**3. High reliability alloy is required**

As discussed in Section 2, there is an evolution of Pb-free solder from conventional high Ag bearing SAC alloy to low SAC alloys because of cost and drop impact resistance. This is the transition of first-generation SAC alloys to second-generation SAC alloys. However, the lower Ag content has compromised in thermal fatigue performance. It is because the strength enhancement of SAC alloy is through the dispersion of very small Ag3 Sn IMC within the tin matrix. This fine Ag<sup>3</sup> Sn IMC has the effect in inhibiting the dislocation movement of the tin matrix. This mechanism makes the SAC alloy to have higher modulus as compared to the SnPb-6337. The more Ag content will increase the amount of fine Ag<sup>3</sup> Sn IMC, but it also increases the possibility of large size Ag3 Sn platelet growth. This large Ag3 Sn platelet can be the stress concentration point and crack will initiate from this point. In extreme case, the large Ag3 Sn plate can even protrude out the solder surface and deform the small solder joint [5]. Therefore, the large Ag3 Sn platelet is not a desirable morphology in a solder joint. However, the lower Ag content in low SAC alloys has tremendous reduction in amount of Ag3 Sn IMC. Although there is another IMC, which is the Cu<sup>6</sup> Sn5 within the bulk solder, this IMC is much fewer compared to Ag3 Sn IMC of conventional SAC alloys. Moreover, a portion of the Cu of bulk solder will be extracted in forming the IMC at Cu pad. This further reduces the amount of Cu6 Sn5 within the bulk solder. Thus, the main strength of the SAC alloy is still the dispersion of fine Ag<sup>3</sup> Sn within the tin matrix.

The compromise in thermal fatigue performance of SAC alloy has been further complicated by the phenomenon of Ag3 Sn IMC coarsening. The Ag<sup>3</sup> Sn coarsening is a process of Ostwald ripening. This is an evolutional process of an inhomogeneous structure where the small particle is growing and drawing the adjacent particle to form a bigger particle over time. This is a spontaneous process because big particle is more energetically stable than small particle. It is because the internal pressure is reversely proportional to the radius of a particle. Hence, the small particles have higher surface energy. The small particles will try to achieve a higher energetic stability by growing to bigger size. This is coarsening. Besides that, the Ostwald ripening is a thermodynamically driven process. If the environment temperature is high, the process of coarsening will increase. This Ag3 Sn coarsening has significant impact to the strength of SAC alloy. As discussed earlier, the strength of SAC alloy is the dispersion of fine Ag3 Sn IMC. **Figure 5** shows an example of the Ag3 Sn coarsening within SAC305 alloy after exposed to high temperature, 125°C for 6 months.

In the coarsening process, two changes will be observed: 1. The amount of Ag3 Sn in the tin matrix, and 2. The distance between Ag<sup>3</sup> Sn IMCs. When the amount of Ag<sup>3</sup> Sn shrinks and the distance between Ag3 Sn gets wider, its ability in inhibiting the dislocation movement reduces. This is a degradation process of SAC alloys. In fact, we can easily demonstrate this mechanism by exposing SAC alloy to high temperature environment over a period of time. The tensile strength of SAC alloy reduces after the thermal aging. **Figure 6** shows an example of this degradation in SAC305 alloy. This is an experimental data of the comparison of tensile strength of tensile test specimen made of SAC305 alloy with and without exposure to isothermal aging (150°C for 500 h).

#### **3.1. High reliability solder alloy for PCBA**

improvements have enabled the adoption of low SAC and SnCuNi alloy in soldering. This has also explained the market share of this low SAC and SnCuNi alloys is expanding in recent years. SN100C® has become one of the preferred choices in Pb-free wave soldering process.

In short, there are two driving forces, which have triggered the proliferation of low SAC and SnCuNi alloys for electronic industries. Someone has named the low SAC as second generation of Pb-free alloy. **Figure 4** summarized the evolution of this Pb-free alloy from conventional SAC to low SAC alloy for electronic industries. This transition is very important to support the IoT era. One of the characteristics of IoT is huge in quantity. The high usage of electronic components in IoT era also prompts the usage of solder. The type of solder for this era must be able to fulfill the specific requirement in manufacturability, reliability, toxicity, cost, and availability. The second generation Pb-free alloy should be a better choice for the

The following section discusses the development of Pb-free alloy into third generation alloy

As discussed in Section 2, there is an evolution of Pb-free solder from conventional high Ag bearing SAC alloy to low SAC alloys because of cost and drop impact resistance. This is

**Figure 4.** Evolution of SAC alloys in Pb-free solution from first generation to second generation.

IoT era.

for high reliability application.

