WLCSP and Flip Chip bumping technologies

WLCSP bumps have traditionally been produced by dropping preformed solder spheres through a metal template onto silicon wafers using modified surface mount stencil printers [1]. The squeegee blades associated with these printers have been retrofitted with a special fixture in which spheres are gravity feed down through a narrow slot. This same stencil printer is often used to apply the flux to the wafer just prior to sphere dropping. This technique is applicable for many applications but there are several issues are associated with this technology that limit its widespread use in high volume and high yield applications. These limitations include: 1) there is a practical lower limit to the size of sphere that can be dropped, 2) the seal between the slotted fixturing and the wafer can fail, causing a release of all the spheres into the tool (often referred to as bursts or escapes), and 3) the yields are statistically low.