Cleaning Guidelines for Fine-pitch Stencil Applications

Estimated reading time: 14 minutes

Recent advances in fine-pitch technology have presented many new printing-related challenges.

By Mike Bixenman and Charlie Pitarys

Stencil cleaning has taken an increasingly important role in surface mount and through-hole technology. Fine- and ultra-fine-pitch parts, together with other advanced packages, have put significant new demands on stencil cleaning. To achieve consistent high quality and reproducible levels of precision in printing fine pitches, the stencils must be free of solder paste residues.

Paste volume is a critical issue. Insufficient and inconsistent solder paste during stencil printing usually is the root cause of solder joint defects following reflow. Clean stencils, on the other hand, are key in delivering correct volumes of material to the printed circuit board (PCB) pads. And because adhesives also may be dispensed via the printing process, assemblers prefer to have one cleaning method suitable for removing both solder paste and adhesive.

Figure 1. A chemical etch is used for SMT, a laser-cut for fine-pitch and electropolishing for wafer bumping.

Chemistry RequirementsModern stencil cleaning chemistries must be practical, effective and safe for workers as well as the environment. They also must be able to remove a wide range of solder paste and flux residues, uncured adhesives and ionic salts on the A- and B-sides of misprinted assemblies. Stencil borders must be compatible with the cleaning parameters of temperature, time, mechanical energy and cleaning chemistry. The border consists of a polyester fabric, which is epoxy-laminated to the frame. Temperatures exceeding 130°F will cause the laminate to soften, rendering the stencil defective. Additionally, the temperature coefficient of expansion between the aluminum frame, stainless steel foil and polyester fabric border could distort the fine-pitch apertures if they are subject to a lengthy high-temperature cleaning process. The effects of chemical incompatibility also can be exacerbated at higher temperatures, prompting concern over adhesive tapes now emerging in new stencil designs.

Understencil WipingReliable high-yield paste deposition for fine- and ultra-fine-pitch devices demands stringent automated control over all materials and equipment. Stencils require in-process cleaning to assure accurate paste height and volume deposition. Automated systems using a safe solvent and line-free paper provide hands-off cleaning of the stencil's bottom side, and can be fitted with a paper-over-plenum cleaning option. This system feature reduces printing-related defects by filtering particulate within the paper while a porous solvent bar wets the paper through osmosis, reducing residual cleaning solvent.

An effective solvent is one that dissolves the flux and binders in the paste and has a flash point above 110°F. The solvent bar applies a controlled amount of solvent across the width of the paper, and it is important that the paper and solvent properties be matched to minimize both solvent wicking across the paper and solvent consumption. Once the solvent has been applied, the programmable vacuum system assists in removing the solder paste from stencil apertures, eliminating opens on final assembly.

There are several contributing factors that determine the stencil profile type required and when and how often underside stencil cleaning is required. Wipe frequency generally is determined by a combination of variables, including stencil type, solder paste, PCB substrate coplanarity and printer set-up. Fine-pitch, high-density stencils are laser-cut and, in many cases, electropolished to provide a smooth finish; they require understencil wiping to maintain high yields. Figure 1 illustrates aperture profiles. A chemical etch is used for standard SMT processing, laser-cut for fine-pitch technology, and E-fab (electropolishing) for wafer-bumping technology.

Figure 2. Examples of irregularities on the board surface that can prevent the stencil from gasketing to the pad.

Stencil alignment to the PCB, and proper aspect ratio of the aperture to the pad help ensure proper gasketing and reduced bleed-out. The aperture size should be 20 percent less than the pad to maintain a minimum aspect ratio of 1.5 width to stencil thickness. Even with the proper stencil design and correct paste selection, there still can be several PCB-related problems, which can cause solder paste bleed-out, requiring more frequent underscreen cleanings. Figure 2 shows examples of irregularities on the PCB surface that prevent the stencil from gasketing to the pad.

Wiping Example: Wafer BumpingUsing stencil printing for wafer bumping offers an attractive, cost-saving alternative to high-volume production. In the few seconds it takes for the print head to distribute solder paste across a stencil, hundreds of thousands of pads can be bumped simultaneously. The stencil contains thousands of very small, closely spaced apertures in extremely tight dimensional and positional tolerances.

