Issue



A Bumpy Trip for Solder Bumping


11/01/2005







A recent study projects a major shift in the relative market shares of the most common solder bumping methods. The never-ending push towards finer-pitch and lower-cost solder bumping conflicts with the requirement to bump 300-mm wafers with ternary or quaternary lead-free solders. As expected in a time of turbulence, market changes will result. Let’s look at the strengths and weaknesses of today’s contending lead-free solder bumping processes.

Pioneered 40 years ago, evaporated bumps were the original flip chip approach, and their age is showing. Evaporators can’t comfortably accommodate today’s larger wafers, and generally waste more material than they deposit. Many lead-free alloys have a high tin content, and the higher material cost compared to lead makes tin more expensive to waste. The geometry of evaporators can’t comfortably accommodate today’s larger wafers. Evaporators appear to be non-contenders for 300-mm lead-free solder bumping, although they will remain important for many specialty applications.

Electroplating presently provides excellent quality, fine-pitch solder bumps at medium-to-high cost. However, plating-bath concentrations and current densities are difficult to control with larger wafers; affecting both cost and bump quality. Plating of ternary and quaternary alloys adds to plating complexity and cost.

Stencil printing of solder paste for reflow to bumps is currently the low-cost, commercial-quality champion. Paste shrinkage during reflow and size limitations of stencil apertures renders printing incapable of fine-pitch bumping. Printing is less threatened by lead-free alloys, since the paste can be mixed in any composition. However, maintaining precise stencil geometry across the wafer is more difficult at 300 mm, affecting cost, quality, or both.

Solder transfer bumping includes any methods where bumps are formed separately before being placed on the wafer. The principal current method is solder sphere deposition. The new contender is injection-molded solder.

Sphere deposition, like printing, allows varying solder compositions. However, spheres are limited to larger sizes and pitches. Sphere placement becomes more demanding as the size shrinks. The care and feeding of small spheres is challenging.

The introduction of C4NP system by two veteran companies* brings injection-molded solder into the commercial arena. C4NP is said to deposit fine-pitch, high-quality bumps of any solder composition - including tertiary and quaternary alloys - on 300-mm or smaller wafers. The companies project the system will do all of this at the lowest cost of any solder bumping method.


Table 1. Worldwide solder-bumped flip chip wafer production, in millions of 200-mm equivalent units. Data courtesy of Prismark Partners LLC.
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Table 1 predicts the 5-year forecast of solder-bumped flip chip wafer production worldwide -including both market and captive sources-for the three major contenders. In total, they show a combined compound annual growth rate of 31% over the five year period. Printing and plating, the older technologies, grow 20% and 23%, respectively. Transfer, driven by the newer technologies, grows 62%, threatening to overtake plating. Transfer’s share increases from 13% to 38%, while plating’s share declines from 73% to 53%.

The great economist, Schumpeter, characterized the process of new challengers displacing old champions as creative destruction; the engine of capitalism. Today we call it disruptive technology, because it causes market turbulence. For the next few years, the “Fasten Seatbelt” sign should be on in solder bumping.

* IBM and SUSS MicroTec

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GEORGE A. RILEY, Ph.D., Advanced Packaging contributing editor, may be contacted at FlipChips Dot Com, 210 Park Avenue #300, Worcester MA 01609; 508/753-3572; E-mail: griley@flipchips.com.