Study On The Corrosion,Electrochemical Migration And Its Inhibition Mechanism Of Tin Based Lead-free Solder Alloys Under Thin Electrolyte Layers

With the trend of miniaturization and light aromatics in electronical industry,integration of the circuit board is continuously improved and people become more dependent on electronics.In this case,the complicated service environments have augmented more and more loads of mechanics and electricity on the interlinkage solder joints.Due to the high density electric field caused by the small line spacing between circuit boards,the degradation of the packaging materials may result in detachment from the interface of solder joints under the synergic effect of the continuously changing temperature field,high density electric filed,relative humid as well as kinds of contaminants.Especially on the surface of solder joints,an absorbed water film may be formed easily and accelerate the occurrence of corrosion and/or electrochemical migration?ECM?.Tin-based solder alloy is the most wildly used interconnect material in the electronic packaging.And corrosion and ECM of tin-based solder alloy are the key points of the electronics failure.Thus,it is of great importance for the theory to study the corrosion and ECM of tin-based solder alloy under a thin electrolyte layer.Based on the theory,it is necessary to analysis the effects of environmental factors on the ECM and establish effective methods to inhibit the ECM of tin-based solder alloy.The main conclusions obtained are as follows:1.The corrosion behavior of Sn-3.0Ag-0.5Cu solder alloy under chloride-containing thin electrolyte layers.The electrochemical measurements,including EIS,potentiodynamic polarization and electrochemical noise,were carried out to study the corrosion behavior of Sn-3.0Ag-0.5Cu?SAC305?solder alloy under electrolyte layers with different thickness containing various chloride ions.Test methods,such as in situ Optical observation,field emission scanning electron microscopy with energy dispersive spectroscope and X-ray diffraction,were used to detect the morphology and composition of corrosion products.Pure tin mainly showed uniform corrosion under electrolyte layers containing NaCl with different thicknesses,while SAC305 solder alloy showed local corrosion under the same corrosion condition.Due to that self-corrosion potential of IMCs was higher than?-Sn,the existence of IMCs facilitated the occurance of galvanic coupling corrosion and?-Sn phase would enjoy optimum dissolution.The crystal boundary between IMCs phase and?-Sn phase provided infiltration channel for Cl^-ions,Cl^-ions could infiltrate into metal/oxide interface through inter-crystalline boundary at oxide layer and form soluble complex compounds with tin.Moreover,in terms of corrosion environment of thin electrolyte layer,as mass transfer process was relatively difficult.Corrosion products would be more easily enriched in solution in pitting corrosion region such as Sn ions,etc.With development of pitting corrosion,tin ions experienced a gradual hydrolysis,and as a result,hydrogen ion concentration in pitting corrosion region increased and corrodibility was enhanced,and then local corrosion would be generated in adjacent region more easily,and corrosion morphology agglomerated with multiple corrosion pits were formed.Tin ions and OH^-formed tin hydroxide precipitates at pitting corrosion boundary,when precipitate layer was accumulated to a certain thickness,ionic mass transfer process in pitting corrosion region was further blocked,which enhanced the development of localized corrosion.2.Evaluation of the influence of environmental factors on the ECM of tin and establish an appropriate kinetic model to predict the failure time of ECM.The ECM behavior of solder alloy under thin electrolyte layers containing chloride ions was investigated.We evaluated the effects of alloy element,electric field intensity,thin electrolyte thickness and typical weak organic acid from the flux residue on the ECM of tin.The conclusions are as follows:?I?The addition of Ag,Cu,Bi,Sb alloy element can suppress the ECM of tin;?II?With the increase of the electrolyte layer thickness,the growth rate of dendrite first decreases and then increases.This phenomenon can be attributed to the local tin ions concentration nearby the electrode.The higher tin ions concentration is,the faster the dendrite growth rate is;?III?At the low weak organic acid?WOA?concentrations levels,the growth of dendrite is accelerated while the rate of dendrite decreased under high WOA concentrations condition.The inhibition effect is attributed to the formation of complexes with electronegativity,which retarded the migration of tin ions.Some complexes can be oxidize to the insoluble tin oxide on the anode surface and the anodic tin-oxide acted as a barrier layer and blocked dissolution of anode during tin ECM process.?IV?Analysis the kinetics of electrochemical migration process of tin,we built a suitable model to predict the failure time for ECM.3.The establishment of effective method to inhibit ECM of tin.From the viewpoint of ECM process:anodic dissolution,migration of metal ions,electrodeposition of metal ions,we introduce three types of inhibitors to the thin electrolyte layer to prevent the ECM of tin.Some conclusions obtained as follows:?I?Complexing type inhibitor?sodium citrate?can react with tin ions to form complexes with electronegativity,which effectively suppress the migration process of tin ions.At the meanwhile,tin electrode became from self-passivation to anodic passivation,which decrease the dissolution rate of anode;?II?Precipitator type inhibitor?sodium sulfide?can precipitate tin ions to form insoluble tin sulfide.At the medium concentrations,only precipitates but no dendrite exist,which act as a barrier to retard the migration of tin ions from anode to cathode.In a high Na₂S concentration level,no dendrite occur but the surface of anode is covered with a black substances,the excess S^2-react with tin ions on the surface of anode and prevent the migration of tin ion to cathode;?III?Deposition type inhibitor?cetyltrimethylammonium bromide,CTAB?can affect the electrodeposition during tin ECM by selective adsorption.The addition of CTAB results in the lesser tendency of tree-like dendrite formation and accelerate the growth of particle-like dendrites,which helps to inhibit the ECM of tin effectively.