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Polymer Stencils for Wafer Bumping
December 31, 1969 |Estimated reading time: 10 minutes
UV lasers have proven to be useful tools In numerous processing areas. Now, state-of-the-art polymer stencils for fine-pitch applications offer a new field for the use of UV lasers.
By Nils Heininger
Meeting the demand for mobile, highly functional systems in communications, medical technology or auto engineering is only possible with packaging technology that provides a very high degree of miniaturization. For this reason, the use of advanced integrated circuit (IC) packaging will become mainstream in printed circuit board (PCB) assembly, as the area-array packages are based on direct contact between the die pads and the PCB.
Figure 1. Prospects for worldwide expansion for flip chip package application.
While there are slightly divergent forecasts about midterm packaging development, it is certain that flip chip, flip chip on board (FCOB), ball grid array (BGA) and chip scale packaging (CSP) will play major roles in the transition to the next IC package generation. One forecast, for example, predicts the use of flip chip packages will increase annually between 200 and 400 percent in the next few years (Figure 1).1,2
Emergence of "Bumping"
Besides a smaller form factor, the main advantages of an advanced package are greater input/output (I/O) and functional density with more pads leading to a decreasing pin/pad pitch. This trend requires higher efforts from interconnection technologies. The first successfully applied advanced packaging method connects the bond pads of the flipped chip (die) to the substrate by evaporated solder bumps. Today, most of the flipped chip-based packages depend on a bumping technique. Contact from the bond pad to the substrate is realized by a solder sphere the so-called bump that compensates for the tension created because of mechanical and thermal stress.
Figure 2. Schematic of a solder bump fabricated via Ni UBM.
The general requirements for a bumping technology include compatibility with existing wafer technologies, high yield, high process stability and low cost. These goals can be reached by an electroless nickel-under-bump metallization (Ni UBM) followed by stencil printed solder application. The principle structure of such a bump is shown in Figure 2. A layer of nickel covered by a thin gold coating is chemically deposited on the aluminum bond pads. The Ni UBM serves as an adhesion layer and a diffusion barrier between Al and solder. Au is required to protect Ni from oxidation before the solder application. Solder then is applied by stencil printing paste and subsequent reflow. Finally, the wafers are cleaned to remove flux residues (Figure 3).3
Conventional Methods Are UnsuitableThe solder bump bumping application on a semiconductor wafer holds a key position regarding the successful and economical use of flip chip technology. Additionally, wafer bumping via solder paste printing combines high existing PCB technology compatibility with remarkably low cost (largely achieved by an already profound understanding of the process basics). The challenge is to develop this technique for flip chip mass production with a suitable pitch of 250 μm and below.Figure 3. Bumping process steps. A gold coating is required to protect the Ni from oxidation before solder is applied. (Source: EKRA GmbH)
Conventional printing methods and materials are not suitable for applications featuring ultra-fine-pitch structures. Thus, achieving reproducible and homogeneous solder deposits requires improvement of the physical properties of solder paste, the stencil materials and the stencil processing technologies as well as the printing equipment. Using solder printing for ultra-fine-pitch applications, solder pastes with very small particle sizes require a nitrogen atmosphere and a well-controlled reflow furnace temperature profile.3
The stencil printing process has many variables. The following factors must be taken into account to achieve high-quality, reproducible ultra-fine-pitch printing:
- Basic equipment printer, wafer holder
- Stencil aperture quality, wall smoothness, thickness, size, geometry
- Machine setup print speed, pressure, snap-off, separation speed, alignment
- Squeegee squeegee, hardness, angle
- Solder paste particle size, distribution, viscosity, thixotropy, flux vehicle, slump characteristics, metal content
- Environment temperatures, humidity, dust
- Operators training, awareness.
Solder Paste DevelopmentIn recent years, fine-pitch printing tools and processes have been improved. Efforts have been focused on developing soldering materials, solder pastes in particular. Because the particle size of solder paste is important in flip chip applications, some suppliers developed materials with homogenous distributions of particles and sizes smaller than 25 μm.
Deposited solder volume and height depends on the thickness and the aperture size of the applied stencil, and on the squeegee material. After printing, the bumps are reflowed in a convection oven under a nitrogen atmosphere. The printed solder paste volume determines the height and the diameter of the reflowed solder sphere.