98 Recent Progress in Soldering Materials

**3. High reliability alloy is required**

The degradation of SAC alloys over time is worrying because it is a challenge for reliability engineer to predict the life of a solder joint. It is especially critical on those life-critical system

**Figure 5.** Ag3 Sn coarsening in high temperature storage test.

**Figure 6.** Tensile strength of SAC305 before and after isothermal aging at 150°C for 500 h.

such as the electronic system in a car or air plane. As mentioned in Section 1, the growth of auto electronics industry is very aggressive recently. This industry demands high reliability and zero defect electronic components. There are some original equipment manufacturer (OEM) parts that have long guaranteed product life ranging from 10 to 15 years. This is the reason why in some of the automotive applications, the SnPb-6337 is still being used. Since Pb-free soldering is getting pervasive even in these kind of high-end applications and SAC305 is still the most common Pb-free alloy, which apparently cannot fulfill the high reliability requirement of such applications, great driving force arises to drive solder manufacturer and researchers to identify new solution of Pb-free alloy.

Besides the Ag<sup>3</sup> Sn coarsening, the homologous temperature of solder system becomes greater due to higher application environment temperature. In order to keep the strength especially in fatigue, solder with lower homologous temperature is needed. The under-hood electronic components are a good example of this case. It has further justified the need of new alloy to support future electronics reliability needs.

There are consortiums and nonprofit organizations such as Universal's Advanced Research in Electronics Assembly (AREA) Consortium and International Electronics Manufacturing Initiative (iNEMI) gathering the experts of solder to jointly identify new Pb-free alloy in solving the issues mentioned above [6]. This new alloys are named as third generation of Pb-free alloy, which has superior performance in both thermal fatigue resistance and drop impact resistance to fulfill the high demand of solder joint reliability. **Figure 7** summarizes the evolution from first generation to third generation of Pb-free alloys.

Apparently, more strengthening of Pb-free alloys is required in order to improve the strength. As discussed earlier, SAC alloys are using particle strengthening technique in enhancing the alloy strength. The main enhancement is coming from the dispersion of Ag3 Sn IMC in tin matrix. Definitely, more Ag can be added to make the alloy modulus higher, but it will increase the possibility of Ag3 Sn platelets formation. Besides that, the high Ag-bearing Pb-free alloy has lower drop impact resistance. High Ag content in the alloy system will add more burden to the material cost too. Therefore, other approach is needed.

Solid solution strengthening is one of the options in strengthening the Pb-free alloy. In fact, solid solution strengthening had been widely applied during the SnPb soldering era where the Pb is added into Sn to improve the alloy characteristics. Somehow, this technique was not

such as the electronic system in a car or air plane. As mentioned in Section 1, the growth of auto electronics industry is very aggressive recently. This industry demands high reliability and zero defect electronic components. There are some original equipment manufacturer (OEM) parts that have long guaranteed product life ranging from 10 to 15 years. This is the reason why in some of the automotive applications, the SnPb-6337 is still being used. Since Pb-free soldering is getting pervasive even in these kind of high-end applications and SAC305 is still the most common Pb-free alloy, which apparently cannot fulfill the high reliability requirement of such applications, great driving force arises to drive solder manufacturer and

**Figure 6.** Tensile strength of SAC305 before and after isothermal aging at 150°C for 500 h.

due to higher application environment temperature. In order to keep the strength especially in fatigue, solder with lower homologous temperature is needed. The under-hood electronic

Sn coarsening, the homologous temperature of solder system becomes greater

researchers to identify new solution of Pb-free alloy.

Sn coarsening in high temperature storage test.

Besides the Ag<sup>3</sup>

**Figure 5.** Ag3

100 Recent Progress in Soldering Materials

**Figure 7.** The evolution of Pb-free alloys from first generation to third generation, high reliability alloy for harsh use condition.

common during the conversion from SnPb to Pb-free soldering. In Pb-free alloys, the Cu and Ag are added to tin, separately or together, to make the most widely used SAC alloys, which have almost no solubility in the β-tin matrix. The Cu and Ag appear in the microstructure only as the intermetallic compounds, Ag3 Sn and Cu6 Sn5 as discussed earlier. Surely, Pb cannot be considered again as solute in solid solution strengthening because of the RoHS compliance. Other solutes should be considered to achieve this objective, for example, Bismuth, Bi; Indium, In; and Antimony, Sb. These three substances are the most common and possible candidates selected by the solder manufacturers and researchers in enhancing the strength of Sn-based Pb-free alloy. Bi and In can decrease the liquidus of Sn-based alloy but Sb can increase the liquidus. It depends on the requirement of end application. If a solder alloy with lower homologous temperature is needed, microalloying with Bi or In may not be appropriate. However, Sb is a banned or restricted used substance in some applications such as mobile and consumer industries. It has limited the use of this substance for microalloying to improve the solder strength.

