Essential Electronics Assembly Cleaning
December 31, 1969 |Estimated reading time: 8 minutes
By Jade Bridges and Amanda Stuart, Electrolube
Requirements for high performance and reliability are stronger than ever. To achieve good insulation resistance, cleanliness of electronics assemblies is essential. Contaminants include flux, solder, adhesives residues, and generic materials.
Cleaning is an essential process within electronics manufacture and has been used for many years to remove potentially harmful contaminants during PCB manufacture. Such contaminants include flux, solder, adhesive residues, and generic materials such as dust and debris from other manufacturing processes. The purpose of cleaning in the rapidly expanding electronics industry is to improve product lifetime, essentially by ensuring good surface resistance and preventing current leakage that could lead to PCB failure. This developing market sees modern and future electronics becoming increasingly smaller. Requirements for high performance and reliability are stronger than ever. To achieve good insulation resistance, cleanliness of electronics assemblies is essential. This can only be achieved by manufacturers of fluxes/adhesives, cleaning chemicals, and cleaning equipment working together with electronics engineers.
When to Clean
There are many stages where cleaning is required: prior to stenciling and soldering a PCB, removing contaminants from the many previous production stages; after stenciling to remove excess adhesive; and after soldering to remove corrosive flux residues and any excess solder paste. In industry today, many manufacturers are turning to no-clean processes, implying that cleaning is not required after soldering. In the no-clean process, the flux’s solids content is lower than traditional types; however, these formulae still contain rosin and activator that are not removed prior to the next process, such as coating or encapsulation.
Figure 1. An operator places PCBs into a benchtop cleaning system.
Such residues – along with any other unwanted elements collected due to the missed cleaning stage – could cause issues with adhesion and possibly affect the performance of the protective media applied. It therefore can be stated that, even with advances in new technologies, such as no-clean fluxes, cleaning remains an essential multi-stage process. There also are cleaning stages required to remove coatings and adhesives when rework is necessary, for cleaning of actual components, and for periodic or emergency maintenance of the production line.
Cleaning Technologies
Two main categories of cleaner dominate the electronics assembly market: solvent- and water-based. Traditionally, solvent-based cleaners such as 1,1,1-trichloroethane and 1,1,3-trichlorotrifluoroethane dominated the market; however, due to their ozone-depleting potential, they were replaced by a more diverse range of solvent cleaners. This category now typically is divided into three subsections: flammable solvent cleaners, non-flammable solvent cleaners, and non-flammable halogenated solvent cleaners such as HFCs and HFEs. All three types have their advantages and disadvantages. Overall, solvent cleaners can be described as fast-evaporating, single-stage cleaners. However, they require specialized equipment and extraction to protect against toxicity and other possible hazards.
Figure 2. Solvent-based cleaners are available in diverse formulations.
Water-based cleaners also were developed to replace ozone-depleting chemicals and reduce solvent emissions during the cleaning process. Water-based cleaning has several advantages over solvent systems, including non-flammable properties, low odor emmission, low/non-VOC contents, and low toxicity. There are many applications for cleaning, all of which depend on the type of equipment available. Water-based cleaners tend to be more complex than their solvent-based counterparts. Whether it be ultrasonic, spray under immersion, or washer-type equipment, identifying the correct water-based cleaner for the specific job at hand is an essential task.
Water-based cleaners use surfactant technology to assist in removing contaminants from PCBs, reducing interfacial tensions and suspending or emulsifying them in solution. Alternatively, water-based flux removers work by saponification, neutralizing flux acids. The major disadvantage of water-based cleaners is that they require multiple stages to complete the cleaning process, including a two-stage rinse process and a final drying stage. A newer type of surfactant-free water-based cleaner is based on glycols. These cleaners combine the advantages of water- and solvent-based cleaners with minimal required rinsing.
Contaminants
With the cleaning market evolving to meet the demands of industry expansion, it is important that the level of cleanliness required is defined clearly. A significant portion of potentially damaging flux residues and contaminants are not visible to the naked eye or even with the aid of magnification. The correct method must be used to determine that the level of cleanliness achieved meets the standards specified by the electronics engineer. There are two types of residues – ionic and non-ionic – and a number of methods can assess the level of contamination after cleaning and accurately describe what is clean.
Image courtesy of CC Hydrosonics, a Crest Group Company.Figure 3. An aqueous cleaning system can require more steps than solvent-based, but has advantages.
