IMS (injection molded solder) is an advanced solder bumping technology with solder alloy flexibility even at very fine pitch and small size. One of key materials for successful fine pitch bumping by IMS is a photoresist material. The photoresist material must be stable at high temperature during the IMS process and be perfectly stripped after the IMS process without any residue on the surface of the substrate. In this study, negative tone liquid photoresist materials were prepared to investigate effects of thermal cure of photoresist on IMS process and stripping performance. With appropriate cure conditions, successful bumping without any film damages at IMS process and any residue at stripping was achieved. Fine pitch bumping down to 40 μm pitch with 20 μm diameter was demonstrated with a Sn-3.0Ag-0.5Cu solder. Also physical and electrical connections for the solder joints of IMS bumps to Ni/Au pads were confirmed using a 80 μm pitch test vehicle.
Novel bumping technology that can realize high density assembly of IC chips and packages with a high number of I/O is required in the field of electronic packaging. Currently, the electroplating method or the solder ball placement method have been widely adopted for the fabrication of solder bumps down to sub-hundred microns in diameter. However, there are some limitations with these current bumping methods. For example, in the case of an electroplating method, the solder bump composition fabricated in this method is limited. In addition, careful maintenance of the plating solution is necessary in order to prevent a gradual compositional shift of the plating solution.
Novel bumping technology that can realize high density assembly of IC chips and packages with a high number of I/O is required in the field of electronic packaging. Recently, a novel bumping process called IMS (Injection Molded Solder) was proposed, which enabled direct injection of molten solder into the holes of a photoresist patterned array. In this paper, the current status of photoresist development and recent achievements for obtaining excellent solder filling rates are described. One of the important factors to obtain a high solder filling rate is reducing the amount of outgases from the photoresists at high temperature, as they prevent smooth solder filling into the photoresist holes. In order to solve such an issue, novel photoresists with high thermal stability have been designed and developed.
Fine pitch interconnection using a flip-chip method is a key technology to achieve ultra-high density packaging on 2.1D/2.5D/3D integrated devices. As the solder bump size gets smaller for fine pitch applications, controlling the properties of solder joints becomes more important for chip package interaction (CPI) and electro-migration (EM) performance. The electro-plating method is widely used to fabricate fine pitch bumps; however solder compositions are limited to pure Sn or some binary solders such as SnAg , Sn-Cu, etc. Hence, a bumping technology with fine pitch capability and solder alloy flexibility is needed.
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