The continuing drive towards finer bump pitch and lower bumping cost is conflicting with the requirement to bump 300mm wafers with more expensive lead-free solder alloys. The shift to lead-free solders will be problematical for some current some flip chip solder bumping methods. Since solder composition and the bump-forming method largely determine bump pitch, cost, quality, and applicability, let’s examine how today’s common solder bumping methods may fare in tomorrow’s brave new lead-free world.
Evaporated bumps were the original flip chip approach pioneered by IBM 40 years ago with high-lead solder. However, the geometry of evaporators, directing a stream of material in straight lines from a small source to an entire wafer, does not readily scale up to accommodate larger wafers.
In addition, many of the proposed lead-free alloys have greater than 90 wt% tin content. Tin’s extremely low vapor pressure makes it evaporate more slowly than lead. These longer deposition times cut throughput and increase operating costs.
Tin’s higher material cost makes tin more expensive to waste than lead. Unfortunately, evaporators generally waste more material than they deposit. Evaporating lead-free bumps onto 300mm wafers appears to be infeasible, although evaporators will remain important for smaller wafers in many specialty applications.
Electroplating has long been the preferred method to produce excellent quality, fine pitch solder bumps. However, some proposed lead-free alloys will make electroplating more difficult. The large electrochemical potential difference between tin and silver in these alloys requires special handling. Electroplating of ternary and quaternary lead-free alloys will further increase process complexity and cost, since the difficulties of controlling bath chemistry and avoiding contamination multiply with the number of baths required.
Larger wafers also increase the difficulty of maintaining uniform plating bath concentrations and current densities across the entire wafer. Non-uniformities cause plating thickness and deposition rate variations, raising bumping costs and lowering bump quality.
Stencil printing of solder paste easily adapts to lead free solders, since the paste is available in a wide range of alloys. However, stencil printing is limited to bump pitches greater than 200μm. Printed solder paste shrinks during reflow, requiring a paste print area substantially larger than the resulting bump. The molten spheres must be well separated during reflow to avoid solder bridging. Figure 1 shows printed solder paste bumps after reflow.
Figure 1. Reflowed printed bumps on electroless nickel UBM. (Pac Tech photo)
The printing process, forcing paste through stencil holes, may introduce voids within the bumps. Variations in the volume of deposited paste cause bump height variations. Maintaining precise stencil alignments across the wafer becomes more difficult for larger wafers, affecting cost, quality, or both.
Solder spheres may be placed on wafer bond pads and reflowed to form bumps. Like solder paste, the pre-formed spheres are available in a wide range of compositions, including lead-free solder alloys. However, spheres must be mechanically handled, limiting them to minimum sizes of about 100μm. Spheres may be placed in tacky flux before batch reflowing, or individually placed and reflowed with a single laser pulse. Laser-jetted spheres may be placed on wafers with non-planar surface features, such as MEMS devices, not bumpable by batch methods. A second bump may be jetted on top of the first to increase bump height. Figure 2 shows laser-jetted double solder bumps.
Figure 2. Laser-jetted double solder bumps. (Courtesy Pac Tech)
Injection molded solder systems have recently become commercially available. Injection molding systems separate the operation of bump formation entirely from that of placing bumps on the wafer. Bumps are formed in cavity molds, matching the wafer bond pads. Because the mold cavities are filled from a reservoir of melted bulk solder, molding accommodates any solder composition, including tertiary and quaternary lead-free alloys.
At some later time, in a different machine, the bumps are transferred in a single step from the mold to the wafer bond pads. Figure 3 shows a portion of a bumped wafer after transfer. Injection molded bumps have been demonstrated on 300mm wafers, with both leaded and lead-free solders. Bump pitch and quality are equal to that of electroplating; cost is projected to be lower.
Figure 3. Portion of a wafer with injection-molded bumps. (Courtesy IBM)
Gold – tin solder has long been preferred for high temperature operation and to eliminate mechanical creep of position-sensitive optical components. The added virtue of lead-freeness may create more applications. Gold-tin solder may be sputtered in successively alternating layers, or alloy electroplated, using special methods to control composition.
A new process for depositing uniform gold-tin solder bumps by jetting gold-tin in vapor form onto patterned photoresist has demonstrated feature sizes well below 10μm, yielding in one example 4 x 4 μm bumps on 8μm pitch.(1) This could open many high-density applications to lead-free gold-tin bumping.
Table 1. Comparison of Lead-Free Solder Bumping Processes
Evaporation provides high quality bumps at medium to high cost but is poorly suited for today’s larger wafers and lead-free solders.
Electroplating produces high quality, fine pitch, solder bumps. It has low flexibility to handle lead-free alloys, and a relatively high cost.
Stencil or screen printing is a low-cost method providing acceptable consumer quality bumps at coarse pitch. It easily handles lead-free alloys.
Solder spheres are a medium to low cost coarse-pitch lead-free method.
Injection molding offers fine pitch, high quality bumps at relatively low cost. It easily accommodates lead-free alloys.
Printing, spheres, and injection molding all should easily adapt to lead-free solders.
Electroplating will be challenged by ternary / quaternary lead-free alloys.
Evaporation is not a contender for large-wafer lead-free solder honors.
Reference (1) Jet Process Corporation