SMT007 Magazine

SMT-Oct2015

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66 SMT Magazine • October 2015 GoLd eMbrIttLeMent MItIGAtIon in a repeatable location. Fixtures also need to be simple to populate as there are some volume re- tinning requirements. The parts of the fixture which will be exposed to the solder need to be manufactured from titanium. There also needs to be drainage built in to the fixture for subse- quent process steps including but not limited to cleaning and subsequent fluxing, soldering and cleaning processes. Two solder pots—one for sacrificial alloy "scrubbing" and for non-porous plating—should be employed in the solder re-tinning process. The first pot should serve as the scrubbing or re- moval pot. The second one should contain the new alloy needed to coat the leads. Both should be flowing type pots, especially the scrubbing pot as the leads will have the solder removed in close proximity to this scrubbing action of the active flowing solder pot. It is important for this pot as the contamination should not be pulled back to and be re-deposited onto the component lead. Nitrogen purging of these pots is essential as the blanket promotes a lustrous finish while mitigating icicle, bridge and dross formation. All process variables should be tightly con- trolled in order to be able to repeatedly re-tin the components in a production setting. Fol- lowing the process guidelines laid out in the ANSI-GEIA-STD-0006 specification will allow for the repeatability that is required. This means process machinery should be able to control at- tributes such as angle of incidence for the com- ponents into the wave, speed of insertion into and out of the wave, precise control of pre-heat temperatures and timing sequence and auto- mated dross removal. In addition, it is critical that the entire processing environment be one in which a nitrogen blanket is present in order to maintain consistent wetting of all of the leads in a production environment. Preheating for flux activation is critical to make sure that a non-porous continuous tinned lead is present. The preheating needs to take into consideration the thermal shock that can be withstood by the components, meaning in some cases sitting over the solder pot is sufficient while in others a forced hot-air knife may be required. Summary The changes to the latest J-STD-001-F are significant to a variety of users who are using gold plating as a finish on their component leads. These latest standards are imploring those with Class 2 and Class 3 assemblies to wash this gold off of the leads using re-tinning processes so as not to impact the reliability of the printed circuit board. This will add cost and lead time to the assembly but will ensure a more reliable solder interconnection. Be pre - pared for it. SMt references 1. P. T. Vianco, "Lead-Free Surface Finishes: Compatibility with Assembly Processes and In- terconnection Reliability" (Edina, MN: Surface Mount Technology Association, SMTA Webinar, January, 2007), pp. 35–36. 2. M. Wolverton, "Quality, Reliability and Metallurgy of ENEPIG Board Finish and Tin- Lead Solder Joints," of SnAgCuX Solder Joints," JOM, 59 (7) (Warrendale, PA: TMS, July, 2007), pp. 38–43. 3. S. J. Krumbein, "Porosity Testing of Con- tact Platings: Part 1," Connection Technology, (January, 1988), pp. 21– 26. 4. W. G. Bader, "Temperature Dependence of the Dissolution Rates of Au, Ag, Pd, Cu, Pt and Ni in Molten 60-Tin-40-Lead Solder," Welding Journal, Research Supplement, 48 (December, 1969), pp. 551S-557S. 5. M. H. Bester, "Metallurgical Aspects of Sol- dering Gold and Gold Plating," Proceedings of figure 2: Hot solder dip. knOCking dOwn The bOne Pile

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