Article: Global SMT & Packaging
New opportunities for controlling pressure in flip chip assembly
Pressure indicating film can immediately produce an image of pressure variations across an entire surface area, allowing significant differences in bond pressure across wafer bonder surfaces to be identified and corrected. Similar applications might be found in flip-chip assembly, particularly given he increasing trend toward larger die. This paper discusses potential pressure-sensing applications in flip chip assembly, such as establishing or verifying die-to-substrate coplanarity, controlling the pressure applied to assure uniform bump heights, and optimizing a number of pressure-sensitive bonding methods.
A recent conference paper¹ showed the advantages of a pressure-indicating sensor film² to establish uniformpressure across the wafers in wafer-to-wafer bonding. The extremely thin (4 to 8 mils) Mylar-based film contains a layer of tiny microcapsules. Applying force to the film ruptures the microcapsules, immediately producing an image of pressure variations across the entire area. The color intensity of the film varies at every point in direct proportion to the local pressure at that point.
Compressing the film between the plates of a wafer bonder produced an irreversible “pressure footprint” of the bonding surfaces. The amount of pressure applied at any point on the map is readily determined by comparing the color at that point to a calibrated color correlation chart. The color comparison is analogous to that made when using litmus paper to determine acidity. Significant differences in bond pressure across the wafer bonder surfaces were revealed and corrected. Repeated pressure samples guided and verified the changes and adjustments made to achieve uniformity. The footprint was also scanned through imaging analysis equipment³ to produce a multicolor pressure map.
Figure 1 shows a typical color map and its pressure interpretation. Visual comparison of a point to the color correlation chart gives ±10% accuracy in determining pressure. Imaging analysis increases accuracy to ±2%. and enables a pressure profile, as shown, to be recorded and interpreted along any potion of the color map. In many applications, the absolute pressure value is less important than relative pressure comparisons, which reveal non-uniform distributions of pressure over the area for correction. A post-correction map provides verification and a permanent record of pressure uniformity.
The success of pressure-indicating film in wafer bonding suggests that similar applications might be found in flip chip assembly, where both the magnitude and the spatial uniformity of applied pressure may be critical. This is increasingly important with the increasing trend towards larger die, which are becoming common. Die larger than 20 mm square are already in routine production³.
Figure I. Color map shows the pressure variations across a semiconductor wafer. The calibrated color correlation chart quickly allows determination of point pressures by visually matching colors; the pressure profile records variations along any path. [Pressure image courtesy of Sensor Products Inc. Madison, NJ www.sensorprod.com]
Potential pressure sensing applications in flip chip assembly include establishing or verifying die-to-substrate coplanarity, controlling the pressure applied to assure uniform bump heights, and optimizing a number of pressure-sensitive bonding methods. Flip chip bonding equipment in general does not directly measure or control pressure. Applied force is set in the machine, and an average distribution of total force over the contact area is assumed to represent pressure. This approach assumes that the force is uniform over the bonding surface. The wafer bonder tests showed that spatial non-uniformities of pressure from tool wear or other causes can be substantial—and can be corrected using pressure-indicating film.
|Figure 2. Misalignment may show acceptable bonds on side A, but create open circuits from failures to bond on side B, where the coplanarity error prevents the bumps from making contact with the pads||Figure 3. Another example of misalignment might create short circuits on side A, where excessive pressure compresses and spreads the burnp material to adjacent pads, while the bumps on side B may still show acceptable bonds.|
Successful flip chip assembly, especially of large die with many bumps in applications such as image sensors, requires that the substrate and the die that is being placed upon it have parallel, coplanar surfaces when they are brought in contact. Any deviation from coplanarity may cause misalignment of the die with the substrate bond pads, resulting in open or poor electrical connections. Extreme cases of misalignment may cause a sideways sliding motion of the die during placement. Pressure differentials across the die may even crack the die. Figures 2 and 3 show some potential effects of non-coplanar bonding4.
