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Step 9: Cleaning By Steve Stach
December 31, 1969 |Estimated reading time: 11 minutes
Defining and implementing a successful cleaning process involves a five-step process, regardless of the product. The technology can range from simple to sophisticated; however, the step-by-step procedure remains the same. Additionally, volume does not affect the steps — one item or a thousand, each must follow the same process.
The first step involves determining the cleaning purpose — a deceptively simple process. Does a potentially corrosive residue need to be removed or does the cosmetic quality of the assembly just need to be improved? To be successful, the standards that will be used to judge the effectiveness of the process must be identified.
The second step involves finding the appropriate solvent system to remove all residues of concern, preferably, a single solvent system. The simplest solvent system is water. Beyond that, there are water/solvent combination systems that range up to pure solvent. In this step, the goal is to find a system that works for all residues that must be removed from a particular object or assembly.
The third step is selecting a cleaning system. This procedure could be as simple as cleaning with a wipe or as complicated as using large automated, high-volume in-line cleaners, multiple cleaners, and advanced chemistries and technology. What equipment will be compatible with and best use the most appropriately selected chemistry for the cleaning application? Note that when ordering equipment, the pump and filter seals must be compatible with the solvent system selected in step two. The selected cleaning operation's complexity and volume should serve as a guide to a cleaning system that provides an appropriate degree of quality assurance and cost efficiency.
The fourth step involves testing and certifying that the new cleaning process meets the standards identified in step one. For example, if surface insulation resistance (SIR) has been identified as a key qualification to validate the cleaning process, SIR testing should be performed at this point to determine if the cleaning process will meet those standards.
The last step in this process is integrating the process control systems that will alarm operators or management if process parameters change, causing an out of control process. Systems here should be simple, easy to use and show a rapid response to change.
Figure 1. Hand cleaning is preferred for low-volume/low cleanliness requirements.
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A Closer Look at the Five Steps
Step 1: Determine Cleaning Purpose and Applicable Standards. The primary reason to clean is to ensure that products meet market expectations, real or perceived. Effective cleaning improves product reliability by increasing mean time to failure.
A customer may view or regard an uncleaned board as cosmetically unattractive, and therefore unreliable because it does not meet the perceived market expectation of being visually clean to the unaided eye. Industry standards require circuit assemblies to be visually clean at 1X inspection unless otherwise specified. Generally, cosmetics are a customer's first reliability concern.
The second concern is electrical failure due to corrosion or electrical leakage. It is likely that if ionic residues are not reduced below a certain level, they eventually will react with moisture from the environment and cause circuit assembly failure in the field.
A third concern is electrical performance. Many high clock speed circuit boards, especially those that operate in above 1 GHz, are vulnerable to contamination; merely the presence of physical residue can cause problems with circuit performance because of radio frequency (RF) skin effects and mismatched impedances in data transmission lines.
Another process yield issue related to cleaning is testability. There are many no-clean fluxes available that still are not easily probe testable. These materials can cover test pads and gum up test points, causing failure even if they function.
Finally, cleaning can improve the encapsulant and underfill adhesion used to protect circuits from environmental and mechanical hazards experienced in use, transportation or storage. A thin film of organic contaminate can prevent proper bonding or sealing of the electrical device, compromising the product's reliability.
The second part of step one involves investigating the standards necessary to guarantee that manufacturability and reliability requirements are met. The answers come from three areas — customer requirements, internal standards (such as company workmanship standards) and general industry standards.
Step Two: Identify and Select the Appropriate Cleaning Solvent System. Typically, a single-solvent system is preferred, beginning with water — the simplest, most economical and environmentally friendly system. Water, however, can be used only approximately half the time. The other half of the time chemistry-based cleaning, either with or without water, is necessary.
Solvent identification begins with identifying all residues on the board or part, not merely the most obvious. This list will include such things as solder paste type (A, B or C), liquid fluxes, wavesolder fluxes, uncured adhesives and even solder balls. Sometimes, residue removal is not a strict function of solvent action, but rather requires a certain amount of kinetic energy in the cleaning process.
