1. Field of the Invention
The present invention relates to a method for manufacturing electronic devices.
2. Description of Related Art
A variety of methods has been developed for joining semiconductor chips together, semiconductor substrates together, and semiconductor chips and boards. JP-A-2002-111191 discloses the following process as a method for joining electronic components to a board.
Ball solder is placed on the electrodes of the electronic component. The ball solder is for example, tin (Sn) with a silver (Ag) content of 3.5 percent by weight. Chromium (Cr), nickel (Ni), and gold (Au) are next metalized onto the board electrodes. The electronic component electrodes are aligned to match the board electrodes, and the ball solder heated below the melting point and pressure applied to make a temporary clamp. The ball solder is next heated to approximately 250 degrees Celsius to melt the ball solder within the metallized elements (permanent clamp). In this case, the position deviation between the electronic components and board is corrected by a self-alignment effect rendered by the surface tension of the ball solder.
JP-A-2004-79685 discloses a method for joining the chip to the semiconductor substrate. In this method, flux is coated onto the solder bumps formed on the semiconductor substrate. After that, the semiconductor substrate and chips are positioned opposite each other, and then temporarily clamped by the flux. The chip is then joined to the semiconductor substrate by heating (permanent clamp or main clamp). During heating, the semiconductor substrate and the chip are essentially rendered a non-load. Consequently the chip can change its position on the semiconductor substrate (self-alignment).
Moreover, JP-A-2003-282819 discloses a method for joining multiple semiconductor chips in one batch after laminating and positioning the multiple semiconductor chips. For joining multiple semiconductor chips in one batch, heating in a reflow process or heating and applying pressure with a bonding tool are utilized.
The following analyses are given by the present invention.
In JP-A-2002-111191 and JP-A-2004-79685, when the self-alignment effect corrects displacements between the electronic component (or chip) and the board during the permanent clamping, no load is applied to the board and electronic component (or chip). This means that there are no fixed electronic component (or chip) and board positions.
Therefore, when using the method of JP-A-2002-111191 and JP-A-2004-79685 with thin boards or electronic components (or chips), a warp or curvature occurs in the board or electronic component (or chip) in accordance with heat applied in the permanent clamping process. The warp causes displacements among the components (or boards) to be joined. This leads to lowering the reliability of the bond part.
The process in JP-A-2003-282819 for positioning multiple semiconductor chips is performed in a state where multiple semiconductor chips are merely stacked on each other. Because of no temporary clamp for multiple semiconductor chips, conveying the positioned semiconductor chips to other place is impossible. Therefore the bonding of the semiconductor chips must be performed at the place for positioning multiple semiconductor chips. In most cases, positioning the semiconductor chip is usually just an additional step on the semiconductor chip production line. This means that the semiconductor chip is positioned in the air. This leads the oxidization of the bond parts during the bonding. The oxidization causes lowering the reliability of the joint.