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Step 8: Cleaning
December 31, 1969 |Estimated reading time: 10 minutes
Many factors drive the decision to clean electronics assemblies. For those that function in harsh or uncontrolled environments, cleaning residue is a common standard. This article discusses the issues driving electronics assembly as they relate to cleaning.
By Mike Bixenman
Forces driving SMT cleaning are centered around cleaning product performance for the specific soil; air, water and solid waste environmental concerns; health and safety; residue left from low-residue soldering elevating end user reliability concerns; cost of ownership; and the conversion to lead-free alloys. Engineered cleaning fluid design must address all of these.
New systems that provide high levels of reliability at reduced cost drive demand for electronic devices. An understanding of changing applications for electronics assemblies and the resulting impact on system design is a prerequisite to understanding the drivers for new technology.
Electronics assemblies require cleaning for reliability, aesthetics and contaminant removal prior to the next process step, as well as prior to coating. The key selection criterion is a cleaning material that performs as designed to perform. Suppliers must innovate next-generation cleaning materials to meet industry demands.
Industry Drivers
Product Performance. Since flux serves multiple functions for reflow and wavesolder applications, the ingredients must be considered from a cleaning perspective. Generally, fluxes used for solder paste comprise resins, activators, solvents and rheological additives. For certain special systems, additives such as tackifiers, surfactants or corrosion inhibitors also may be needed.
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Flux is the primary soldering aid. During soldering, the flux removes metal oxides as well as other surface tarnishes such as grease or metal carbonates, allowing the coalescence of solder powder and parts wetting by the molten solder. The second function of flux is as a vehicle for solder powder. The rheology of the flux vehicle must provide not only a stable suspension of solder powder in this vehicle during storage and handling, but also a solder paste that can be handled by paste deposition equipment. Additionally, solder paste rheology needs to sustain the subsequent reflow process without slumping and bridging issues. With properly formulated solder paste, the material can be homogeneous, thus allowing the composition of the mixture to be consistent from point to point during paste deposition. Cleaning material suppliers must study the leading flux materials to ensure consistent cleaning performance over a wide range of products.
Peak soldering reflow temperature also impacts cleaning performance. Optimizing the thermal profile to reduce peak temperature and time to the lowest level possible will improve cleanability. Hand soldering commonly is troublesome. Lead-free soldering likewise presents challenges due to higher reflow temperatures for high-tin alloys. When cleaning is needed, these processes require close attention, as well as work to optimize the process conditions.
Environmental. Demand for environmental properties is related directly to governmental and public concern with clean air, water discharge and solid waste. The table presents some of these concerns.
Generally, there are two types of air emissions from a cleaning process: volatile organic compounds (VOCs) and water vapor. VOCs are generated from cleaning with organic solvents and aqueous cleaners that contain organic solvents. In certain areas, VOCs are highly regulated because of their photochemical reactivity with nitrogen oxides, producing smog. The worldwide trend is toward increasing stringency of VOC regulations.
In the United States, discharge of the wastewater stream, both by total flow (gallons per day) and by chemical makeup, likely will be regulated by the local public-owned treatment works (POTW) under the Clean Water Act (CWA). It is important that any new or additional wastewater flow from a post-solder cleaning process step be evaluated prior to wastewater discharge.
Disposal of spent or waste-cleaning chemicals is regulated as well. Spent cleaning chemistries, filter cartridges, activated carbon or others must be analyzed and characterized by someone knowledgeable in hazardous waste generation, storage and shipment regulations. In the United States, the federal Toxic Characteristic Leachate Procedure (TCLP) test is the minimum requirement. Applicability of the hazardous waste rules subject a facility to extensive regulations on personnel training, storage/handling requirements and record keeping.
Health & Safety. HMIS is a complete program that helps employers comply with OSHA's Hazard Communication Standard (HCS). The program uses a numerical hazard rating system, labels with colored bars, and training materials to inform workers of chemical hazards in the workplace. Personal protective equipment information gives employees the tools to protect themselves from hazardous materials on the job.
