Study of Sn-Zn solder alloy as a conventional Sn-Pb solder replacement

Abstract

Due to the lElherenttoxicity of lead (PI'), environme"tal regulations around Ihe world have bCCT1:.rgeted to el1mlT,alCthe usage of Pb-beming solder, iTlelectronic ""emillies, rlus has tl'1ggcrodthe development of "Pb-free" solder, fOl the elec{ronic indust"e,. In ordcr to become a successful solder motenal, Ph-free soldeL'alloy, need to he reliubl. over a long tenn, Even though many Ph-free <old"r alloys po"e5S h\gller strength thm the tradLtional Sn- Pb ones, there ,till c~i,t some reliability problems ,uch us low solderability and h'gh interfacial rea"tion on melalli7.atlOnpad ofp"ekaging substrate. tt is the mm Oflhis study to develop new Pb-free soldcrs and to charactcflze some of {heir thermal and mechamcal properties, In thi, study, the TlcwPb-liee ,older, WCT~de"eloped by doping trace amounts at AI and Cu ,nl0 Sn-9Zn eutect,c ,ol,le!, TIlese ,old~r alloys were Sn-gZn-O,5AI and Sn-9Zno 7Cu. In general, the rc\ulting AI-doped Pb-f,ee solder was found to have beller plOperhe" thalllhc original Sn-9Zn eulc",tic solde,. The miero'ITuctme, of .11 the Pb-free ,,,ldeIS were refined and become more uniform with the addition ofbo!h AI and Cu. For ,o<tanc", with the addition of 0 5wt% 1\1in Sn-9Zn, the coar,e iJ-Sn gram S17Cwa, reduced and {he intermetallic compound (IMC) parttclc\ became tiner tn cutectic colome, ~in"e only trace amount AI wa, added into the .utechc S,,-9Zn ,older, the melting tompemtures werc only shghtly chaOb'edcomparing with the cuteetie melung tcmpcTmure. 1t wa, al,o found that with slower cooling mte the microstructure of the Sn-9Zn solder contained th,ck enleeli" 1amelln, ! e Tod-like Ci-Znpha<c in the (1-~nmatrlx, wh,le fa,{ cooling resulted in a tine needle like ",-Zn phase di<per:scdIII{3- Sn matri:" more unifonnly. A chnnge of mLcro'lru~wre ill turn atrecic,l the mechanical propel1ie, of lhe ",Ide,,;, TIms when Sn-9Zn solder" dopc'd WIthAl and Cll, for both ca,e, the miclOhardnelS was improved due 10 rormation of ,nleTmo",llic compOlH\(L\%en 511-920 ,,,lder wos doped with a small perceotagc of 1\1. the te""lc properties ",as al", improved. TI,t improvemetl{ in {enoli. strength wa, I(mnd to be traded "ffwjth the decrease in c1ol]gatLo'L Composite approach is a very new teChtlLqucto 'mpro,," th~ propenies of Pb_rre" solder Ag m\cro-partLdes conlon! in the ra,,~e bCl"cen 0 _ 4.0 WL% with Sl1-9Zn eulc'di" 'ystem, were examined in order to lmdcT"and the efTec{ of Ag addition on the micm,tructural and mecbamcal properties a, well a< the thermal behavIOr of {he comp",ite solders, TIle shear 'tT'L'Tlgthsand the LlllLTfaeial 'CaCl101lSof ~n-Zn mlcro-<:ompo,ite eutectic solders witll AwNilCu ball ),.,-jd anay (EGA) pad melall!zotlOn "erc systemallcally investigated. The three distJOCIlTllCrm"loliiccompound (lMC) loyers were rormcd or the solder mterface of the All/electl'Olyhc Nilell bond p~d w1\h Sn-Zn compOSl\Calloys, "l11e more Ag panicles atlded to the So_Zn sold~T,the mo]'e Ag_7n compound formed 10 Ihieken the uppermo,t !'vIC layer, TI,e dissolved Ag-Zn IMCs formed in the bulk solder rcdcpnsiled Over the imtJally formed interfacial Au_7.nI"lC layer, preveoted the whole IMC layer hfl--off from the pad slIlface. Cro,,_,ectionol stud!es of 1l1e inkrfoce, were "1<0 conducted to corretate WIththe frac!un; ,uTiaces.