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Process Verification: Cleaning Inspection
December 31, 1969 |Estimated reading time: 4 minutes
By Tom Forsythe, Kyzen Corporation
Process cleaning inspection is an inherent part of the electronic assembly verification process. The importance of inspection amplifies as new lead-free designs gain global market acceptance. Gaining an understanding of inspection options, and the critical information each provides, gives technicians knowledge of how these inspection techniques build on each other to enable statistical process control (SPC). Lead-free materials tend to leave higher residue levels, with the flux residue being more difficult to remove (Figure 1). The preferred lead-free profile increases thermal gradients across the board while leaving tenacious, hard-to-clean flux residue on the board surface. Electronic assemblers use both quantitative and qualitative inspection procedures to assure high cleanliness levels.
Visual Inspection: Operations large and small routinely use visual inspection as a "final QC" following the cleaning process. At this point, inspectors are screening for obvious material defects, and for complete removal of all flux residues. Inspectors are also on the lookout for any attack or etching of the solder joints.
Highly alkaline cleaning technologies, though effective cleaners, react chemically with the metallization, and attack or etch the solder joints (Figure 2). Since the days of CFCs, inspectors have been trained to accept bright and shiny solder joints, and to be suspect of gray or dull finishes. This step is often combined with a "light-rework" operation that may complete assembly of connectors or other hand-attach devices not designed to survive the reflow process.
Tools used for visual inspection generally include a microscope (10-15×) or jeweler's loop, as well as the normal tools required for any hand-soldering operations. Operator training can take time due to the subjective judgments that must be made. The tools are quite easy to master, but that judgment requires operator experience. Obviously, any manual operation is subject to the skill and attention to detail of each individual inspector, which can vary from operator to operator, and throughout the day for individual operators.
Ionic Cleanliness: Visual cleanliness is always the first line of defense, but trace ionic contamination is not visually detectable, despite its potential for causing field failures. Ionic-cleanliness devices flood a chamber containing a single assembly with an isopropyl alcohol and a DI-water mixture. The purpose is to dissolve residual ions as compared to sodium chloride ion equivalents. Ionic contamination equivalents, compared to the surface area of the substrate (data provided by the user), provide a contamination measurement in units of micrograms (µg) of ionic contamination to the unit area of the assembly. In the CFC era, levels up to 14-µg NaCl/in2 were acceptable. Today, most assemblies achieve levels below 1 µg/in2 of board surface, with only the most complex, dense assemblies registering in the mid single digits.
These devices are quick and easy to use, and require little training. This quantitative procedure provides the operator with a quick measure to determine if the process is in control. Ionic extraction is subject to handling risks, as well as errors in the input surface area of the board and attached devices. But with routine care, they can be used effectively.
These devices (available from a variety of manufacturers) are designed for single-assembly testing on a batch basis. For this reason, high-volume shops will evaluate samples of production, while low-volume operations may test each assembly. This technique provides a "gross measurement" of ionic contamination on the assembly; more detail will be needed to troubleshoot a problem assembly thoroughly.
Ion Chromatography (IC): Quantitative extraction procedures are a useful tool to qualify the cleaning process. When you discover a problem with an assembly, ion-extraction analysis identifies trace levels of constituents to determine a root cause and corrective action. Ion chromatography detects anion residues that represent detrimental species that lead to corrosion and the potential for electromigration. This is when ion chromatography becomes a helpful tool.
IC is many levels of sophistication above the ion-contamination testers, in that it will identify the exact ions (sodium, chlorides, bromides, and acids) present on the assembly. By identifying the species present, IC enables root-cause analysis to proceed. Step one with IC is a gross, whole-board analysis to identify species. IC also provides focused analytical techniques for more targeted, spot analysis.
These devices are expensive and are not well suited to manufacturing operations. An IC detects specific ions; therefore, the assemblies must be handled with care prior to undergoing IC evaluation to prevent data corruption. This type of work is either provided by outside labs, or by cleaning materials suppliers with this capability in-house.
ConclusionCleaning and inspection go hand-in-hand. As lead-free truly emerges and sweeps the industry, keeping assembly lines running will be more important. Just as new devices and packages are key to the success of lead-free assemblies, using modern, designed-for-lead-free cleaning materials and equally modern, effective inspection techniques is essential for success. SMT
Tom Forysthe, vice president, Kyzen Corporation, may be contacted via e-mail: tom_forsythe@kyzen.com.