METHOD OF PLACING SOLDER BALLS ON A SUBSTRATE USING A STENCIL WITH SPACING MEANS AND REVERSELY TAPERED APERTURES AND CORRESPONDING APPARATUS

摘要

The present invention relates to a method of placing solder balls (400) on a substrate (500) containing a plurality of semiconductor devices having input/output terminals (510) using a ball placement stencil (100) and to a corresponding ball placement stencil (100), the method comprising: - providing a substrate (500); - providing a plurality of balls (400) comprising solder; - providing a ball placement stencil (100) having an upper side (110) and a lower side (120), the ball placement stencil (100) comprising a plurality of apertures (105) for receiving the balls (400) and positioning the balls (400) on the substrate (500), and having attached thereto a plurality of discrete spacing means (200) (e.g. pillars) on the lower side (120); - positioning the ball placement stencil (100) adjacent the substrate (500) so that the ball placement stencil (100) is spaced from the substrate (500) by the spacing means (200); and - depositing the balls (400) onto the substrate (500) via the apertures (105) of the stencil (100), wherein the apertures (105) reduce in width from the upper side (110) towards the lower side (120). The reverse taper of the ball placement stencil (100) provides an upper opening (115) that readily accepts a solder ball (400), whilst providing a lower opening (125) that allows accurate positioning of the solder balls (400) on the substrate (500). The discrete spacing means (200) are arranged on the lower side (120) of the ball placement stencil (100) such that there is a clearance (210) between the edge of the discrete spacing means (200) and the lower opening (125) of each aperture (105). The cleareance (210) allows the discrete spacing means (200) to be located away from flux deposit (300) helping to retain the solder balls (400) in position on the substrate (500), thereby reducing the risk that flux (300) would adhere to the ball placement assembly and increase the need for intensive cleaning. Moreover, a plurality of markers may be located on the lower (120) and/or upper side (110) of the ball placement stencil (100) for alignment of the stencil (100) while positioning it on the substrate (500).