Mastering Challenges of Lead-free Alloys
BY GEORGE A. RILEY
The first new method for solder bumping to reach the market in the 21st century has arrived, providing low-cost, lead-free solder bumping on 300 mm wafers. Four decades ago, IBM invented C4 solder bumping. Using C4, die could rotate 180° to face downward before connection, thereby forming the flip chip. In recent months, C4’s successor - C4 New Process (C4NP) - was launched and is expected to overcome today’s challenges of wafer size and lead-free alloys. C4NP does not resemble the original C4; the name and use of solder are about the only process similarities. C4NP is a unique, radically different approach that claims to provide a smaller, faster, lower-cost, and more versatile process for lead-free solder bumping of larger wafers.
While the C4NP concept is easy to describe, it is difficult to accomplish. Instead of forming bumps on the wafer, C4NP creates them using injection molding. Molton solder is injected into a mold with cavities in wafer bond locations. In a separate bumping step, the pre-molded solder bumps are transferred to wafer bond pads.
This process was difficult to adapt to an automated system supporting high-volume 300 mm wafer bumping. Solving the mechanical puzzles of a practical system involved the collaboration of two veteran companies*.
The chart in Figure 1 shows pre-molding bumps separately from bump transfer to the wafer. The wafer process flow in the left-hand column includes the normal steps of depositing and inspecting the bump-limiting metallization (BLM); also known as under-bump metallization (UBM). Wafers of 300 mm or less can be processed with no special passivation or protective coating. A conventional evaporated or electroless-plated UBM may be used with no changes required in UBM composition.
Figure 1. Process flow chart for bump molding and wafer bumping.
The right-hand column shows the separated process of bump formation. The C4NP mold is a glass plate containing etched cavities matching the wafer bond pads. Mold cavities are filled with molten solder and, after cooling, pass through an automated visual inspection system that detects filling deficiencies. Plates that fail the inspection process are recycled and the mold reused. Satisfactory plates are stored in dry nitrogen until the wafers are ready for bump placement. At the process merge point, molded bumps are transferred to wafer bond pads. Wafers continue in the normal flow to final packaging, while the empty mold plates are cleaned and reused.
Molds are borosilicate glass plates, with the same coefficient of thermal expansion (CTE) as silicon wafers. Cavities are etched in the plate, and solder-bump volume and pitch are determined by cavity size and spacing. Cavities give precise control of bump solder volume, resulting in excellent bump height uniformity. Typical applications might call for 75-μm diameter bumps on 150-μm pitch. Smaller bumps down to 25-μm in diameter on 50-μm pitch have been demonstrated, matching the fine-pitch capability of electroplated bumps. For a higher board-to-die standoff, jumbo BGA solder bumps could be molded to 500-μm diameter.
Figure 2. Mold filling.
After bump transfer, molds are cleaned in a simple wet-chemistry process, preparing them for re-use. Pilot line data shows no detectable degradation in a mold-plate that has gone through 60 repetitions of cleaning and reuse. Each mold should last through several hundred reuses. Two or three identical mold plates might suffice for small-lot wafer bumping, while high-volume bumping might require several dozen of the reusable plates. Qualified regional mold suppliers provide and pattern unique molds for each wafer type, but mold pricing depends on the quantity, the cavity size and the number of cavities.
During this stage, the filling head maintains close contact with the mold-plate, using an O-ring seal to prevent solder leakage. As the filling head moves across the mold plate, all of the empty cavities are automatically detected and uniformly filled with solder to the level of the mold plate surface. The solder reservoir temperature typically is maintained at 10° to 20°C above the solder melting point, while the mold plate temperature is kept at or below the solder melting point. Once the solder solidifies, it creates the bump. Solidifying forms a bond between bump and mold plate, which holds bumps securely in the mold.
The process design prevents voiding in the solder bumps. Because bulk solder is used in the reservoir, any solder composition that can be melted can be molded, including ternary and quaternary lead-free alloys. The injection method fills only the cavities, with no wasted solder; an important consideration when using expensive lead-free solder alloys.
Figure 3. Transfer process steps.
Filling a 300 mm wafer mold takes about four minutes. The fill reservoir contains enough solder to fill 50 mold plates, and refilling the reservoir takes about 10 minutes. To change solder alloys, a different fill head may be substituted in less than an hour. An array of several fill heads with different alloys might also be available. Alternatively, any fill head can be emptied, cleaned, and filled with another alloy. A single set of bump molds and transfer equipment can provide a line throughput of up to 300 bumped wafers per day.
