Shaoming Zhou

Effect of electromigration on intermetallic compound formation in line-type Cu/Sn/Cu and Cu/Sn/Ni interconnects

In this study, the line-type Cu/Sn/Cu and Cu/Sn/Ni interconnects were used to determine the growth kinetics of interfacial intermetallic compounds (IMCs) under electromigration (EM), and the current crowding effect and thermomigration are expected to be avoided in this line-type interconnects because of their symmetric structure. The Cu/Sn/Cu interconnect was under the current density of 5.0×10³ A/cm² at 100 °C and 150 °C, and the Cu/Sn/Ni interconnect was under the same current density at 150 °C. For the purpose of comparison, the Cu/Sn/Cu and Cu/Sn/Ni interconnects were aged at the same temperatures for the same durations. In the case of Cu/Sn/Cu interconnect, the same types of IMCs, Cu<inf>6</inf>Sn<inf>5</inf> and Cu<inf>3</inf>Sn, formed at the Sn/Cu interface, which was independent of electric current. EM caused a polarity effect, i.e., the interfacial IMCs on the anode side were significantly thicker than those on the cathode side. The growth kinetics of the interfacial IMCs on the anode side during EM were significantly enhanced compared with that of the aging (the no-current case), and still followed a t^1/2 law with time. The temperature was one of the critical factors that influenced the EM. The effect of EM became more significant at higher temperature under the same current density. The growth behavior of the interfacial IMCs on the cathode sides was complicated. When the initial interfacial IMCs were very thin, the inward atomic fluxes were larger than the outward fluxes and thus the interfacial IMCs grew. After the IMCs reached a critical thickness, the inward atomic fluxes were less than the outward fluxes and thus the thickness of the interfacial IMCs decreased. In the case of Cu/Sn/Ni interconnects, Ni<inf>3</inf>Sn<inf>4</inf> and Cu<inf>6</inf>Sn<inf>5</inf> IMCs formed at the as-soldered Sn/Ni and Sn/Cu interfaces, respectively. The Cu content in the IMCs at the Sn/Ni interface increased with the increasing aging time, and the original Ni<inf>3</inf>Sn<inf>4</inf> IMC at the Sn/Ni interface transformed into (Cu<inf>0.56</inf>Ni<inf>0.44</inf>)<inf>6</inf>Sn<inf>5</inf> after aging at 150 °C for 200h; while the IMC at the Sn/Cu interface remained Cu<inf>6</inf>Sn<inf>5</inf>, which contained less than 0.5 at% Ni even after aging at 150 °C for 200h. When electrons flowed from Cu side to Ni side in the Cu/Sn/Ni interconnects during EM at 150 °C, the original interfacial Ni<inf>3</inf>Sn<inf>4</inf> IMC at the Sn/Ni interface (anode side) had already transformed into (CuNi)<inf>6</inf>Sn<inf>5</inf> type after EM for 100h. After EM for 200h, (Cu<inf>0.60</inf>Ni<inf>0.40</inf>)<inf>6</inf>Sn<inf>5</inf> formed at the Sn/Ni interface and Cu<inf>6</inf>Sn<inf>5</inf> (containing less than 0.1 at% Ni) formed at the Sn/Cu interface. When the direction of electron flow was reversed, after EM at 150 °C for 200h, the types of IMCs remained unchanged, i.e., Ni<inf>3</inf>Sn<inf>4</inf> (containing 2 at% Cu) and Cu<inf>6</inf>Sn<inf>5</inf> (containing less than 2 at% Ni) formed at the Sn/Ni and Sn/Cu interfaces, respectively. The diffusivity of Cu atoms in Sn is two orders of magnitude higher than that of Ni in Sn. Thus, Cu atoms would easily diffuse across the bulk solder than Ni atoms and influence the interfacial reactions on both the anode and cathode sides when electrons flowed from Cu side to Ni side. However, the diffusion of Cu atoms was blocked while against the electron wind, i.e., when electrons flowed from Ni side to Cu side.

