Usually, when mounting semiconductor chips on a substrate, the substrate is presented on a horizontally oriented support surface and the semiconductor chips are presented on a wafer table whereby the electrical contact areas of the semiconductor chip point upwards. The semiconductor chip is removed from the wafer table by a bondhead of an assembly machine, a so-called Die Bonder, and placed onto the substrate. This assembly method is known in the trade as epoxy die bonding or softsolder die bonding depending on whether the semiconductor chip is glued to the substrate with epoxy or soldered to the substrate with solder. The flip chip method differs from this assembly method in that the electrical as well as the mechanical connection between the semiconductor chip and the substrate is made through the bumps. So that the semiconductor chip with the bumps can be mounted, it has to be turned (flipped) by 180° after removal from the wafer table, hence the name flip chip.
With the flip chip method, the bumps on the semiconductor chip have to be brought into contact with the electrical connection areas of the substrate, the so-called pads. The demands on the placement accuracy are therefore somewhat greater with the flip chip method than with epoxy die bonding. Today, in order to be able to build such precise assembly machines, a lot of effort is put into the accuracy of the mechanical axes of motion. Such an assembly machine comprises for example a flip device that removes the semiconductor chip from the wafer table and turns it, a pick & place system with a bondhead that removes the flipped semiconductor chip from the flip device and places it on the substrate, and three cameras whereby the first camera makes an image of the semiconductor chip presented on the wafer table, the second camera makes an image of the already turned and picked up semiconductor chip—and therefore flip chip—by the bondhead, i.e., an image of the surface of the semiconductor chip with the bumps, and the third camera makes an image of the substrate with the pads. The images made by the second and third cameras are processed in order to determine the position of the flip chip and the position of the substrate in relation to the axes of motion of the bondhead so that the bondhead can place the flip chip in a positionally accurate manner onto the substrate. Temperature fluctuations cause linear expansion and have the effect that the position of the cameras changes relative to each other and to the axes of motion of the bondhead. In order to minimize the influence of temperature fluctuations on the placement accuracy, the distances between the second and third camera and the mechanical transport system are kept as short as possible. Hence an assembly machine is known for example with which the bondhead with the flip chip is brought into a position above the substrate, then the second and third camera are swung in between the flip chip and the substrate, the bondhead is repositioned based on the images delivered by the second and third camera, the second and third cameras are swung out again and the bondhead lowered. With this assembly method however, maintaining the placement accuracy is achieved at the cost of the throughput.
USPTO Backgrounds
USPTO Backgrounds