An automated optical inspection (AOI) of every cavity is completed after filling. The system maps missing or partial filling, overfilling, or solder bridging areas. While minor defects may be reworked manually under a low-power microscope, an unacceptable defect level will require cleaning and refilling the mold. Filled molds can be stored indefinitely in dry nitrogen.
During the transfer process, bumps and wafer meet for the first time. The transparent mold material simplifies the alignment of bumps and pads without requiring split-image optics. An alignment accuracy of 10 to 20-μm should suffice for typical bond pads because the surface tension of the molten solder will self-center the bumps on the UBM. After aligning, the mold and wafer are brought into close vertical proximity, and the solder is heated to about 20°C above the solder reflow temperature.
Heated solder liquefies and expands beyond the mold, creating a solder meniscus above the mold surface. As molten solder contacts the oxide-free UBM, the wetting force between solder and UBM overcomes the surface tension and separates solder and mold, transferring the bumps to the wafer. The mold is cooled and removed vertically from the wafer.
The bump transfer process takes approximately four minutes per wafer. It is fluxless, conducted in a controlled atmosphere, and is designed to prevent the entrapment of gases in the bump-to-bond pad interface during solder transfer. Transferred cooled bumps are hemispheric with slightly flattened tops, allowing easier probing.
Figure 4. Solder bumps after being transfer to the wafer.
Separating bump formation from wafer processing provides several direct and indirect advantages including the reduction of handling fully processed wafers, which can decrease yield losses. Processing wafers and bumps in parallel, rather than sequentially, decreases throughput time. For fast-turn lots, bump molding can be completed while wafers are still in process; so bumping is just a reflow away from wafer fab. In addition, injection molding of bumps has advantages beyond those of line separation. The versatility of easy adaptation to all types of solders and wafer sizes is another benefit.
With this process, final assembly yield should increase simply because bumps are fully inspected prior to transfer and can be reworked as needed. The molding process provides void-free bumps and controlled bump heights, which can further raise yields. Standard environmental stress tests were passed with no failures, matching or exceeding the performance of other bumping methods.1
Figure 5. Bumped wafere after transfer from mold plate.
Using bulk solder can also save money. Solder may be available in wires, pellets, or any other convenient form, rather than more expensive and less convenient pastes, preforms, plating solutions, or spheres. Molding saves solder and offers zero waste in the filling process. Eliminating solder waste is a major cost saving with lead-free alloys.
C4NP demonstrates high quality, great versatility, and fine pitch. Based upon these process savings and yield advantages noted above, C4NP is projected become the lowest cost bumping method. Compared to the common bumping methods of evaporation, plating, and printing, C4NP displays several advantages. As in Table 1, the evaporation process produces high-quality bumps, but is not adaptable to 300 mm wafers or to most lead-free solders. Evaporation also has a high capital equipment cost and wastes as much as 99% of solder per run.
Table 1. Comparison of solder bumping processes.1
Electroplating produces high-quality bumps on fine pitch with good yields, but is a high-maintenance process that uses messy wet chemistry and secret recipes. Plating baths call for close attention and constant care, and are also difficult to maintain and control. Plating has high capital costs, and high consumable acquisition and disposal costs, with limited flexibility.
While solder paste allows great flexibility in solder composition, the 50% shrinkage factor from paste to bump eliminates fine pitch, challenges uniformity, and makes voids inevitable. Printing lends itself best to high-volume, low-cost wafers with consumer-level reliability requirements.
Evaporation is high-quality and medium-to-high cost. Plating is also high-quality with fine pitch, low flexibility, high complexity, and high cost. Printing offers lower but acceptable commercial quality, coarse pitch, and low cost. C4NP is projected to be the most viable solution; with quality and pitch equal to plating and cost below printing2. If these projections prove true in the real world of volume manufacturing, we may well be entering the best of both worlds.
* IBM and SUSS MicroTec
- Peter A. Gruber et al., “Injection Molded Solder Technology for Pb-free Wafer Bumping,” Proc. IEEE 54th Electronics Components and Technology Conference, 2004.
- Klaus Ruhmer et al., “C4NP: New Solder Bumping Technology - Low Cost and Lead Free,” IMAPS Flip Chip Advanced Technology Workshop, Austin, Texas, June 20, 2005.
GEORGE A. RILEY, Ph.D., contributing editor of AP, may be contacted at FlipChips Dot Com. 210 Park Avenue #300, Worcester MA 01609; 508 753-3572; email: firstname.lastname@example.org.