Effects Of ZnCl₂on Interface Behavior Of Lead-free Micro-joints In Size Effect

Lead-free micro-joints have been developed as environmental friendly solderssince “Waste Electrical and Electronic Equipment”（WEEE） and “Restriction of the use ofcertain Hazardous Substances in electrical and electronic equipment”（RoHS） wereimplemented by European Union in2003. Nowadays, in microelectronics industry,SnAgCu solders have been replaced the SnPb solders gradually because of SnAgCusolders have several excellent properties, such as low melting point, outstandingmechanical properties and better wetting properties. However, the reliability of lead-freemicro-joint in size effect has become the key problem in microjoining.500μm diameter and100μm diameter lead-free micro-joints were used as soldersto connect Cu pads in this subject, in order to simulate the microjoining process in ThroughSilicon Via （TSV） technology. The interface behaviors of Cu substrate brazed with500μmdiameter solders and two Cu wires brazed with100μm diameter solders were discussed inthis paper. From the experience of research and analysis earlier, different brazingtemperature, holding time and brazing flux were the variable quantity in the test. Thedissolution and diffusion behavior of brazed joints were measure by optical microscopy（OM）, scanning electron microscopy （SEM） and its accompanying spectrum analyzer （EDS）, etc. Meanwhile, the effects of oxidation and ZnCl₂on interface reaction ofmicro-joints were analyzed.The results showed that interface behavior of lead-free micro-joints was closelyconnected with ZnCl₂in size effect. The smaller solders were influenced by oxidation to agreater extent in a slow heating brazing process without protection atmosphere. When thebrazing system brazed with a flux which could not wipe off oxidation adequately, the fluxresidue would accumulate around the solder, which might hinder the wetting. And part oftest samples were emerged “shrink waist” phenomenon.Generally, the better weldability ofsolders formed with the rise in brazing temperature and the diffusion of interfacial reactionzone dissolve more abundant with the increase of brazing hold time. But the effect of thecontent of ZnCl₂was more obvious than brazing temperature and holding time. ZnCl₂added could improve wetting velocity and wetting reaction area size effectively. Theresults of experiment showed that the best value of ZnCl₂was5%in the flux. The effectwas good in removing oxidation film, which caused the better wetting reaction betweenlead-free micro-joints and base metal by using5%ZnCl₂added flux. The interfacebehavior was severely and Intermetallic Compound （IMC） morphology was almost rodlike Cu6Sn5, while it was easy to form the large plate Ag3Sn by using the flux withoutZnCl₂, which was a large hidden trouble for the reliability of joints. Contrary to fluxwithout ZnCl₂, the IMC morphologies were scallop type mainly and little rod like Cu6Sn5in the middle. However, tiny Ag3Sn was discovered in the exchange area of wetting front.In addition, the dissolved load on unit area was reduced by the increase of the reaction area,which could result in decreasing the thickness of IMC layer with ZnCl₂added flux.