Circuits Assembly - June 2008 - (Page 38)

Process Doctor Figure 2. Positive color reaction through the flux test method. Figure 3. Conductive residues between two solder joints. 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. Another important criterion for leakage current protection is surface insulation resistance. SIR is usually measured according to IPCTM-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 38 Circuits Assembly JUNE 2008 Figure 4. Residues invisible (left) and visualized by the resin test (right). 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). 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. n circuitsassembly.com 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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