A power module for controlling power includes a power semiconductor element such as a diode, an insulated gate bipolar transistor (IGBT), or a metal oxide semiconductor field effect transistor (MOSFET). 
In each of these power semiconductor elements, respective electrodes are formed on two surfaces thereof facing each other.  A circuit board or interconnection material is electrically connected to each of the electrodes and the power semiconductor element is insulatively sealed with a sealing resin, thus manufacturing the power module. 
The power semiconductor element is electrically connected to the circuit board by joining the power semiconductor element onto the circuit board with a joining material, such as a solder, being interposed therebetween, for example.  Moreover, for example, in the case of a power module for large current, the power semiconductor element is electrically connected to the interconnection material by joining a plate-like interconnection material onto each of the electrodes of the power semiconductor element with the joining material, such as a solder, being interposed therebetween. 
The plate-like interconnection material has a linear expansion coefficient greatly different from that of the power semiconductor element.  For example, copper, which is frequently used as such a plate-like interconnection material, has a linear expansion coefficient of about 17 ppm/° C. (17 μm/° C./m).  On the other hand, silicon, which is frequently used as such a power semiconductor element, has a linear expansion coefficient of about 3 ppm/° C. (3 μm/° C./m).  When a temperature (environmental temperature) around the semiconductor module or a temperature of the power semiconductor element itself is fluctuated, the difference in linear expansion coefficient therebetween leads to application of thermal stress to the joining material for connecting the plate-like interconnection material to the power semiconductor element.  Repeated application of thermal stress to the joining material causes a generation of a crack in the joining material, thus resulting in a deteriorated function of the power module, disadvantageously. 
In order to overcome such a problem, the following method is employed: a curable sealing resin is used as an insulation sealing for the power module to mechanically bind the power semiconductor element, the joining material, and the plate-like interconnection material, thereby reinforcing the joining material.  The insulation sealing by the sealing resin is performed by: surrounding, with an uncured insulation resin, the power semiconductor element connected to the circuit board, the interconnection material, and the like; and curing the resin in that state. 
As one example thereof, there is the following method: a power semiconductor element connected to a circuit board, an interconnection material, and the like is set in a metal mold, then an uncured sealing resin is poured into the metal mold, and the sealing resin is cured under application of pressure.  As another example, there is the following method: a circuit board to which a power semiconductor element is joined is joined to a base plate; a case having a shape surrounding the circumference of the power semiconductor element is adhered onto the base plate; and then an uncured sealing resin is poured into the case and is cured. 
However, generally, since the linear expansion coefficient of the sealing resin is larger than the linear expansion coefficient of the power semiconductor element, shear stress is generated at an interface between the sealing resin and the power semiconductor element when an environmental temperature or temperature of the power semiconductor element itself is fluctuated.  When adhesion force of the sealing resin to the power semiconductor element is weak, the sealing resin may be detached from the power semiconductor element, thus resulting in a decreased insulating property between the electrodes of the power semiconductor element, disadvantageously. 
For example, as a method for preventing such detachment of the sealing resin from the power semiconductor element, each of Patent Document 1 and Patent Document 2 proposes the following method: a polyimide film having strong adhesion force to a sealing resin is formed on a power semiconductor element connected to a circuit board, an interconnection material, and the like, and they are sealed with a sealing resin from above the polyimide film. 
Furthermore, Patent Document 2 proposes a method for detaching the sealing resin from a portion of the surface of the interconnection material at a position distant from each of the power semiconductor element and the joining material.  In this method, the sealing resin at the position distant away from the joining material is detached to relax stress, thereby suppressing detachment of the sealing resin from the surface of the joining material and the surface of the power semiconductor element.