In early period of Pb-free soldering adoption which was about 2006, there was a working group formed to develop a more robust Pb-free alloy for automotive industries. Since then, the industries understand the need of a stronger alloy for future auto electronics requirement. This working group members include Siemens, Bosch, Heraeus, Alpha Metals, Infineon, the Fraunhofer Institute, and etc. 90iSC was the outcome of this group's effort. The 90iSC is a high Ag SAC alloy with addition of Bi, Sb, and Ni. It is a very complex system in which the characteristics and reliability cannot be fully understood and assessed. Solder manufacturers such as Nihon Superior has adopted a simpler approach by just micro-alloying Bi into SnCuNi system in order to achieve a stronger alloy. It is SN100CV®. Without the Ag addition into SN100CV®, the worry of alloy degradation due to Ag3 Sn coarsening can be lifted. Many users may still be skeptical on the Pb-free alloy without Ag addition. They are worrying about the strength of this Ag-free Pb-free solder. In fact, the solid solution strengthening with Bi addition is even more effective in increasing the tensile strength of the alloy. For example, in the SN100CV®, the 1.5% Bi microalloying can significantly increase the tensile strength of the SnCuNi alloy and make it even higher strength when comparing to SAC305 which containing 3% Ag. **Figure 8** is the comparison of tensile strength of three alloys, which are SnCuNi (SN100C®), SnCuNiBi (SN100CV®), and SnAgCu (SAC305).

Besides the as-cast comparison, the three different alloys shown in **Figure 8** have been subjected to thermal aging at 150°C for 500 h. After the thermal aging, tensile strength data were collected. As expected, there is significant degradation in strength for SAC305 sample due to the Ag3 Sn coarsening. The drop in tensile strength for both SnCuNi and SnCuNiBi is minimum. **Figure 9** summarizes the comparison data after thermal aging.

However, choosing a right ratio of Bi into SnCuNi system is challenging. If too little of Bi added, the effect of solid solution strengthening is negligible. But if too much of Bi added, precipitation of Bi out of β tin will occur. **Figure 10** shows the tensile strength of different alloys before and after thermal aging at 150°C for 500 h.

**Figure 8.** Tensile strength comparison for three alloys in as-cast condition.

common during the conversion from SnPb to Pb-free soldering. In Pb-free alloys, the Cu and Ag are added to tin, separately or together, to make the most widely used SAC alloys, which have almost no solubility in the β-tin matrix. The Cu and Ag appear in the microstructure

Sn and Cu6

be considered again as solute in solid solution strengthening because of the RoHS compliance. Other solutes should be considered to achieve this objective, for example, Bismuth, Bi; Indium, In; and Antimony, Sb. These three substances are the most common and possible candidates selected by the solder manufacturers and researchers in enhancing the strength of Sn-based Pb-free alloy. Bi and In can decrease the liquidus of Sn-based alloy but Sb can increase the liquidus. It depends on the requirement of end application. If a solder alloy with lower homologous temperature is needed, microalloying with Bi or In may not be appropriate. However, Sb is a banned or restricted used substance in some applications such as mobile and consumer industries. It has limited the use of this substance for microalloying to improve

In early period of Pb-free soldering adoption which was about 2006, there was a working group formed to develop a more robust Pb-free alloy for automotive industries. Since then, the industries understand the need of a stronger alloy for future auto electronics requirement. This working group members include Siemens, Bosch, Heraeus, Alpha Metals, Infineon, the Fraunhofer Institute, and etc. 90iSC was the outcome of this group's effort. The 90iSC is a high Ag SAC alloy with addition of Bi, Sb, and Ni. It is a very complex system in which the characteristics and reliability cannot be fully understood and assessed. Solder manufacturers such as Nihon Superior has adopted a simpler approach by just micro-alloying Bi into SnCuNi system in order to achieve a stronger alloy. It is SN100CV®. Without the Ag addition into SN100CV®, the worry of alloy degradation due to