Laser-cut stencils, which are electroplated to provide smooth tapered walls, are used for this technology. Stencil foil thickness is important for determining the opening size needed to achieve the required paste volume and the target reflowed bump height. Thicker stencils are attractive because the openings need not be as large to achieve the same aperture volume requirements as required with thinner foils. However, paste-transfer efficiency suffers with thicker stencils because more paste adheres to the aperture walls.

Some more important issues to be considered for successful wafer printing include solder paste, cleaning options, printing environment, printer machine settings and reflow profile. The lowest pressure for wiping the paste cleanly across the apertures during a printing stroke is recommended. To achieve the most uniform printed deposit distribution, it is best to start with a clean stencil and a slow separation speed to gain the highest transfer efficiency and to obtain finer print definition.

Stencil cleanliness is critical to bumping process success. The combination of tacky solder paste encompassing small particles and tiny stencil apertures can reduce the transfer ratio of the extruding paste. Even after a single printing stroke, the stencil apertures can accumulate a significant lining of paste residue, which may dry quickly and contaminate the depositions of subsequent printing strokes. For this reason, thorough stencil cleaning between each print is recommended. Understencil wiping using lint-free cloths with a solvent is advised.

Process ContaminantsUncured Solder Paste. The residue encountered on stencils generally is the solder paste itself. The advent of low-residue (i.e., no-clean) solder paste has caused a dramatic shift in flux technology among solder paste manufacturers. Whereas optimum cleaning results are achieved when the cleaning chemistry is matched to the residue, the residues from solder pastes are more complex because of the incorporation of various thixotropic control and tack-time increasing ingredients. As a result, one chemistry will not fit all processes.

As no-clean technology has evolved, the challenge was to design a solder paste to print very-fine-pitch patterns, achieve long stencil life and compatibility with the new solder pump technology emerging from various printer manufacturers. The solder paste had to include very low flux volume that was active enough to ensure effective solderability.

Thixotropic material design to hold the powder and flux in composition for efficient printing is benign in nature to be left on the board. (Bellcore and other standards are available to ensure this.) Thixotropic materials, however, create an added difficulty when removing no-clean solder pastes in both raw and reflowed conditions.

Reflowed No-clean Flux Residue. As the industry successfully moved toward no-clean technology, manufacturers spent considerable effort removing cleaning from the assembly process. Cleaning the primary side of an assembly if the secondary side was misprinted was an area commonly overlooked. The cleaning process must be able to remove all reflowed residues from the primary side as well as the misprinted paste. This is more difficult than any of the traditional stencil and "A-side" cleaning challenges because the no-clean residues usually are not visible after the reflow process but, indeed, are there. The elevated temperatures of the cleaning process alone, without the effect of the cleaning agents, are enough to turn the clear residues opaque or white.

Many no-clean pastes use paraffin-like materials to improve tack times and to provide the "ideal" tall bricks of printed paste. If a PCB manufacturer has purchased equipment dedicated to ultrasonic energy, hand cleaning will be required. If the equipment is of an atomized spray design, considerable pressure and flow may be needed to remove the reflowed residues. Because these options typically were not taken into account when the equipment was purchased, removing reflowed no-clean flux residues from a B-side misprint must incorporate a cleaning chemistry capable of removing this difficult residue. This task generally requires a cleaning agent with a high degree of solvency rather than an inorganic, high-pH blend. High solvency, together with good surface activation, will remove the A-side residues without ultrasonic agitation or extremely high pressures typically absent within most batch systems.

SMT Adhesives. Surface mount encompasses two fundamentally different manufacturing processes: solder paste and adhesive use. Bottom-side components may be placed before wave soldering using an adhesive to prevent them from falling into the solder pot.

Adhesives can be applied to the PCB using one of several methods: stencil printing, pin transfer or syringe dispensing. Those used for stencil printing typically are epoxy-based with a single heat-curing system. Stencil printing provides a rapid dispensing technique. Although less than two percent of adhesives presently are applied this way, interest in this method has increased. Assemblers using this technique generally use existing equipment to clean stencils and misprinted PCBs.

