Formation Mechanism of Porous Cu₃Sn Intermetallic Compounds by High Current Stressing at High Temperatures in Low-Bump-Height Solder Joints

Abstract

Electromigration tests of SnAg solder bump samples with 15 μm bump height and Cu under-bump-metallization (UBM) were performed. The test conditions were 1.45 × 10⁴ A/cm² at 185 °C and 1.20 × 10⁴ A/cm² at 0 °C. A porous Cu₃Sn intermetallic compound (IMC) structure was observed to form within the bumps after several hundred hours of current stressing. In direct comparison, annealing alone at 185 °C will take more than 1000 h for porous Cu₃Sn to form, and it will not form at 170 °C even after 2000 h. Here we propose a mechanism to explain the formation of this porous structure assisted by electromigration. The results show that the SnAg bump with low bump height will become porous-type Cu₃Sn when stressing with high current density and high temperature. Polarity effects on porous Cu₃Sn formation is discussed.

1. Introduction

In the pursuit of higher operation speed and improved performance of microelectronic devices [1], the application of flip chip solder joints has become a key technology for high-density packaging. The solder joints have been used in high power devices, such as central processing unit (CPU) and application processors (AP). The specification of operation temperatures for these devices range from 100 to 110 °C and the required lifetime is usually 10 years. To meet the demand for portable devices, the input/output pin numbers continue to increase while the size of the solder joints continues to shrink. This inevitably leads to higher current densities and operating temperatures in the joints. Therefore, it raises serious reliability issues such as electromigration and thermomigration [2].

Over the past decades, electromigration behavior and metallurgical reactions of flip chip solder joints have been widely studied [3,4,5,6,7,8,9]. The intermetallic compounds (IMC) of Cu₃Sn and Cu₆Sn₅ are commonly formed in solder reactions on Cu under-bump-metallization (UBM). The Cu column-type UBM (a structure with a thicker Cu) was developed to alleviate both the Joule heating and current crowding effects in flip-chip solder joints under normal device operating conditions, having a longer lifetime [10].

Dimensional shrinkage of the solder joints in portable devices may cause new reliability issues. The smaller solder joints tend to transform completely into IMC joints during electromigration tests [11]. Additionally, in low-bump-height solder joints, when they are combined with a thick column-type Cu UBM, the solder reaction on the side wall of the Cu-column becomes one of the new reliability issues of concern. Liang et al. found that in reflow tests, the effect of side wall reaction would lead to the formation of large voids in the solder joints [12].

Panchenko et al. reported a porous Cu₃Sn structure in 50 μm pitch Cu/Sn microbump interconnects during reflow due to the degradation of the Cu₆Sn₅ IMCs layer [13]. The proposed explanation of the porous Cu₃Sn formation is the dissolution of Sn atoms from the Cu₆Sn₅ matrix owing to the reaction between Cu₆Sn₅ and flux residues. However, the formation mechanism of the porous Cu₃Sn structure has not yet been confirmed.

In this study, we investigated the microstructure evolution of solder joints with Cu column UBM, especially the porous Cu₃Sn formation, under current stressing at 1.45 × 10⁴ A/cm² at 185 °C and 1.20 × 10⁴ A/cm² at 170 °C. We observed the porous Cu₃Sn formation enhanced by current stressing.

2. Experimental Methods

Typical flip-chip solder joints were used in our electromigration tests. Figure 1a shows the schematic diagram of a bump with Cu column UBM. On the chip side, 100 nm Ti was sputtered as the adhesion layer. Then a 2 μm Cu layer was sputtered as the seed layer for the subsequent electroplating of a 50 μm Cu UBM column and SnAg solder. The composition of the solder was Sn-2.3Ag. The diameter of UBM and passivation opening is 145 and 85 μm, respectively. The Cu trace on the flame retardant 5 (FR5) substrate is 100 μm wide and 27 μm thick. Pre-solder of Sn-2.3Ag was used on the substrate side. The chips were flipped over to align with the substrates, and they were reflowed at 260 °C for 1 min to form flip-chip solder joints. Figure 1b illustrates the cross-sectional SEM images of a flip-chip bump before the Electromigration EM tests. A scallop-type Cu₆Sn₅ layer was found at the interface between the Cu metallization and the solder on the chip side, as well as on the substrate side [14].

Figure 1. (a) Schematic of the flip-chip solder joints with Cu column UBM. The redistribution layer (RDL) on the Si chip is 5-μm thick Cu; (b) Cross-sectional SEM images; (c) Layout for electromigration tests and four-point structure for measuring bump resistance.