Electron microscope pictures of a pad after the different process steps are shown in Figure 4. The stability of the paste volume for every bump (which mainly influences yield) chiefly depends on the stencil quality. This is similar to the solder paste printing process of PCB manufacturing where nearly 64 percent of the reject rate is related to printing defects.4 However, even with laser-cut stainless steel stencil technology, printing reliability is not assured. Improvements and adjustments to this very special wafer bumping technology are necessary. The following describes a new technology in which the production of a laser-cut polymer stencil suitable for solder paste printing is intended for fine-pitch applications.Figure 4. Micrographs of solder bumping processing steps: a) bond pad in initial state; b) with Ni/Au UBM; c) with printed solder paste; d) after solder reflow.
Experiments The experiments concentrate on the major factors of the solder paste printing process. They include the stencil's material, the laser to cut the stencil, printing equipment and the solder paste.
Substrate must meet several criteria to fulfill process requirements. It must be qualified for industry, have constant material thickness, good electrical, thermal and mechanical parameters, and be suitable for both metallization and laser structuring (but not suffer damage from ablation).
The polyimide film Kapton* is a suitable material for these applications. This high-performance material offers good dimensional stability and adherence with most adhesion systems. Its mechanical parameters are important with respect to the stencil preparation process. The foil is spread onto a metal frame at a specific tension. The polyimide's good physical and thermal properties generally provide a stencil layout without deviation from the original, even under production conditions (Table 1).
The metallization's suitability is another important criterion. After a drying process (the 75 μm thickness required 270°F, 8 rh), the film is provided with a very thin Cr layer of 20 nm. This pretreatment prevents the foil from static charging during the printing process. Either sputtering or vapor deposition technology are suitable. The peel strength (>15 N/cm) is measured after electroplating with Cu to 35 μm (0.0014"). Reliability tests (1,000 h) at 270°F showed a 25 percent peel strength decline after 100 hours, after which it remains constant. The same is noted during the 85°C/85 percent rh test.
Laser equipment for producing stencils comprises a laser source and a positioning unit. The source generates a highly focused light whose beam is suitable for cutting polymer foils by centering the foil under the focused laser beam with the positioning unit to make cutouts. The principle of the system is shown in Figure 5.Figure 5. Principle of the laser system: 1) observation optics; 2) interlock switch; 3) cutting head; 4) clamping frame; 5) X-Y positioning unit; 6) granite base; 7) transport eyes; 8) pneumatic box; 9) compressed air prep; 10) laser source.
The laser source is a solid-state ultraviolet (UV) unit based on a Q-switched Nd-YAG laser in combination with an optical module. The laser's frequency being tripled, the wavelength is changed from the infrared (1,064 nm) to the UV. The laser works at 355 nm with a maximum power of 2.5 W at 20 kHz. The pulse repetition rate ranges from 10 to 50 kHz.
The positioning system is an air-bearing precision X-Y table on a massive granite stand. Both axes are guided with air bearings along granite paths on highly polished surfaces. The substrate is fastened to the table by means of a clamping frame. The servomotor-driven positioning unit moves the foil into the laser's focus point with the respective positioning coordinates being sent from a PC to a control system, which controls the cross table as well as the laser. Table 2 lists the parameters of this system.
The printing equipment is necessary for the automatic handling of the wafer from the cassette into the printer and then into a cassette or the oven conveyor. To guarantee this sophisticated print process, an automatic stencil cleaner, 2.5-D inspection and a vision system are used (Figures 6a and 6b).
The solder paste, developed especially for printing ultra-fine pitches, contains an extremely fine powder (Sn63/Pb37 type 6). Typical values are 10 percent below 7 μm, 50 percent below 11 μm and 90 percent below 16 μm, determined by laser granulometry. Table 3 lists the printing parameters.
Results and DiscussionThe tests are performed on 8" wafers with benchmark layouts. The final application is a wafer with more than 20,000 I/Os. There are two different types of ICs on the wafer: one layout with a 250 μm pitch, which requires a bump height of 125 μm; and the other with a 200 μm pitch, requiring a bump height of 100 μm. Stencil thickness is 75 μm and its apertures are 250 x 170 μm and 180 x 130 μm.