Conventional approaches for establishing and verifying coplanarity depend upon optical or laser equipment. The optical system establishes coplanarity by adjusting the tilt of one surface relative to the other until the images are aligned in an optical collimator. The laser system compares reflections from different locations while the die or wafer is tilted to establish coplanarity. Reflective gold spots must be added at several locations on the die or wafer for laser reflections. However, most flip chip bonders are not equipped with either of these expensive add-on accessories. Without them, manually establishing coplanarity often requires repeated trial-and-error adjustments to establish a common focal distance to several points on the die with a high-powered microscope. Multiple sample assemblies may be required for coplanarity verificationCoining
Gold stud bump flipchip assembly places gold bumps on the die using a modified wire bonder. In normal wire bonding, the bonder makes a connection on a chip bond pad, and then extends the wire to make a second connection onto a substrate or package lead frame. In stud bumping, the wire bonder makes the first connection, and then breaks the wire.
First, a small gold sphere formed by melting wire at the wire tip is metallurgically bonded to a chip bond pad by heat, pressure, and sonic energy. The bonding tool next clamps on the wire, and pulls it to break the wire somewhere near the bump. The tool then moves to the next bump location, creates a new ball by melting the end of the wire with spark or electric flame-off in normal wire bonding, and repeats the bump placement sequence on another bond pad. The bumps as deposited have wire tails of varying length, and the bumps themselves may vary in height. Figure 4 shows a bump as deposited.
Figure 4. In this gold stud bump, the gold ball has been pressed flat on the bond pad, with the wire stub protruding from the top of the bump.
Stud bumps may be “coined” by pressing the bumped die against a flat surface as a simple method to reduce height variations and create larger contact areas. Figure 5 shows a cross-section of a coined bump. Again, inadequate or non-uniform coining pressure across the die surface may cause some bumps to have height differences, leading to open or poor contacts. Pressure-indicating film could be applied to verify uniform bump heights after coining.
Figure 5. Cross-section of a gold stud bump, with the top surface flattened by pressure.
Many common methods of flip chip bonding require controlled, uniform pressure to avoid open contacts, poor contacts, and die cracking:
- Thermosonic stud bump bonding experiments determined that there is an optimum pressure for maximum bond adhesion. Bonding pressure was varied across a range from 40 grams to 100 grams per bump. Die shear measurements demonstrated that bump adhesion to the bond pad at the optimum placement pressure was more than 40% greater than at the extremes4
- Thermocompression bonding requires higher bonding pressures than thermosonic bonding, raising the hazards of inadequate pressure control. Copper/ copper thermocompression bonding of copper “nails” on a die to copper bond pads on a wafer has been demonstrated for high density 3D assemblies.5
- Copper/tin interdiffusion bonding of chips to wafers in 3D heterogeneous assemblies depends upon proper pressure at 300°C to form a stable copper-tin intermetallic, with copper upper and lower bonding surfaces.6
- Gold/tin wafer bonding tests show that too much pressure sometimes causes squeeze-out of solder, potentially leading to open or short circuits. Non-uniform pressure may squeeze solder out in some areas, but not in others.¹ A similar problem could occur with large die in chip-to-substrate or chip-to-wafer gold/tin Bonding.
While published papers may not have yet reported the results of using pressure-indicating film for controlling flip chip assembly pressure uniformity, the importance of control in all of the above rxamples suggests that pressure indicatng film might significantly improve yield, costs, and set-up time.References
- D. Spicer et al, Pressure Indicating Film Characterization of Pressure Distribution in Eutectic Au/Sn Wafer-to-Wafer Bonding, Proceedings International Wafer-Level Packaging Conference, pp. 135-139, October 27 - 30, 2009.
- Sensor Products Inc. Pressurex® Film.
- R. Asgari, Copper Pillar and Micro Bump Inspection Requirements and Challenges, Proceedings International Wafer-Level Packaging Conference, pp. 186 - 188, October 27 - 30, 2009.
- L.K. Cheah et al, Gold to Gold Thermosonic Flip-Chip Bonding, Proceedings HDI 2001, April 2001, pp 165 - 175. See also www.flipchips. com/tutorial09.html
- P. De Moor et al, “Recent Advances in 3D Integrations at IMEC,” MRS Fall Meeting, November 27 - December 1, 2006, Boston MA. See also www. flipchips.com/tutorial71.html
- M. Jurgen Wolfe, et al, Technologies for 3D Heterogeneous Integration, Proceedings of SMTA Pan Pacific Symposium 2008. See also www.flipchips.com/tutorial90.html