Figure 1. (a) Schematic of the flip-chip solder joints with Cu column UBM. The redistribution layer (RDL) on the Si chip is 5-μm thick Cu; (b) Cross-sectional SEM images; (c) Layout for electromigration tests and four-point structure for measuring bump resistance.

Four-point probes were employed to monitor the resistance change during the EM tests of the bump. Figure 1c presents the schematic of the test layout. There are 6 nodes (n1 to n6) in the test layout, and the direction of the electron flow is pointed by the red arrows which shows the two bumps (b1 and b2) were stressed with opposite direction of electron flow. The electrons enter from n1, go upward through b1, then downward through b2, and finally flow out from n6. The resistance change in b1 (b2) can be measured by the voltage drop between n2 and n3 (n5 and n6). In the microelectronic industry, a failure is typically defined when the resistance change increases to 20% of its initial value [15]. In our study, the early stage of the tests was defined when the bump resistance increment is smaller than 20% during the EM tests, while the later stage of test was defined when the increment was larger than 20%.

The solder joints were stressed with 2.4 A and 2 A at 150 °C. The calculated current densities were 1.45 × 10⁴ A/cm² and 1.20 × 10⁴ A/cm², respectively, based on the UBM opening size. The real temperature in solder joints may be higher than the ambient temperature during current stressing due to Joule heating effect in the stressing condition [6]. Therefore, in our study the temperature coefficient of resistivity (TCR) were employed to measure the real temperature in the joints. The calibrated temperatures were about 185 °C and 170 °C when the test sample was stressed with 2.4 and 2 A, respectively.

Solder joints were cross-sectioned by grinding using abrasive papers #400, #1000, #2000, #2500, and #4000, and then polished by Al₂O₃ of 1 and 0.3 μm. The microstructure and composition were examined with a JEOL 6500 field-emission scanning electron microscope (SEM) (JEOL Ltd., Tokyo, Japan) and energy dispersive spectroscopy (EDS) (Oxford Instruments, Oxfordshire, UK). The SEM EDS was operated at 15 KeV with a current of 1.0 × 10⁻⁵A and a beam size of 1000 nm. Focused ion beam (FIB, FEI Nova 200, FEI Company, Hillsboro, OR, USA) technique was adopted for cross-sectional observation, and transmission electron microscopy (TEM, JEOL-2100F, JEOL Ltd., Tokyo, Japan) and electron probe micro-analyzer (EPMA, JXA-8800M, JEOL Ltd., Tokyo, Japan) were utilized to verify the microstructure results. The operation conditions for the EPMA were at 12 KeV with a current of 1.0 × 10⁻⁸ A and a beam size of 500 nm.

3. Results and Discussion

3.1. Current-Enhanced IMC Formation

The formation of Cu-Sn IMCs can be significantly enhanced by current stressing. Figure 2 shows cross-sectional SEM images of a pair of solder joints stressed with 1.45 × 10⁴ A/cm² at 185 °C. At the early stage, the bump resistance increased by 5% (after 283 h in the upward electron flow) and 10% (after 283 h in the downward electron flow) of its initial value, are shown in Figure 2a,b, respectively. The current enhanced the dissolution of Cu UBM to react with the solder joint and form a large volume of IMCs. The layer-type Cu₆Sn₅ formed in the middle of the bump and the layer-type Cu₃Sn formed at the interfaces between the Cu and Cu₆Sn₅. The entire joint almost completely transformed into IMC joints at the early stage in the EM tests, although there was still some Sn remaining in the bump, as indicated in Figure 2b. The results showed that a large volume of IMCs formed at the early stage regardless of the direction of the electron flow.

Figure 2. Cross-sectional SEM images of solder bumps with Cu column UBM stressed at 1.45 × 10⁴ A/cm² at 185 °C with bump resistance increases (a) 5% with upward electron flow and (b) 10% with downward electron flow.

Figure 2. Cross-sectional SEM images of solder bumps with Cu column UBM stressed at 1.45 × 10⁴ A/cm² at 185 °C with bump resistance increases (a) 5% with upward electron flow and (b) 10% with downward electron flow.