OSHA recently confirmed the acceptability of HMIS as an in-plant hazard communication tool. In the preamble to the 1994 revised HCS, OSHA indicated that this type of system continues to be an acceptable means of complying with the standard.
Figure 1. The data represents the cleaning agent1 at 9 to 10 percent run in the aqueous inline cleaning equipment2 inline. The red dot represents the mean cleaning performance on water-soluble fluxes
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Figure 2. The data represents at 9 to 10 percent run in the cleaning agent1 inline. The red dot represents the mean cleaning performance on the rosin-based fluxes in the test matrix.
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Figure 3. The data represents the cleaning agent1 at 9 to 10 percent run in the aqueous inline cleaning equipment2 inline. The red dot represents the mean cleaning performance on the low residue synthetic fluxes in the test matrix.
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Residue left from low residue soldering elevates end user reliability. The two types of remaining residues from the soldering process are ionic or non-ionic. Ionic residue conducts electrical current, and non-ionic residue acts as an insulator. These residues can cause malfunctions if they are not cleaned from the assembly.
High-reliability applications in the medical, automotive, telecommunications, computing, aerospace and military industries continue to be drivers for cleaning. In products where continued high performance or performance on demand is critical, equipment downtime cannot be tolerated and the end-use environment may be uncommonly harsh, cleaning will continue to be a driver.
Cost of Ownership. Factors that play into the cost of ownership model are operating temperature, concentration, and cost of the cleaning agent, bath life, energy and water conditioning.
Lower operating wash bath temperature will reduce exhaust losses, internally lowering operating costs. Lowering the operating temperature from 150° to 140°F may reduce evaporative losses significantly. This, in combination with a stack demister, will improve recovery and cut operating costs.
Cleaning chemistry concentration is important, but not nearly as much as operating temperature. Many cleaning compositions are formulated with low-vapor pressure materials that will evaporate at a lower rate than water. Therefore, operating costs are more closely tied to operating temperature. Secondly, cleaning solutions that clean with solvency consistently provide longer bath life.
Lead-free Soldering. Cleaning flux residue of lead-free solder pastes is more challenging than that of Sn/Pb systems. Primarily, this is because of higher reflow temperature, higher flux capacity, higher flux-induced side reactions and more tin-salt formation. Water-soluble flux technologies are more difficult to clean due to the higher soldering temperatures, and may require an additive with water to remove all residues.
Initial cleaning studies reveal that longer resident time in the cleaning chemistry may be required. Additionally, higher cleaning chemistry concentrations may be needed. Current cleaning technologies have been improved over the last five years. These materials exhibit good performance on the flux subsets tested to date.
SMT Cleaning Fluid Restraints
Challenges faced when designing SMT cleaning fluids that meet industry drivers are performance over a wide range of fluxes and meeting the 25 g/liter VOC constraint. Many leading cleaning chemistries clean the majority of the flux packages, but some are incompatible with the flux chemistry. Additionally, designing a fluid that will clean all these flux packages and still meet the 25 g/liter VOC constraint compounds the task.
Saponification has been a common aqueous cleaning process by chemically reacting with the acidic flux to form soap. This is purely a chemical process involving a neutralization reaction. The saponifier agents in use may be organic or inorganic. The commercial saponifiers may contain some organic solvents or surfactants to assist in removing the non-saponificable residues. Saponifier formulations, without proper inhibitors, may attack some metal surfaces, resulting in a dull solder joint. Effects can be magnified with higher temperatures and exposure times.
The organic saponifier often is composed of alkaline amines. Some common limitations are the potential attack of the solder joint, chemical oxidation that reduces bath life, limited effectiveness on some of the new flux compositions, and a contributor to the VOC content.
The inorganic saponifier addresses the VOC concern by using salts that do not volatilize at elevated bath temperatures. The key limitation is water solubility at operating temperatures. The inorganic alkaline saltwater saturation point typically decreases with temperature and concentration. This causes the salt to drop out of solution, with the net effect being a harden scale.
