Step-by-Step: Cleaning

Estimated reading time: 12 minutes

By Mike Bixenman

Many see cleaning simply as an added process step. Generally, only high-reliability products are cleaned today. However, the question now becomes: Will cleaning be a reliability driver facing more manufacturers as the demands for increased miniaturization, higher speeds and tighter board space constraints become more urgent? Will industry adopt no-clean or water-soluble technology when assembling with lead-free alloys? Because industry standards in both design and manufacturing must change to keep pace, cleaning may come full circle.

Lead-free alloys will change soldering technology as well as the entire manufacturing process, including assembly materials and components. Additionally, this change highlight questions relating to awareness and dissemination of existing information, materials availability, soldering technology, and implementation requirements. The first step in evaluating the implications of changing to lead-free solders is the accumulation, evaluation and dissemination of as much relevant data as possible.

Alloy Compositions

Many lead-free alloys have been developed and applications for more than 100 patents on various compositions have been filed. Although not all are commercially available, there is a wide range to choose from. The question of which lead-free alloy is the "best" often is asked but difficult to answer, because there is no absolute drop-in replacement for eutectic Sn/Pb solder with identical melting temperature, cost, wetting and strength properties. The usual choice is a lead-free alloy with similar melting characteristics to eutectic Sn/Pb. Alloys that have a liquidus range of 180° to 200°C consist of mixtures of tin, bismuth, indium and zinc. Unfortunately, these possess certain notable challenges. For example, alloys with significant indium levels are very expensive and can be subject to low-temperature phase formation, which may result in solder joint fatigue and cracks. Alloys containing zinc are subject to rapid oxidation; little research is available examining potential corrosion problems associated with this metal.

Figure 1. Examples of flux residues used in the study. Residues are more intense than traditionally seen with eutectic Sn/Pb solders.

Most of the best known lead-free solders, as well as those that have been developed more recently with complex quaternary (four-part) compositions, fall into the mid-temperature range. These compositions generally are combinations of tin with copper, silver, bismuth and antimony. Their liquidus temperature range is 205° to 230°C with peak reflow ranges of 250° to 260°C. The higher temperatures will be incompatible with some materials and components while equipment and processes will need to be modified to accommodate the higher thermals.

High-tin alloys require increased flux activation to remove oxidation layers, and flux technology may require modification. The goal is to provide a low-residue, no-clean flux that is clear or invisible to the naked eye. The flux-activation time/temperature during heating and soldering must be designed to fit lead-free process settings. As a result, the higher tin levels, peak reflow temperature and flux activation will produce more (and unfamiliar) residues.

Lead-free Wave Soldering

Wave soldering with lead-free materials is probably the most demanding of the changes in traditional assembly methods. New flux formulations may be required because of higher preheat and soldering temperatures as well as the need to deal with slower solder-wetting properties. The optimum alloy for wave soldering is still debated. Because bar solder represents a significant part of the wave soldering cost, some have opted for Sn99.3/Cu0.7 (melting point 227°C) as the least expensive alloy. And while others prefer a Sn/Ag/Cu alloy, there are process reservations because of the difficulty controlling a ternary alloy.

The added challenges stemming from higher process temperatures and time are significant. When processing different board designs, such as multilayer, double-sided or single-sided printed circuit boards (PCB), the thinner boards will experience more than the specified thermal exposure if the immersion depth in the solder wave is not optimized. The result may be a degraded ("white") residual flux that is difficult to remove. Rosin and water-soluble fluxes particularly are prone to degradation via excessive heat.

Many wavesoldering processes use water-soluble flux. However, the question remains whether existing in-line cleaning processes using deionized water will remove residue. Generally, the higher processing temperatures required for lead-free processes degrade the water-soluble residues, resulting in additive or saponification chemistry use to remove them. Lead-free processing will drive wavesoldering fluxes to meet the demand for defect-free soldering, and sophisticated chemistry must be used to ensure compliance with industry flux standards.

Surface Mount Reflow

Lead-free soldering's primary impact on SMT assembly will be on the reflow process. Designed experiments will determine the ideal reflow parameters for optimum reliability at the lowest temperature. A significantly hotter oven profile as compared to eutectic Sn/Pb solder will be required. Preheat zone temperature will be higher and exposure times longer to obtain the necessary temperature uniformity across the board at reflow. (The worst-case peak reflow temperature of 260°C is well outside the acceptable maximum for many SMT components.)

Reflow oven performance will receive the greatest scrutiny. Design improvements must reduce the temperature difference across complex boards. The challenge will be compounded by the requirement for faster heating and cooling with higher peak temperatures. Improvements in efficiency also will be needed.

The trend toward high-density components and substrates will be a key concern. Standoff heights and lead pitches are decreasing in size, as is the spacing between line traces. Area-array package size continues to decrease while the number of solder connections increases. The increase in the area-array density decreases the pitch and, hence, the standoff. These changes, combined with higher reflow temperatures, make cleaning challenging.

