Interconnect Solutions for Multi-Stacked Packages
BY BOB CHYLAK, STEPHEN BABINETZ, AND LEE LEVINE
The market for advanced, stacked-die packages continues to grow as portable electronic devices become more advanced. Features, such as the Internet, e-mail, music, and television incorporated within the “cellular” system, require the production of stacked-die packages with as many as five die levels, and packages with up to 12 levels are being developed.
Interconnection between these levels is predominantly performed by wire bonding. The advanced looping controls provided by today’s automatic wire bonders allow for flexibility and adaptability that other technological processes cannot provide. Figure 1 shows an advanced, stacked-die package with four die levels and three wire-loop shapes, including die-to-die bonding. Die-to-die bonding saves substrate space and costs while decreasing interconnect length. While these package-types challenge wire-bond capabilities, necessary process improvements are advancing.
Figure 1. Advanced stacked-die package with die-to-die bonding.
The ability to shape a wire-bond loop with well-controlled bends and kinks has been in continuous development for over 12 years.1, 2 With the advent of multiple-level stacked-die packages with thin profiles, the industry is driving to new, even lower loop-height levels. Today’s state-of-the-art wire bonders may offer the capabilities to support as many as 20 premium-process loop shapes, and new loop shapes are continually developed to accommodate packaging design requirements.
The reverse-bonded stand-off-stitch bond (SSB) is the earliest shape developed for ultra-low loop wires. A flat-topped bump is first bonded, and then followed by the formation of a new ball, which is bonded to the substrate. The second (stitch) bond of that wire is bonded on top of the first bump. This reverse-bonded SSB produces very low, 50- to 75-µm loop heights. However, the additional bump in the process lowers the machine productivity by nearly 40%.
Recently, new loop shapes that demand forward bonds with loop heights of <75 µm without sacrificing throughput have been introduced to the market. Achieving these ultra-low loop shapes is important for the lowest die of a stacked die, especially when the wires are underneath an overhanging cantilever. Production die, commonly thinned to 100-µm thickness, are now down to 75 µm. Even 50-µm die are being introduced into mass production. This presents an increasing control problem as loop shapes must be made lower, while the variation in loop height control tolerance must be more rigorous.
Wire bonders move in smooth, continuous, coordinated motions, with all three axes (X, Y, and Z) moving simultaneously. Wire is fed through the capillary and bent by the machine motion to provide the required shape. Motions are calculated on-the-fly and coordinated so that each machine axis travels along a precisely calculated trajectory, which must be controlled to within a few microns of tracking error. Variation of a few microns can significantly affect loop shape and repeatability because of the geometries. Wire payouts and bends need to be adjusted on-the-fly to account for changing wire lengths caused by die-placement variation. All this must be accomplished while bonding up to 16 wires/second at acceleration rates exceeding 12 Gs in XY, and 150 Gs in Z. The latest wire-bonding equipment employs new, faster, and more accurate servo control, enabling more repeatable looping and more complex shapes.
The simplest standard wire-bond loop employs four separate motions from the ball bond to the second bond. More complex loop shapes may employ more than 12 motions for loop formation. Kinks, bends, flat portions of the loop, and smooth, curved portions are all developed based on programmable loop parameters within the software. Wire lengths are controlled and adjusted based on both the modeled loop shape and actual calculated distances between the ball and second bond. Actual distances change for each device because of die attach and other manufacturing variability. As each die is indexed to the bond site, the machine vision system uses pattern recognition to locate the die and leads. Machine intelligence corrects the location of each bond. The corrected locations are also used to provide metered wire length for repeatable loop shape and height.
Wire and Wire Properties
Wire properties have undergone continuous improvements over the years to enable the production of longer, lower, and straighter loops required by today’s packages. Gold bonding wire is normally specified as 99.99% purity, while chemistry within the residual 100-ppm impurities is carefully controlled to provide the required mechanical and electrical properties. New alloys* recently introduced are in the high 99.9% purity range to provide improved long-term reliability in very-fine-pitch (<50-µm ball diameter) applications without significantly sacrificing electrical properties.
Ball formation is accomplished by firing a spark that melts the wire tip. Wire adjacent to the molten ball – the heat-affected zone (HAZ) – undergoes rapid heating and cooling as the heat from the melting process is transferred from the ball up the wire. Wire within the HAZ re-crystallizes, forming new grains with different mechanical properties that are lower strength, stiffness, and higher ductility than the rest of the wire. During the looping process, much of the bending will naturally occur within the HAZ. Wire chemistry can play an important role in providing a short HAZ with high strength and ductility, which will provide lower, more repeatable loops. A newly introduced wire** allows for higher strength and stiffness for advanced wire bonding.
