Reliability Study of Low Silver Alloy Solder Pastes

Sn3.0Ag0.5Cu solder paste is a common alloy for lead-free solder paste in the industry. However, the price of silver has been rising in the past few years. This has promoted the demand for alternative low/non-silver alloy materials and has led to the development of many alternative alloys. Today, there are many low/non-silver alloy solder pastes available on the market. Publication on replacing lead-free alloys [1-3]. However, most research has focused on alternative alloys for BGA solder balls and their reliability. Publications on the thermal reliability of low/silver-free alloy solder pastes are very limited. In our previous publication [4], the alternative low/non-silver alloy solder paste performed well in the process evaluation. As shown in Figure 1 and Figure 2, many alternative solder pastes have good printability and wettability.

Printability of low/no silver alloy solder paste. Solder paste J is SAC305 solder paste. The other material is low-silver alloy solder paste. The results show that many alternative alloy materials have printing performance comparable to SAC305.

Wetting test results of various lead-free alloy solder pastes. Shows the wetting test images of SAC305, SAC0307 and SnCuNi alloy solder pastes. These three solder pastes have similar spread diameters on average.

From a process point of view, it is feasible to use alternative alloy solder pastes. However, information about its reliability is lacking. In this article, we will discuss the structure, reliability and failure analysis of lead-free solder joints reflowed using alternative low-silver solder pastes.

As shown in Table 1, six different lead-free solder pastes were studied. Type 3 does not use clean solder paste. The material A SAC305 was used as a control. Material B, SAC0307, ​​is an alternative low-silver solder paste without adding other alloys. Material C, SAC0307, ​​contains some microalloying additives, which may affect the coarsening of tin grains. There is no Ag in the composition of material D (SnCuBiCo) and material E (SnCuNi). In addition, a near-eutectic SnBi alloy (material F) with a small amount of Ag added has also been studied. The liquidus temperature of this alloy is about 138°C, which is much lower than other tested lead-free alloys. The melting temperature of Sn3.0Ag0.5Cu alloy is about 217ºC. The melting point of another low-silver superalloy is about 227-228ºC. It is known that low-temperature Sn-58Bi eutectic is fragile. In order to refine the microstructure of the Sn/Bi eutectic and promote creep deformation through brittle fracture upper grain boundary sliding, a small amount of Ag is required to be added [5]. The alloy composition of the solder paste material is listed in Table 1.

Solder paste material and its alloy composition.

The company's multi-function test vehicle was used in the study (Figure 3). The size of the circuit board is 225mm x 150mm x 1.67mm. The surface finish of the board is OSP. The test vehicle has many different SMD component types, such as BGA (0.8mm and 1.0mm pitch), CSP (0.5mm, 0.4mm, 0.3mm), QFN component (0.5mm and 0.4mm pitch), leaded component (SOIC) , QFN100, QFN208, etc.), chip components (0201,0402, 0603, 0805), through-hole components, etc...In addition, the test vehicle has different areas, which are designed for printability testing, slump testing, and lubrication Wetness test, solder ball test, pin testability, etc.

The company's multifunctional test vehicle, revised version 1.0.

As shown in Table 3, components of different types and sizes were assembled for reliability testing. The daisy chain components are monitored during the thermal cycling test.

Components in reliability testing.

All samples using solder paste A to E were reflowed with a typical lead-free curve, with a peak temperature of about 245°C (Figure 4). The SnBiAg paste (material F) was reflowed using a low-temperature reflow profile (peak temperature is about 170°C) (Figure 5). The reflux is carried out in an atmospheric environment.

Solder paste material and its alloy composition.

The company's multifunctional test vehicle, revised version 1.0.

High temperature lead-free reflow curve.

Low temperature lead-free reflow profile.

The thermal cycling test is carried out in an air-to-air thermal cycling chamber, with a temperature ranging from 0 to 100°C, staying at each peak temperature for 10 minutes, and the heating rate is about 10°C per minute. The chamber curve of temperature versus elapsed time is shown in Figure 6.

Thermal cycle temperature profile-0°C to 100°C.

The thermal cycle test was terminated after 3000 cycles. Measure the resistance of all components before and after the thermal cycle test for fault detection. The samples were subjected to cross-sectional analysis before and after the thermal cycling test.

