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"Lessons Learned and Best Practices Developed for Cleaning Lead-Free Flux Residues from Printed Circuit Assemblies and Advanced Packages"

Authors: Mike Bixenman, Erik Miller and Fernando Rueda;

Abstract: This paper reports data from designed experiments and customer projects that studied the removal of a wide range of Pb-free flux residues by optimizing chemical and mechanical driving forces. The research studied the static cleaning rate (rate at which the cleaning fluid removed flux residue without mechanical forces), dynamic rate (flow, directional forces, spray patterns and pressure), time (residence time to remove all flux residue), and temperature (thermal energy). This paper reports 10 lessons learned and best practices for environments that require cleaning of Pb-free flux residue. (IPC/Jedec International Conference on Lead Free Electronic Components and Assemblies, December 2006)

Component Reliability

"Effect of Cu Leadframe Substance on Whisker Initiation from Electrodeposited Sn/Cu Coating"

Authors: Dr. Takahiko Kato, Haruo Akahoshi, Masato Nakamura, Tomoaki Hashimoto and Asao Nishimura

Abstract: Drastically different tendencies of whisker initiation have been observed from the same electrodeposited SnCu coating on two copper leadframe materials: CUFE and CUCR. After longterm storage at room temperature, no whisker initiation was observed at the coating on CUCR, whereas long whiskers with a maximum length of more than 200 [micro]m were formed from the coating on CUFE. Using FE-STEM and FE-TEM microstructural observations for vertical sections of each Cu leadframe with the coating, cross-sectional morphologies of CuSn intermetallic compound (IMC) formed at the interface between the coating and leadframe were found to be wedge-shaped for the Sn/Cu-CUFE sample and in the shape of the teeth of a comb for the Sn/Cu-CUCR sample. EDX analysis results confirmed the formation morphologies of the CuSn IMC were affected through minor elements in copper leadframes. Iron particles with large diameters of mainly 50-200 nm precipitated nonuniformly in CUFE leadframes (which had minor amounts of iron, zinc and phosphorus). Very small Cr-rich particles, 10-20 nm in diameter, were precipitated in CUCR leadframe (minor elements were chromium, tin and zinc). Iron particles suppressed the growth of the CuSn IMC as the obstacles, whereas grain boundaries in the SnCu coating acted as the preferential formation sites of the IMC. Superimposition of those two effects of iron particles and grain boundaries for IMC formation were thought to result in the configuration of the IMC's cross-sectional, wedge-shaped structure. On the other hand, the Cr-rich particles did not have such a suppression effect on CuSn IMC growth, and only grain boundaries in the SnCu coating kept acting as the IMC formation site. As result, a structure shaped like teeth of a comb for IMC formed in Sn/Cu-CUCR sample. The difference in whisker initiation tendency was inferred to be a result of the difference in compressive stress according to the CuSn IMC formation morphology in the SnCu coating. The stresses were measured using x-ray diffraction method to understand the correlation between whisker initiation tendency and formation morphology of CuSn IMC. Based on the results, a model for controlling whisker initiation is described for SnCu coating-Cu leadframes. (IPC/Jedec International Conference on Lead Free Electronic Components and Assemblies, December 2006)

Screen Printing

"Process Development for 01005 Lead-Free Passive Assembly: Stencil Printing"

Authors: Joe Belmonte, Vatsal Shah, Dr. Rita Mohanty, Tim Jensen, Dr. Ron Lasky and Jeff Bishop;

Abstract: The volume consistency of the solder paste brick is among the critical metrics in determining high yields in assembly. The advent of 0201 and 01005 components made meeting the area ratio target of 0.66 a challenging task, especially with a 0.005" thick stencil and Type 3 powder solder paste. A series of designed experiments (DoEs) evaluated Type 3 and Type 4 solder pastes, varying aperture designs for 0201, 01005 and 0.4 mm CSPs, and 0.003" and 0.004" thick laser-cut and E-Fab stencils. In these experiments we determined optimum print parameters for various combinations of pastes, components, aperture design and stencil fabrication mentioned above. The results also suggested new criteria for area ratios. This work showed that Type 4 solder paste and a 0.003" thick stencil delivered solder paste aperture release values greater than 100%. Type 3 solder paste worked almost as well with a 0.003" thick stencil, producing releases values around 90%, although Type 3 paste seemed to be more sensitive to stencil printer parameters. With a 0.004" thick stencil, Type 4 paste performed reasonably well achieving release values around 80%, even though the area ratio was less than 0.5. Type 3 paste did not perform well with a 0.004" thick stencil, having release values of less than 70%. (IPC/Jedec International Conference on Lead Free Electronic Components and Assemblies, December 2006)

CIRCUITS ASSEMBLY provides abstracts of papers from recent industry conferences and company white papers. With the amount of information increasing, our goal is to provide an added opportunity for readers to keep abreast of technology and business trends.
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Title Annotation:Technical Abstracts
Publication:Circuits Assembly
Date:Feb 1, 2007
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