The laser ablation of the polymer material is evaluated optically during the tests. It shows that extensive tests for determining an optimum choice of power, frequency and cutting speed are necessary. All three parameters influence the energy density on the substrate. If the power density is too low, the ablation will not cut through the complete material sufficiently. If power densities are too high (>500 mJ/cm2), substrate material damage may result. An optimum combination of parameters to cut a polyimide film of 75 μm thickness is given in Table 4.
Figures 6a and b. Micrographs of a laser-cut polymer film (left) and a stainless steel foil. Note the absence of melting traces in the film material.
Figure 6 compares a metal stencil and a polymer stencil, which has been cut using the parameters mentioned. There are no melting traces in the polymer material, demonstrating the so-called "cold ablation" of the UV laser in plastics. The ablation created by the high photon energy of UV laser light is a kind of photochemical process in which the long carbon chains of the polyimide are cracked and leave a sharp nonmelted edge.
Figures 7a and b. Printing results (left) with a polymer stencil (250 μm @ 200 μm pitch) and a stainless steel stencil (200 μm pitch).
Inner wall measurements of the apertures show approximately a 50 percent decrease in roughness. Another positive result is that there is no phase at the substrate's backside (laser exit), which is typical in appearance when cutting holes in metal sheets. Both characteristics significantly influence paste release. The lower roughness enables a faster and thorough paste release from each aperture. The sharp edges of each hole (without an angle at the laser exit) result in a sharp contour of the paste volume. Increasing the printing speed to >70 mm per second (five times higher than that of a conventional stencil) is possible while still providing a constant bump geometry. Typical printing results with a wafer bump stencil are shown in Figures 7a and 7b.
A minimum diameter of 50 μm and an aspect ratio of 1:1.6 represent the actual limits of the solder paste printing process with polymer stencils. Generally, the limits are not feasible, but
an average printing defect rate of more than 50 parts per second makes this technology uneconomic for wafer bumping.
Ongoing tests show the solder paste to bear a significant influence on the printing process. The flux especially shows a reaction with the polymer substrate even if the polyimide has excellent chemical resistance. Solder pastes with water-soluble flux indicate a detectable effect on the polyimide (proven by FTIR spectrum analysis). The combination of water-soluble flux and ultra-fine-solder granulation deteriorates the paste release. To cover these results, further investigations are in progress.
ConclusionThin polymer foils can be cut very precisely and accurately with a UV laser. Hole diameters down to 50 μm with exact roundness can be realized. This precise technology combined with the advantages of the polymer substrate (e.g., reduced inner wall roughness) make the polymer stencil a suitable tool for fine-pitch applications.
While it must be done very precisely, wafer bumping via stencil-based solder paste printing on electroless Ni/Au UBM is a familiar technique. Advantages of the cost-effective stencil printing process, combined with those of the polymer stencil, make this the recommended technology for low-cost wafer bumping processing featuring high yield.
- Kapton is a registered trademark of DuPont.
ACKNOWLEDGEMENT
The German Federal Ministry of Education and Research (BMBF) is acknowledged for enabling the research project, Ökobump, on the described technology, to proceed. Also thanked are the other project partners, especially EKRA GmbH Bönnigheim, Heraeus GmbH Hanau, IZM Berlin and OKM Messtechnik GmbH Jena.
REFERENCES
1 Advanced IC Packaging Markets and Trends, Electronic Trend Publications, San Jose, CA 1999.
2 R. Kuracina, Flip Chip Packaging for the Year 2000, IBM Microelectronics, Endicott, NY.
3 K. Heinricht, J. Kloeser, L. Lauter, A. Ostmann, A. Wolter, H. Reichl, "Quality and Yield of Ultra-Fine-Pitch Stencil Printing for Flip Chip Assembly," Eupac 1998, Germany.
4 J. Kickelhain, Mikroschneiden von SMD: Schablonen für den Lotpastendruck, LEF 2000, Germany.
NILS HEININGER may be contacted through Suzanne Seco, marketing coordinator at LPKF Laser & Electronics, 28220σW. Boberg Rd., Wilsonville, OR 97070; (503) 454-4200; Fax: (503) 682-7151; Web site: www.lpkf.de.