Because aqueous cleaning chemistries are ineffective for removing epoxy adhesives, cleaning-chemistry design requires advanced innovation to address this requirement. As stated earlier, solvency is key. Solvent-stable emulsions have proven effective in managing both pastes and adhesives.

Solder Balls. Another area commonly overlooked is the equipment's capability to remove solder balls from the process tanks and chamber. This is not as easy as it sounds. Centrifuge and in-line filtration are common methods used in this task. But in a dedicated stencil cleaner, this is not necessarily an issue. Rather, equipment for cleaning misprints (or especially, double-sided secondary-side misprints) is a major concern, i.e., the equipment's capability to filter solder paste from the process chambers and spray manifolds.

Owing to greater applications of double-sided reflowed assemblies, solder balls must be removed from the process chambers lest the high pressure and high flow created by the pumps "whip up" the solder paste and redeposit it on the PCB. This can create production packaging problems and field reliability issues.

Paste Removal from Stencils and MisprintsRemoving uncured solder paste from stencils and misprinted assemblies may take place using a hand-wipe, immersion or spray-in-air process. Each has inherent benefits and tradeoffs:

Lint-free Wipes. Presaturated lint-free wipes use a wide array of cleaning solvents to address most contaminants. Wipes quickly remove uncured solder paste and adhesives with relative ease. Key advantages are low cost, controlled solvent application, contained waste and ease of use.

There are many tradeoffs that must be considered, however. As lead pitches become finer, print quality must be improved. Lint-free presaturated wipes will not provide consistent solder paste or adhesive removal from fine-pitch apertures. If the solder paste dries in the aperture before re-using the stencil, misregistered boards will result. Using these wipes for removing solder paste from a misregistered board also is discouraged. The risk is spreading the paste over the board's surface with the potential of depositing solder balls in vias and in the areas between the solder pads and the surface coating. Removing solder balls trapped in these areas can lead to an extremely difficult cleaning application.

Immersion. Ultrasonic agitation, coupled with an aqueous detergent process, is viable for cleaning fine-pitch stencils and misregistered boards. Impingement energy must deliver the cleaning solution effectively to remove contaminants from apertures and etched-back areas of fine-pitch stencils. Aqueous detergents may be used at low concentrations and temperatures, protecting the stencil from delamination and expansion. Solder paste will drop to the bottom of the cleaning tank, helping prevent redeposition on cleaned surface areas.

A limitation of ultrasonic immersion systems emerges when additional cleaning is required. Here, aqueous cleaning detergents are not very effective in removing epoxy-based materials. While ultrasonic agitation in combination with detergents will remove the adhesive from the surface, it also will cause the material to ball up and float to the top of the wash solution or remain suspended in a still bath. This incurs possible redeposition on the surface of a stencil or misregistered PCB. A second limitation occurs during the removal of reflowed flux residue from the B-side of a misprint. These limitations can be overcome in the cleaning chemistry design, which must sufficiently emulsify the adhesive and remove no-clean flux residues. The cleaning chemistry must be selective to the process contaminants.

Spray-in-Air stencil cleaning systems are designed for solvent, semi-aqueous and aqueous cleaning chemistries. These systems typically use a single chamber for washing and rinsing. A rotating wand provides spray impingement to the surface of the stencil or assembly. Selecting the right chemistry will provide a process capable of removing uncured solder paste, adhesive and reflowed flux residue.

Spray-in-air systems typically feature subsystems to filter out solder balls to prevent their redeposition. It is important to select the "right" filter capable of removing the highest percentage of solder spheres from the bath. As with most processes, there are efficiencies in filters and filtration systems.

Like ultrasonic immersion systems, spray-in-air systems carry inherent tradeoffs that must be considered. For example, single-chamber systems tend to create chemical "drag-out" problems (wash solution entering the rinse tank). This is most commonly an issue when the process chamber is large and when common plumbing exists between the wash and rinse. Chemical drag-out makes close looping of the stencil cleaner economically difficult. Accordingly, for aqueous systems, an open-loop final rinse to drain or an evaporator is recommended.