Effect of surface finish (OSP and ENEPIG) on failure mechanism induced by electromigration in Sn-3.0Ag-0.5Cu flip chip solder interconnect

The different effects of OSP and ENEPIG surface finishes on the electromigration-induced failure mechanism of Sn-3.0Ag-0.5Cu flip chip solder joint were investigated at 150°C under a current density of 1×10⁴ A/cm². In as-soldered state, the interfacial (Cu<inf>0.55</inf>Ni<inf>0.45</inf>)<inf>6</inf>Sn<inf>5</inf> IMC formed on Ni UBM at the chip side in both OSP and ENEPIG joints. However, the EM resistance of the two joints was greatly different when electrons flowed from chip to PCB though they had the same composition of interfacial (Cu, Ni)<inf>6</inf>Sn<inf>5</inf> and the same Ni UBM. For OSP joint, the interfacial (Cu, Ni)<inf>6</inf>Sn<inf>5</inf> and the Ni UBM displayed an excellent EM resistance; and the Cu content of interfacial (Cu, Ni)<inf>6</inf>Sn<inf>5</inf> IMC at the chip side was slightly higher than that of as-reflowed joint. While for ENEPIG joint, the interfacial (Cu, Ni)<inf>6</inf>Sn<inf>5</inf> IMC and Ni UBM were seriously consumed during EM, and the joint failed. The obvious difference of EM-induced failure between the OSP joint and the ENEPIG joint was due to the different effects of surface finishes. Compared with the ENEPIG joint, the OSP joint could offer a Cu source to improve the stability of interfacial (Cu, Ni)<inf>6</inf>Sn<inf>5</inf> IMC, which effectively inhibited the dissolution of Ni during EM.

Effect of electromigration on interfacial reaction of Cu/Sn3.0Ag0.5Cu/Ni solder joint at high temperature

The Cu/Sn3.0Ag0.5Cu/Ni (Cu/SAC305/Ni) solder joints were designed to investigate the Cu, Ni atoms diffusion behavior and the interfacial reaction during electromigration (EM) at 180 °C under a current density of 1.0×10⁴ A/cm². For comparison, the Cu/Sn3.0Ag0.5Cu/Ni solder joints were aged at 180 °C for the same durations. In as-soldered state, the (Cu<inf>0.55</inf>Ni<inf>0.45</inf>)<inf>6</inf>Sn<inf>5</inf> and (Cu<inf>0.92</inf>Ni<inf>0.08</inf>)<inf>6</inf>Sn<inf>5</inf> IMCs formed at the SAC305/Ni and SAC305/Cu interfaces, respectively. The interfacial IMC thickness increased with increasing aging time. During EM, the current direction played an important role on Cu consumption. When electrons flowed from PCB side to chip side, the Cu pad dissolved into the solder and microcrack formed at the solder/Cu interface. The dissolved Cu atoms were driven toward the anode side and precipitated as large Cu<inf>6</inf>Sn<inf>5</inf> IMCs in the solder matrix along the flowing direction of electrons. While when electrons flowed from chip side to PCB side, no Cu pad consumption was observed and a thick layer-type Cu<inf>6</inf>Sn<inf>5</inf> IMC formed at the SAC305/Cu interface; the thickness of the interfacial IMCs increased with increasing EM time and increased to 15.90 μm after EM for 100 h. As to Ni UBM, no significant consumption of Ni was observed even electrons flowed from chip side to PCB side. Ni UBM was more resistant than Cu UBM during EM. After EM for 143 h, the solder joint failed due to the melting of the SAC305 solder bump.

Failure mechanisms of Ni/Sn3.0Ag0.5Cu/OSP flip chip solder under high current stressing

The electromigration-induced failure of Ni/Sn-3.0Ag-0.5Cu/ organic solderability preservatives (OSP) flip chip solder joints was investigated under a current density of 1 × 104 A/cm2 at 150 °C for 1000 h. Three-dimensional (3-D) finite element simulations on current density and temperature distribution in the test structure were carried out. Current density simulation implied that current crowding effect obviously existed at the electron-entry point and electron-exit point. While temperature distribution simulation implied that the temperature was quite uniform inside the entire solder bump. During EM, when electrons flowed from the PCB to the chip, i.e., the Cu pad on the PCB was the cathode, the Cu pad on the PCB was almost completely consumed and the voids extended across the entire cathode interface, which induced the failure of the solder joint. The dissolved Cu atoms were driven toward the anode side and precipitated a large amount of Cu6Sn5 in the solder matrix near the electron-exit corner. When electron flowed from the chip to the PCB, i.e., Ni UBM on the chip was the cathode, no serious consumption of Ni UBM and underneath Cu pad occurred, and no large voids formed at the cathode interface. Furthermore, no large numbers of Cu6Sn5 IMCs formed in the solder matrix. The growth of (Cu,Ni)6Sn5 IMCs at the Ni/solder interface (the cathode) was retarded and the growth of Cu6Sn5 IMCs at the Cu/solder interface (the anode) was enhanced.

