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Pure Tin Color Change Concern under IR Reflow for Lead –Free Chemistry in Connector Application

RAM-CHEM


Abstract
    This study describes the most recent concern with color change concern under higher reflow temperature profile required by the lead free Application due to the solder temperature for pure tin is higher than tin lead . With higher and higher peak temperature profile demand , from 250℃ to 260℃ for now , color change concern has become a very major concern . In our study , we found in order to solve this color change concern , we must first try to stop from the diffusion from the substrate , as our founding has shown that without Nickel Barrier, tin plated direct on copper , color change is not as serious , hence , color change origin might re-lated to the Nickel Barrier . Hence , our study has fo-cused on Nickel Barrier which is required as a barrier to retard whisker .
Introduction
    Under the WEEE and RoHS regulation force , lead metal are being considered as harmful ,  as Tin Bismuth plating has the concern for toxicity of bismuth under acidic , Tin copper is to be abundant for both US and japan within 2 years is relatively more brittle , where as pure tin has proven to be a more stable , earlier to con-trol and low whisker process.
    A high temperature resistant Nickel is evaluated to overcome the high temperature reflow profile with low whisker impact , and still meet the entire functional requirement as solder-ability , contact resistance and other functional test for lead free connector applicaton .
    Nickel layer works as a barrier layer for eliminat-ing the whisker formation on the top tin finished layer . without the nickel barrier layer , the substrate element , especially the copper metal will then form a complex  alloy with Tin which caused whisker formation as a re-sult after a period of time storage . Functional failure will then happen after assembly-short circuit problem . However , with present high temperature reflow profile , general nickel plated layer cannot tackle the discolora-tion subject , even the dissolution subject as well . High temperature Nickel process is then introduced to over-come this IR reflow need , while without affecting the other functional requirements such as whisker concern , solder-ability concern and bending requirement concern . considering this high temperature Nickel layer has rather high internal stress , risking will be happened on those functional requirement , therefore double Nickel are being introduced for lead free electronic parts application .