Second- and third-generation cleaning fluids address the limitation of saponifiers. These materials use stable organic alcohols that dissolve or solvate with mild saponification. These cleaning fluids expand the performance window, lower the pH, increase the bath life and allow multiple passes without solder joint attack. Because these materials contain organic solvents, they fail to meet the 25 g VOC constraint. Therefore, the challenges facing SMT cleaning material suppliers are the development of cleaning agents that meet the performance criteria, long bath life, low salt levels that contribute to scale and VOC compliance at the 25 g per liter level.
Generation Four SMT Cleaning Agents
Some of the newer flux packages present significant cleaning challenges. In some cases, the trade offs outweigh the benefits, with the net result being a part that is not cleaned. But in most cases, the performance objective can be accomplished while still being cleanable. New fourth-generation cleaning agents perform well on the vast majority of these flux materials and meet the stringent 25 g per liter limitation.
Today's "design for the environment" cleaning formulator thinks within the realm of an engineered cleaning fluid that performs well with less chemistry. The cleaning fluid must be balanced properly with ingredients that provide solvation, mild saponification and stability. Formulating with these objectives provides the balance needed to meet the constraints. The net result is innovative technology that meets the design criteria; performance, environmental, health and safety; cleaning under low standoffs; cost of ownership; and next-generation lead-free cleaning.
Performance. There are hundreds of flux formulations under three classifications: water soluble; rosin (R, RA and RMA); and synthetic low residue (no-clean). The synthetic low-residue fluxes present the most difficult cleaning challenge. Figures 1, 2 and 3 represent a useful graphical method of summarizing the performance data with the fourth-generation design for the environment formulation. The right and left of the box are at the third and first quartiles. Therefore, the length of the box equals the interquartile range (IQR), and the box itself represents the middle 50 percent of the observations. The vertical line inside the box indicates the location of the median. The point inside the box indicates the location of the mean. Horizontal lines are drawn from each side of the box. They extend to the most extreme observations that are not further than 1.5 IQRs from the box.
Data were generated using aqueous inline cleaning equipment1 and a cleaning agent. 2 Key process parameters were a belt speed of 2 ft/minute, cleaner concentration of 9 to 10 percent, and temperature of 125°F. At these process parameters, the cleaning chemistry meets and exceeds the design for the environment criterion.
Environmental. To be a designed-for-the-environment clean air solvent, the formulator must achieve maximum performance with less chemistry. This requires a cleaning agent that has less than 2.5 percent VOC organic content at use conditions. Additionally, the solvent shall not contain ozone-depleting solvents, global warming substances or hazardous substances.
The DFE fourth-generation cleaning material meets this criterion. The material also has a near neutral pH, no flash point, a low/no chemical oxygen demand, and is odorless.
Health & Safety. The HMIS rating for the DFE fourth-generation cleaning material is Health = 0, Flammability = 0 and Physical Hazard = 1. When formulating with less chemistry, the intrinsic benefit is fewer hazards.
Cost of Ownership. An interesting feature of the DFE fourth-generation cleaning material is its effectiveness at lower operating temperatures and wash bath concentration. Both features reduce consumption. When the bath solution is maintained properly, data have revealed that extended bath life can be expected.
Conclusion
DFE clean air solvents require formulators to design cleaning agents that meet performance standards with less chemistry. This requires a balanced composition of materials that are targeted at a broad range of soils. Product performance in a changing environment is of key importance. If the cleaning agent has all the attributes for the environment but cannot remove the soil, its success in the marketplace will be short lived. Some process parameters may need to be adjusted to achieve desired performance criterion. By achieving maximum performance with less chemistry, environmental, health and safety, and cost of ownership goals can be achieved.
1 Electrovert A2002 Aquanox A4512
Mike Bixenman, CTO, may be contacted at Kyzen Corp., 430 Harding Industrial Dr., Nashville, TN 37211, (615) 831-0888, (615) 831-0889; Fax: (800) 845-5524; Web site: www.kyzen.com