To deal with pin multiplication, fine pitches, and increase in signal speed of semiconductor chips, there are used semiconductor devices adopting flip chip connection as a mounting method having short wiring and connection lengths. A semiconductor chip which is used for flip chip connection has electrode pads in an area shape and solder bumps formed thereon. A wiring substrate on which the semiconductor chip is mounted has connection pads formed at positions corresponding to the electrode pads of the semiconductor chip. The flip chip connection is a method for electrically and mechanically connecting the electrode pads of the semiconductor chip and the connection pads of the wiring substrate by aligning them to face each other and heating and melting the solder bumps.
In a gap between the wiring substrate and the semiconductor chip, an underfill resin formed of a thermosetting resin is filled for preventing a fatigue failure of the solder bumps due to thermal stress. By curing the thermosetting resin filled into the gap, a connection part with the solder bumps is protected. In the underfill resin, protection for the solder bumps decreases in a temperature region exceeding its glass transition temperature (glass transition point) Tg. In this aspect, it is considered that the glass transition temperature Tg of the underfill resin is preferred to be not lower than the upper limit value (125° C. for example) of operation guaranteed temperatures of the semiconductor chip (see JP-A 2008-042077 (KOKAI)).
However, in an underfill resin formed of a thermosetting resin having a high glass transition temperature Tg, thermal stress (residual stress) based on a temperature difference between the curing temperature and a temperature after cooled down increases. Accordingly, stress converges excessively to the electrode pads of the semiconductor chip, and thereby a crack can easily occur in an insulating protection film (such as a polyimide resin film) and an interlayer insulating film of the semiconductor chip. Particularly, when a low dielectric constant insulating film (low-k film) capable of reducing an inter-wiring capacity is used for the interlayer insulating film of the semiconductor chip, the low-k film itself has low strength and hence causes a problem that a crack or delamination can easily occur.
An underfill resin formed of a thermosetting resin with a glass transition temperature Tg lower than the upper limit value of operation guaranteed temperatures of the semiconductor chip (see JP-A 2006-313826 (KOKAI)) has a weakness of low protection for solder bumps. Particularly, when a *thermal cycle is applied to a semiconductor device adopting the flip chip connection for connecting a semiconductor chip and a wiring substrate, fatigue resistance of the solder bumps deteriorates because crystal grains grow by thermal stress and cause decrease in rigidity. The under fill resin having the low glass transition temperature Tg has poor protection for such solder bumps embrittled by thermal stress, and thus an open failure can easily occur due to a fatigue failure of the solder bumps.