Circuits Assembly - August 2008 - (Page 40)

Getting Lean Faster Throughput Through Reliability Analysis Thorough reliability analysis helps minimize process variability. apitalizing on the benefits of Lean manufacturing requires a companywide focus on elimination of non-value added activity and enhanced throughput. Reliability analysis resources can be great tools in supporting this effort. At EPIC Technologies, an in-house reliability laboratory supports new process definition and validation; new product process validation; and resolves internal, supplier and customer quality issues. What are ways this activity supports process improvement, speeds new product introduction and resolves hard-to-identify quality issues in the production process? The lab is co-managed by quality and process engineering to ensure eye-to-eye vision on support. A process engineer trained in reliability lab processes runs the lab. Data are collected in production per lab request; appropriate analysis is performed in the lab; reports are generated; appropriate stakeholder reviews are performed, and corrective actions are implemented. New process definition and validation. EPIC is a long-term user of vapor phase technology. Detailed reliability analysis was used in both initial validation and subsequent fine-tuning of processes to better support the requirements of Pb-free processing and BGAs. Cross-section analysis using a scanning electron microscope is part of extensive failure analysis. Other key tools include shear analysis to analyze joint strength, 2-D and 3-D x-ray inspection, and strain gauge analysis to measure the strain in each process. Circuits Assembly AUGUST 2008 C Ryan Wooten is process engineering manager at EPIC Technologies (epictech. com); ryan.wooten@ epictech.com. Study of the vapor phase process determined two preferred PCB finishes: immersion silver and electroless nickel immersion gold. Immersion silver has better solderability, but it is not as flat and tarnishes. ENIG is flatter and doesn’t tarnish. Oxidation is not a problem in the current process because suppliers are required to package PCBs in silver saver paper, and the Lean Kanban process used with the supply base ensures PCBs are consumed before adverse oxidation levels form. Tests were conducted using VP reflow. Test vehicles were used with SnPb HASL or Pb-free immersion silver, immersion tin or ENIG surface finishes, as appropriate. Boards were populated with SnPb or Pb-free components, printed, assembled and soldered using standard reflow or VP production equipment. The solder pastes selected for testing included SnPb and Pb-free no-clean and watersoluble formulations. Assembled test boards were thermal-shocked between -45o and +125oC for 20 min. duration at each limit for 500, 1000 and 2000 cycles in EPIC’s FA lab. Other test boards were subjected to accelerated aging at 85oC and 85% relative humidity for 1000 hrs. Vapor phase soldered boards were soldered in an EPM-IBL SLC500 vapor phase soldering chamber using Galden LS/230 perfluorinated heat transfer fluid. The vapor phase profiles developed provided a time above liquidus (TAL) of about 90 sec. and a maximum temperature of 230oC, a temperature governed by the vapor temperature. After a vapor phase profile is established, TAL can be modified to achieve any time required without exceeding the 230oC maximum temperature. Visual inspection for solder balls, tombstones, bridging, voids and dewetting indicated no apparent difference between the two methods of solder joint creation. No tombstones were experienced on the test vehicles in either case – a positive result of proper pad, aperture and reflow profile design. Visual inspection also indicated that while vapor phase solder joints performed well and microsection appearance was good, it might be a good idea to explore increasing the Pb-free TAL above the 60 to 90 sec. recommended by solder paste manufacturers to accommodate thorough heat transfer to larger components or clusters of large components. Another option is to increase the peak temperature of the vapor phase fluid. Larger thermal load components, especially in clusters, tended to retard the complete melting of Pb-free paste. It is more difficult to ensure good joints on components with high thermal mass in convection processing because while trying to achieve a sufficient TAL on larger components, smaller components in less populous areas may tend to overheat. Another area of focus has been to match the size of the BGA to the BGA pad. This facilitates a better solder ball joint because variance in pad size and ball increases stress. Additionally, internal design guidelines oversize corner balls and pads on BGAs to provide more strength in the place likely to crack first. circuitsassembly.com 40 http://www.epictech.com http://www.epictech.com http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - August 2008

Circuits Assembly - August 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Screen Printing Better Manufacturing Auditing a Fabricatior Cutting Machine Programming Time Simultaneous Acoustic Imaging and Surface Mapping Tech Tips Soldering Test and Inspection Process Doctor Pb-Free Lessons Learned Getting Lean Component Advances Product Spotlight Ad Index Assembly Insider Techincal Abstracts

Circuits Assembly - August 2008

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