Effect of electromigration on the Cu-Ni cross-interaction in line-type Cu/Sn/Ni interconnect

The line-type Cu/Sn/Ni interconnects were used to determine the effect of electromigration (EM) on the Cu-Ni cross-interaction under the current density of 1.0×10⁴ A/cm² at 150 °C for 100 h and 200 h. For the purpose of comparison, the line-type Cu/Sn/Ni interconnects were also aged at 150 °C for 100 h and 200 h. After soldering, Ni<inf>3</inf>Sn<inf>4</inf> and Cu<inf>6</inf>Sn<inf>5</inf> IMCs formed at the Sn/Ni and Sn/Cu interfaces, respectively. No cross-interaction occurred during soldering. The Cu concentration in the IMCs at the Sn/Ni interface increased with the increasing aging time, and the original Ni<inf>3</inf>Sn<inf>4</inf> IMC at the Sn/Ni interface transformed into (Cu<inf>0.56</inf>Ni<inf>0.44</inf>)<inf>6</inf>Sn<inf>5</inf> IMC after being aged at 150 °C for 200 h; while the IMC at the Sn/Cu interface remained Cu<inf>6</inf>Sn<inf>5</inf> even after being aged at 150 °C for 200 h. A thermal-electric finite element simulation showed that no thermomigration (TM) effect occurred. During EM, the direction of electric current played an important role. When Cu atoms were under downwind diffusion and Ni atoms were under upwind diffusion (electrons flowed from Cu side to Ni side), more Cu atoms were driven to the opposite side than that of the aging case. The original interfacial Ni<inf>3</inf>Sn<inf>4</inf> IMC at the Sn/Ni interface (anode side) had already transformed into (Cu<inf>0.53</inf>Ni<inf>0.47</inf>)<inf>6</inf>Sn<inf>5</inf> after EM at 150 °C for 100 h, which was faster than the aging case. After EM for 200 h, (Cu<inf>0.60</inf>Ni<inf>0.40</inf>)<inf>6</inf>Sn<inf>5</inf> IMC layer formed at the Sn/Ni interface. When Cu atoms were under upwind diffusion and Ni atoms were under downwind diffusion (electrons flowed from Ni side to the Cu side), Cu atom diffusion was retarded, while Ni atom diffusion was enhanced. More Ni atoms were dissolved into the Sn. The interface between Ni and Sn became uneven after EM for 100 h and became rougher after EM for 200 h. Large Ni<inf>3</inf>Sn<inf>4</inf> particles precipitated in the Sn near the Sn/Ni interface (cathode side). The size of Ni<inf>3</inf>Sn<inf>4</inf> particles that precipitated in the Sn increased with the increasing EM time. Cu atoms were very sensitive to the direction of electron flow. When Cu atoms were under downwind diffusion, more Cu atoms could diffuse across the solder to the Ni side and alter the interfacial IMC type. When Cu atoms were under upwind diffusion, few Cu atoms could diffuse across the solder to the Ni side and no interfacial IMC transformation occurred at the Ni side. Regardless of electron flow direction, few Ni atoms could diffuse across the solder to the Cu side due to its high diffusion activation energy.

Electromigration-induced interfacial reactions in line-type Cu/Sn/ENIG interconnect