Oxidized or polymerized flux residues can occur because of excessive assembly heating during preheating and soldering. This will be a key concern when switching to lead-free technology. Residues from the reflow process, even with the same flux packages used today, are greater in lead-free applications. Ball grid array (BGA)-type devices and intrusive reflow show the largest residues. The assembler will need to control heating profiles for assemblies of varying size, density or PCB type.

Temporary soldermask comes in soluble and insoluble compositions. A temporary solvent or water-soluble soldermask can be used to block holes or pads for testing or for later attachment of sensitive or missing components. The higher lead-free processing temperatures can make the residues very difficult to remove. Over-cured soldermask will require aggressive cleaning to remove trace materials and most likely necessitate formulation revisions.

Evaluating Residue Formation and Cleaning

Table 1 represents some leading lead-free solder paste alloys, formulated with no-clean technology. These pastes are stencil printed and assembled on a mixed-technology printed circuit assembly (PCA). The degree of flux residue and cleaning using standard surface mount cleaning processes are evaluated.

The fluxes used in the pastes are composed of different materials, including:

• Solvent is the liquid carrier for the flux ingredients, permiting the flux material's even assembly distribution. During board preheat, the solvent is intended to evaporate.
• Vehicle is a thermally stable material that acts as a weak activator.
• Activator is one or more ingredients in the flux that creates a wettable surface for solder by removing oxides and other contaminants when coming in contact with the alloy.
• Antioxidant is a material that prevents metal surface reoxidation after the activator has prepared them for soldering.

Lead-free alloys require higher levels of activator to remove oxides and to properly wet the surface. As a result, the process window for lead-free soldering is narrower than eutectic Sn/Pb. Alloys containing high tin levels require more aggressive flux formulations. Their residues will be more intense, darker and increasingly difficult to remove. Cleaning processes may require modification. Figure 1 displays examples of flux-residue formations of the four pastes used in this study. The residue level is more intense than traditionally seen with eutectic Sn/Pb.

Cleaning Lead-free Residues

Regardless whether cleaning is performed at the assembly stage, the finished assembly must not contain residues that could result in premature field failure. If a low-residue (i.e., no-clean) soldering process is desired, reliability studies determining the mean-time-to-failure (MTTF) should be performed. If cleaning is not performed at the assembly stage, the incoming parts (fabricated boards) must be free of potentially harmful residues. Also, contamination must not occur during hand soldering or repair.

Cleaning requirements and defluxing ease start with the soldering operation's nature and the components to be assembled. Normally a foamed-fluxed, through-hole wavesoldered assembly would be expected to be easier to clean than a high-density, fine-pitch surface mount assembly (SMA). However, relative cleaning ease could differ if the through-hole assemblies are held for an extended period before defluxing and the SMAs cleaned immediately after assembly. Similarly, BGAs would be expected to be easier to clean than ultra-fine-pitch devices. Overheated assemblies can produce more stubborn residues. Obviously, the more aggressive the flux and the tighter the spacing between leads or tracings, the greater the cleanliness degree required.

Figure 2. Grading scale of residues in the study: 0 (no cleaning) to 4 (completely cleaned).

In addition to the ability to deflux, choosing cleaning media and equipment is determined by numerous factors. Apart from its main job, the system also must meet economic, worker safety and environmental considerations. Local volatile organic compounds (VOC) emissions could determine the cleaning media choice and equipment. Similarly, wastewater regulations may require low-biochemical oxygen demand (BOD) cleaning media. If an assembler cannot safely store and handle flammable materials, nonflammable media would be the preferred choice. Additionally, the cleaning agent must be compatible with the assembly materials and the washing equipment. To understanding lead-free alloy cleaning, some current cleaning options (Table 2) were sampled using standard SMT process parameters.

Aqueous

Aqueous formulations consist of water only or water with additives less than 50 percent total composition. The aqueous chemistries evaluated consist of solvent sprayable aqueous alcohol and low-VOC aqueous formulations. The solvent sprayable alcohol formulation is a proven aqueous cleaning solution designed for removing resin-based and low-residue fluxes from surface mount or through-hole technology. The solution is for use in high-pressure in-line cleaning equipment. Operating concentration is in the range of 20 to 30 percent.

The low-VOC formulation is a low-solids mixture, which is certified for use in California's South Coast Air Quality Management District (CSCAQMD). These cleaning agents, which must contain less than 50 g per liter VOC in their use condition, remove rosin-based and water-soluble flux residues from surface mount and through-hole assemblies.

Semiaqueous

Semiaqueous is the solvent cleaning/water rinsing process. The chemistries evaluated were nonlinear-alcohol and synthetic-alcohol compositions. The nonlinear alcohol blends combine low and mild reactivity, and have proven effective on difficult solder fluxes such as high-temperature and synthetic resins, and water-soluble and no-clean fluxes from hybrid, flip chip, and SMAs.

The synthetic blend contains a branched linear organic alcohol that has an ether linkage. This oxygenated molecule is highly effective on both polar and nonpolar residues as well as low-solids, synthetic-flux resins.

