1. Field of the Invention
The present invention relates to a printing head and a manufacturing method of a printing head, and particularly, to a manufacturing method of a printing head for bonding an electrode of a printing element substrate to a lead and a printing head manufactured by this manufacturing method.
2. Description of the Related Art
In recent years, an ink jet printing device which ejects ink droplets from a printing head to carry out the printing has been rapidly widely used. Such an ink jet printing device has an advantage that the downsizing is easy, color printing is relatively easily carried out, and the like.
Japanese Patent Laid-Open No. 2005-101546 has proposed a method in which in a printing head used in such an ink jet printing device and in a manufacturing method of the printing head, an electrode pad as an electrode is connected electrically through a stud bump to a flexible film wiring substrate. Japanese Patent Laid-Open No. 2005-101546 discloses the method in which the stud bump is arranged and bonded on the electrode formed as a film on a printing element substrate and the stud bump is bonded and connected to the flexible film wiring substrate equipped with a lead by an ILB method.
Hereinafter, among methods of electrically connecting this electrode to the flexible film wiring substrate, an example of the conventional method will be explained. FIGS. 4A to 4C and FIGS. 5A to 5E show an example of a conventional manufacturing method of the ink jet printing head. FIG. 4A is a perspective view showing a structure of an electrical connection portion in the printing head. FIG. 4B is an enlarged side view showing a key part in the printing head. FIG. 4C is an enlarged plan view showing the key part of the printing head. FIGS. 5A to 5E are explanatory diagrams showing and explaining a flow of processes in order in regard to an example of the conventional manufacturing method of the printing head upon manufacturing the printing head shown in FIGS. 4A to 4C.
First, as shown in FIG. 5A, a stud bump 103 is formed on an electrode pad (electrode) formed as a film on a printing element substrate (hereinafter, referred to as bump forming process also). At this point, the stud bump 103 is formed in a state where the printing element substrate 101 is fixed by vacuum absorption and is heated. The stud bump 103 to be formed on the electrode pad 102 on the printing element substrate 101 is formed by forming a gold (Au) wire to be in a spherical shape with spark. In a state where the stud bump 103 is put on the electrode pad 102 arranged on the heated printing element substrate 101, the bonding between the stud bump 103 and the electrode pad 102 is carried out. The bonding therebetween is carried out by applying a supersonic wave to the stud bump 103 in a state where the stud bump 103 is put on the electrode pad 102 and the stud bump 103 is pressed against the electrode pad 102. At the time of applying the supersonic wave to the stud bump 103, the bonding is carried out using a ball bump bonding tool. In this example, the formation of the stud bump 103 is made by a single point bonding method. In general, in the single point bonding method, a method of applying both of heat and supersonic wave is adopted in a semiconductor industry.
Next, as shown in FIG. 5B, in the stud bump 103 formed between an inner lead (lead) 108 and the electrode pad 102 by the bump forming process, an upper surface of a part of the stud bump 103 in an opposing side to the bonding part of the stud bump 103 with the electrode pad 102 is smoothed (hereinafter, referred to as smoothing process). At this point, in the same way as the bump forming process, the printing element substrate 101 is fixed by the vacuum absorption and is then heated. The smoothing process is carried out by applying a supersonic wave to a cutting part of the gold wire produced at the time of forming the stud bump 103 in a state of pressing a bonding tool for smoothing against the stud bump 103. This smoothing is carried out by the single point bonding method.
Next, as shown in FIG. 5C, the printing element substrate to which the smoothing process has been executed is bonded and fixed to a support member and the flexible film wiring substrate 109 having the inner lead 108 is bonded and fixed to a support plate 107 fixed on a support member 106. At this point, the electrode pad 102 on the printing element substrate 101 and the inner lead 108 are respectively arranged in such a manner as to be accurately positioned and thereafter, bonded and fixed (hereinafter, referred to as mount process).
Next, as shown in FIG. 5D, the electrode pad 102 and the inner lead 108 positioned in the mount process are bonded through the stud bump 103 (hereinafter, referred to as ILB bonding process). At this point, the support member 106 after the mount process is fixed by vacuum adsorption and is then heated. In the ILB bonding process, the inner lead 108 is pushed in toward the electrode pad 102 in a state where the stud bump 103 is sandwiched between the inner lead 108 and the electrode pad 102. The electrode pad 102 and the inner lead 108 are bonded by applying the supersonic wave to the stud bump 103 using the bonding tool for ILB. At this time, the ILB bonding process is carried out by the single point bonding method.
The single point bonding is one of the ILB (Inner Lead Bonding) methods and is a method of individually, selectively and in sequence bonding objects one by one. The other of the ILB methods is a gang bonding method of bonding plural bonding objects together at a time. In either case, the bonding objects are bonded in a highly heated state. When two methods thereof are compared in a case where the bonding objects are bonded on the same condition, the bonding by the single point bonding method is possible at a lower temperature than by the gang bonding method. Even in a case of the single point bonding method, however, the minimum temperature required for bonding is a relatively high temperature and it is required to heat the bonding object to approximately 180° C. By this heating, the printing element substrate 101 and the flexible film wiring substrate 109 are bonded in a thermal expansion state.
Next, as shown in FIG. 5E, after the printing element substrate 101 and the flexible film wiring substrate 109 are bonded by the ILB process, these elements are cooled by natural convection of the surrounding environment or the like (hereinafter, referred to as a cooling process). At this point, a thermal expansion coefficient of the printing element substrate 101 formed of silicon (Si) is much smaller than that of the inner lead 108 formed mainly of copper (Cu) or the flexible film wiring substrate 109 formed mainly of resin material. Therefore, when the cooling process is completed, a difference in contracting amount between the printing element substrate 101 and the inner lead 108 or between the printing element substrate 101 and the flexible film wiring substrate 109 occurs. In consequence, stress occurs between the electrode pad 102 and the inner lead 108 after the cooling process. This stress occurs between the stud bump 103 and the inner lead 108, but more stress occurs between the electrode pad 102 and the stud bump 103. When the stress occurring and remaining inside these members at this point is excessively large, this stress possibly causes the bonding part between the electrode pad 102 and the stud bump 103 to be damaged.
For overcoming such a problem, Japanese Patent Laid-Open No. 2005-101546 discloses means for forming a through bore or a recessed groove in an inner lead to reduce rigidity of the inner lead, thereby alleviating stress occurring between an electrode pad and the inner lead.
In the field of the ink jet printing head, a demand for the further shrinking of the printing element substrate is increasing. This demand is true of an electrical connection part of the ink jet printing head and therefore, technologies for downsizing of a pad and downsizing of a stud bump corresponding to the downsizing of the pad become an important factor for meeting this demand.
However, when the inner lead and the electrode pad are bonded by the ILB bonding in a state where the stud bump is downsized, a ratio of the stud bump formed of a relatively flexible and easily deformable material is lowered to seemingly increase rigidity of the inner lead bonded by the ILB bonding process. In consequence, stress caused by a difference in thermal expansion coefficient between the printing element substrate and the flexible film wiring substrate occurs between the electrode pad and the inner lead in the cooling process, and this stress possibly causes damages at the bonding part between the electrode pad and the stud bump.
As a result of the downsizing of the stud bump, it is required to increase a bonding area between the stud bump and the inner lead by more strongly pushing the inner lead into the stud bump at ILB bonding. Therefore, an external force applied to the electrode pad results in increasing.