Wafer Bumping Solder Paste Selection
02/01/2004
Getting it Right
BY FRITZ BYLE
How do you choose the right wafer bumping paste? Emerging technologies are often difficult to define, and even more difficult to implement on the factory floor. There are no specifications or standards for such products. Current users of leading technology often are not willing to openly share information. Manufacturers of such materials boast about amazing performance attributes, but often provide little detail on specific application. This article is intended to provide insight into selection criteria for wafer bumping solder pastes.
Solder Paste Particle Size Matters
Size makes a big difference when depositing very small amounts of solder paste. Solder paste deposit size is critical in determining the required characteristics of a solder paste. The size of deposits affects the selection of a solder alloy powder and will have an impact on the flux technology chosen. Solder powder typically is characterized by size, using particle distributions. Industry standards exist for solder particle sizes, such as IPC's J-STD-005, which describes powder size using different classifications for various powder distributions. For wafer bumping solder pastes, particle distributions of Type 4 (25-38 µm), 5 (15-25 µm) or 6 (5-15 µm) are common. Types 4 and 6 are most commonly used for bumping. Due to market forces, Type 5 has been bypassed somewhat. The availability and quality of Type 6 powder has increased dramatically during the past two years, and the higher production volumes often make it an overall better choice than Type 5. Selection of powder distribution is based mainly on stencil aperture size. The mean particle size in the paste should be less than ~ 12 percent of the smallest aperture dimension (Figure 1). In other words, at least eight average particles should fit across the narrowest part of the aperture.
 Figure 1. This image shows the applicability of the various powder particle size specifications by bump pitch. |
State of Flux
The flux in a bumping paste has several critical roles. In addition to providing rheological properties that enable very-fine-pitch printing at high yield, the small size of the deposits also places a strain on the fluxing action. The small particle size of the metals implies much greater surface area, meaning that there are more oxides to be removed. The small size of the deposits means that there is less flux to do the job. Oxygen in the reflow environment also can react with the flux and reduce its effectiveness. The smaller the deposit and the finer the powder, the more severe these will be. Flux must be designed to accommodate these effects without exhibiting poor behavior.
For semiconductor packaging applications, it is critical that flux residues be removed completely. A water washable formulation is normally used for bumping, and sophisticated techniques such as SEM/EDX and ion chromatography are used to verify cleaning performance. Of particular concern are precipitates of the metal salts formed during oxide removal. Organic compounds that form as a result of the high temperatures involved in reflow are also a concern. This is especially of concern where Pb-free bumping is performed, since reflow temperatures can reach >250°C. It would not be advisable to repurpose a flux system from standard SMT-grade pastes; a flux system designed specifically for bumping is necessary.
The Role of the Stencil
Solder bump size and pitch directly influence the stencil design. Square apertures with slightly rounded corners are preferred for full array printing, while elongated apertures can be used to good effect on depopulated arrays where space permits their use. The advantage of elongated apertures is a larger bump size in relation to the pitch. Aperature design is normally based on known relationships between the aperture opening and the aperture wall area. Aperture size, relative to spacing, is limited by the amount of foil left between apertures, referred to as web width (Figure 2). Ratios over 2.0 are considered nearly unprintable, and for high-yield bumping, ratios well below 1.5 are preferred.
 Figure 2. As an aperture is made smaller, with stencil thickness held constant, the ratio of Aw to Aa increases. |
For fine pitch bumping (<250-µm pitch), an electroformed stencil is required. For pitches <150 µm, even electroformed stencils are challenged to meet the need (Figure 2). The stencils must be very thin (50-70 µm), and the fine "webs" between the apertures become delicate. The foil is easily stretched and deformed or "coined," at the same time the requirements for registration are tighter because of the reduced pitch. The move from 200-mm to 300-mm wafers further complicates matters. The increased wafer size creates a more difficult alignment task, because of the greater stretch. A paste with exceptional release characteristics can enable use of a slightly thicker, more robust stencil. The paste is the key to successful bumping at fine pitches. Over the long term, stencil technology will need to catch up with paste and printing technology. Stencil materials with greater rigidity and new manufacturing technology will be required to take stencil printing into the sub-150-µm realm.
