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A technique commonly used in surface-mount-technology could prove highly effective in forming the interconnects for advanced semiconductor packaging. DEK president Richard Heimsch discusses mass imaging for wafer bumping.

Semiconductor vendors are excited by advanced packages such as flip chip, several variants of chip scale package (CSP), and system in package (SiP). Packaging overheads are low, leading to highly miniaturised components, and electrical performance is also enhanced.

These are ideal attributes for broadband applications such as IP-based voice and data communications, Gigabit and 10G Ethernet, and 3G mobile. SiP, on the other hand, offers a potentially more flexible, lower cost alternative to the system on chip, more tolerant of redesign and inherently able to support mixed technologies.

But component assemblers need improved package assembly processes and techniques to create large area arrays, which can feature several hundred interconnects. As ever, the objectives are greater throughput, higher end of line yield, and lower capital expenditure and cost of ownership, to minimise the cost per package.

DEK has addressed these challenges with a wafer bumping solution based on mass imaging that is capable of creating interconnects on a pitch as fine as 200ìm. The technique works with platforms that are much faster and less expensive to buy and run than traditional vapour phase deposition equipment.

The tools are at least an order of magnitude less expensive, and can be delivered within just 48 hours turnaround. In addition, the process is more tolerant of ambient conditions, occupies less factory real estate, consumes less energy and generates less waste.

But high accuracy mass imaging must now prove that it can keep pace with future packaging advances. With forthcoming generations of chip scale packages aim at bump pitches significantly below 200ìm and even below 150ìm, the established stencil design rules are already stretched.

In addition, several parts of the world are seeking to eliminate lead (Pb) from electronic products - either compulsorily or voluntarily. Some process issues have already come to light when screen printing with Pb-free pastes at normal surface-mount technology (SMT) resolutions, but so far little is known about how these issues will manifest themselves, or be overcome, at the wafer level.

In its favour, the mass imaging technique already has important users, as well as a couple of tricks up its sleeve. High accuracy mass imaging for wafer-level applications exploits recent advances in screen printing for SMT preplacement to create large numbers of area array interconnects at high speed.

There are three key enablers: enclosed printhead technology has achieved paste volume repeatability well beyond the requirements of ordinary SMT, into wafer-level territory; precision manufacturing of electro-formed stencils, with high dimensional stability and excellent paste release characteristics; and motion controls based on linear motor technology, with new position encoders that enable high repeatability at waferlevel resolution.

First generation high accuracy mass imaging processes have also delivered much greater flexibility, for instance being able to deposit both solder flux and paste, or solder balls, with handling interfaces for several input formats including wafers singulated substrates, or substrates in carriers such as Auer boats.

In addition to these automated handling systems, further new enabling technologies have emerged to meet the needs of the semiconductor production environment, including paperless cleaning systems suitable for clean room conditions and new evolutions of the fully enclosed print head. The low waste these systems achieve is particularly important when using specialised, expensive low-alpha solder pastes for ultra-fine pitch bumping.

Wafer bumping by high accuracy mass imaging requires the use of unique design guidelines tailored for each application, and much work has already been done to establish suitable guidelines for bump pitches and solder paste characteristics in common use.

In the imaging step, an over-printing strategy is used to achieve bump height targets of 80 to 150 microns on pitches of 150 microns up to 500 microns (figure 1). This technique requires rigorous application of design rules when creating the stencil. It is also necessary to pay careful attention to the design of bond pads, allowing sufficient contact area to achieve sufficient solder joint strength for a given stand-off.

The size of the pads on the chip has a direct influence on solder volume requirements. Smaller pads require less paste than larger pads to achieve the same target reflowed bump height. When the pad size is too small, however, less bonding area is available to support a relatively large volume of solder. Thus, bump and solder joint strength may be sacrificed for only slight gains in stand-off distance.

Solder bump sizes also are affected by the shape of the wettable bonding area of the chip pads. This shape is not necessarily dependent on the geometry of the pads, but is chiefly determined by the passivation opening overlying the pads.

In a typical example, solder paste is deposited through apertures measuring 6 x 19 mils and 3 mils deep. On reflow, the paste retracts onto the pad, forming spheres 5 mils high and 6 mils in diameter. Generally, this process is suitable for bumping pitches down to 200 micron for full array die and 150 micron for peripheral array designs.

As bump pitches shrink, the volume of each solder bump will also reduce. Paste volume repeatability therefore becomes even more important, if coplanarity is to be maintained. There are two aspects to achieving high paste volume repeatability: 100% aperture filling, which the enclosed printhead delivers; and optimal paste release. A new technique has been developed to enhance paste release for waferlevel applications.

When screen printing with a traditional emulsion screen and squeegee, the stencil tends to peel away from the substrate surface, resulting in a progressive reduction of adhesion between the paste and the stencil until the stencil separates fully from the substrate.

This has a beneficial effect on both paste transfer efficiency and repeatability. But the majority of todays mass imaging processes - including semiconductor assembly processes - feature a metal stencil, such as a laser cut stencil. This is brought into direct contact with the substrate before the squeegee or enclosed head begins its excursion. Afterwards, the substrate is moved directly downwards away from the stencil. The combination of forces that result in the peeling action are thus replaced by a vertical pulling force.

By investigating the movement of metal stencils during separation, DEK has noted that separation actually begins at each outside edge of the stencil and converges towards its centre. The centre is the last part of the stencil to release. This separation also accelerates towards the centre of the stencil, which does not make for consistent paste release.

Regaining the peeling action displayed when a conventional emulsion screen separates (figure 2) is crucial to developing a repeatable process for bump pitches of 150ìm and below.

To achieve this, DEK has developed an optimised stencil tensioning mechanism, combined with a radically different separation action.

The frame is capable of automatically adjusting the tension within the foil to optimise both the deposition and separation phases of the process. The tension is adjusted pneumatically, using the standard air supply available on any automated mass imaging platform.

Wafer bumping must also respond to the industry-wide migration to lead (Pb)-free solder alloys. Pastes manufactured with these alloys are now known to display a different rheology from their Pb-rich predecessors.

SMT assemblers are now finding that they must subtly alter their screen printing parameters in order to optimise processes for Pb-free printing. The same will be true at the wafer level, where the higher viscosity and greater metal content of the new pastes will have implications for aperture filling, paste on pad, and shrinkage during reflow.

Responses will likely come in the form of revised design rules for both pads and apertures. Research into Pb-free screen printing for SMT applications has shown the process is now sensitive to stencil separation speed.




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