Abstract:
A semiconductor device includes a solder dam for restricting the flow of solder during manufacturing. The device includes a semiconductor chip bonded to a first side of a circuit board, a metal base for dissipating heat produced by the semiconductor chip, the metal base being bonded to a second side of the circuit board, and a dam material disposed on the metal base in a predetermined pattern for restricting the flow of solder used in bonding a plurality of the circuit boards to the metal base. By employing the solder dam, solderability is not impaired, device contamination can be avoided, and a highly reliable semiconductor device can be produced.

Description:
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT  
       [0001]     This invention relates to a semiconductor device and a manufacturing method thereof, and particularly to a semiconductor device such as a power module made by bonding semiconductor device chips to one side of a circuit board and bonding a metal base for dissipating heat produced in the semiconductor device chips to the other side, and a manufacturing method thereof.  
         [0002]     In the manufacture of semiconductor devices such as power modules, bonding of multiple heat-sinking, electrically insulating boards to a metal base is carried out by soldering. At this time, in related art, a commercially available solder resist has been printed onto the metal base and dried to prevent solder flow (see, for example, JP-A-6-244224). And, in assembly environments where solder resist cannot be used, solder bonding has been carried out with the boards fixed with a positioning jig or the like.  
         [0003]     There is also the method of forming an oxide film on or in depressions in the metal base with a laser beam to prevent solder flow, but when the amount of solder is large (in terms of thickness, 0.05 mm or greater) this is largely ineffective.  
         [0004]     However, with the solder resist of related art there has been the problem that cost and time are entailed in making up a screen for printing, and in process steps from printing to drying.  
         [0005]     And, with solder resist, because it is generally an organic substance such as epoxy resin, its resistance to solder heat is not exceedingly high. When, for example, fluxless soldering is carried out in a hydrogen gas atmosphere, solderability has been impaired by the production of outgas from the solder resist. There are also problems such as contamination of the device, and thus use has been limited.  
         [0006]     The present invention was made in view of such prior art problems. It is an object of the invention to provide a semiconductor device and a manufacturing method thereof with which the creation of a dam material for preventing solder flow is easy, and furthermore, with which reliability is high.  
         [0007]     Further objects and advantages of the invention will be apparent from the following description of the invention and the associated drawings.  
       SUMMARY OF THE INVENTION  
       [0008]     To solve the problems described above and other problems, the invention provides a semiconductor device having semiconductor chips bonded to one side of a circuit board and a metal base for dissipating heat produced in the semiconductor chips bonded to the other side, wherein a dam material is disposed on the metal base by being painted in a predetermined pattern so as to restrict the flow of a solder used in bonding a plurality of the circuit boards to the metal base.  
         [0009]     With this construction, by means of the dam material disposed on the metal base by being painted in a predetermined pattern, flow of the solder used in bonding the multiple circuit boards to the metal base is restricted.  
         [0010]     And, a second aspect of the invention provides a semiconductor device using an inorganic substance with a high solder heat resistance as the dam material, and by this means the production of outgas during soldering is prevented.  
         [0011]     In the invention, as opposed to the solder resist of the above-described related art, the flow of the solder used in bonding multiple circuit boards to a metal base is restricted by means of a dam material disposed on the metal base by being painted in a predetermined pattern. It is possible, therefore, to easily make a semiconductor device in which solder flow during soldering can be prevented. And, because making up of a printing screen and printing are not carried out, reductions in cost can also be expected.  
         [0012]     And, because an inorganic substance with a high solder heat resistance and no solderability is used as the dam material, the production of outgas during soldering can be prevented. Consequently, at the time of soldering, solderability is not impaired, device contamination can also be avoided, and a highly reliable semiconductor device can be produced. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a perspective view of a main part of a semiconductor device according a preferred embodiment of the invention;  
         [0014]      FIGS. 2A and 2B  are plan views showing a dam part pattern according to one embodiment of the invention;  
         [0015]      FIGS. 3A and 3B  are plan views showing a dam part pattern according to another embodiment of the invention; and  
         [0016]      FIG. 4  is a perspective view showing a carbon jig fitted to a metal base. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]     A preferred embodiment of the invention will be described in detail below with reference to the drawings.  
