SMT007 Magazine

SMT-Feb2014

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30 SMT Magazine • February 2014 abstract As reliability requirements increase, espe- cially for defense and aerospace applications, the need to characterize components used in electronic assembly also increases. OEM and EMS companies look to perform characteriza- tions as early as possible in the process to be able to limit quality related issues and improve both assembly yields and ultimate device reli- ability. In terms of BGA devices, higher stress conditions, RoHS-compatible materials and in- creased package densities tend to cause prema- ture failures in intermetallic layers. Therefore it is necessary to have a quantitative and quali- tative test methodology to address these inter- faces. Typically, solder ball shear or pull testing is employed to measure the interfacial strength, sometimes requiring very high speeds to do so. While there is no current industry-accepted specification on proper test speeds, strength or energy metrics, procedures do exist which allow for relevant comparisons. These tests are always run on unassembled BGA devices, so the inter- action with the PCB is completely removed. While the data is useful for the component manufacturer, the risk is that the test does not fully represent the final assembly in terms of metallurgical condition. Specifically when BGA components using a nickel-gold surface finish are soldered to PCBs with a Cu-based pad (i.e., Cu-OSP, ImmAg, ImmSn or HASL), there will be additional Cu dissolved into the solder joint. The addition of this copper can have an impor- tant effect on the intermetallic structure at the ENIG pad. Current mechanical solder ball test- ing procedures on unassembled BGA devices do not accurately duplicate the condition of this intermetallic structure. The test results on ENIG pads will then not necessarily correlate to ac- tual manufacturing reliability. by Martin K. anselm, Ph.D. and brian roggeman UNIVErSAl INSTrUMENTS CorP. Testing Intermetallic Fragility on ENIG upon Addition of Limitless Cu FEATUrE

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