| Solder paste is the key m aterial of surface mounting technology.At present,the real problem that troubles solder paste is the poor stability.The problems lead to the deterioration of printing and welding performance of solder paste,such as leak print,reduction/uneven tin content,solder pearl,grape balls,back wetting and other production defects,especially for the CSP(Chip Scale Package)solder joint with compact spacing.Therefore,the physical and chemical properties of solder paste need to be consistent in the long storage and using process,It means th at the solder paste needs high stability,but there is still a big gap between domestic solder paste and foreign solder paste.Starting from the failure mechanism of solder paste,the relationship between the corrosion rate of solder flux to solder powder and solder paste stability was studied by potentiodynamic method.Based on this,the effects of activator,solvent,corrosion inhibitor and storage temperature on solder paste stability were studied by means of spreading wetting experiment and viscosity test.At the same time,the effects of physical coating on the stability of solder paste were investigated by means of the microencapsulation of activator and coating of welding powder with rosin alcohaol solution.The main conclusions are as follows:(1)The corrosion rate of flux to solder powder is related to the combination of solvent and activator.The corrosion rate of welding aids made of several organic acid-type activators and tetrahydrofurfurfuryl alcohol to solder powder from small to large is:2-decyltetradecanoicacid,dodecanoic acid,adipic acid,sebacic acid,suberic acid,azelaic acid,glutaric acid,malic acid and salicylic acid.The corrosion rates of welding aids prepared with glycol ether solvents and adipic acid to solder tin powder are propylene glycol phenyl ether,tetraethylene glycol dimethyl ether,ethylene glycol monohexyl ether,dipropyleneglycol dimethyl ether,polyoxyethylene glycerol ether(MF-200A),tetrahydrofurfurfuryl alcohol,diethyleneglycolmonobutylether,polyethylene glycol 400(PEG 400),polyethylene glycol monomethyl ether 250.(MPEG 350).(2)The stable storage time of solder paste depends on the corrosion rate of active medium in flux to solder powder,and the smaller the corrosion rate,the longer the stable storage time.The time of solder joint retraction of the solder paste prepared by adipic acid,sebacic acid,octanedic acid,azelaic acid and glutaric acid is 250 days,150 days,130 days,120 days and 40 days with the decrease of corrosion rate.The time when the solder paste viscosity increased to 200 pa·s at 40℃ was 35 days,30 days,30 days,12 days and 10 days,respectively.(3)Temperature is positively correlated with the corrosion rate of solder powder by welding aids.The solder paste prepared by adipic acid type simple flux has a stable storage time of 42 days,40 days,36 days and 1 day at 20,30,40 and 50 ℃.(4)Microencapsulation with KE-604 resin can effectively slow down the decreasing trend of solder paste spreading rate with storage time at room temperature.When the encapsulation ratio of microcapsules was 4:1,the pavement retention rate of solder joints was 99.4%after 220 days storage,while that of uncoated samples was 96.7%.(5)The experimental results of accelerated oxidation show that:KE-604 resin,AX-E resin and water-white rosin resin coating can effectively improve the oxidation resistance of welding powder,and 0.5%KE-604 rosin coating powder has the best oxidation resistance.(6)SEM results show that:KE-604 resin was used to coat the solder powder by soaking in Rosin alcohol solution.The uniform coating was obtained and the corrosion resistance of the solder powder in flux medium was improved.(7)A solder paste which can be stored at room temperature was developed.The flux formula was 40%KE-604 resin,14%tetraethylene glycol dimethyl ether,14%propylene glycol phenyl ether,10%polyoxyethylene glycerol ether,9%adipic acid,6%2-decyltetradecanoicacid,6%modified hydrogenated castor oil and 1%1H-Benzotriazole.The solder paste prepared with powder ratio 88:12 has excellent wettability on copper plate and can meet the requirements of 0.06mm fine spacing welding and 0.2mm CSP package welding. |
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