         [0018]      FIG. 1  is a perspective view of a main part of a preferred embodiment of a semiconductor device according to the invention.  
         [0019]     The semiconductor device  10  of this preferred embodiment is made by bonding semiconductor chips  12  to one side of a circuit board  11  and bonding to the other side a metal base  13  for dissipating heat produced in these semiconductor chips  12 . A dam material  15 , for restricting the flow of a solder  14  used in the bonding of a plurality of the circuit boards  11  to the metal base  13 , is disposed on the metal base  13 .  
         [0020]     The circuit board  11  is a heat-dissipating electrically insulating board made by bonding conductor patterns to the front and rear sides of a ceramic board by direct bonding or active metal bonding or the like, and in the following description it will be assumed that a DCB (Direct Copper Bonding) board made by bonding copper to the front and rear sides of a ceramic board is being used.  
         [0021]     The semiconductor chips  12  are, for example, devices for use in a power module such as IGBTs (Insulated Gate Bipolar Transistors), and are bonded to the circuit boards  11  with solder (not shown).  
         [0022]     The metal base  13  is a heat sink having the function of dissipating heat produced in the semiconductor chips  12  mounted on it, and, for example, Cu (copper) plated with Ni (nickel) is used.  
         [0023]     The dam material  15 , which is a characteristic feature of this preferred embodiment, is formed on the metal base  13  as a dam part in a predetermined pattern by painting, thermal spraying, or plating.  
         [0024]      FIGS. 2A and 2B  and  FIGS. 3A and 3B  are views showing patterns of the dam part.  
         [0025]     The pattern of the dam part  15   a  shown in  FIG. 2A  is a typical one, and is a pattern in which the dam part  15   a  is formed so as to surround the copper plate peripheries  11   a  of the rear sides of the circuit boards  11  (shown with dashed lines (and similarly in the other drawings)).  FIG. 2B  shows a pattern in which a dam part  15   b  is formed so as to surround the copper plate peripheries  11   a  of six circuit boards  11 . This is used when the quantity of solder is small and the outflow of solder between the circuit boards  11  is small.  
         [0026]     The pattern of a dam part  15   c  shown in  FIG. 3A  is an example in which the dam part is formed to the same dimensions as or slightly smaller than the copper plate peripheries  11   a  of the rear sides of the individual circuit boards  11 . And the pattern of the dam part  15   d  shown in  FIG. 3B  is an example in which the dam part is formed only between the copper plate peripheries  11   a  of the circuit boards  11 , and is the same as the one shown in  FIG. 1 . When the quantity of solder used is large, if the dam part  15   d  having a large width of pattern is formed, as shown in this  FIG. 3B , the outflow of solder can be prevented effectively. And if no pattern is formed on sides where any outflow of solder is not problematic, solder will flow to those sides during soldering and the outflow of solder on sides where outflow must be prevented can be prevented effectively.  
         [0027]     A method of forming the dam material  15  will now be described.  
         [0028]     As the substance used for the dam material  15 , one having an inorganic substance with a high solder heat resistance and no solderability as a main component is used. Specifically, one having particles of carbon or a ceramic as a main component and made by mixing these particles with a volatile binder is used. As a dam material having a ceramic as its main component, a ceramic adhesive used for bonding ceramic members can be applied. Ceramic adhesives have silicic acid and boric acid as main components, and can withstand temperatures in excess of 1000° C. or higher.  
         [0029]     However, among ceramic adhesives, one in which no organic binder remains after drying is used. In this case it can be used for soldering in a hydrogen reducing atmosphere. In the case of a ceramic material, by choosing one with good electrical insulating properties and by coating it thickly, it is possible to further increase the dam effect and electrical insulation.  