Bonder motions within the HAZ can be used for cold-working and bending functions. Standard wire-bond loops use a reverse motion in the HAZ to bend the wire away from second bond initially, cold-working the HAZ. When the looping trajectory bends the wire in the HAZ back into a vertical position, the cold-worked HAZ is stiffer and the wire loop more erect than without the cold work from the reverse motion. Other motions can be used to make additional bends in the wire, creating useful shapes for additional standoff near second bond, as required in BGA packages with power and ground rings in the vicinity of second bond, or to provide flat (parallel to the die surface) portions of the loop with sharp bends descending to the second bond, as in the CSP or worked-loop. Wire stiffness must be optimized to maintain uniform bends. New wire types are being developed to provide both a high stiffness above the HAZ and a soft ball for improved bondability.
Capillaries provide the necessary features that form the bond. They transfer ultrasonic energy to the bond, control weld size and shape, and are a source of friction during wire feed. While conventional bonding tools contribute little to the looping process, one capillary*** positively affects the looping response in challenging packages by providing greater control in wire-loop height and shape stability, significantly reducing looping failures found on wires formed with a conventional capillary.
Over 20 premium process loop shapes are currently available. Table 1 compares major loop families and their capabilities.
Table 1. Looping family comparisons.
Stand-off-Stitch Bond (SSB 1 and 2). The reverse-bonded SSB is composed of three bonds. First, a ball is welded to the die and the wire is terminated as either an accu-bump (SSB1), or as a flex-bump (SSB2). The flex-bump is higher and provides additional standoff. After the bump is terminated, a new ball is formed and bonded, with the second bond placed on top of the initial bump. Because they require three bonds, both SSB loops are slower than most other bond shapes and impact productivity (Figure 2).
Figure 2. SSB1 and SSB2 bonds.
Figure 3. Worked-loop family.
Worked Loop, CSP, and CSP Long Loop. Worked (WL), chip scale package (CSP), and long chip scale package (CSPLL) loops all have a flat portion of the loop that is approximately parallel to the die surface (Figure 3). They have controlled loop height and can have a steep-kink approach angle (>60°). These loops are used for long inboard bonds (DDR2 memory in FBGA packages), and applications where they must span over lower wire layers in multi-tiered packages. The worked-loop has superior thermal cycling performance in hermetic applications because the outer kink can absorb the strain of thermal cycling better than the HAZ in a standard loop shape.
Figure 4. BGA looping.
BGA Looping (BGA, BGA 2, BGA3, BGA4, and BGA5). BGA looping (Figure 4) is a high-productivity loop with a programmable kink near the second bond that provides standoff over ground or power rings in BGA packaging. It can be shaped laterally and vertically, with variants such as the “J” loop, the “M” loop, and the “Spider” loop. When tailored with lateral curvature, it reduces sweep due to mold flow by as much as 46% in long-loop, difficult-to-mold applications.
Figure 5. Very low loop.
Very-Low Loop (VL). VL loops are new, highly productive loop shapes developed for lower levels of thin, stacked-die packages (Figure 5). By controlling the Z-axis, the chamfer region of the deformed ball can be angled towards the second bond, achieving very low loops (<70 µm), without damaging the neck (HAZ) region.
Figure 6. Folded-forward loop and Pseudo FFL.
Folded-Forward Loop (FFL) and Pseudo FFL (PFFL). One type of ultra-low, forward-bonded wire loop (Figure 6) is produced after the ball is bonded. During subsequent motions, the capillary descends and pushes the wire down onto the ball surface with the capillary face before producing the loop to second bond. This process produces wire loops in the 75-µm height range without sacrificing significant process throughput, but has manufacturing stability issues and cannot be set up effectively without an SEM inspection. Standard optical inspections cannot detect a difference between over-deformed and acceptable bonds. A good alternative, The Pseudo FFL, avoids the deformation issue, but sacrifices a small increase in loop height.
The demand for stacked-die, die-to-die, and multi-tiered packages has produced a wide array of loop-shape choices for packaging engineers. As new packaging requirement are defined, equipment makers and wire and capillary producers work together; providing advanced tools for better, more productive, leading-edge technology solutions.
*Kulicke & Soffa’s Radix wire
- Holdgrafer, L.R. Levine, and D.L. Gauntt, “Method of Making Constant Clearance Flat Link Fine Wire Interconnections,” U.S. Patent 5,205,463, (4/27/1993)
- Y. Alcobi, “Stacked Die & Multi-tier Applications” Advanced Packaging, October 2005, p.10-12
BOB CHYLAK, VP engineering; STEPHEN BABINETZ, senior process engineer; and LEE LEVINE, senior member of the technical staff, may be contacted at Kulicke & Soffa Industries Inc., 2101 Blair Mill Road, Willow Grove, PA 19090; 215/784-6014.