Circuits Assembly - September 2008 - (Page 55)

The Great Testing Migration As 0201s and 01005s pop up, cleanliness monitoring progresses as well. Process Doctor T raditional resistivity of solvent extract (ROSE) is a quick board-level assessment of the entire immersed board surface and components in an alcohol water solution. This solution (75% isopropyl alcohol/25% deionized water) is a recirculating system of a monitored resistivity, both static (not using cleaned solution) and dynamic (using cleaned solution), that decreases when ionic and some organic material is exposed to the probe. After test, the contaminated solution is easily cleaned: Once the solution is passed through a mixed bed of resin, it is ready for the next test. The ROSE test is a measure of ionic residue in solution. Automated ROSE Tester Automated ROSE Tester Automated Localized Extraction Automated Localized Extraction C3 C3 Pump Detector Detector Pump Figure 2. Localized extraction diagram. The principle behind all automated ROSE testers (Figure 1) that perFigure 1. Automated ROSE form to IPC-TM-650 test diagram. method 2.3.25 is to use the recirculated solution to rinse ionic residues from the board and component surfaces and measure the resistivity change. As this change occurs over the testing time (15 to 60 min.), it reports the amount of ionic residue cumulatively detected from the board surface in the 3-5 gal. testing tank with a 2 gal. solution reserve. Because no standard operating protocol has been developed for determining the ratio of solution to board surface area, it is generally accepted to use the largest tank and volume with a single bare board or assembly to determine ionic residues. The principle of localized extraction (Figure 2) is to isolate a single area (cell #1 = 0.1 in2 area) and to administer heated extraction solution (18.3 MΩ DI water with less than five TOC level) to the specific area. The extraction has three phases: 1. Apply heated solution to the spot (this is a microburst of steam). 2. Allow the solution to soak for at least 5 sec. 3. Aspirate the solution of the spot surface. 4. Repeat a total of nine times to solublize and remove the residue in the specific location. Now the solution can be electrically assessed and analyzed by ion chromatography with a small ratio of Mixed Mixed Resin Resin Switch Switch Figure 3. Sample anion analysis. 0.1 in2 to 2.5 mls of solution. Now the topside repaired part cleanliness can be compared against the cleanliness of the first reflow step, or the cleanliness of a soldered area versus the bare board area. Using localized extraction to assess the residue, we can see the contamination is from the specific area, and understand the electrical effect of the amount and type of ionic residue. Contamination found in an area of failure also can be compared to other assemblies in the same area that are not failing. This allows us to understand processing variables that impact performance. Using a Dionex ICS 2000 system to run anion and cation analysis in 17 min., we have great separation for the following ionic and organic residues (Figure 3): • Anions by AS22 separate fluoride, formate, acetate, MSA, chloride, nitrite, bromide, nitrate, phosphate, sulfate, succinic, malic, maliec, glutaric and adipic acids. • Cations by CS12a separate sodium, potassium, amine, lithium, calcium and magnesium. Continued on pg. 56 Terry Munson is with Foresite Inc. (residues.com); tm_foresite@ residues.com. This column appears monthly. circuitsassembly.com Circuits Assembly SEPTEMBER 2008 55 http://residues.com http://circuitsassembly.com

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Circuits Assembly - September 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Focus on Business Global Sourcing On the Forefront Screen Printing Better Manufacturing Reflow Soldering with a SnCu Eutectic Pb-Free Alloy Improving OEE in High Mix Facilities Effectively Managing RF Design in Utility Metering Applications Solder Joint Reliability of Different BGAs Reworked Using Low Melting Point Pb-Free Alloys Tech Tips Wave Soldering Pb-Free Lessons Learned Materials World Process Doctor Equipment Advances Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - September 2008

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