the Ni layer  114  is gone. In that case, the strength of the solder joint will be very low, making the electronic device fails easily if subject to external force.  
         SUMMARY OF THE INVENTION  
         [0007]    Therefore, an object of the present invention is to provide a solder joint having a Cu element contained therein, and by means of controlling the concentration of Cu in the solder a more protective reaction product at the interface between the solder joint and the Ni layer of the contact region is selected to from so as to reduce the consumption rate of the Ni layer.  
           [0008]    Further, there are three more alternative ways to provide an effective Cu concentration in the solder other than aforesaid typical way to have Cu in the solder joint directly. The first way is to coat an extra Cu layer on the contact region which is opposite to the contact region with the Ni layer. The second way is to dispose an extra Cu layer between the solder joint and the Ni in the contact region. The third way is to alloy Cu into the Ni layer of the contact region directly. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The above and further objects, features and advantages of the invention will become clear from the following more detailed description when read with reference to the accompanying drawings in which:  
         [0010]    [0010]FIG. 1 is a cross-sectional view of a conventional solder joint;  
         [0011]    [0011]FIG. 2 is a cross-sectional view of a solder joint according to the present invention;  
         [0012]    [0012]FIG. 2A is a metallographic photograph of a Pb—Sn alloy solder joint without adding Cu after 2 min. of soldering of the present invention;  
         [0013]    [0013]FIGS. 2B, 2C and  2 D are metallographic photographs of Pb—Sn alloy solder joints with Cu added in different concentrations after 2 min. of soldering of the present invention;  
         [0014]    [0014]FIG. 3A is a metallographic photograph of a Pb—Sn alloy solder joint without adding Cu after reacting at 225° C. for 4 hours of the present invention;  
         [0015]    [0015]FIG. 3B is a metallographic photograph of a solder joint with Cu added after reacting at 225° C. for 4 hours of the present invention;  
         [0016]    [0016]FIG. 4A is a metallographic photograph of a Pb—Sn alloy solder joint without adding Cu after aging at 160° C. for 2000 hours of the present invention;  
         [0017]    [0017]FIG. 4B is a metallographic photograph of a solder joint with Cu added after aging at 160° C. for 2000 hours of the present invention;  
         [0018]    [0018]FIG. 5 is a diagram showing the growth rate of different reaction products during heat treatment of the present invention;  
         [0019]    [0019]FIG. 6A is a metallographic photograph of a solder joint using the Sn-3.5Ag (wt %) solder, taken after 2 min. of soldering of the present invention;  
         [0020]    [0020]FIG. 6B is a metallographic photograph of a solder joint using the Sn-4Ag-0.5Cu (wt %) solder, taken after 2 min. of soldering of the present invention;  
         [0021]    [0021]FIG. 6C is a metallographic photograph of a solder joint using the Sn-4Ag-0.75Cu (wt %) solder, taken after 2 min. of soldering of the present invention;  
         [0022]    [0022]FIG. 7A is a metallographic photograph of a solder joint using the Sn-3.5Ag (wt %) solder after aging at 180° C. for 300 hours of the present invention;  
         [0023]    [0023]FIG. 7B is a metallographic photograph of a solder joint using the Sn-3.5Ag-0.5Cu (wt %) solder after aging at 180° C. for 300 hours of the present invention;  
         [0024]    [0024]FIG. 7C is a metallographic photograph of a solder joint using the Sn-3.5Ag-0.75Cu (wt %) solder after aging at 180° C. for 300 hours of the present invention;  
         [0025]    [0025]FIG. 8 is a cross-sectional view showing an alternative way of incorporating Cu by coating a Cu layer on the contact region which is opposite to the contact region with the Ni layer according to an embodiment of the present invention; and  
         [0026]    [0026]FIG. 9 is a cross-sectional view of another embodiment showing another embodiment of incorporating Cu by coating an extra Cu layer over the Ni layer of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    Referring to FIG. 2, a typical embodiment according to the present invention comprises a Cu doped solder  220 , which the solder  220  can be an alloy of Pb—Cu—Sn or an alloy of Ag—Cu—Sn, and a contact region  210  seated on a electronic component  200  such as a chip, a substrate or a motherboard. The contact region  210  is composed of a Cu layer  212 , a Ni layer  214  and an Au layer  216 , in which the Ni layer  214  is deposited over the Cu layer  212  by means of electroplating, electroless plating, sputtering or evaporation. The thickness of Ni can be from 50 nm to 15 μm. The Au layer  216  is deposited over the Ni layer  214  by electroplating or electroless plating. The solder  220  is formed onto the Au layer  216  by means of screen-printing followed by reflowing, or by solder ball plating followed by reflowing.  
         [0028]    Referring to FIGS. 2A, 2B,  2 C and  2 D, there are metallographic photographs after 2 min. of soldering. The Cu concentration in these solder joints is form 0.0 wt % to 1.5 wt %. FIG. 2A shows a diagram of a Pb—Sn alloy solder joint; FIG. 2B shows a diagram of a Pb—Sn alloy solder joint with 0.1 (wt %) Cu added; FIG. 2C has a Cu concentration of 0.5 (wt %); FIG. 2D has a Cu concentration of 1.5 (wt %). Before soldering, the thickness of a Au layer of the contact region is 0.8-1.2 μm, and the thickness of a Ni layer and Cu layer is 6-8 μm and 7 μm, respectively. As shown in FIG. 2A, the reaction product at the interface is Ni 3 Sn 4 . As shown in FIGS. 2C and 2D, when the Cu concentration becomes higher, the reaction product at the interface becomes a simple and continuous (Cu, Au, Ni) 6 Sn 5 .  
