| What Is 'Reliably' Clean? |
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| Written by Ravi Parthasarathy, Dr. Harald Wack and Dr. Harald Wack | |||
| Saturday, 31 May 2008 19:00 | |||
Quick analytical tests can produce reliable conclusions on surface cleanliness.
Engineers are typically confronted with the question of cleanliness levels when assessing assemblies. A wide range of standards and test methods exist. Assemblies exposed to critical, frequently alternating climatic influences require high surface cleanliness. Malfunctions due to impurity-related leakage current and electrochemical migration can otherwise arise. Conformal coating and wire bonding processes are among the most cited reasons for cleaning. Standards related to cleanliness include:
Initial qualification of cleanliness should be completed by visual inspection at 10X to 40X magnification. IPC-A-610, section 10.4, shows visual cleanliness examples. Measuring ionic contamination indicates the presence of hygroscopic residues, according to IPC-TM-650, method 2.3.25, using a 75 or 50% 2-propanol/water solution, respectively (Figure 1). {mosimage} The evaluation is based on J-STD-001D, 8.3.6.2. It should be noted the indicated threshold of 10.06 µg/sq. in. eq. NaCl (1.56 µg NaCl Eq./cm²) refers to ROL0 and ROL1 fluxes. Modern flux systems typically generate much less ionic contamination. For example, cleaned assemblies show residual contamination of 0.6 to 2.58 µg/sq. in. eq. NaCl (0.1 to 0.4 µg NaCl Eq./cm²) and 2.58 and 4.5 µg/sq. in. eq. NaCl, respectively (0.4 and 0.7 µg NaCl Eq./cm², respectively) for no-clean fluxes, mainly as a result of the encapsulation of hygroscopic activators. The requirement for reliable coatings is a contamination level of <2.58 µg/sq.in. eq. NaCl (<0.4 µg NaCl Eq./cm²). Measuring ionic impurities does not permit conclusions concerning the local distribution of these conductive contaminants, as it measures only the total level of ionic contamination across the entire surface area. However, the precise distribution of the residues on assemblies is extremely important when assessing potential short- or long-term hazards they may cause. Alternative test methods – for example, ion chromatography – provide analyses of “local ionic distribution.” Unfortunately, this latter technique also employs IPA as the extracting solvent. IPA is a chemically poor solvent (Kauri-Butanol value), and therefore, the overall measurement accuracy might not correlate to the actual contamination level. Charge contrast images through SEM now have been complemented by easy-to-use analytical tools, such as the flux test, which selectively visualize any hygroscopic activator residues through a color reaction (Figure 2). The flux test permits detection of the local distribution of residues onsite. {mosimage} Another obstacle for post-cleaning processing is the amount of resin residues on the assembly. If a protective coating is applied on top of the resin residues, for example, wettability is impaired and adhesion becomes insufficient because of the different temperature-dependent expansion coefficients. The residual amount of resin is specified by J-STD-001D, section 8.3.6.1. For Class 3 assemblies, a level <257.95 µg/sq. in. (<40 µg/cm²) is required, which corresponds to the typical amount of resin left around a single solder joint. IPC-TM-650, section 2.3.27.1, calls for an extraction method followed by HPLC (High Performance Liquid Chromatography) to determine the resin amount. Depending on the coating system, organic tin salts, which manifest themselves through a chemical reaction of metallic solder and fluxes (i.e., tin organic salts), can act as polymerization inhibitors. These impurities are generally identified by EDX using a SEM. A more suitable alternative to an actual lifecycle test (IEC-68-2, up to six months), which can be used at any stage during product development, is the so-called Coating Reliability (CoRe) test, described in guidelines for conformal coating use and processing. Through complete immersion of the coated assembly in DI water, the mechanism of electrochemical migration is strongly accelerated. Therefore, conclusions can be drawn within a few hours regarding the reliability of the coating. To prevent leakage currents (Figure 3), the assembly should be inspected visually and checked for hygroscopic residues. {mosimage} Another important criterion for leakage current protection is surface insulation resistance. SIR is usually measured according to IPC-TM-650, using specified comb structures. The threshold limit is 108 Ω. Resin films and oxides primarily impair the bonding process. For example, resin can splash onto pads during soldering and result in insufficient bonding. These impurities can reduce molding materials adhesion. Extraction methods and HPLC analyses or analytical tests can be used to detect resin residues (Figure 4). {mosimage} Although surface activation is not a criterion specified in the standards, it represents an important variable for reliable bonding. To evaluate the oxidation status for metallically pure surfaces, the “interference contrast” is used. Metallically pure surfaces appear black and impurities can be identified by any other color but black. Maintaining the described thresholds in the aforementioned standards will eliminate typical failure patterns. To evaluate assembly cleanliness, expensive and time-consuming test methods are not always necessary. By using quick alternative analytical tests, reliable conclusions regarding surface cleanliness can be drawn. Ravi Parthasarathy is senior process engineer and Dr. Harald Wack is vice president and CEO, Zestron America (zestron.com); and Helmut Schweigart is head of application technology, Zestron Europe; This e-mail address is being protected from spambots. You need JavaScript enabled to view it .
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| Last Updated on Tuesday, 30 June 2009 13:39 |
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