         [0030]     By means of these materials, for example using fine coating technology using a dispenser, a dam material  15  with a predetermined pattern can be formed on the metal base  13  by painting. A screen printing method may alternatively be used. By this means it is possible to manufacture a large volume of mass-produced product efficiently by in-line automation. High quality, high reliability, low cost, and shortening of the production time of the semiconductor device  10  can also be achieved.  
         [0031]     And, as the dam material  15 , a solid carbon such as the core of a pencil may alternatively be used. Although the core of a pencil contains clay besides carbon, this is not a problem in soldering, and the effect of repelling solder is the greater. In painting with a pencil, for example, a template is superimposed and the pattern is painted through the template. In order to deposit more carbon on the metal base  13 , the hardness of the core of the pencil is preferably  2 B to  6 B. After painting, unwanted residue (carbon) (i.e., residue that is loose on the surface of dam material  15 ) is removed with air or the like.  
         [0032]     Although pattern painting may be carried out automatically using a dispenser on the basis of digital data, particularly in the case of pencil painting, painting may be carried out by hand. This is suitable for high-mix low-volume production. In this case also, quality, reliability, low cost, and shortening of production time can be achieved.  
         [0033]     And, a dam material  15  having a metal with a low solder wettability (i.e., such that solder does not spread) or a ceramic as a main component may be thermally sprayed onto the metal base  13  to form a dam part. Thermal spraying is a method of forming a film by melting or softening a coating substance by heating it, accelerating it in fine particle form, and causing it to impact with the surface of the object to be coated, and setting and accumulating particles having collapsed flat. Thermal spraying methods include room-temperature thermal spraying and plasma thermal spraying. Examples of metals of low solder wettability include Al (aluminum), Mo (molybdenum), W (tungsten), and Cr (chromium). Thermal spraying increases cost slightly, but can be applied to a variety of materials, and the thickness can be easily set so that materials suitable to the application of products can be selected.  
         [0034]     And, with dam materials such as chromium, it is possible to form a dam part with a predetermined pattern by plating. Because with plating the thickness can be kept below 0.01 mm, it is used when it is desirable to limit the thickness. And Cr plating in a predetermined pattern can be carried out following a step of Ni-plating the metal base  13 .  
         [0035]     By forming a dam part like this on the metal base  13 , it is possible to restrict solder flow, and there is no production of outgas during soldering as there is when a solder resist made from an organic substance such as an epoxy resin is used. Consequently, at the time of soldering, there is no impairment of solderability, and device contamination can also be avoided.  
         [0036]     When a carbon jig for board positioning and a partition board such as a carbon material are further used for a semiconductor device  10  of the construction described above, it is possible to prevent outflow of solder still more effectively.  
         [0037]      FIG. 4  is a view showing a carbon jig attached to a metal base.  
         [0038]     The carbon jig is made up of an outer frame  16   a  and an inner frame  16   b,  and fixes the positions of the circuit board  11 , the semiconductor chips  12 , the metal base  13  and the solder  14  (not shown here) . And, a partition plate  17  is disposed above the dam part shown in  FIG. 1 , and when the quantity of solder is large, solder flow can be effectively prevented.  
         [0039]     Because the partition plate  17  is disposed in the proximity of the solder bond parts, it is necessary to choose a material having a low solder wettability for the partition plate. Materials having a low solder wettability include ceramics. However, because ceramics have a high rigidity compared to carbon, the circuit boards  11  may break when the partition plate  17  is sandwiched between the circuit boards  11  due to contraction of the metal base  13  after soldering. Therefore, a partition plate  17  made of carbon is preferable. Weights  18  are for pressing down the circuit boards  11 .  
         [0040]     Bonding of the parts (the circuit boards  11 , the semiconductor chips  12  and the metal base  13 ) is carried out by performing thermal soldering in a hydrogen atmosphere with the construction as shown in  FIG. 4 . By this means it is possible to prevent outflow of solder effectively, and prevention of mutual mechanical damaging of the circuit boards  11  and electrical insulation are achieved.  
         [0041]     The disclosure of Japanese Patent Application No. 2005-027164 filed on Feb. 3, 2005, is incorporated herein.