After identifying all contaminants and residues present on the substrate, it is necessary to find the simplest single-solvent system to remove these effectively. It is best to screen potential cleaning agents by simulating the cleaning process in a laboratory environment, whether soaking the product in a beaker of solvent, or using some type of hand spray or immersion technique. The goal is to discover if a solvent system is effective in removing the offending residues and what parameters (time, temperature, physical energy) and methods will be needed to use this solvent system.
Step Three: Select the Appropriate Cleaning System. When selecting a cleaning system, there really are only two choices: hand cleaning or machine cleaning. Under machine cleaning, there are batch and in-line cleaning systems; the deciding factor is volume requirement. With very low-volume and low cleanliness requirements (simple metal parts, boats, etc.), soaking and hand cleaning may suffice (Figure 1). Tougher jobs requiring higher cleaning standards (stencils, boards, assemblies) need process automation and control to ensure consistent results.
In terms of process cleaning parameters, i.e., the key process parameters affecting cleaning, temperature, time and physical action must be attended to and controlled. When selecting a cleaning machine, one should be chosen that will meet volume requirements, provide for control over the key parameters of the cleaning process, as well as offer the correct delivery systems, i.e., spray under immersion, high impingement air spray, ultrasonics, or vapor degreasing.
Step Four: Test and Verify. A test vehicle is assembled containing all required residues that will have to be cleaned. Run it through the cleaning process and then test the vehicle to see if it meets the identified standards (Figure 2). There are four basic tests specified by IPC — two process-checking tests and two qualification tests. In addition to these tests, and meeting IPC requirements, it also is necessary to meet any additional customer requirements that may be required of the cleaning process; thus, one must meet a combination of industry standards and customer requirements in the cleaning process. It also is necessary to recertify the process occasionally.
Step Five: Keep the Process in Control. Most people are familiar with statistical process control (SPC) techniques, defect rate, and yield measurements to keep processes under control, ensure process consistency and produce the highest yields possible. The same techniques also apply to cleaning. In this case, process control systems are integrated into production lines to monitor key process indicators. The items selected to do so vary occasionally; however, they basically default to industry standards and customer requirements.
Figure 2. Once a test vehicle is assembled, all residues that are required to be cleaned must be run through the cleaning process and tested to ensure the vehicle meets the key standards.
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Hidden Cleaning Costs
Once a cleaning operation has been established, keep in mind that the total cost of cleaning involves more than simply investing in technology, i.e., the cleaning machine or equipment. Many factors influence the overall cost including equipment cost, equipment/process efficiency, chemical costs, disposal costs, power consumption, floor space occupied, labor cost, maintenance and more.
Surprisingly enough, the highest cost in the cleaning equation is that of operating the cleaning process — including labor, chemicals and water. These costs typically account for 70 to 80 percent of the total cost of cleaning. Equipment cost is relatively low compared to overall operation cost, particularly the labor of personnel required to operate the cleaner(s). Operating cost, not equipment cost, traditionally has been the most significant element in the cost-of-cleaning model. Conversion to no-clean assembly processes has driven up the costs of cleaning because of an efficiency loss in converting from in-line to batch processes. If overall cleaning costs in a low- to medium-volume batch cleaning environment seem overly expensive or appear to be rising, contract cleaning can be a good alternative.
Steve Stach, president, may be contacted at Austin American Technology Corp., 12201-160 Technology Blvd., Austin, TX 78727-6102; (512) 335-6400; Fax: (512) 335-5753; Web site: www.aat-corp.com.
Lead-free and No-clean: A Contradiction of Terms?
By Andreas Muehlbauer and Helmut Schweigart
Studies on the climactic reliability of various lead-free pastes reveal an increasing threat at ambient conditions to the long-term reliability of lead-free solder joints. Thisarticle highlights the potential quality problems that can be detected and avoided.
Circuit failures due to electrochemical migration in silver-containing solder joints have been documented as early as the 1950s. During this period, the components were wired using soldering pins with spacing usually in the order of centimeters. Also, the first circuits were exposed neither to wind nor weather as they are nowadays, but were operated mostly in closed areas generally not air-conditioned.
