In a power semiconductor device, soldering is achieved by one of the following three ways. In the first method, preliminary soldering is formed in a continuous furnace (a tunnel furnace) in a reducing atmosphere to provide solder on electrodes on a bottom face of a silicon chip. Subsequently, the silicon chip is soldered on an insulator substrate with the solder placed between them. Afterward, wire bonding is done. Then, an element with the silicon chip soldered on the insulator substrate is soldered in the atmosphere onto a metal base made of copper using a flux. In the second method, a silicon chip and an insulator substrate are soldered using the continuous furnace in a reducing atmosphere. Afterward, wire bonding is carried out. Then, an element with the silicon chip soldered on the insulator substrate is soldered onto a metal base using the continuous furnace in the reducing atmosphere. In the third method, by using a reduced pressure furnace in an inert atmosphere, a silicon chip, an insulator substrate, and a metal base are soldered with flux cored solder positioned among them. Afterward, wire bonding is carried out.
In a power semiconductor device, such as a power module, a large current flowing generates significant heat in a silicon chip, as much as several tens to several thousands of watts. To dissipate heat, the power semiconductor device needs to have an excellent heat dissipating capability. Presence of voids in a solder between the silicon chip and the insulator substrate or between the insulator substrate and the metal base prevents heat dissipation, which can damage the semiconductor device. Therefore, it is important to prevent voids in the solder bonded layer.
One of the causes of formation of voids in the solder bonded layer is that gas, such as carbon dioxide gas, dissolved in the solder material remains in the voids formed in the solder when the solder melts. Another cause is that, during soldering, materials adsorbed, on the surfaces of solder, or components to be bonded, such as an insulator substrate, or tin oxide on surfaces of the solder, copper oxide on surfaces of copper pattern or metal base (i.e., copper), nickel oxide on surfaces of nickel plating formed on surfaces of the copper pattern and metal pattern or the like, are reduced, and H2O produced thereby is gasified and remains in the voids. Another cause is that gasses generated by the flux vaporization also can form voids. Moreover, the flux itself remaining in the solder bonded layer can form voids.
Therefore, to reduce voids in the solder bonded layer, the surfaces of the components to be bonded need to be free of oxidization to keep the surface clean, the solder materials need to be free of dissolved gas, and the solder materials need to have good wetting ability. Moreover, it is desirable to optimize the soldering profile, to control deformation of components to be bonded, and to carry out soldering in a depressurized atmosphere.
In this regard, there have been a number of proposals to improve the soldering technique. For example, JP-A-11-154785 discloses a method in which soldering is done inside a sealed vessel containing an atmosphere of gas having a higher thermal conductivity than that of the air, while reducing pressure prior to heating and melting of the solder, and increasing the pressure higher than the prior pressure before the solder is solidified. In this method, voids are compressed during solidification to reduce their volumes.
In addition, JP-A-779071 discloses a method in which solder is provided on a substrate and electronic parts are temporarily mounted on the solder sections before the solder is heated and melted under an atmosphere of a vacuum to solder the electronic parts. Here, however, no reduction gas, such as hydrogen gas, is used.
Moreover, JP-A-11-186331 discloses a method of manufacturing a semiconductor device, where an insulator substrate having a conductor layer on a metal base is solder bonded and a semiconductor chip is mounted on the insulator substrate. The solder bonding is achieved by melting solder under an atmospheric pressure to form melted solder, reducing the atmospheric pressure on the melted solder, returning the pressure on the melted solder to the atmospheric pressure, and solidifying the melted solder. While this method applies a vacuum operation during solder bonding using a flux, no reference is made about using hydrogen or a reducing atmosphere.
Furthermore, JP-A-5-291314 discloses a method in which, in soldering of a bare chip or the like and a heat spreader, the heat spreader, to which the bare chip is soldered beforehand, is placed in a vacuum heat treatment furnace. Then, furnace is heated while evacuating the furnace to melt the soldered part again. Again, while this method uses a soldering method using flux, no reference is made about using hydrogen or a reducing atmosphere.
In addition, JP-A-8-242069 discloses a method of solder bonding using a soldering apparatus. The soldering apparatus includes a processing vessel, means for controlling an atmosphere and pressure in the processing vessel by producing a low oxygen concentration atmosphere through evacuation and introduction of a high purity gas, and heating means provided in the processing vessel. With the use of the soldering apparatus, the solder bonding is carried out by heating a circuit board with the heating means, and by controlling the pressure of the atmosphere in the processing vessel.
In the above-described first to third methods of manufacturing power semiconductor devices, however, following problems exist. In the first method, the silicon chip is mounted on a jig and the preliminary soldering and soldering of the silicon chip to the insulator substrate are carried out. Because the silicon chips are handled many times, probability of damaging silicon chips is increased, which can degrade electric characteristics. Moreover, in the first and second methods, by a bimetal effect due to difference in coefficient of thermal expansion among the silicon chip, the metal circuit board, and the ceramic, the insulator substrate can warp after soldering. The produced warping creates nonuniform stress in the silicon chip at wire bonding, which can degrade electric characteristics.
Moreover, in the first and third methods, flux must be removed after soldering. Contaminants, such as residues left after cleaning, adhered on the surface of the silicon chip can degrade electric characteristics, such breakdown voltage. Particularly in the third method, since the wire bonding is done after cleaning, adhered flux residue or residue left remaining after the cleaning operation prevents wire bonding sections from forming strong bonding. Thus, it can degrade reliability.
Moreover, the first and third methods use a continuous soldering furnace having a total length close to 10 m. Thus, when the furnace is in operation, gas necessary for soldering such as hydrogen or nitrogen must continuously flow. Moreover, differences in heat capacity among different materials fed into the furnace necessitate control of temperature in the furnace whenever a material is fed. At the start of the furnace operation, much time is spent on reaching uniform temperature and atmosphere in the furnace. This is costly.
Moreover, in the methods of manufacturing power semiconductor device by the above-explained first to third methods, the following heat dissipation problem exists. In the first and second methods, the soldering is carried out under the atmospheric pressure (normal pressure) condition using the continuous furnace. While the solder is melting, the viscous solder or the components to be bonded cuts off void escaping paths in the solder. Therefore, the voids are liable to be left in solder bonded layers. Thus, as explained above, it is necessary to use solder material with less dissolved gas, or to store or pack the solder materials or components to be bonded so that they do not oxidize. This, however, raises the cost of the materials. Moreover, precise control is required for an oxygen concentration, a dew point, and a temperature profile in the soldering furnace, which all significantly increase the operation cost of the soldering furnace.
Furthermore, in the third method, insufficient depressurization leaves traces of removed fluxes, i.e., traces of traveled fluxes, and voids in the solder bonded layer. In addition, removal of all the fluxes by depressurization is very time consuming, thus decreasing productivity in the case of a batch furnace. Moreover, rosin, which is the main ingredient of the flux, can adhere onto the inside of the depressurized chamber or onto piping. This necessitates frequent cleaning of the inside of the apparatus, again raising the cost of maintaining and controlling the apparatus. Further, soldering carried out at 300° C. or higher causes sticking of scorched flux. Thus, soldering is limited to below 300° C.
Accordingly, there is a need for a better and more economical way of manufacturing a semiconductor device, where soldering results in fewer voids. The present invention addresses this need.