Sn coarsening can be lifted. Many users may still be skeptical on the Pb-free alloy without Ag addition. They are worrying about the strength of this Ag-free Pb-free solder. In fact, the solid solution strengthening with Bi addition is even more effective in increasing the tensile strength of the alloy. For example, in the SN100CV®, the 1.5% Bi microalloying can significantly increase the tensile strength of the SnCuNi alloy and make it even higher strength when comparing to SAC305 which containing 3% Ag. **Figure 8** is the comparison of tensile strength of three alloys, which are SnCuNi (SN100C®), SnCuNiBi (SN100CV®),

Besides the as-cast comparison, the three different alloys shown in **Figure 8** have been subjected to thermal aging at 150°C for 500 h. After the thermal aging, tensile strength data were collected. As expected, there is significant degradation in strength for SAC305 sample due to

However, choosing a right ratio of Bi into SnCuNi system is challenging. If too little of Bi added, the effect of solid solution strengthening is negligible. But if too much of Bi added, precipitation of Bi out of β tin will occur. **Figure 10** shows the tensile strength of different

mum. **Figure 9** summarizes the comparison data after thermal aging.

alloys before and after thermal aging at 150°C for 500 h.

Sn coarsening. The drop in tensile strength for both SnCuNi and SnCuNiBi is mini-

Sn5

as discussed earlier. Surely, Pb cannot

only as the intermetallic compounds, Ag3

102 Recent Progress in Soldering Materials

the solder strength.

and SnAgCu (SAC305).

Ag3

the Ag3

**Figure 9.** Tensile strength comparison for three alloys in as-cast and after thermal aging.

#### **3.2. High reliability solder alloy for die attach**

Solder usage is not limited to the interconnects between electronic components and PCB at the PCBA process. Within the electronic components such as integrated circuit (IC), solder has been used as die attach material especially in the device where there is a need of high thermal and electrical connectivity between the die backside and lead frame or substrate. This kind of IC includes the high power devices and high speed switches such as Power MOSFET, insulated-gate bipolar transistor (IGBT), high power diode and transistor, rectifier, and inverter. Due to high operating temperature of such IC, which is usually above 260°C, the industries are still exempted from RoHS compliance, and they are still using the high Pb solders such as Pb5Sn and Pb5Sn2.5Ag in die attach process. It is because the conventional Pb-free Sn-based alloys have melting point lower than 260°C. The RoHS committee allows another 5-year extension for this exemption in 2016 because of technical limitation with current technology. So far, there is still no straightforward drop-in solution in replacing the high Pb solder for such application.

**Figure 10.** Tensile strength of test alloys as-cast and after thermal aging at 150°C for 500 h.

Since most of the Sn-based Pb-free alloys have solidus lower than 260°C, other Pb-free alloys such as gold tin (AuSn), bismuth silver (BiAg), and zinc aluminum (ZnAl) are being considered as candidates to replace the high Pb solder, which is the current high-temperature die attach material. However, there are some limitations in these alloys which the manufacturers need to overcome before the mass adoption is possible. The limitations include high cost (AuSn), low thermal conductivity, poor wettability (BiAg), and low workability due to brittleness (ZnAl). Indium Corporation has innovatively improved the characteristics of BiAg to make it more user-friendly product for the die attach process. To improve the wettability of BiAg, this company has introduced a solder paste consists of two types of solder powder. The first type is the major powder which is the BiAg and the minority powder which is the additive powder. The additive powder has lower melting point than BiAg and better wettability on common soldering pad. The additive powder can be the familiar SnBi eutectic, SAC alloy, or SnAg alloy. In the reflow process, the additive powder will melt first and react with soldering pad then follows by the melting of majority powder which is the BiAg. It is an irreversible process because the minority powder will be fully consumed or reacting with the majority powder during the reflow process. The joint formed using this solder paste will have similar properties as the BiAg, which is the majority part of this solder paste. More studies are required to assess the suitability of this solder paste in replacing the high Pb alloy. The solidus of BiAg is still lower than the high Pb solder. Therefore, the homologous temperature is different for these two solders.