3.2. Formation of Porous Cu₃Sn

It is interesting that porous Cu₃Sn IMCs may form at later stages of electromigration tests. Figure 3 and Figure 4 illustrate the cross-sectional SEM images at different stages in the EM tests with opposite direction of electron flow. Figure 3 shows the EM test results with upward electron flow. Figure 3a–c are the cross-sectional SEM images with bump resistance increase, 8% (after 517 h), 32% (after 217 h), and 85% (after 429 h) of its initial value, respectively. When the bump resistance increased to 8% of its initial value, the layer-type Cu₆Sn₅ in the middle started to transform into porous-type Cu₃Sn, as shown in Figure 3a. We note that the porous-type Cu₃Sn first formed on the periphery of the bump. As the change in bump resistance increased to 32%, shown in Figure 3b, the layer-type Cu₆Sn₅ IMC had fully transformed into porous-type Cu₃Sn IMC. Figure 3c shows similar results, from a different bump, as Figure 3b which means the Cu₆Sn₅ IMC had also totally transformed into porous-type Cu₃Sn IMCs in the later stage of the EM tests.

Figure 3. Cross-sectional SEM images of solder bumps with Cu column UBM stressed at 1.45 × 10⁴ A/cm² with upward electron flow at 185 °C with bump resistance increases (a) 8%, (b) 32%, and (c) 85% of its initial value.

Figure 3. Cross-sectional SEM images of solder bumps with Cu column UBM stressed at 1.45 × 10⁴ A/cm² with upward electron flow at 185 °C with bump resistance increases (a) 8%, (b) 32%, and (c) 85% of its initial value.

Similarly, Figure 4 shows the EM tests results with downward electron flow, where Figure 4a–c illustrate the cross-sectional SEM images with the bump resistance increase 15% (after 517 h), 20% (after 217 h), and 100% (after 429 h) of its initial value, respectively. The results are similar to those in Figure 3. As the bump resistance increased, the layer-type Cu₆Sn₅ in the middle started to transform into porous-type Cu₃Sn.

In order to rule out the possibility that the porous structure was generated in the polishing process or by other external forces, FIB were utilized to provide a deeper cross-section in a selected solder joint, stressed at 1.45 × 10⁴ A/cm² for 429 h as illustrated in Figure 5. The downward direction of the electron flow was labeled in the figure. This second cross-section view indicated that the porous structure not only formed on the polished surface but in the entire bump.

In the bumps with porous Cu₃Sn, we observed that the effect of side wall reaction is very serious. Park et al. reported that a high stressing current may lead to a serious side wall reaction effect [16]. Figure 6a shows the side wall of a bump. The bump has had porous-type Cu₃Sn IMCs formation. We can clearly observe that there are IMCs formation on the side wall, indicated by the ellipses. Since the thickness of the IMCs formed on the side wall is only about 3 μm, it is hard to verify the composition with EDS. Hence, EPMA was used instead. Figure 6b illustrates the results that all the IMCs formed on the side wall are Cu₃Sn. The compositions of the layer-type and porous-type structures were also confirmed with EPMA to be Cu₃Sn, identical with the results measured with EDS. Therefore, the whole joint has transformed completely into Cu₃Sn with two different morphologies.

Figure 7 shows the cross-sectional TEM image and the electron diffraction patterns of point “a” in a porous Cu₃Sn. In the diffraction pattern, the spots correspond to the principle reflections of a basic hexagonal lattice. The zone axes are indexed with [01$\overline{1}$2]. The results show that the porous-type structure is hexagonal Cu₃Sn.

Figure 4. Cross-sectional SEM images of solder bumps with Cu column UBM stressed at 1.45 × 10⁴ A/cm² with downward electron flow at 185 °C with bump resistance increases (a) 15%, (b) 20%, and (c) 100% of its initial value. Cross-sectional SEM images of solder bumps with Cu column UBM stressed at 1.45 × 10⁴ A/cm² with downward electron flow at 185 °C with bump resistance increases (a) 15%, (b) 20%, and (c) 100% of its initial value.

Figure 4. Cross-sectional SEM images of solder bumps with Cu column UBM stressed at 1.45 × 10⁴ A/cm² with downward electron flow at 185 °C with bump resistance increases (a) 15%, (b) 20%, and (c) 100% of its initial value. Cross-sectional SEM images of solder bumps with Cu column UBM stressed at 1.45 × 10⁴ A/cm² with downward electron flow at 185 °C with bump resistance increases (a) 15%, (b) 20%, and (c) 100% of its initial value.

Figure 5. Cross-sectional FIB image of Cu₃Sn IMC structures.

Figure 5. Cross-sectional FIB image of Cu₃Sn IMC structures.

