Circuits Assembly - June 2008 - (Page 37)

What Is ‘Reliably’ Clean? Quick analytical tests can produce reliable conclusions on surface cleanliness. The evaluation is based on J-STD-001D, ngineers are typically confronted with the 8.3.6.2. It should be noted the indicated threshquestion of cleanliness levels when assessold of 10.06 µg/sq. in. eq. NaCl (1.56 µg NaCl Eq./ ing assemblies. A wide range of standards cm²) refers to ROL0 and ROL1 fluxes. Modern and test methods exist. Assemblies exposed to flux systems typically generate much less ionic critical, frequently alternating climatic influences contamination. For example, cleaned assemblies require high surface cleanliness. Malfunctions show residual contamination of 0.6 to 2.58 µg/ due to impurity-related leakage current and sq. in. eq. NaCl (0.1 to 0.4 µg NaCl Eq./cm²) electrochemical migration can otherwise arise. and 2.58 and 4.5 µg/sq. in. eq. NaCl, respectively Conformal coating and wire bonding processes (0.4 and 0.7 µg NaCl Eq./ are among the most cited Table 1. Parameters for Evaluating Surface Cleanliness cm², respectively) for noreasons for cleaning. StanTest Unit Target Value clean fluxes, mainly as a dards related to cleanliness <2.58 µg/sq. in. result of the encapsulation include: Ionic contamination µg/cm² ( 40 mN/m The requirement for reliments for Soldered ElecFlux test Residue-free able coatings is a contamitrical and Electronic Residue-free nation level of <2.58 µg/ Assemblies,” section 8 Resin test µg/cm² <257.95 µg/sq. in. (<40 µg/cm²) sq.in. eq. NaCl (<0.4 µg describes methods and Polymerization NaCl Eq./cm²). requirements for elecResidue-free inhibitors Measuring ionic impuritronic assemblies. Uniform black ties does not permit conclu• IPC-A-610F, “AcceptInterference contrast coloration sions concerning the local ability of Electronic distribution of these conAssemblies,” section 10.4 ductive contaminants, as it meais visual documentation specisures only the total level of ionic fying quality requirements of contamination across the entire electronic assemblies using bestsurface area. However, the preand worst-case scenarios. cise distribution of the residues on • IPC-TM-650, “Test Methods assemblies is extremely important Manual,” lists various test methwhen assessing potential shortods regarding electronic assemor long-term hazards they may blies. cause. Alternative test methods – • IPC-TP-1113, "Circuit Board for example, ion chromatography Ionic Cleanliness Measurement: – provide analyses of “local ionic What Does it Tell Us?" describes distribution.” Unfortunately, this methods and limitations of latter technique also employs IPA ionic contamination measureas the extracting solvent. IPA is a ment and influences of various chemically poor solvent (Kauriflux systems. Butanol value), and therefore, the • IEC-68-2 is a standardized overall measurement accuracy method for testing climatic relimight not correlate to the actual ability of assemblies. contamination level. Initial qualification of cleanliCharge contrast images through ness should be completed by visual Figure 1. Ionic contamination SEM now have been complementinspection at 10X to 40X magni- measurement process. ed by easy-to-use analytical tools, fication. IPC-A-610, section 10.4, such as the flux test, which selecshows visual cleanliness examples. tively visualize any hygroscopic activator residues Measuring ionic contamination indicates the through a color reaction (Figure 2). The flux test presence of hygroscopic residues, according to permits detection of the local distribution of IPC-TM-650, method 2.3.25, using a 75 or 50% residues onsite. 2-propanol/water solution, respectively (Figure Another obstacle for post-cleaning processing 1). circuitsassembly.com Circuits Assembly JUNE 2008 Process Doctor E 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; r.parthasarathy@ zestron.com. 37 http://circuitsassembly.com

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Circuits Assembly - June 2008 Contents Letters Caveat Lector Industry News Market Watch Talking Heads Screen Printing Better Manufacturing China Goes ‘Upmarket’ Out of the Garage Improving Production Line Performance Manufacturing Steps Onto the Enterprise IT Stage Measuring and Controlling Wave Height and Dwell Time Wave Soldering Tech Tips Test and Inspection Process Doctor Pb-Free Lessons Learned Getting Lean Eastern Advances Semicon West Product Preview Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - June 2008

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