Contrary to tin and lead, silver-containing solder forms hydroxides that are readily soluble in water. Formed as a result of electrochemical migration, the hydroxides can be diffusible in thin moisture films at a relative humidity (rh) as low as 60 percent and as such, are inherently susceptible to migration at rh levels above 60 percent (i.e., in the upper moisture range of closed rooms that are not air-conditioned; see figure). Silver also is prone to sulphur-compound reaction (sulphurization), which can cause shorts and interruptions particularly in hybrid circuits. This process is likely at the 60 percent rh level.
Discontinuous Resistance
Climate-resistance measurements revealed the formation of dendrites having brief life spans of 10 to 15 minutes owing to electrochemical migration.1 The tests most likely were conducted under maximum accelerated conditions on uncleaned comb structures at 85°C and 85 percent rh
with a permanently applied voltage of 50 VDC.2 Silver-free, eutectic tin/lead solders used for comparison did not exhibit such a failure pattern. Brief resistance failures have not been observed in the past because the climate tests were limited to discontinuous (interrupted) surface-resistance measurements, as defined in the corresponding standards.
The phenomenon can be explained by the uniform distribution of the silver with a maximum of 4 percent in the soldered joint. The electrochemical migration mechanism only activates the silver close to the surface of the joint. This means that only low concentrations of silver hydroxides are available and, thus, only filigree dendrites with low current-carrying capacity can be formed. The dendrites lower the surface resistance primarily in the final stage of their growth, but then are burnt off by the short circuit so that the original surface resistance is re-established.
The rate of renewed silver supplied to the surface of the solder joint is slower than the attack of electrochemical migration at normal temperatures, which is the probable reason why short-life bridges that never transform into permanent short circuits are formed repeatedly. Nevertheless, this results in inexplicable fault patterns in assemblies that cannot be reproduced systematically or discovered via discontinuous resistance measurements.
Continuous Resistance
New measuring systems perform continuous parallel-resistance measurements of test specimens during air-conditioned storage that clearly can detect short-life dendrite formations. Also, ongoing research, focused primarily on the climactic reliability of lead-free solders vs. their Cu content and level of contamination, aims at clarifying the influence of solder-alloy compositions.3 It is conceivable that a low copper content is sufficient to prompt a shift in corrosion potential. The failure mechanism indicates that even traditional tin/lead/silver solders can be affected. Compared to traditional tin/lead alloy solders, a much greater corrosion-favoring effect exists with silver-based solders because of the high solubility rate of silver hydroxide in moisture films and the low critical rh (60 percent). In particular, carboxylic acid-based activators in solder pastes can result in a steady increase of the absorbed moisture film. Therefore, it is possible that the value of critical humidity could drop below levels typically found in air-conditioned rooms. However, removing hygroscopic residues (carboxylic acid, salts, etc.) through assembly cleaning can contribute to a significant improvement in resistance to elevated humidity levels and to a reduced susceptibility to electrochemical migration.
Conclusion
The introduction of lead-free solders continues adding to the troubled history of silver-containing solder usage and the failures caused by electrochemical migration. Even though the solders have been further developed, the expected demands, particularly with regard to climate reliability, have risen disproportionately. Moreover, the increased sensitivity toward electrochemical migration as well as damage mechanisms rarely observed in the past generally has been the result of the ultraminiaturization of designs and the increased use of lead-free (Cu/Ni/Au) solders.
Higher soldering temperatures and lead-free-joining techniques require correspondingly upgraded cleaning processes. Whether or not cleaning is actually necessary must be verified individually on the basis of expected quality specifications. Moreover, the susceptibility of assemblies to failure is not only determined by electrochemical migration and other corrosion-induced damaging mechanisms, but also by circuit design. It is recommended that the latest climactic reliability information be reviewed. These findings will help avoid costly return of failed assemblies.
REFERENCES
- Tabai Espec, Japan.
- As established by Dr. Wege, et al. at the Technical University, Munich, Germany.
- Work Group for Lead-free Joining, Technical University, Augsburg, Germany.
Andreas Muehlbauer, Ph.D., may be contacted at Zestron Corp., 21641 Beaumeade Circle, Suite 315, Ashburn, VA 20147; (703) 589-1198; E-mail: A.Muehlbauer@zestron.com. Helmut Schweigart, Ph.D., may be reached at Zestron Europe, Bunsentrasse 6, 85053, Ingolstadt, Germany; 49 841 635 29; E-mail: H.Schweigart@zestron.com.