In case you missed it.

Cleaning

"Applied Research for Optimizing Process: Parameters for Cleaning Pb-Free Flux Residue"

Authors: Dirk Ellis and Mike Bixenman; speed-linetech.com/docs/Applied-Research-Flux-Residue.pdf

Abstract: Both cleaning equipment and cleaning chemistry must be modified to enable effective, consistent cleaning of Pb-free flux residues. The best overall cleaning solutions will be provided by cleaning equipment and cleaning material companies working together to optimize their products. This paper discusses extensive cleanliness testing of over 70 Pb-free soldering materials. The data suggest increased difficulty when cleaning Pb-free post-soldering residues. Based on the data, this paper discusses advances in cleaning material and cleaning equipment mechanical designs that will optimize the cleaning of Pb-free flux residues. It details the changes made in cleaning equipment for optimizing the cleaning material advancements in standard cleaning equipment.

Conformal Coatings

"The Role of Permeability and Ion Transport in Conformal Coating Protection"

Authors: Angela Mensah and Dr. Christopher Hunt; npl.co.uk/ei/publications/

Abstract: The level of protection offered by conformal coatings on assemblies has been evaluated, with the primary interest the role of permeability and ion transport. Testing was conducted on six generic coatings currently used by industry, either conformally coated onto FR-4 laminate boards or as free films. Methods used were surface insulation resistance, sequential electrochemical reduction analysis (SERA), PermeGear diffusion cells and gas chromatography mass spectrometry (GC-MS). Conformally coated boards were used for the SIR and SERA measurements while free films of the selected coatings were used for the diffusion and GC-MS measurements. Each method revealed aspects of the level of protection offered by the coatings as well as the extent to which the coatings are permeable to contaminants in high environmental stress regimes. The coatings acted as an effective barrier to NaCl penetration but were permeable to dibasic acids found in electronic fluxes. The investigations also highlighted the importance of board cleanliness. (NPL Report DEPC-MPR 032, September 2005)

Reliability

"Preliminary Measurements of Solder Flux Residues in an AC Environment"

Authors: Alan Brewin, Ling Chunxian Zou, Milos Dusek and Dr. Christopher Hunt; npl.co.uk/ei/publications/

Abstract: This exploratory work studied the effects of flux residues on the impedance of interdigitated combs under the application of AC voltage. These effects are considered to be more relevant to digital and AC circuit designs currently in use. The effects of the presence of generic flux types (rosin, water-based, and glutaric acid) on SIR style interdigitated combs have been measured and compared, using the parameters of impedance (real and imaginary) and capacitance. The study indicates that the use of impedance properties as a tool for differentiating the effects of flux and their potential effect on AC circuits is promising, and warrants further study. (NPL Report DEPC-MPR 029, August 2005)

"Measuring the Reliability of Electronics Assemblies during the Transition Period to Lead-Free Soldering"

Authors: Martin Wickham, Ling Chunxian Zou, Milos Dusek and Dr. Christopher Hunt; npl.co.uk/ei/publications/

Abstract: Over 200,000 solder joints on assemblies incorporating the main types of SMT and through-hole components have been manufactured with SnPb and Pb-free terminated components using SnPb, Pb-free and mixed alloy systems. The work included manufacture of joints with controlled levels of Pb-contamination from 1 to 10%. All assemblies were subsequently thermally cycled (-55[degrees] to 125[degrees]C) to 2000 cycles, and tested for continuity, shear, pull and vibration. The work indicates that there should be few solder joint reliability problems when mixing SnPb and Pb-free components and solder alloys (with lead contamination from 1 to 10%). Uncontaminated SAC systems or those contaminated with low levels of Pb showed fewer failures. Failures in Sn-plated QFP components were probably due to batch-related solderability issues. Work with hot peel testing of SOIC components has indicated that Pb contamination may cause problems during processing. The addition of small quantities of Pb (<10%) to a Pb-free joint significantly reduces its peel strength above 180[degrees]C. This may cause the top-side solder joint to separate or deform during second-side reflow, if the assembly is subjected to mechanical forces during assembly. Also noted: four test kits tested did not provide a repeatable method for detecting Pb. (NPL Report DEPC-MPR 030, August 2005)

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.