Vapor Phase and Grading System

Vapor-phase cleaning also commonly is referred to as vapor degreasing. The parts are cleaned in the solvent and rinsed in the solvent vapor. An n-propyl bromide (nPB)/alcohol azeotrope was used in the evaluation. This solvent azeotrope has strong cleaning properties for both polar and nonpolar soils and has been proven effective on difficult soldering fluxes such as synthetic resins, organic acid and rosin.

The boards are graded on a scale of 0 to 4 with 0 representing no cleaning performed and 4 referring to a totally cleaned board. Examples of these residues are shown in Figure 2.

Results

Table 3 lists the alloy, cleaning process parameters used and cleaning results. They indicate that residue removal was more difficult when compared with current surface mount processing. Technology development is needed to support lead-free processing.

Conclusion

Changing to lead-free soldering highlights questions relating to awareness and dissemination of existing information, materials availability, soldering technology, and implementation. Moreover, the time scale involved leaves little room for maneuvering. Japanese companies are beginning to market lead-free products, and many intend to convert to lead-free technologies within the next two years.

The leading alloys have higher melting points. The alloy oxide level requires increased flux activator levels, leading to increased residues. Thus, it may be difficult to achieve true no-clean processing. Nevertheless, cleaning is a required additional process development step.*SIA (Spray in air). ** SA (Semiaqueous). *** SUI (Spray under immersion).

WORKS CONSULTED

1. Mike Bixenman, "How Will Electronic Precision Cleaning be Impacted when Adopting Lead-free Solder Technology," SMTA Lead-free Symposium, NEPCON East, June 2000.
2. J. Kloescer, C. Dallmayer, E. Jung, P. Coskina, R. Aschenbrenner and H. Reichl, "Lead-free Solders from Area-array Packaging," Fraunhofer Institute FhG/IZM-Berlin, SMTA Lead-free Symposium, NEPCON East, June 2000.
3. R. G. Robertson and J. Smetana, "Some Fundamental Concerns in Lead-free Implementation," SMTA Lead-free Symposium, NEPCON East, June 2000.
4. P. Biocca, "(Two) Lead-free Alloys for Wave and SMT Assembly," SMTA Lead-free Symposium, NEPCON East 2000.
5. C. Jorgensen, "Lead-free Electronics: Full Steam Ahead," SMT, May 2000.
6. Guidelines for Cleaning of Printed Boards and Assemblies, IPCCH-65A, 1999.
7. W. Boyd, Lead, CleanTech 2000.
8. Lead-free Assembly & Soldering Cook Book, National Physical Laboratory, 2000.

MIKE BIXENMAN is CTO at Kyzen Corp., 430 Harding Industrial Drive, Nashville, TN 37211; E-mail: mike_bixenman@kzyen.com.

ABCs of SMT

Azeotrope: A mixture of two or more polar and nonpolar solvents that acts as a single solvent to remove board contaminants.

Contamination: A foreign material (dirt, oil, etc.) on a component lead or pad that acts to deter solderability.

Dispersant: A chemical added to water to improve its particulate-removal ability.

Halides: Compounds containing fluorine, chlorine, bromine, iodine or astatine. These materials may be part of a solder flux system acting as the activator. The residues are corrosive and must be removed.

Saponifier: An aqueous solution of organic or inorganic base and additives for dispersing and removing rosin and water-soluble flux residues via chemical reaction as a detergent solution.

Solids: The percentage by weight of rosin in a flux formulation.

Surfactant: A chemical added to water to lower its surface tension to improve wetting for cleaning.

Checklist for Designing a SuccessfulLead-free Cleaning Process

1. Design the soldering and cleaning requirements:a. Alloy b. Solder profilec. Nature of flux residued. Manufacturing process (batch or continuous)e. Cleaning requirementsf. Benchmark existing cleaning technologyg. Application lab evaluationsh. Results reviewi. Field verification

2. High-reliability parts that require cleaning:a. Select a cleaning technologyb. Seek services of a chemical supplier who can provide a demo or application labc. Check parts for cleaning process compatibilityd. Determine the environmental impacte. Implement waste management and conservation procedures

3. For a no-clean process:a. Set cleanliness requirements on incoming boards and componentsb. Develop an incoming cleanliness quality control procedurec. Educate and set up procedures for handling assemblies during manufacturingd. Prove out the soldering process that leaves minimal residuee. Control subsequent operations, e.g., inspection, touch-up, rework, etc.

4. Select application equipment:a. How many parts will be manufactured?b. Is the process an in-line or batch requirement?c. Floor space available?d. Is the equipment compatible with the cleaning agent?e. Equipment supplier to run parts in selected machines and chemistryf. Evaluate consumption, ventilation, chemical isolation, time in wash, rinsing and drying to ensure time allotted is met

5. Define testing required for qualification, control and monitoring:a. Test methodsvb. Equipment requiredc. Personnel requiredd. Outside services required

6. Conduct pilot run:a. Optimize process conditionsb. Qualify the processc. Optimize equipment featuresd. Issue process/material specificationse. Optimize waste-management proceduresf. Train production personnelg. Optimize conservation procedures

7. Implementation:a. Customer approvalsb. Vendor support