To determine the conditions under which porous Cu₃Sn could form, we stressed the sample at a lower current density and, hence, a lower temperature. Figure 8 shows cross-sectional SEM images of a pair of solder joints stressed with 1.2 × 10⁴ A/cm² at 170 °C. The bump resistance increases 140% (after 5094 h) and 530% (after 5094 h) of its initial value as indicated in Figure 8a,b, respectively. It can be seen that only layer-type Cu₃Sn and layer-type Cu₆Sn₅ have formed, with no evidence of porous Cu₃Sn even in the case of Figure 8b. The results demonstrate that the solder joints do not transform into porous structures at an insufficient current density and temperature.

Figure 6. Cross-sectional SEM images of (a) IMCs formed on the side wall and (b) EPMA analysis. The composition data were shown in atomic percent.

Figure 6. Cross-sectional SEM images of (a) IMCs formed on the side wall and (b) EPMA analysis. The composition data were shown in atomic percent.

Figure 7. (a) Cross-sectional TEM image of porous-type structure and (b) diffraction patterns of point A in (a).

Figure 7. (a) Cross-sectional TEM image of porous-type structure and (b) diffraction patterns of point A in (a).

To further examine the temperature effect on the of porous Cu₃Sn formation, another set of bumps were aged at 185 °C in the oven without current stressing. In the case of 1000-h aging (Figure 9a), the joints have fully transformed into IMC joints. Layer-type Cu₆Sn₅ has formed in the middle of the bump and layer-type Cu₃Sn has formed between Cu and Cu₆Sn₅. In the case of 2000 h aging (Figure 9b), the layer-type Cu₆Sn₅ in the middle has completely transformed into porous Cu₃Sn. The results show that porous Cu₃Sn could form without current stressing at a sufficiently high temperature, which gives an independent confirmation of temperature’s critical role in the formation of porous Cu₃Sn.

Figure 8. Cross-sectional SEM images of solder bumps with Cu column UBM stressed at 1.20 × 10⁴ A/cm² at 170 °C with bump resistance increases (a) 140% with upward electron flow and (b) 530% with downward electron flow.

Figure 8. Cross-sectional SEM images of solder bumps with Cu column UBM stressed at 1.20 × 10⁴ A/cm² at 170 °C with bump resistance increases (a) 140% with upward electron flow and (b) 530% with downward electron flow.

Figure 9. Cross-sectional SEM images of solder bumps with Cu column UBM aging at 185 °C for (a) 1000 h and (b) 2000 h.

Figure 9. Cross-sectional SEM images of solder bumps with Cu column UBM aging at 185 °C for (a) 1000 h and (b) 2000 h.

So far, we have observed that the porous-type Cu₃Sn IMCs can form during high current stressing at high temperatures and the effect of side wall wetting is obvious. To explain its formation mechanism, a further discussion is presented in the next section.

3.3. Formation Mechanism of Porous Cu₃Sn IMCs

On the formation mechanism of porous Cu₃Sn, first, the temperature must be high enough. Second, the solder height must be low enough so that the Cu supply is enough for the whole joint to transform into IMC. This is because when there are unreacted solders in the joints, the major IMC is Cu₆Sn₅ after the metallurgical reactions, as illustrated in Figure 1b. Only a thin layer of Cu₃Sn presented between the Cu metallization layer and the Cu₆Sn₅ IMCs. The transformation process is schematically illustrated in Figure 10. In the early stage of the EM tests, layer-type Cu₆Sn₅ and Cu₃Sn formed first, as shown in Figure 10a. As the bump resistance increased, the layer-type Cu₆Sn₅ started to transform into porous-type Cu₃Sn, as illustrated in Figure 10b. The IMCs formed on the side walls due to the effect of side wall reaction are also shown in Figure 10b, labeled by the ellipses.

According to the experimental results, we have developed the following mechanism to explain the formation of the porous-type Cu₃Sn. Previously, the Kirkendall void formation mechanism in Cu₃Sn was proposed [17]. When the layer-type Cu₆Sn₅ decomposes into Cu₃Sn, it will release 3 Sn atoms:

Cu₆Sn₅ → 2Cu₃Sn + 3Sn

(1)

The Sn atoms will then attract nine Cu atoms to form three more Cu₃Sn.

3Sn + 9Cu → 3Cu₃Sn

(2)

Figure 10. Schematic diagrams of transformation process at (a) early stage and (b) final stage in the electromigration tests.

Figure 10. Schematic diagrams of transformation process at (a) early stage and (b) final stage in the electromigration tests.

