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32 SMT Magazine • February 2014 From this research we have determined that generating an intermetallic morphology that is similar to a standard mass reflow surface mount process is not straightforward. The method used to add Cu to the ENIG pad and lead-free solder system will affect the morphologies at the electroless Ni substrate and therefore the mechanical properties of the intermetallic. Data is presented on the intermetallic strengths and failure modes of two bond pull test meth- ods. Specifically hot bump pull (HBP) and cold bump pull (CBP) testing are compared where Cu is added by the copper pins of the HBP tes- ter or by Cu power in a second reflow followed by CBP testing. Introduction In this study we present results using a ther- mo-mechanical test technique which develops a similar intermetallic condition as seen during second level assembly. A careful study was con- ducted which examines the influence of solder alloy, reflow condition and test technique on the interfacial behavior for the most accurate replication of second-level attach without ac- tually performing the attachment process. The above variables are used to qualitatively vary both the Cu and Ni concentrations within the solder joint, and the interaction between the formation of Ni3Sn4 and Cu6Sn5. Microstruc- tural analysis was conducted and shows a dif- ference in intermetallic morphology as a func- tion of the additional copper. The testing re- sults show that when we simulate second-level reflow onto a Cu-based board, the failure mode and ultimate interfacial strength are significant- ly affected. The consequence of this work sug- gests a more rigorous testing approach can be employed for specific condition. background The soldering of ENIG components to Cu substrates generates a condition of elevated Cu concentrations with the solder joint during sur- face mount assembly. Typical ENIG intermetal- lic composition consists of primarily Cu6Sn5 due to the high concentration of Cu in the solder system. However, although composition is known, the intermetallic morphology is of- ten not. Morphology is dependent on the con- centrations of the various elements in the sys- tem, diffusion and dissolution rates of the pad metallurgy, and reflow soldering profile. These morphologies will have varying mechanical strengths and therefore may be more suscep- tible to failure during manufacturing and reli- ability testing. Typical intermetallic morpholo- gies seen on the ENIG surface can be seen in Figure 1, where areas of thick Cu6Sn5 scalloped structures are adjacent to thin areas of interme- tallic that may be Ni3Sn4 or Cu6Sn5. Typically in these mixed systems Ni and Cu atoms can substitute for one another with the matrix mak- ing the more accurate description of the inter- metallic formed (Cu,Ni)6Sn5 and (Ni,Cu)3Sn4. For simplicity within this paper the former will be referred to in all future discussions. It has been shown in this research that the test method one selects to test the intermetallic can have profound results on the results. In ad- dition a method must be developed to generate morphologies that better represent the struc- tures shown in Figure 1. Solder alloys All solder balls were acquired from a single supplier and are 99.9% pure. The alloys used for this research were: 1. Sn/Ag (3.5wt.%) - (SnAg) 2. Sn/Ag (3.0wt.%)/Cu(0.4wt.%) - (SAC304) 3. Sn/Pb (37wt.%) - SnPb FEATUrE TeSTING INTerMeTaLLIC FraGILITy ON eNIG uPON aDDITION OF LIMITLeSS Cu continues Figure 1: SEM cross-sectional micrograph of typical lead-free solder joint Cu6Sn5 formation at ENIg surface following assembly to Cu oSP board.

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