Achieving Fine-pitch Ball Placement
07/01/2008
BY TOM FALCON, DEK
Demands to reduce the dimensions of even the most highly miniaturized semiconductor packages are placing pressure on IC vendors and packaging specialists to develop faster and more productive package-assembly processes. Ball placement techniques are being fine-tuned to address these issues.
Since the wafer-level chip-scale package (WLCSP) has become the package technology of choice for devices such as DRAMs, flash memories and small FPGAs, research has focused here to find suitable miniaturization measures. The next evolutionary step for WLCSP is to reduce the ball-grid interconnect array pitch to 0.3 mm, which allows semiconductor manufacturers to increase the number of die per wafer by more than 50%, for little extra manufacturing cost. However, continued shrinkage of pitches and ball diameters brings challenges of implementing reliable and repeatable processes for placing 0.2-mm solder balls on a 0.3-mm pitch at high yield and throughput.
Ball Placement Techniques
Over the last 10 years, several methods for the mass transfer of solder balls to wafers has been developed, as well as complementary processes for rework and single-ball placement. An accurate and repeatable stencil printing ball placement process for 0.2-mm solder balls on a 0.3-mm pitch was developed, with the goal of demonstrating greater than 99.99% placement yield at a throughput of 60UPH or more. These are the fundamental criteria for a ball-attach process supporting cost-effective mass production of 0.3-mm pitch WLCSPs.
Equipment and Process Overview
Solder-ball placement using screen printing begins with tooling the wafer in a conveyorized aluminum pallet. This pallet is transported into the flux-printing machine where the wafer is visually aligned with an emulsion-mesh fluxing screen with reference to laser-marked fiducials. Flux is then screen-printed onto all solder-bump pads. After flux printing, the wafer and pallet are transported to the ball-placement machine. Following accurate visual alignment with the metal ball-placement stencil, the wafer is brought into contact with the underside of the stencil before the ball transfer head traverses the topside of the stencil. This deposits a single solder ball into each of the stencil’s apertures (Figure 1). The alignment process ensures that the apertures coincide accurately with the fluxed solder-bump pads.
 Figure 1. Solder ball placement. |
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Screen and Stencil
The specification of the emulsion-mesh screen used for the fluxing process is derived from established formulas based on the pitches and ball diameters to be placed. Typically this will be a stainless-steel, 45?? mesh with several microns of photoimageable emulsion on the wafer side. The screen’s mesh wire diameter, mesh pitch, and emulsion aperture size are calculated according to the ball diameter to achieve the optimal ratio of printed-flux volume to solder-ball volume. Once fabricated, the screen is checked against specifications and parameters including aperture size, mesh tension, emulsion integrity, and image-stretch or shrinkage.
The ball placement stencil is composed of two layers. The top layer is either stainless steel or electro-formed nickel, and the bottom (or stand-off) layer is a photoimageable dry-film resist. The purpose of this stand-off layer is to hold the top metal layer away from the pre-printed flux. Another benefit of this material is that it provides a relatively soft surface in contact with the active side of the wafer, thereby minimizing any possibility of wafer damage.
Wafer Tooling
The wafer tooling is a two-part system consisting of a universal base plate, with a user-configurable series of vacuum rings suitable for wafers from 100 to 300-mm.
This is coupled with a top plate, or shim, unique to the wafer diameter and thickness. Generally, the shim aperture in which the wafer sits is 1-mm greater than the wafer diameter, and the shim thickness is lower than the wafer thickness, but by no more than 50 µm. This creates a smooth and flat platform to support the metal stencil and solder-ball transfer head during the ball-placement process.
Equipment Modifications
The placement process requires that the wafer, its pallet, the stencil, and the transfer head are all flat to within a fraction of the ball diameter. Clearly any reduction in ball diameter makes this even more difficult to achieve. For this experiment great care was taken to ensure these parts surpassed their specifications.
Wafer tooling was checked with a dial-gauge indicator and was within 30 µm. This was well inside the normal specification of 50 µm in the wafer area. The shim was matched to the wafer thickness within 20 µm rather than the usual 50 µm, and the base of the ball place transfer head was precision-ground to within ??10 µm, in contrast to the usual ??25 µm.
