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. 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. Flip Chip bumps have traditionally been produced by electroplating or paste printing processes. Both technologies have been implemented in high volumes for PdSn bumping at many facility across the world. The electroplating technique is somewhat limited for use in smaller facilities due to the high capital and operation costs. In addition, ternary alloys, like SnAgCu are difficult to plate with consistent results. There is also a practical upper limit to the size of the bump that can be produced, and most applications rare for fine pitch bumping.. The paste printing technologies are very versatile with respect to the alloy composition that can be use, but is limited to pitches around 200μm for 100μm tall bumps. One new WLCSP technology that is showing high promise toward eliminating these limitations for both WLCSp and Flip Chip, is Wafer Level Solder Sphere Transfer (also called Gang Ball Placement). This technology uses a patterned vacuum plate to simultaneous pick up all of the preformed solder spheres, optically inspect for yield, and then transfer them over to the wafer. This paper will discuss this technology and the process parameters for producing WLCSP bumps. Throughput levels of 25 to 30 wafers per hour were measured. Yield losses of less than 10ppm were realized for placing 300μm spheres onto 200mm wafers with ~80,000 I/Os. Similar yields have been observed for placing 60μm flip chip sized spheres onto semiconductor wafers.