Printed Circuit Design & Fab - October 2008 - (Page 35)

PlatinG Metallurgy on Copper dissolution WABLE, QUyEn ChU, ShIAnG TEnG, KEITh SWEATMAn, and KAzUhIrO nOGITA, Ph.D. CoPPeR eRoSioN: The influence of The rate of copper dissolution in lead-free soldering may be linked to the physical characteristics of the electroplated copper. by ChryS ShEA, JIM KEnny, JEAn rASMUSSEn, GIrISh Lead-free solder alloys, particularly those with higher silver and lower copper contents, have been shown to react with the copper on circuit boards, dissolving the copper much more quickly than tin-lead solders. Rapid dissolution of copper into tin-rich alloys can create hidden defects and reliability issues, and as copper is washed off the PCB, the exposed traces, annular rings and barrel knees can become thinner or even nonexistent. Solid copper reacts readily with molten tin to form a layer of intermetallic compound, Cu6Sn5, at the contact surface. That is what makes tin an essential ingredient in most solders because it is that intermetallic layer that provides the metallurgical connection to ensure good electrical and thermal conductivity through the solder joint. The intermetallic compound dissolves quickly in molten tin, and as a result of these sequential dissolution processes, the underlying copper can quickly be eroded. In eutectic “63/37” SnPb solder, the inert lead diluent slows the rate of intermetallic dissolution. The effect of the lead’s presence in the eutectic SnPb is enough to slow the copper dissolution rate such that extreme copper erosion has not been a significant problem. In lead-free soldering, the high tin content (typically >95%), coupled with the absence of lead in modern lead-free solders, raises the rate of dissolution to the extent that the copper erosion occurring in normal soldering and rework processes may be sufficient to compromise the integrity of the circuit. Under certain conditions, the copper can dissolve too fast, resulting in the removal of most - or even all - of the copper from the circuit board features. Traces and plated through holes that are exposed to wave soldering and rework processes are at a high risk for excess dissolution. Knees, the OCTOBER 2008 area where the barrel of the PTH meets the annular ring, are particularly susceptible to erosion problems. The actual plated copper surface is suspected to play a part in the erosion process as well. This is indicated by the fact that two identical circuit boards from different suppliers can demonstrate vastly different dissolution properties in the same soldering process. It is believed that the dissolution rate may be affected by the grain structure of the copper because loose, open-grain structures, typically associated with columnar grain morphology, are perceived to dissolve more readily than tighter, fine, equiaxed grain structures. Generally, finergrain structures exhibit a higher bond strength between the grains, and thus require more energy to “break” loose or dissolve. Tensile elongation at break is an indicator of grain structure that is used as a quality assurance measure by PCB fabricators. Elongation of 16% or higher indicates good grain structure, whereas elongation in the 12% to 14% range indicates that there could be potential quality issues with the copper plating. Elongation that is less than 12% is considered unacceptable and can be a criterion for rejection. If elongation at break is an indicator of grain structure, and grain structure is a factor in dissolvability, then a relationship may exist between elongation and erodability of the electrodeposited copper. The metallurgical properties of the copper, including tensile strength and elongation, were evaluated on plated copper to see if these characteristics could be used to determine the severity of copper dissolution during wave soldering in the lead-free environment. printEd CirCuit dESign & fAB 35

Table of Contents Feed for the Digital Edition of Printed Circuit Design & Fab - October 2008

Printed Circuit Design & Fab - October 2008 Contents Our Line Market Watch Around the World Happenings Test and Inspection ROI Tip Jar Interconnect Strategies Final Finish Forum The New Wave in High-Speed Modeling The PCB Design Library Mixed Signal Design Considerations Modeling Conductor Surface Roughness Copper Erosion: The Influence of Metallurgy on Copper Dissolution The Wave of the Future Ad Index Building a Profitable Niche Marketplace Off the Shelf BGA Bulletin

Printed Circuit Design & Fab - October 2008

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