Measure the thickness of the intermetallic layer of solder joints assembled with various lead-free alloy solder pastes. For SAC 305 materials and other lead-free high melting temperature alloys, the thickness of the intermetallic layer on the PCB side after the reflow process is between 2 μm and 2.5 μm. After the SnBiAg reflow process, the thickness of the intermetallic layer on the PCB side is less than 1 μm. The thin layer formed when using this alloy is due to the use of a low temperature reflow profile. It is worth noting that during the thermal cycle test, the IMC layer of SnBiAg significantly increased to about 2μm, which is similar to the thickness of the intermetallic layer of other lead-free alloys tested. After the thermal cycle test, the IMC thickness of SAC 305 and other low/silver-free superalloys did not change significantly. After reflow and thermal cycle testing, the thickness of the intermetallic layer on the PCB side is shown in Figure 7.

 After reflow and thermal cycling tests, measure the thickness of the intermetallic layer on the PCB side.

The thickness of the intermetallic layer on the component side after the reflow treatment is about 1.5 μm to 2 μm. It is slightly thinner than the IMC on the PCB side. Similarly, after the thermal cycle test, the IMC thickness on the component side of the replacement low-silver superalloy did not change significantly. After the reflow process, the IMC thickness of low-temperature SnBiAg is very thin (~1.2μm). After the thermal cycle test, the thickness increased to about 1.7 μm. The thickness of the intermetallic layer on the component side is shown in Figure 8.

The thickness of the intermetallic layer at the component interface after reflow and thermal testing.

The cross-sections of all components are performed after the reflow process. Generally, good solder joints are observed for alternative alloys. For most BGA components of SnBiAg solder paste reflow, it was found that the mixing was not complete. Due to the low temperature profile used for SnBiAg solder paste, uneven solder joints were observed. You can also see that there are large components on the headrest (HiP) solder joints, such as BGA1156 reflowed with SnBiAg solder paste. Figure 9 shows a cross-sectional view of a BGA solder joint reflowed with an alternative alloy solder paste.

Cross-sectional image of the BGA 196 component after the reflow process.

A scanning electron microscope (SEM) was used to further analyze the alloy's solder joint microstructure. The Cu6Sn5 intermetallic compound layer is a common feature of all alloy solder joints. For SAC305 solder joints, Sn IMC and Ag3Sn and Cu6Sn5 intermetallic compounds are mostly found in solder joints (Figure 10a). Sn / Ag3Sn binary eutectic and Sn / Ag3Sn / Cu6Sn5 ternary eutectic regions are both visible at the grain boundaries of Sn dendrites. The existence of Sn / Ag3Sn binary eutectic indicates that Cu6Sn5 is the final stage of solidification. In the samples reflowed with SAC0307 solder paste, the reduction of Ag content in SAC0307 solder paste resulted in a decrease in the number of Ag3Sn particles in the solder joint microstructure and the formation of larger Sn dendrites (Figure 10b). The large area of ​​Ag3Sn/Sn binary eutectic is not visible. The microstructure of the solder joints of material C after reflow shows that most of the Cu6Sn5 particles in the solder appear smaller in material C than in SAC0307 (Figure 10c). However, compared with SAC0307, ​​the doped alloy did not show any statistically significant difference in the composition or thickness of the intermetallic layer. The BGA solder joints of the two alloys are also similar, very similar to the all-SAC305 component, although the size of the Ag3Sn particles is slightly reduced in both cases.

The microstructures of the solder joints of SnCuNi and material D are shown in Figure 10d and Figure 10e, respectively. The solder pastes of both alloys do not contain silver. A Cu6Sn5 intermetallic compound layer was seen in the solder joints. Compared with other superalloys, the solder joints of material D show a uniform distribution of Cu6Sn5 particles, and the grain size of Sn dendrites is smaller. Cu6Sn5 grains are usually larger than those in SnCuNi, which is an Ag-free alloy with a higher Cu content than material D. This means that Cu6Sn5 is formed as the main phase, and the degree of supercooling is lower than expected.

In the microstructure of SnBiAg solder joints, most of the solder joints are composed of Bi and Sn dendrites (Figure 10f). Small grains of Ag3Sn can be seen at the grain boundaries, and Cu6Sn5 particles can also be seen near the pads. Note that after the reflow process, the Bi dendritic boundary is not well defined.

SEM images of QFN solder joints are reflowed with various alloy solder pastes. a) SAC305; b) SAC0307; c) material C; d) SnCuNi; e) material D; f) SnBiAg.

Table 3 summarizes the thermal cycle test results of different components reflowed with different lead-free alloy solder pastes.

Thermal cycling test results.

Generally, SAC305 has better thermal reliability than other low-silver alloy materials. Thermal reliability performance depends on component type and component design/material. After 3000 cycles (0°C to 100°C), some BGA 196 components, BGA228 components, BGA97 components and resistor 2512 components were observed to fail completely. A large crack was found at the solder joint. After the thermal cycle test, the cross-sectional view of the BGA196 solder joint is shown in Figure 11.

