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Flip-chip Assembly Using Ultra-fine-pitch Solder Pastes
December 31, 1969 |Estimated reading time: 6 minutes
In electronics manufacturing, keeping pace with technology is not enough to ensure long-term viability. One must constantly plan for future applications. This quest for increased circuit density and miniaturization will make use of flip-chip technology.
By Steve Dowds
Keeping up-to-date with technology is not enough to ensure long-term viability; one must constantly stay ahead of the game, planning for future applications. Products requiring increased circuit density and further miniaturization will make extensive use of flip-chip technology. By enabling the design of products with solder joint structures down to 50 or 60 µm in diameter, flip-chip technology can help achieve geometries similar to semiconductor chip I/Os.
Cost is also a major issue in this competitive industry. Therefore, making use of existing production facilities when possible is vital. If minimal changes to existing SMT production lines are required, including conversion to lead-free processing, process economy can be achieved. The low-cost/high-volume joint-making ability of stencil-printed solder paste makes it a logical choice over less-versatile conductive adhesives optimized for dispensing and limited in deposit pitch. To test the viability of this idea, the first step is to specify a suitable fine-particle-size lead-free solder paste. The second step presents a challenge: understand and manipulate the sub-processes dominating fine-pitch solder paste behavior, so as to ensure reliable interconnections.
The sub-processes involved in paste roll, aperture filling, and aperture release are of particular importance. Paste roll, which produces the hydrodynamic pressures required to force solder paste into stencil apertures, is optimized through a suitable combination of solder paste rheology, print speed, print pressure, and stencil-surface texture. Proper aperture filling is achieved using a solder paste particle-size distribution (PSD) compatible with the aperture size. For ultra-fine stencil apertures smaller than 50 µm, this means using a solder paste formulated with a Type 6 (15- to 5-µm ) or Type 7 (12- to 2-µm ) PSD powder to maintain the preferred ratio (≥5) of aperture diameter to particle size. This is no small adjustment; swapping from an industry-standard Type 3 to Type 6 PSD increases the number of particles per unit volume by a factor of 15, which leads to considerable differences in the particle packing of Types 6 and 7 solder pastes. This can change the nature of particle motion during paste roll, so rheology modifiers should be considered in paste formulation to achieve desired properties.
Paste release from stencil aperture to substrate depends on the interaction of forces between solder paste, circuit pad, and aperture wall. Designing the aspect ratio (aperture width/stencil thickness) and area ratio (aperture area/aperture-wall area) of stencil apertures to be greater than 1.5 and 0.6, respectively, can improve release, but necessitates a fine web between apertures. Paste rheology modifications may be required to avoid paste slump and bridging defects.
A practical approach can test stencil-printing limits of fine-particle, lead-free solder pastes at ultra-fine pitch. Research sought answers to the following questions:
- How does a fine-particle solder paste behave during stencil printing compared to an industry-standard Type 3 (45- to 20-µm) PSD paste?
- What pitch and web limits are achievable; what factors govern them?
- Are solder paste design, stencil quality, aperture design, or aperture-array design responsible for controlling printing limits?
- Can reliable interconnections be assured using low paste volumes at fine pitch?
Two solder pastes were used in the printing trials using an industry-standard stencil printing machine fitted with twin polyurethane squeegees angled at 60° toward the print stroke. An electroformed-nickel wafer-bumping stencil was selected to provide aperture-size control, aperture-wall smoothness, and a shape suitable for an ultra-fine-pitch application. The design incorporated square and circular apertures from 30- to 90-µm wide, arranged in peripheral and full area-array patterns ranging in pitch from 150 µm down to 30 µm. Both square and circular apertures were used because they yield different volumes for the same aperture dimensions, and the web area between circular apertures is slightly larger. This provided a variety of web spacings to observe the printing limits of fine-particle solder paste. The substrate used was FR-4 laminate with Cu and Ni/Au coating (Figure 1).
Figure 1. SEM image of an electroformed nickel-stencil aperture.
Achieving consistently sized paste deposits over the whole print pattern without bridging, skipping, or smudging were defined as acceptable printing results. Printing limits were defined as the pitch and/or web at which these criteria were no longer met due to incorrect printing parameters, too fine a pitch, web-in-stencil aperture patterns, or unsuitable paste rheology.
Printing Parameters and Limits
Using the setup for Types 6 and 7 pastes, optimal printing parameters were: print speed of 10-30 mm/s, squeegee pressure of 5-6 kg, and a print gap of 0.2 mm. These parameters were used to identify minimum dimensions that pastes would print successfully.
For Type 6 paste, printing limits were 50-µm deposits at 90-µm pitch for peripheral-array patterns, and 110-µm pitch for full-array patterns. At finer pitches, print-pattern smearing occurred, which was attributed to paste bleed between apertures on the stencil underside. Adjusting the parameters showed no improvement. While marginal slump was evident in full-array patterns at 90-µm pitch, 40-µm web spacing in peripheral arrays did not cause smearing or bridging. Solder paste remains in the web area between adjacent apertures after the stencil has been removed, indicating that solder paste bleeds into capillaries between apertures during print stroke.
Type 7 paste results were different - printing limits and aperture sizes that the paste could fill were much finer, and no smearing was observed. This may be explained by rheological properties of the paste, which exhibited higher viscosity and tack, implying that it is less inclined to flow into capillaries between adjacent stencil apertures at fine web.
It was observed that for a given aperture, deposits using Type 7 paste were smaller than those achieved with Type 6 paste. The same viscosity and tack properties that allow finer-pitch printing caused poor aperture release. When the stencil is drawn away from the substrate, paste release occurs when the pull force exerted by the paste tackiness (surface tension) overcomes the drag force exerted by friction between aperture walls and solder paste. Thus, increasing the aspect and area ratios to yield a lower drag force could improve paste release. Using square rather than circular apertures may also aid paste release, because square apertures produce a stronger stress gradient at the corners. This may be significant when considering eventual assembly requirements as solder volume is an important factor in the assembly and reliability of flip-chip devices (Figure 2).
Figure 2. Aperture release comparison of Type 6 (left) and Type 7 (right) solder pastes.
As a further experiment, both pastes were used to bump a 100-mm-diameter silicon wafer comprising 576 flip-chip-sized dies, offering a total of more than 90,000 pads on which to print the paste, ranging from 60-80 µm in size in full-array and peripheral patterns at 125- and 150-µm pitch. Printing at the coarser geometries presented no problem for either paste, and paste behavior was similar to that on the FR-4 test PCB (Figures 3a and b).
Figures 3 a/b. Fine-pitch solder paste deposits on a silicon wafer.
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Conclusion
Adjusting solder paste rheology enables successful printing at ultra-fine-pitch geometries which, when combined with appropriate printing parameters, allows a practical printing process window. This advancement is made possible through refinements to solder paste design and stencil-manufacturing technology. While both solder pastes tested are suitable for fine-pitch stencil printing and flip-chip wafer-bumping assembly, Type 7 paste prints at finer limits and fills smaller stencil apertures. However, Type 6 paste offers more complete aperture release, allowing the potential to control solder paste volume deposited by selecting between the two types of solder pastes. These results are encouraging and support the view that implementing a low-cost, lead-free wafer-bumping process is a possibility that merits further investigation.
Steve Dowds, global product manager, the electronics group of Henkel, may be contacted at 44(0) 7909876851; e-mail: steve.dowds@uk.henkel.com.