Speed and Physical Attributes
During stencil printing operations, the squeegee speed and physical attributes such as edge quality, angle and flexure have significant impact on how well the aperture is filled. Optimization of the printing process through designed experiments including squeegee speed, blade angle and force applied is critical to process success. Slower speeds, less than 25 mm/second, typically are required to properly fill apertures. The viscosity and release characteristics of the solder paste interact with printer parameters. Solder paste viscosity needs to be relatively high for bumping applications, but the paste needs to be fluid enough at the shear rates present during printing to form a good "roll" in front of the blade and fill apertures fully. At the same time, the paste needs to recover enough stiffness to form a well-defined brick upon removal of the stencil. Again, the design of the paste is critical to the success of the bumping process. Strong interactions between paste properties and printer variables dictate that optimization experiments be performed to precisely determine proper combinations that will generate the best possible bump yields. Enclosed print head systems can potentially provide benefits to the bumping operation. The enclosed head allows independent control of aperture filling time and pressure, whereas these variables are linked for squeegee-based systems.
Avoiding Voids
Solder bumps are about the size of a pinhead, about 100 to 200 µm in diameter. Voids within solder bumps need to be kept to a bare minimum. The level of voids should preferably be less than 5 percent of total joint volume, and eliminated if at all possible. This is a differentiating characteristic for wafer bumping solder pastes. The best pastes on the market are capable of using small particle distributions (Type 6), while generating minimal voids during the reflow operation. Minimizing voids within the solder bump generates wafers with better bump yield and improved reliability. To measure the amount of void area in a solder bump, x-ray inspection of the bump typically is required. Void content within the bumps can be directly linked to the solder paste flux vehicle used (Figure 3). Selection of the appropriate solder paste flux vehicle is critical to minimizing voids in the solder bumps.
Alpha Emissions
A state change in a semiconductor circuit can occur due to the impact of particles emitted by radioactive materials. An example would be changing a single bit from "0" to "1" in a memory array. The array does not sustain permanent damage, but the stored data is corrupted. This is referred to as a "soft" error. Of specific concern are elements that decay by "alpha emission," which refers to the ejection of a helium nucleus from the radioactive parent atom as it decays. Alpha particles are massive and carry significant energy. They deposit this energy in a localized manner and can change the state of an electronic circuit.
Alpha-emitting elements may be present in electronic packaging materials as either contaminants or intentional additions. The most notorious is Pb; this element is present at high concentrations in eutectic SnPb solders, (it is also a component or contaminant of ceramics and glasses used in electronics). Pb is not an alpha emitter, but has isotopes that decay into elements that are alpha emitters (bismuth, polonium, astatine, thallium). The proximity of the bumps to the active surface of the semiconductor and the high concentration of Pb in the bumps makes alpha emission a problem.
 Figure 4. Void content of solder bumps is measured by X-ray. Voids within the bumps can be directly linked to the solder paste flux vehicle. |
Not all applications require low-alpha emission wafer bumping solder pastes, but it is important to understand the needs of the specific application. Low-alpha pastes provide alpha emission levels at or below 0.02 counts/hr/cm². Ultra-low-alpha materials provide emission levels at or below 0.002 counts/hr/cm². Pb-free alloys can contain Pb as a contaminant at up to 0.1 percent, so if low-alpha or ultra-low-alpha emission is required, the alpha-emission level of the Pb-free materials must be specified.
Conclusion
Selection of the proper wafer bumping solder pastes requires attention to fundamental-engineering principles. Foremost are the mechanical requirements for the stencil, which dictates the aperture design and the particle size required. Paste manufacturers provide insight into aperture design limitations imposed by paste performance. Stencil fabricators should review proposed designs to provide guidance.*
Designed experiments to optimize printing and printer parameters will be required. Analyze the bumps produced from a yield, void and consistency perspective. Unlike SMT printing processes, it is difficult to measure the printed deposits (prior to reflow) with sufficient accuracy to gauge process performance.
Lastly, understand the need for special characteristics such as low-alpha emission materials. Specifying a lower-than-required level can add significant unnecessary cost. Total process cost contribution for low-alpha materials can be minimized if the process and paste are specified to minimize paste waste.
FRITZ BYLE, Advanced Technology Products manager, may be contacted at Northrop Grumman-KESTER, 515 E. Touhy Ave., Des Plaines, IL 60018; (847) 699-4637; e-mail: [email protected].
*The author can make available some basic spreadsheet-based tools for aperture design analysis; contact the author via e-mail.