Cleanliness is Critical for Wafer Bump Stencil Printing
SOLDER PASTE DEPOSIT REMOVAL
BY MIKE BIXENMAN AND MICHAEL KONRAD
Stencil or screen printing solder paste is a well-known and proven process technology for surface mounted components. Application of this technology for deposition of solder bumps offers an attractive, cost-saving and high-volume production alternative. The cleanliness of the stencil is critical to the success of the bumping process. Insufficient solder is the primary cause of defects from stencil printing, therefore stencil cleanliness is an essential factor in delivering the proper amount of solder paste. Stencil cleaning must remove all solder particles and flux vehicle from apertures without damaging the stencil, bonding adhesives and elastomer frame (Figure 1).
Figure 1. Inserting a large stencil into a cleaning machine.
Modern stencil cleaning processes integrate the cleaning agent with the cleaning equipment. Cleaning material suppliers work closely with cleaning equipment manufacturers to provide an integrated process that meets the design criterion. Cleaning the wafer bumping stencil provides the challenge of removing paste from tiny apertures, while not damaging the thin foil. To achieve reproducibility, the process requires a mild chemistry integrated with stencil cleaning equipment that provides mild impingement and exceptional residue removal.
Stencil printing offers an efficient low-cost method of fine-pitch solder deposition. To achieve reproducible and homogeneous solder deposits, ultrafine-pitch paste depositions demand control over all sub-processes involving material and equipment. Cleanliness of the aperture places new demands regarding the removal of residual solder paste deposits. Developing a stencil cleaning procedure for wafer bumping creates a design challenge because of the need to remove dried solder paste from tiny apertures on a thin foil.
Stencil cleaning is an art that is placing an increasingly important role for surface mount and advanced packaging manufacturing processes. Stencil printing offers quick and efficient in-line method of sold deposition, but may have difficulty in delivering precise ultrafine-pitch solder volumes at less than 150 µm. To achieve reproducible and homogeneous deposits, the process techniques for fine-pitch printing require an improvement of the solder paste particle size, stencil materials and cleaning processes.
The use of stencil printing for wafer bumping offers a quick in-line method of solder deposition. To achieve suitable yields, the stencil printing procedure must deliver sufficient solder volume to form an adequate bump height. Reduced pitch necessitates the use of finer solder powder and apertures free of residue. The excessive solder paste oxide caused by large solder surface area requires highly active flux capacity to prevent void formation.
Process conditions must operate seamlessly to achieve acceptable yields. Numerous variables such as pad size, stencil thickness, aperture shapes, flux, aperture geometry and cleanliness must be synergistically in control. The pad size, pitch and aperture cleanliness influence solder paste volume. Constricted apertures reduce both volume and bump height. The stencil must be clean and capable of producing evenly distributed solder deposits to the under bump metallurgy. Tight dimensional and positional aperture tolerances leave no room for error.
The thickness of the stencil metal foil is significant in determining the size of the opening needed to achieve the required volume of paste to produce a target reflowed bump height. Thicker stencils are attractive because the openings do not need to be as large to achieve the same aperture volume requirements as they do with thinner foils. Paste transfer efficiency suffers with thinker stencils, because more paste adheres to the aperture walls. Thicker stencil foil can be helpful when printing tight pitches. To address the transfer efficiency problem, cleanliness between prints is an essential requirement.
Cleaning Chemistry Equipment
Removal of solder deposits requires a cleaning material that wets, dissolves, saponifies and displaces the flux vehicle (Figure 2). Cleaning process development hinges on the cleaning chemistry. Wetting occurs by reducing surface and interfacial tension by using surface-active agents that allow the cleaner to penetrate and undercut the soil-substrate bond. Dissolution of the flux vehicle allows the metallic spheres to separate and drop from the aperture. Saponification is the reaction of free alkalinity that reacts with the flux resin to form a water-soluble soap. Displacement occurs by bombarding the containment with mechanical force that facilitates the cleaning process and impacts reproducibility. Four process variables influencing cleanliness include chemistry concentration, bath temperature, cleaning time and mechanical action imparted by the equipment used.
Figure 2. Checking chemistries.