Taking these extra precautions ensures that the wafer, stencil, and transfer head are all co-planar within a few microns of each other over their entire area, thereby enabling the smooth and accurate transfer of solder balls from the transfer head, through the stencil, to the wafer. Failure to ensure this level of flatness in each element of this stack could promote placement yield problems later on; particularly damaged or sheared spheres.
Experiment
Having prepared all equipment to a greater level of accuracy than is normally required, the placement experiment commenced. To verify achievement of the desired throughput of 60 UPH and ball yield of 99.99%, an entire cassette of 25 wafers was processed, with no operator intervention and at a cycle time of 60 seconds or less.
Flux Print
Flux printing for ball placement uses established screen printing techniques coupled with state-of-the-art printing machines and precision mesh screens to create a robust process with a wide operating window. Provided the mesh screen is made correctly and the flux is well formulated for screen printing, it is usually not difficult to find a set of process parameters that produce excellent results. In most cases a print gap of 1.5 to 2 mm, and a print speed of 25 to 50 mm/sec, with a print pressure of 2 kgs per 100 mm of (60 degree polyurethane) squeegee blade length will produce uniform flux deposits (Figure 2).
 Figure 2. Typical flux deposits on blank 200-mm wafer. |
This process is generally reliable enough not to warrant a separate inspection stage. Established practice is to perform inspection after ball placement. The same protocol was followed for this experiment: following initial setup, no flux-print inspection took place. Interestingly, none of the defects later captured by the ball place inspection could be directly attributed to a flux printing defect.
Ball Placement
It is feasible to take a batch-processing approach using this equipment, and flux all 25 wafers before subsequently performing ball placement on each fluxed unit in the complete 25 wafer cassette. Alternatively, units in one cassette could be fluxed simultaneously with ball placement on a previously fluxed cassette. However, since the equipment used in this experiment was not configured for high volume manufacture, all three operations (flux print, ball placement, and inspection) were performed sequentially for each wafer, before the next wafer was processed.
In the industry generally, ball placement inspection is sometimes performed post-reflow. However, in this case it was carried out post placement so that no reflow-generated defects could influence the data. Therefore the placement process alone was measured. Inspection was performed manually and with both stereo zoom and video measuring microscopes.
Results
The results of the wafer-processing experiment are shown in an SPC control chart (Figure 3). Defects recorded were missing, misplaced, or damaged balls.
 Figure 3. SPC analysis. |
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 Figure 4. Ball-placed wafers in cassette. |
In this test configuration, the machines are not capable of 60 UPH, because they are configured for manual loading. Automated wafer handling options are available that have already been proven to meet 60 UPH for 0.3-mm ball diameters at this wafer diameter. It follows that 60 UPH would also be possible for this 0.2-mm process. Figure 4 shows the wafer cassette and ball-placed wafers part of the way through the run. Figure 5 shows a complete wafer immediately after ball placement.
 Figure 5. 200mm wafer populated with 269,108 0.2mm solder balls on 0.3mm pitch. |
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Conclusion
This experiment has proven that it is possible to successfully automate the placement of 0.2-mm balls for WLCSPs with ball yield greater than 99.99%. It has also proven that the process is relatively stable and controllable, with no data points below the lower control limit, and no more than four continuous data points either above or below the mean yield.
By recording the machine cycle time and comparing it with current high-volume equipment installations for larger balls, we can also conclude that the throughput requirement of 60 UPH can be met for this ball diameter, provided robotic wafer handling is used.
References:
- M. Whitmore, M. Staddon, D. Manessis: “Development of a Low Cost Wafer-Level Bumping Technique.” International Wafer-Level Packaging Conference 2004.
- M. Whitmore, M. Staddon, D. Manessis: “The Development Of Balling Technologies For Wafer Level Devices With Pitches Down To 0.4mm.” International Wafer-Level Packaging Conference 2005.
Tom Falcon, senior process development specialist, may be contacted at DEK Printing Machines Ltd, Granby Idustrial Estates, Weymouth, Dorset, DT4 9TH, U.K.;+44/1305 208415; [email protected].