The effect of electromigration (EM) on the solid state interfacial reactions in line-type Cu/Sn/ENIG interconnect was investigated under the current density of 5.0×10³ A/cm² at 150 °C for 100 h and 200 h. The Cu/Sn/ENIG specimens were also aged at the same temperature and durations for comparison. After soldering, Cu<inf>6</inf>Sn<inf>5</inf> and Ni<inf>3</inf>Sn<inf>4</inf> IMCs formed at the Cu/Sn and ENIG/Sn interfaces, respectively. During aging time, a thin Cu<inf>3</inf>Sn layer formed beneath the Cu6Sn5 at the Cu/Sn interface, and the original Ni<inf>3</inf>Sn<inf>4</inf> transformed into (Cu,Ni)<inf>6</inf>Sn<inf>5</inf> IMC at the ENIG/Sn interface. A thin Ni<inf>3</inf>P film was detected at the ENIG/Sn interface after aging for 200 h. During EM, when the electrons flowed from the Cu side to the ENIG side, the type of interfacial IMCs kept unchanged at Cu/Sn interface for 100 h and 200h, while the origin Ni<inf>3</inf>Sn<inf>4</inf> transformed into (Cu,Ni)<inf>6</inf>Sn<inf>5</inf> IMC at the ENIG/Sn interface for 100 h and 200h. When the electrons flowed toward the Cu side, high current density induced Ni-P consumption and EM-induced the formation of nearly all (Ni,Cu)<inf>3</inf>Sn<inf>4</inf> IMC in solder near the ENIG/Sn interface were observed. The EM-assisted crystallization of electroless Ni-P was more noticeable with increasing time, and a thin NiSnP layer formed on the thick Ni<inf>3</inf>P layer at the ENIG/Sn interface for 200 h. Little Ni was detected at the Cu/Sn interface when it acted as the anode side.

Dissolution of substrates in line-type Cu/Sn/Cu and Cu/Sn/Ni interconnects under current stressing

The line-type Cu/Sn/Cu and Cu/Sn/Ni interconnects were used to investigate the dissolution of substrates under a current density of 2.0×104 A/cm2 at 150 °C for 50 h, 100 h and 200 h. For comparison, the line-type Cu/Sn/Cu and Cu/Sn/Ni interconnects were also aged at 150 °C for 50h, 100 h and 200 h. According to the experimental results, as the cathodes, no matter Cu or Ni, the dissolution of substrates in both Cu/Sn/Cu and Cu/Sn/Ni interconnects was in linear relationship with time. The dissolution rate of the Cu cathodes in Cu/Sn/Cu interconnects was approximately equal to that in Cu/Sn/Ni interconnects, indicating that the anode material had little effect on the dissolution of Cu cathodes. While in Cu/Sn/Ni interconnects, the Cu substrates dissolved more easily than the Ni substrates under the same conditions, i.e., Ni had a better electromigration (EM) resistance than Cu.

Electromigration-induced failure of Ni/Sn3.0Ag0.5Cu/ENEPIG flip chip solder joint

The failure mechanisms of Ni/Sn3.0Ag0.5Cu/ENEPIG flip chip solder joint were investigated during EM at 150 °C under a current density of 1×10⁴ A/cm². In as-soldered state, (Cu,Ni)<inf>6</inf>Sn<inf>5</inf> IMCs formed at the both Ni/solder and electroless Ni-P/solder interfaces, and the spalled (Cu<inf>0.58</inf>Ni<inf>0.42</inf>)<inf>6</inf>Sn<inf>5</inf> was observed in the solder at the chip side. After aging for 600 h, the interfacial IMCs at the both Ni/solder and electroless Ni-P/solder interfaces transformed from the initial (Cu,Ni)<inf>6</inf>Sn<inf>5</inf> into (Ni, Cu)<inf>3</inf>Sn<inf>4</inf>. The composition of spalled IMC was nearly unchanged. In the solder, (Au,Pd,Ni)Sn<inf>4</inf> phases coarsened during aging. During EM, the failure mechanisms could be classified to two types. When electrons flowed from the chip to the PCB, at the chip side (the cathode) the current crowding effect induced the serious localized dissolutions of interfacial Cu-Ni-Sn IMC, Ni UBM and Cu trace at the electron-entry corner. The solder joint would fail due to that the complete dissolution of Cu trace induced an open circuit at the electron-entry point. While when electrons flowed from the PCB to the chip, at the PCB side (the cathode) EM induced the consumption of electroless Ni-P to form Ni<inf>3</inf>P and Ni<inf>2</inf>SnP. After the complete consumption of electroless Ni-P, the poor attachment between Ni<inf>3</inf>P and Cu pad induced a crack across the cathode interface, and then the failure of the solder joint occurred. Furthermore, during EM (Au,Pd,Ni)Sn<inf>4</inf> phase tended to deposit at the anode interface and in the solder near the anode interface.