Non-ionic residues include rosin, oils, and grease. They are non-conductive and usually organic species that remain after board fabrication or assembly. Their insulative properties cause problems where plug-in contacts or connectors are placed on assemblies. These residues contribute to poor soldermask, conformal coating, and potting compound adhesion, and also encapsulate ionic contaminants and various kinds of foreign debris.
Ionic contaminants typically are flux residues or harmful materials left behind after soldering. Water-soluble organic or inorganic compounds can disassociate in a solution as charged ions, increasing the overall conductivity of that solution. They degrade electronic components’ and assemblies’ reliability by contributing to current leakage between the circuitry, causing corrosion and promoting dendrite growth. While ionic and non-ionic contaminations both impact the operation and reliability of the device on which they are present, ionic contamination accounts for the larger proportion of failures.
There are several methods available for monitoring levels of ionic and non-ionic contamination. The simplest method, suitable for both, is a visual inspection. Although this does not provide quantitative data, it should be used along with other methods. Magnification at 10 to 15× should be sufficient for quality purposes and will provide information on production processes, including handling and packaging and their contributions towards contamination.
Other than optical inspection, there are no simple methods of measuring non-ionic residue. Fourier transform infrared spectroscopy (FTIR) is a widely used analytical method for efficiently determining the precise contamination identity. High-performance liquid chromatography (HPLC) and UV-vis spectroscopy can be used to identify residual rosin. Scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis, and Auger analysis also suit determination of residues and contaminants on a PCB.
Each method has its own specific advantage. The equipment required to set up these types of experiments is costly and requires extensive maintenance; therefore, these rarely are used in electronics production environments.
A common method of determining the degree of ionic contamination is to measure resistivity of solvent extract (ROSE), also known as solvent extract conductivity (SEC). The theory of ROSE is that, as the concentration of ions in a solution increases, the resistivity decreases. Simple automated versions of ROSE testing are used by a number of electronic assembly houses for quality control (QC) testing.
The industry-standard test IPC-TM-650 employs a solution of isopropanol and deionized (DI) water to extract the contaminants while the meter measures the change in conductivity. This type of testing is generally accepted and offers rapid results, but it can be restrictive.
Originally designed to test residues from traditional rosin-based fluxes and use a cheap, readily available solvent (the solvent IPA), the scope of this method now is somewhat outdated and may not alert users to possible changes that result in non-soluble residues.
Evolving Standards
The change in accepted cleanliness levels also highlights the development of the cleaning industry. Traditionally, for the CFC-113-type cleaners, an accepted limit of 1.56-μg/cm2 (10 μg/in2) equivalent NaCI is detailed as per ANSI/J-STD-001. Today, most assemblies are achieving well below this level, typically in the region of 0 to 1 μg/in2. This method also only is capable of measuring ionic contamination and cannot define exactly where or what that contamination is.
Two further methods provide valuable data: measurement of surface insulation resistance (SIR) and ion chromatography (IC). The former involves measuring the change in electrical current over time using an interleaved comb pattern PCB and typically is performed at elevated temperatures and humidity levels. The presence of contamination lowers the insulation resistance of the material between the conductors on the board.
The latter is a newer method for cleanliness evaluation that can be used for identifying and quantifying specific ionic species present on an electronic device. The test method details a specific list of ionic residues, which can be removed by specific media. Subsequent analysis of the fluid can separate, identify, and quantify the residue. Substrate handling and preparation are critical for this method, making it particularly expensive and time consuming; therefore, it is not used for general QC, but instead as a more specific analytical technique.
Conclusion
The effective cleaning of PCBs and associated surface mount and thru-hole components is an essential part of electronics manufacture. It increases the reliability of assemblies and allows operators to carry out coating and encapsulating operations with full confidence. The type of cleaner chosen depends heavily on the manufacturing conditions. Whether solvent- or water-based technology is chosen, correct application method and setup are imperative for successful cleaning.
Many specifications have been outlined for cleanliness evaluation; IPC TM-650 is the generally accepted industry standard. It details methods for many of the cleaning tests described above, giving precise guidelines for analysis. It is clear that some methods are costly and rather time-consuming; however, they can provide accurate data on the type, location, and quantity of the residue.
Other, less-intense methods can be employed for fast and efficient quality control. Selection of the most suitable cleaning process, which in turn provides the required level of cleanliness, is the key to ensuring maximum reliability of end products and assemblies at minimum costs.
Jade Bridges, R&D manager, and Amanda Stuart, development chemist, Electrolube, may be contacted at Kingsbury Park, Midland Road, Swadlincote, Derbyshire, U.K., DE11 0AN; jade.bridges@hkw.co.uk and amanda.stuart@hkw.co.uk.