The vacancies which enable the diffusion of the Cu atoms tend to form Kirkendall voids in Cu₃Sn. It implies that the growth of layer-type Cu₃Sn IMCs is at the expense of layer-type Cu₆Sn₅ IMCs. However, in our experimental results, it can be observed that as the layer-type Cu₃Sn IMCs grew to certain thickness, the layer-type Cu₆Sn₅ would not transform into the layer-type but porous-type Cu₃Sn. Hsiao et al. found that Cu-Sn IMCs can become a diffusion barrier for the Cu/solder reaction [18]. We assume that the layer-type Cu₃Sn served as a diffusion barrier and inhibited the Sn atoms released from the decomposition of Cu₆Sn₅ to react with Cu. Since the bumps were stressed with high current density at high temperature, the released Sn diffused to the side wall to form Cu₃Sn due to the effect of the side wall reaction. The place originally occupied by Sn became empty and led to the porous-type structure.

3.4. Polarity Effect

We have observed the polarity effect in the final stage of the EM tests; namely, more porous structures formed on the anode side than on the cathode side. By the mechanism theorized above, the phenomenon was caused by an abundant Cu supply on the cathode side than on the anode side. Hence, there was more Cu₃Sn formation on the cathode side. However, Cu atoms were less likely to diffuse to the anode side to react with the released Sn. As a result, the porous structure was more obvious on the anode side. The polarity effect is, thus, a consequence of the proposed mechanism.

3.5. Theoretical Calculation of Pore Volume

This mechanism can be further verified by volume calculations. One mole of Cu₆Sn₅ decomposes into two moles of Cu₃Sn plus three moles of Sn. Approximately, one mole of Cu₆Sn₅ has a volume of 117.7 cm³, two moles of Cu₃Sn has a volume of 69.5 cm³, and three moles of Sn has a volume of 48.4 cm³. In our observation, the volume before and after the Cu₆Sn₅ to Cu₃Sn conversion was nearly unchanged (117.7 to 117.9 cm³). Calculations also indicate that the volume change in phase transformation is negligible. On the other hand, the diffusion of Sn to the side walls would leave the pores behind. Assuming that the pore volume equals that of the released Sn, we have calculated the volume occupied by pores to be 41.2% (48.4/117.9) of the porous Cu₃Sn volume, when all the released Sn atoms diffuse to the side walls of the UBMs. As a verification, we have used software to calculate the volume percentage of the pores in the actual bumps. Six bumps were input into the software to ensure accuracy. The average volume percentage of the pores was found to be 38.2%, a number close to the theoretical 41.2%. The volume of pores (or the volume of the released Sn) in the bumps is approximately 63,879 μm³. We assume that all of released Sn atoms have diffused to the side walls to form Cu₃Sn. This implies that 63,879 μm³ of Sn would form 137,978 μm³ of Cu₃Sn on the side walls. This is again close to the calculated value of 157,932 μm³ for the volume of Cu₃Sn formed on the side walls in the actual bumps.

The above experimental results and calculation both indicate Sn diffusion to side walls of Cu UBM may be a possible mechanism for the formation of the porous Cu₃Sn. The porous Cu₃Sn will be a reliability issue for solder joints with Cu UBM. Yet, no solutions have been proposed so far to prevent it from happening. More studies need to be performed to solve this problem.

4. Conclusions

We have carried out a systematic study on an important reliability issue related to high-density packaging of microelectronic devices. In this study, SnAg solder bump samples with Cu UBM were stressed at current densities of 1.45 × 10⁴ A/cm² and 1.20 × 10⁴ A/cm² and operating temperatures of approximately 185 °C and 170 °C, respectively. A porous Cu₃Sn structure, unseen in traditional flip-chip solder joints, was observed in the process. Cu₃Sn IMCs were also observed on the side walls of Cu column UBM due to side wall reactions. We have proposed a theory to explain the formation mechanism of the observed porous Cu₃Sn and side-wall Cu₃Sn. In the reaction: Cu₆Sn₅→ 2Cu₃Sn + 3Sn, one Cu₆Sn₅ molecule converted into three Cu₃Sn molecules and three Sn atoms were released. The released Sn left pores behind to form the porous Cu₃Sn. This is because when the early formed layer-type Cu₃Sn becomes a barrier to Cu diffusion, the released Sn atoms, taking an alternative path, diffuse to the side walls to form the Cu₃Sn IMCs by side wall reaction. Results of volume calculations were consistent with the proposed processes and consequently provided further evidence of this mechanism. In the past, the layer-type Cu₃Sn is regarded as the terminal phase for the solid-state Cu-Sn reactions; however, our results conclude that the SnAg bump with low bump height would lead to porous-type Cu₃Sn formation when it is stressed with high current densities and high temperatures.