Aqueous cleaning agents provide users with a variety of options that effectively remove water soluble, rosin and low residue flux. High technology manufacturing environments demand materials that operate at low concentration and temperature, while meeting clean air requirements. The ultimate goal is to maximize performance with less active chemistry. In an effort to meet these requirements, the chemistry design must operate in an aqueous media that provides suitable performance at low concentration and temperature.
Aqueous cleaning chemistries also need to be designed for spray and immersion cleaning equipment. Cleaners designed for spray in air must not foam when processing through the equipment cycle. Foam causes a number of issues that upset the cleaning process. To address this issue, aqueous materials contain tiny micelles that break or destabilize foam. Heating the aqueous solution activates these tiny micelles. When the bath sits idle, these micelles sit on the liquid surface. This is undesirable for immersion cleaning processes. For immersion, the aqueous cleaning design must remain as a homogeneous solution. This prevents phase separation, which eliminates carryout of active materials when removing the stencil for rinsing.
To meet design goals, engineered cleaning fluid design necessitates a material for spray and one for immersion processes. Both cleaning materials provide a mild cleaning agent specifically designed to wet, dissolve and saponify uncured flux. The materials provide compatibility on yellow metals, aluminum and other precious metals. The materials are suitable for the optimum cleaning temperature range of 100° to 120°F, work at concentration levels of less than 10 percent, have a VOC content of less than 25 g/L and provide maximum safety within the workplace environment.
Automated stencil cleaning equipment transport the stencil from wash tank to rinse tank while processing wash and rinse solutions into and out of a single process tank. Using PLC controller, the operator simply loads the substrate into the carrier mechanism and starts the machine cycle. Two common system designs — immersion ultrasonic and spray-in-air — provide the displacement energy needed to remove solder paste from apertures.
Most stencil cleaners operate with a single chamber used for both washing and rinsing operations. Single chamber systems provide fluid management to remove residual solder paste, rinse and return a dry stencil that is free of contamination. Single chamber units have a tendency to contaminate the rinse, which leads to the need for a cleaning medium with a less active cleaning chemistry. Proper integration provides a simpler process that meets design criterion.
Spray-in-air systems use a rotating arm that creates zones of constantly changing force for improved cleaning performance. The cleaning solution provides wetting, dissolution and saponification. Spray pressure provides impingement energy that displaces the soil from the tiny aperture. Spray pressure, cleaning concentration, cleaning temperature and time represent process variables that define the process window.
Ultrasonic agitation creates high-frequency sound waves that vibrate through a liquid medium. The cavitating liquid forms microscopic vapor pockets and breaks onto the surface of the part. As the vapors break throughout the liquid interface, agitation recesses into tightly spaced configurations. Performance and reliability of the system depend on the transducer frequency and generation design. Wafer bumping stencil foil is thin and fragile, requiring proper frequency.
To address additional environmental concerns and lower cost of ownership, closed-loop ultrasonic stencil cleaning systems offer a design advantage. These systems are fully contained in the solder paste, chemical effluent and water waster, which produce no liquid waste stream. This eliminates the need for drain or an evaporator. The system design ideally integrates with the cleaning chemistry to provide wash-solution filtration that automatically filters the solder paste from the wash solution, lengthening the wash solution's useful life and eliminates the need for a drain connection. To purify rinse water, absorption bed technology removes organic and inorganic particulate. By integrating the chemistry with the machine, the process provides environmental, cost of ownership and cleaning benefits.
Wafer bumping stencil cleaning processes offer an efficient low-cost method of fine-pitch solder deposition. Cleanliness of the aperture places new demands regarding the removal of residual solder paste deposits. There is a need for integrated processes of the cleaning chemistry with the cleaning machine. Repeatable process control requires stencil cleanliness. Cleaning process development hinges on the cleaning chemistry. Fully integrated aqueous cleaning systems offer design advantages when addressing environmental issues and lower cost of ownership.
MIKE BIXENMAN, CTO, may be contacted at Kyzen Corp., 430 Harding Industrial Dr., Nashville, TN 37211. MICHAEL KONRAD, president, may be contacted at Aqueous Technologies Corp., 9785 Crescent Center Dr., Building 302, Rancho Cucamonga, CA 91730.