         [0029]    Referring to FIGS. 3A and 3B, the effect of Cu concentration on the reaction product is illustrated more clearly. The solder joints in FIGS. 3A and 3B has been reacted at 225° C. for 4 hours. There is no Cu added in FIG. 3A, and the reaction product is Ni 3 Sn 4 . In FIG. 3B, 1.5 wt % Cu has been added, and the reaction product is (Cu, Au, Ni) 6 Sn 5 . Comparing FIGS. 3B and 3A, it is clear that adding Cu into solder joints can reduce the consumption rate of the Ni layer during soldering.  
         [0030]    Refer to FIG. 4A FIG. 4B, which shows the result of thermal aging at 160° C. for 2000 hours for solder joints with and without Cu added, respectively. FIG. 4A shows a layer of Ni 3 Sn 4  with a thickness of 13 μm, and a layer of (Au, Ni)Sn 4  with a thickness of 14 μm. The remaining Ni layer thickness is only 1.7 μm. While FIG. 4B is a metallographic diagram of a solder joint with 0.5 wt % Cu added. Here, the interface product is a layer of (Cu, Au, Ni) 6 Sn 5 , and there are no (Au, Ni)Sn 4 . More importantly, the thickness of the remaining Ni layer is 7.1 μm. This result shows that the consumption rate of Ni layer in FIG. 4B has been greatly reduced in comparison to that shown in FIG. 4A. This is because of the fact that the Ni concentration in Ni 3 Sn 4  is much higher than that of (Cu, Au, Ni) 6 Sn 5  compound.  
         [0031]    Refer to FIG. 5, which is a diagram showing the different growing rate of Ni 3 Sn 4  in comparison to(Cu, Au, Ni) 5 Sn 6  at 160° C. In FIG. 5, line  510  shows the thickness (μm, vertical coordinate) of the Ni 3 Sn 4 , and line  520  shows the thickness of same state of a (Cu, Au, Ni) 6  Sn 5  compound. It is clear that the growth rate of Ni 3 Sn 4  is much higher than the growth rate of (Cu, Au, Ni) 6  Sn 5 .  
         [0032]    Refer to FIGS. 6A, 6B and  6 C, which show the metallographic diagrams of a Sn—Ag3.5 (wt %) solder joint, a Sn-4Ag-0.5Cu (wt %) solderjoint, and a Sn-3.5Ag-0.75Cu solder joint, respectively. These three pictures are solder joint after 2 min. reflowing. The reaction product at the interface in FIG. 6A is a simple and continuous Ni 3 Sn 4 . The reaction product at the interface in FIG. 6B is a mixture of (Ni, CU) 3 Sn 4  and (Cu, Au, Ni) 6 Sn 5 . The reaction product at the interface in FIG. 6C is a simple and continuous (Cu, Au, Ni) 6 Sn 5  layer. These results shows that behavior for the Sn—Ag solders is very similar to the behavior for the Pb—Sn solders shown in FIGS. 2A, 2B and  2 C. Furthermore, this research has been extended to various Ag concentration, including 1, 3 and 4 wt %, and the results did not show that the Ag concentration has no obvious effect on the interfacial reaction. Therefore, Cu concentration is the major factor for selecting the interfacial reaction product for reducing the consumption rate of the Ni layer.  
         [0033]    Now please refer to FIGS. 7A, 7B and  7 C, which show the metallographic diagrams for a Sn-3.5 Ag (wt %) solder joint, a Sn-4 Ag-0.5 Cu (wt %) solder joint, and a Sn-3.5 Ag-0.75 Cu (wt %) solder joint, respectively. These three solder joint had been aged at 180° C. for 300 hours. In FIG. 7A, the reaction product is a Ni 3 Sn 4  and (Au, Ni)Sn 4 ; in FIG. 7B, the reaction product is (Ni, Cu) 3 Sn 4  and (Cu, Au, Ni) 6  Sn 5 ; in FIG. 7C, the reaction product is the same as in FIG. 7B, but the thickness of the remaining Ni layer (3.9 μm) is much thickness than the remaining Ni layer in FIG. 7B (2.6 μm). This means that increasing the Cu concentration in a solder joint can also reduce the consumption rate of the Ni layer for the Sn—Ag and Sn—Ag—Cu solders.  
         [0034]    [0034]FIG. 8 is a cross-sectional view showing the alternative way of incorporating Cu into solder to produce the desirable compound at the interface. In FIG. 8, the solder  820  is soldered between a first contact region  810  seated on a substrate or a motherboard  800  and a second contact region  810  is similar to the structure of the contact region in FIG. 2, and the contact region  840  can have a layer of Cu exposed to the solder  820 . The Cu in contact region  840  can diffuse into the solder  820  and provide the necessary Cu atoms to induce the formation of the desirable compound.  
         [0035]    [0035]FIG. 9 is a cross-sectional view showing another alternative way of incorporating Cu into solder. In FIG. 9, the contact region  910  has four layers, a Cu layer  990 , an Au layer  916 , a Ni layer  914 , and a Cu layer  912 . During soldering, the first Cu layer  990  will dissolved into the solder and provide the necessary Cu atoms to induce the formation of the desirable compound. The Cu layer  990  can also locate between the Au layer  916  and the Ni layer  914 .  
         [0036]    Therefore, the foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the present invention.