Cross-sectional image of BGA196 component after 3000 thermal cycles. a) SAC305; b) SAC0307; c) Material C; d) SnCuNi; e) Material D; f) SnBiAg.

Most cracks occurred on the component side of the package, although some cracks were also observed on the PCB side. The solder joints reflowed with SnBiAg also had cracks at the interface between the SAC305 solder ball and the SnBiAg solder paste. The cross-sections of the 2512 resistor and QFN88 components after the thermal cycle test are shown in Figure 12 and Figure 13, respectively. The solder joints of these components showed severe cracks.

Cross-sectional view of R2512 solder joint assembled with different lead-free alloy solder paste after 3000 thermal cycles a) SAC305; b) SAC0307; c) material C; d) SnCuNi; e) material D; f) SnBiAg .

: Cross-sectional images of QFN solder joints assembled with different lead-free alloy solder pastes after thermal cycle testing. a) SAC305; b) SAC0307; c) material C; d) SnCuNi; e) material D; f) SnBiAg.

After the thermal cycle test, no failures of some BGA components were observed, such as BGA1156 (1.0mm pitch, 35mmx35mm) and BGA64 components (0.8mm pitch, 8mm X 8mm). Only small cracks were observed. The cross section of the BGA1156 component is shown in Figure 14. In this study, QFP components also showed slight cracks, and no failure was observed (Figure 15).

Cross-sectional image of BGA1156 solder joint after thermal cycling test. a) SAC305; b) SAC0307; c) material C; d) SnCuNi; e) material D; f) SnBiAg.

Cross-sectional images of QFP208 solder joints assembled with different lead-free alloy solder pastes after thermal cycle testing. a) SAC305; b) SAC0307; c) material C; d) SnCuNi; e) material D; f) SnBiAg.

SEM equipment was used to analyze the microstructure of various alloys after thermal cycle testing. In the SAC305 solder joint, the formed bulk Cu6Sn5 particles and Ag3Sn particles have begun to elongate (Figure 16). Some intergranular cracks can be seen between the Sn dendrites and the boundaries between dendrites and intermetallic species. Cracks along the intermetallic layer were also observed.

After thermal cycle testing, the Ag3Sn board in QFP solder joints assembled with SAC305 solder paste was formed.

The use of SAC0307 solder reduces the number and size of Ag3Sn crystal grains. Unlike SAC305, thermal cycling does not result in the formation of larger Ag3Sn plates (Figure 17). However, the dislocations along the grain boundaries are visible and are more extensive than those of SAC305, which indicates that the coarsening of Sn dendrites does increase its tendency to crack.

After thermal cycle testing, the microstructure of QFN88 solder joints assembled with SAC0307 solder paste.

Material C behaves similarly to SAC0307, ​​showing a large number of cracks along the grain boundary in almost all samples. Compared with SAC0307, ​​BGA solder joints show a decrease in the growth rate of Ag3Sn particles. In the lower half of the material C BGA solder joints, larger Cu6Sn5 particles were observed (Figure 18).

The microstructure of QFN88 solder joints assembled with material C solder paste after thermal cycle testing.

For solder joints composed of SnCuNi solder, the Cu3Sn layer adjacent to the PCB pad is the thinnest, indicating that the insertion of Ni into the intermetallic layer reduces the diffusion rate within the crystal structure (Figure 19).

The microstructure of QFN88 solder joints assembled with SnCuNi solder paste after thermal cycle testing.

After the thermal cycle test, the microstructure of the solder joint of material D is relatively not rough. You can see tin dendrites and Cu6Sn5 (Figure 20). There are several possible explanations for the little coarsening observed. First of all, it has been previously proved that Co can replace Cu in Cu6Sn5 grains, thereby introducing substitution defects that inhibit coarsening [6]. It is known that the presence of low concentrations of Bi will produce a Zener effect on Sn grains, thereby increasing the energy barrier to grain growth [7]. BGA cracks mainly occur at the bottom of the solder joint, rather than the top. Under strain, the more brittle solid-solution tin base formed by material D is easier to crack than the more ductile tin base at the top of the solder joint (its composition is close to SAC305).

After thermal cycle testing, the microstructure of QFN88 solder joints assembled with material D solder paste.

Figure 21 shows the typical microstructure of SnBiAg after thermal cycling. So far, SnBiAg solder joints have grown the most in intermetallic compound layers, some of which have more than doubled their thickness in thermal cycling tests. The thickness of the intermetallic layer becomes comparable to that formed by other solder alloys, which indicates that the thin layer observed before the thermal cycle test is at least partly a result of the low reflow temperature. The concentration of Sn in the solder seems to have little effect on the thickness of the intermetallic layer. The thickness of the formed Cu3Sn layer is similar to that of the SnAgCu alloy. Sn and Bi dendrites are coarsened. The sizes of Ag3Sn and Cu6Sn5 particles both increase. Cu6Sn5 particles are also found more frequently near the Cu pad, which indicates that during the thermal cycle test, Cu further dissolves from the pad into most of the solder.

