Abstract:
A method for soldering an electronic component to a substrate is provided. The method includes the steps of forming a metal layer on the substrate; applying a solder material on the metal layer; and performing a thermal process to transfer the solder material into a solder joint so as to connect the electronic component with the substrate. During the thermal process, a portion of the metal layer is introduced into the solder joint, thereby elevating the eutectoid temperature of the solder joint. This invention also provides an electronic device made by this method.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the right of priority based on Taiwan Patent Application No. 095116703 entitled “Method of Soldering Component to Substrate and Electronic Device Made by the Same,” filed on May 11, 2006, which is incorporated herein by reference and assigned to the assignee herein. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to a soldering method, and more particularly to a method capable of elevating the eutectoid temperature of the solder joint and the electronic device made by the same. 
       BACKGROUND OF THE INVENTION 
       [0003]    Soldering is an important connection technique in the electronic industry. The solder material is typically a metal with low melting point and good malleability, and the tin alloy is a most common one. During the soldering process, the solder material contacts the substrate material to be connected, and usually a portion of the substrate material would dissolve into the melt and be transferred into an intermetallic phase at the interface of the melt and the substrate material to be connected. 
         [0004]    The melting of the alloy typically has a start melting temperature and a complete melting temperature that define a temperature interval, wherein the start melting temperature is called the eutectoid temperature, and the complete melting temperature is called the liquidus temperature. During the soldering process according to the prior art, some ingredients of the substrate material may dissolve into the melt, and the eutectoid temperature or the liquidus temperature of thus formed solder joint may decrease. Take the Sn—Cu eutectic solder material with an eutectoid temperature, namely an eutectic temperature, of 227° C. as an example, the eutectoid temperature of thus formed solder joint may decrease to 217° C. as Ag in the substrate material dissolves into the solder material. Similarly, for a Sn—Ag hypoeutectic solder material with a silver content less than 3.5 wt %, the liquidus temperature of thus formed solder joint may decrease as Ag in the substrate material dissolves into the solder material. Furthermore, when a common Sb solder material of Sn-5 wt % contacts the substrate material Ag and makes Ag dissolve into the solder material, the eutectoid temperature of thus formed solder joint may decrease. 
         [0005]    The eutectoid temperature decrease of the solder joint may have ill effects on the stage-by-stage soldering process. For example, in a two stage soldering process, the operation temperature of the second stage cannot be higher than the eutectoid temperature of the solder joint resulted from the first stage, so as to prevent the solder joint from melting. However, in the prior art mentioned above, the substrate material may dissolve into the melt and make the eutectoid temperature of the solder joint decrease. Therefore, it is not easy to control the operation temperature of the second stage soldering, and the product quality is impacted. Accordingly, it is necessary to provide a soldering method to solve problems caused in the prior art. 
       SUMMARY OF THE INVENTION 
       [0006]    To solve the problem, the present invention provides a method for forming a solder joint on a substrate. The method includes forming a metal layer on the substrate, and introducing a portion of or entire the metal layer into the melt during a thermal process to form the solder joint. The thermal process connects the substrate with an electronic component, and meanwhile, elevates the eutectoid temperature of the solder joint. 
         [0007]    In one embodiment, the present invention provides a method for soldering an electronic component to a substrate, and the electronic component has a pin. The method includes forming a metal layer on the substrate; applying a solder material on the metal layer; performing a first thermal process to transfer the solder material into a solder joint and introduce a portion of the metal layer into the solder joint, so that an eutectoid temperature of the solder joint is higher than an eutectoid temperature of the solder material; and connecting the pin and the solder joint. 