Cleaning Chemistry OptionsThe ideal approach is to have a single, environmentally friendly chemistry that is effective in removing all the contaminants in question, is compatible with the cleaning equipment and substrates, and is cost effective. It is recommended that the process engineer focus on the cleaning chemistry for removing the contaminants in question:

VOC Compliant. This technology uses detergents enhanced by inorganic builders. At suggested-use concentrations of 3 to 10 percent, these cleaning agents are effective on most uncured solder pastes. Extensive trials with various solder pastes have been performed using both immersion ultrasonic agitation and spray-in-air processes. Compliant cleaning agent technology wets the solder paste, dissolves the resin binder into the cleaning solution and allows solder balls to dislodge from the surface. These solutions work at a temperature range of ambient to 125°F.

However, when running in spray-in-air equipment, temperatures of 125°F are required to eliminate foaming issues. This chemistry is limited by its moderate capacity to remove reflowed, no-clean flux residues and adhesives. If an A-side misprint requires cleaning, the reflowed flux residues on the B-side may yield incomplete removal that can result in noticeable white residue on the PCB.

Solvent-continuous Macro-emulsion technology removes the broad range of soils (i.e., uncured solder paste, reflowed no-clean flux residues, adhesives, solder balls, etc.) encountered in today's stencil cleaning processes. To achieve a truly aqueous process, the cleaning chemistry must be designed to use phase solution. When agitated using ultrasonic or spray-in-air processes, the phase solution intermixes to form a solvent-continuous macro-emulsion.

Why is a solvent-continuous macro-emulsion required? SMT adhesives are epoxy-based materials that are insoluble in aqueous detergents. To make the adhesive soluble, an inverse solvent emulsion is formed, wetting the adhesive with a solvent and dissolving it. The surfactant in solution couples the solvent with the water matrix. This novel approach provides a hybrid cleaning solution that acts like a solvent in an aqueous phase.

Figure 3. Schematic representation of a surfactant molecule, characterized by a polar head and a non-polar tail.

Solvent-continuous emulsions begin with surfactant chemistry, which is a surface-active agent that migrates to surfaces and interfaces with complex organic chains to create new molecular surfaces. They are characterized by having two functional groups: a polar head and a non-polar tail. Figure 3 shows the general representation of a surfactant molecule.

Figure 4. With solvent-continuous macro-emulsion technology, solution may be used in batch ultrasonic and spray-in-air processes.

The product design requires a stable micro-emulsion to prevent a spontaneous separation when shipped. Upon diluting and elevating temperature to about 105°F, the cleaning solution forms an inverted macro-emulsion that tends to separate into two phases. The solvent-continuous phase wets the "oil-loving" soils (adhesive and reflowed flux residues), permitting their removal within the existing stencil cleaning process (Figure 4). This unique approach meets most design criteria. The cleaner operates at a temperature that does not affect the stencil laminate, yet removes the adhesive, uncured solder paste and reflowed no-clean solder pastes. The solution may be used in batch ultrasonic and spray-in-air processes (it has a flash point of none to boiling). Limitations include VOC noncompliance within California, a tendency to generate foam in spray applications (at process temperatures below 105°F), and cleaning performance on adhesive and reflowed flux residues will be subpar below 105°F.

ConclusionStencil printing fine- and ultra-fine-pitch requires that the stencil's bottom side be cleaned in process. This is needed to prevent small amounts of solder paste from drying and accumulating residue around the base of the aperture. Using lint-free rolls with a specifically designed solvent will break down the paste and permit its removal.

Many surface mount processes have adopted no-clean technology; most successfully have eliminated process cleaning equipment. Stencil cleaning processes have been asked to take on additional tasks of removing solder pastes and adhesives from misprinted assemblies. This has transformed a simple process of cleaning unreacted solder paste from stencils to a more complex, multi-task cleaning procedure. With the wide range of variables associated with the printing process, application-specific cleaning agents have been designed for stencil cleaning. Accordingly, selecting the cleaning solvent to meet the requirement should be verified carefully for its benefits and tradeoffs before selecting equipment.

This article originally was presented at APEX 2001.

For a list of reference sources in this article, contact authors MIKE BIXENMAN and CHARLIE PITARYS at Kyzen Corp., 430 Harding Industrial Drive, Nashville, TN 37211; (615) 831-0888; Fax: (615) 831-0889; Web site: www.kyzen.com.