Study Of Influence Of Substrates With Surface Finishes On The Electrochemical Migration And Mechanisms

Electrochemical Migration(ECM)is one of circuit short failure phenomenon,occurred on electric product.This failure has a strong relationship with working voltage,environment circuit size level and so on.Nowadays,the material used in PCB is getting more complex,components have larger amount of I/O pins and size of multi-substrate of PCB smaller than it used be.Those trends make the risk of ECM failure on PCB higher.This severe problem is an unavoidable challenge in microelectronics packaging reliability field.So it is meaningful to investigate ECM phenomenon and mechanism in order to obtain important instruction to the electronic industry in circuit design,PCB manufacture and material selection and so on.This dissertation is mainly about investigation of ECM phenomenon and mechanism on three substrates with four kinds of surface finish.The Water Drop(WD)test had been selected for the test method.Scanning Electron Microscope(SEM),Energy Dispersive Spectrometer(EDS)were used to observe the microstructure and analyze the element distribution,respectively.Different surface finish group test results showed that for the ECM phenomena and mechanism of OSP,immersion Sn(iSn)and electroplate Ag(eAg),metallic dendrites grew from the cathode edges towards the anode,until the end of the test.The whole dendrite structure is similar to fractal geometry,and the tiny structure likes clusters and trees.The main elements in dendrites were Copper,Nickel and Silver,respectively.For the Electroless Nickel/Immersion Gold(ENIG)test,the phenomenon and mechanism are totally different from former groups test: dendrites in this case were precipitation of Ni's oxidase or hydroxide with black and gray color accumulated at the edge of anode.These precipitations are distributed on test area uniformly.After test,in some sample Copper's dendrites were found in ENIG test group.In the whole test duration,current in circuit was in a very low level because the dendrite have hindered contract of electrode with solution.Results show that different substrates had no influences on ECM phenomena in the same surface finish test group,but have different failure time.After comparing the Mean Time To Failure(MTTF)of each test group,a rank of resistance to ECM can be given: Resistance of different surface finish(with same substrate)ranking from the weakest to the strongest: eAg,OSP,iSn and ENIG.Resistance of different substrate(except for the ENIG)ranking from the weakest to the strongest: FR-4,Ceramic and BT substrate.Resistance of different substrate with ENIG surface finish ranking from the weakest to the strongest: Ceramic,FR-4 and BT substrate.Through ECM mechanism analysis,the two models for test had been introduced: one is the classical ECM model,which is suit for OSP,iSn and eAg test group.Another is the Anodic Dendrite Growth;this model can interpret the ECM phenomenon in ENIG test group very well.