         [0008]    Another aspect of the present invention provides an electronic device formed by the method mentioned above. In one embodiment, the present invention provides an electronic device having a solder joint. The electronic device includes a substrate over which the solder joint located; and a metal layer between the substrate and the solder joint, contacting the solder joint. The solder joint includes a portion of the metal layer introduced during a thermal process, so that an eutectoid temperature of the solder joint is higher than an eutectoid temperature of a solder joint without the portion of the metal layer. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  to  FIG. 4  are schematic diagrams illustrating the steps of a thermal process and a soldering process for a substrate and an electronic component of a circuit board according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0010]    The preferred embodiments of the present invention would be illustrated below referring to the attached drawings. It should be noted that, to present the invention clearly, the layers and the elements in the attached drawings are not depicted to scale, and the following description would omit well-known components, materials and process techniques for avoiding obscuring the present invention. 
         [0011]      FIG. 1  to  FIG. 4  illustrate a method for soldering an electronic component to a substrate to form an electronic device according to the present invention. This illustration takes a circuit board as a preferred embodiment of the electronic device. As shown in  FIG. 1 , a circuit board  100  is provided. The circuit board  100  includes a substrate  110  and an electronic component  120  to be connected to the substrate  110 . The substrate  110  typically has a pattern or a line consisted of metal (not shown). A surface treatment layer  111  may be included on the substrate  110  on demand, serving as a barrier layer for preventing the solder material from overreacting with the surface of the substrate  110 , or a wetting layer for wetting the solder material/substrate  110  interface. Typical materials of the surface treatment layer are Au, Cu, Ni, Pd, or their combinations. The electronic component  120  may be a resistor, a capacitor, or an IC chip, and includes a pin  121  preferably made of metal. Similarly, a surface treatment layer  111  may be applied to the surface of the pin  121  on demand. 
         [0012]    Next, as shown in  FIG. 2 , a metal layer  210  is formed on the substrate  110 , preferably by electroplating. Then a solder material  220  is applied on the substrate  110  and contacts with the metal layer  210 . It should be noted that, the composition of the metal layer  210  should match the solder material  220  to elevate the eutectoid temperature of the solder joint to be formed. For example, the metal layer  210  may preferably be made of Sn as the solder material  220  includes Sn and Sb. Alternatively, the metal layer  210  may preferably be made of Te as the solder material  220  includes Se. Besides, persons skilled in the art would understand that the solder flux, the surfactant or other ingredients may be added into the solder material  220  on demand. 
         [0013]    Next, as shown in  FIG. 3  and  FIG. 4 , a first thermal process and a first soldering process are proceeded to transfer the solder material  220  into the solder joint  320  connecting the pin  121  with the substrate  110 . And a portion of or entire the metal layer  210  is introduced into the solder joint  320  to make the eutectoid temperature of the solder joint  320  higher than the eutectoid temperature of the solder material  220 . Furthermore, through the solder joint  320  connecting the pin  121  with the substrate  110 , the metal patterns or lines on the substrate  110  are electrically connected to the pin  121 . As shown in  FIG. 3 , there is a remaining metal layer  210  on the substrate  110 , which indicates only a portion of the metal layer  210  is introduced into the solder joint  320 . As to  FIG. 4 , there is no metal layer  210  on the substrate  110 , which indicates the entire metal layer  210  is introduced into the solder joint  320 . Additionally, it should be noted that the method of the present invention may not induce the intermetallic phase of the solder material  220 /substrate  110  interface, and the metal layer  210  is kept at the solid solution phase. 
         [0014]    After the first thermal process and the first soldering process mentioned above, a second thermal process and a second soldering process may be performed on demand. As discussed above, the operation temperature of the second thermal process should be carefully controlled to prevent the solder joint  320  formed in the first thermal process from melting. Since the present invention elevates the eutectoid temperature of the solder joint  320  formed in the first thermal process to be higher than the eutectoid temperature of the solder material  220 , the eutectoid temperature decrease in the prior art would not occur. Thus the operation temperature of the second thermal process could be precisely held below the eutectoid temperature of the solder material  220 . In other words, an advantage of the present invention lies in the easy control of the operation temperature of the second thermal process. 