The microstructure of QFN88 solder joints assembled using SnBiAg solder paste after thermal cycling.

For single alloy QFP and QFN components, four different typical failure modes are observed. For SAC305 solder joints, cracks mainly propagate along the Cu6Sn5 and Ag3Sn intermetallic compounds in most of the solder (Figure 22). As shown in Figure 23, the single alloy solder joints assembled from SAC0307, ​​material C and SnCuNi (almost no intermetallic compounds) all show signs of ductile fracture at the grain boundary between Sn dendrites. Cracking observed with SAC305 in the same component. Compared with these three alloys, the degree of cracking of material D is smaller. The crack seems to spread in the Sn grain boundary (Figure 24). SnBiAg solder joints showed different failure modes. The cracks are mainly formed due to the sliding of grain boundaries (Figure 25). The embrittlement caused by the dissolution of bismuth in the tin matrix also cracks the tin grains.

Typical cracks in QFP208 solder joints assembled with SAC305: ductile fractures propagating along the grain boundaries of intermetallic species.

In QFP208 solder joints assembled using SAC0307, ​​cracks along the Sn grain boundary.

QFN88 solder joints assembled with material D cracked along the tin grain boundaries.

QFN88 solder joints assembled with SnBiAg solder paste have cracks.

In BGA solder joints, due to the mixing of alloys, different effects have been observed. For most components, cracks that propagate along the grain boundaries of intermetallic species within most solder joints are observed. However, the severity of this cracking depends on the solder paste alloy. When using any of the four high-temperature low-silver alloys, more cracking seems to occur.

When using material D solder, an increase in ductile fracture at the bottom of the solder joint was observed. It can be expected that the solder joints formed by this alloy are more brittle than other alloys. As a result of the addition of Bi, the upper part of the SAC305-based solder joint has greater plastic deformation than the lower part, and the content of Bi may be higher.

After the thermal cycle test, many BGA solder joints assembled with SnBiAg solder paste are still uneven. The creep deformation of the BGA solder joints can be seen. Grain boundary sliding occurs at the bottom of the solder ball, mainly composed of Sn and Bi dendrites. It was observed that for some BGA components, SnBiAg solder joints had fewer cracks than low-silver superalloys, while for other BGA components, cracks were larger. The cracks in the SAC305 area are not as common as the cracks in the SnBiAg area, which is easier to deform under strain. In some cases, cracks can be seen at the interface between the SnBiAg and SnAgCu regions of the BGA

There is no significant difference in the thickness of the intermetallic layer of the solder joints assembled with SAC305 and other alternative high-temperature, low-silver, lead-free alloy solder pastes (SAC0307, ​​SnCuNi, material C, material D). Generally, the IMC thickness of these materials slightly increases after the thermal cycle test, but the change is negligible. After the reflow process, the IMC thickness of SnBiAg solder joints is usually thinner than that of high-temperature lead-free alloys. During the thermal cycle test, the IMC layer of the SnBiAg solder joints increased, reaching an IMC thickness similar to other lead-free alloys. During the thermal cycle test, the thickness and composition of the intermetallic compound layer have not been determined to affect the reliability of solder joints.

The thermal reliability of the optional lead-free solder joints depends on the package type and component size. In our research, the influence of this factor on the thermal reliability of solder joints is greater than the influence on the composition of solder paste alloys. Compared with other test components, the 2512 resistor failed first. After 3000 thermal cycles (0°C to 100°C), most 2512 resistors failed completely and severely cracked. After the test, no complete failure of small chip components (such as 0603, 0402, 0201 components) was observed. After the thermal cycle test, it was also found that BGA196, BGA228, BGA97 and QFN88 had severe cracking and some failures. BGA1156, BGA64, QFN32, QFP208 and QFP100 components were not slightly cracked, and no malfunctions were found. Generally, solder joints assembled with SAC305 solder paste are still better than low-silver alloy solder paste. Unexpectedly, when the low temperature SnBiAg solder joint is a single alloy in the solder joint, it performs well after the thermal cycle test. When SAC 305 BGA and SnBiAg solder paste were reflowed together, more defects and malfunctions were found. For other lead-free alloy solder paste materials, further reliability studies should be conducted.

The author would like to thank Elissa McKay and Tu Tran of the AEG laboratory for their help in the cross-section and failure analysis of this study.

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