         [0015]    In addition, the eutectoid temperature of the solder joint  320  may be elevated to be higher than the liquidus temperature of the solder material  220  by adjusting the composition of the metal layer  210 . Therefore, the operation temperature of the second thermal process may have a broader range, which brings another advantage of the present invention. 
         [0016]    Non-limiting embodiments of eutectoid temperature elevation of the present invention are provided below. 
       Embodiment 1 
       [0017]    200 mg of Sb solder material with Sn-5 wt % is placed on a pure Sb slice and sealed as a whole in a quartz tube with an inner diameter of 3 mm and an outer diameter of 4 mm. This embodiment is heated to 265° C. and kept for 5 minutes by a differential thermal analysis (DTA) equipment, then cooled to room temperature, and the melting temperature interval is observed and measured. Next, the heating, keeping, cooling, observation and measurement mentioned above are repeated. The temperature intervals obtained by the two measurements are 240° C., 244° C. and 250° C., 257° C. respectively. It could be found that the eutectic temperature has been elevated from 240° C. to 250° C., and the liquidus temperature has been elevated from 244° C. to 257° C. 
       Embodiment 2 
       [0018]    A Sb layer with a thickness of 50 μm is electroplated on an Ag substrate with a thickness of 500 μm. A Sb solder material with Sn-5 wt % is placed on the Sb layer on the substrate and reflowed at 260° C. to form a solder joint. Next, the substrate with the solder joint is placed at 244° C. and observed for 1 hour. No melting occurs. 
       Embodiment 3 
       [0019]    An Ag layer with a thickness of 100 μm is electroplated on a Si substrate, and then a Sb layer with a thickness of 50 μm is electroplated on the Ag layer. A Sb solder material with Sn-5 wt % is placed on the Sb layer on the substrate and reflowed at 260° C. to form a solder joint. Next, the substrate with the solder joint is placed at 244° C. and observed for 1 hour. No melting occurs. 
       Embodiment 4 
       [0020]    500 mg of Se metal particles are placed on a pure Te slice and sealed as a whole in a quartz tube with an inner diameter of 3 mm and an outer diameter of 4 mm. This embodiment is heated to 225° C. and kept for 5 minutes by a differential thermal analysis equipment, then cooled to room temperature, and the melting point is observed and measured (Se metal particles has no temperature interval since they are not alloy). Next, the heating, keeping, cooling, observation and measurement mentioned above are repeated. The melting point/temperature interval obtained by the two measurements are 221° C. (namely the melting point of Se) and 225° C., 232° C. respectively. The temperature interval 225° C.-232° C. obtained by the second measurement indicates that a potion of Te melted into Se, and the eutectic temperature is elevated from 221° C. to 225° C. 
       Embodiment 5 
       [0021]    200 mg of Se metal particles are placed on a pure Te substrate and reflowed at 260° C. to form a solder joint. Next, the substrate with the solder joint is placed at 221° C. and observed for 1 hour. No melting occurs. 
         [0022]    The method of the present invention applies to any electronic devices that need soldering to form interconnections between its components and the substrate.  FIG. 1  to  FIG. 4  illustrate the method of the invention with a circuit board as an example of the electronic device, however, it should be noted that the electronic devices herein are not limited to the circuit boards. 
         [0023]    It should be noted that the method of the present invention does not utilize an alloy solder material containing the kind of metal employed in the present invention directly. Since if this kind of alloy solder material is directly utilized, the process temperature would certainly be elevated. In other words, according to the present invention, the appropriate process temperature of the first thermal process depends on the ingredients of the solder material but not on the temperature to be elevated to. As shown in  FIG. 3 , the dissolution of the metal layer  220  into the melt occurs after the wetting of the substrate  110 . At this time, the interface between the solder material  220  and the substrate  110  has been formed, and the elevation of the melting point of the melt would not influence the thermal process. 
         [0024]    The above description is only for preferred embodiments, but not to limit the scope of the present invention. Any other equivalent changes or modifications performed within the spirit disclosed by the present invention should be included in the appended claims.