Besides solder alloys, the industries are also working actively to develop sintering material to replace high Pb solder. The driving force to develop sintering material is not only to replace the existing high Pb solder but also to prepare a solution for future application. Based on the power IC roadmap, the IC operating temperature can go up to as high as 600°C especially for the silicon carbide (SiC) die. With such high operating temperature, it will be a challenge for conventional solder alloy. Sintering is an atomic diffusion process. Heat and pressure can be applied to accelerate this diffusion process. The atoms in the powder particles diffuse across the boundaries of the particles, fusing the particles together and creating one solid piece (bulk material). In other words, a lower process temperature than the material melting point is required to complete the sintering process. In sintering process, there is no melting involved. The joint formed after sintering will have the characteristics similar to the bulk material. Since Ag is the most common major ingredient for sintering material, the joint formed with Ag sintering material will re-melt only when the temperature exceeds the Ag melting point which is 961°C. This is the main attraction of Ag sintering material to be used in this application. In fact, the industries had explored Ag sintering material in 1980s. But, it still required high sintering temperature and pressure to complete the process during that period of time. It made the acceptance of this material low. With the recent development of nano Ag particle, the sintering process can be completed within an acceptable sintering temperature range. Nihon Superior has developed a Ag sintering paste, Alconano®, which has demonstrated good quality of silver joint at die attach layer with sintering temperature as low as 200°C. This is only possible because of the nano size Ag particles which have high surface energy. The challenge of manufacturing Ag sintering paste is not only in producing the Ag particles, but also in the development of right passivation system to keep the particles in good condition until the sintering process starts. Alconano® has utilized alcohol as the passivation system, which forms alkoxides with silver atoms on the surface of the nanoparticle. The advantage of these chemicals in this application is that the oxygen-silver bond, which is strong enough to stabilize the nanoparticle during manufacturing processes and subsequent storage and handling, is weak enough that it can be broken at a relatively low temperature to expose the active surface of the nanoparticle so that it can bond to adjacent particles. Another advantage of using alcohol as the passivation system is it leaves no harmful residue behind after the sintering process. The residue is free from sulfur and nitrogen compounds that can interfere with the performance of the sintered silver and contribute to corrosion problems in service. The challenge of mass adoption of Ag sintering paste does not limit to paste manufacturing only. Application of this paste at die attach process is also challenging. Many engineering works are required in developing an appropriate set of process parameter. Sintered Ag joint is a porous layer. In fact, this porosity is necessary to make the Ag be a reasonable die attach joint. Bulk Ag joint without any porosity could be too rigid to support the IC architecture. In such case, die crack will be the most prominent defect in the field. Getting a right properties of this Ag joint should be a joint effort between the user and supplier. One of the wonder of this Ag sintering paste is the porosity can be adjusted via sintering process. By varying the pressure and temperature during sintering, different range of porosity can be achieved. **Figure 11** shows an example of Ag sintered joint of Alconano® with porosity less than 10%. This joints were formed with pressurized sintering parameters: 10MPa, 300°C and 5 min sintering time. The consistency of porosity percentage is satisfying. Besides satisfying the processibility, the reliability of joint made of this Ag sintering material must be on par or even better than the performance of high Pb solder. There are two main characteristics of such die attach material, which the users are emphasizing. They are thermal conductivity and electrical conductivity of the joint. Based on a recent industry wide survey on Pb-free high-temperature die attach material, thermal conductivity stands out to be the most required property for this material [7]. It is crucial to

Since most of the Sn-based Pb-free alloys have solidus lower than 260°C, other Pb-free alloys such as gold tin (AuSn), bismuth silver (BiAg), and zinc aluminum (ZnAl) are being considered as candidates to replace the high Pb solder, which is the current high-temperature die attach material. However, there are some limitations in these alloys which the manufacturers need to overcome before the mass adoption is possible. The limitations include high cost (AuSn), low thermal conductivity, poor wettability (BiAg), and low workability due to brittleness (ZnAl). Indium Corporation has innovatively improved the characteristics of BiAg to make it more user-friendly product for the die attach process. To improve the wettability of BiAg, this company has introduced a solder paste consists of two types of solder powder. The first type is the major powder which is the BiAg and the minority powder which is the additive powder. The additive powder has lower melting point than BiAg and better wettability on common soldering pad. The additive powder can be the familiar SnBi eutectic, SAC alloy, or SnAg alloy. In the reflow process, the additive powder will melt first and react with soldering pad then follows by the melting of majority powder which is the BiAg. It is an irreversible process because the minority powder will be fully consumed or reacting with the majority powder during the reflow process. The joint formed using this solder paste will have similar properties as the BiAg, which is the majority part of this solder paste. More studies are required to assess the suitability of this solder paste in replacing the high Pb alloy. The solidus of BiAg is still lower than the high Pb solder. Therefore, the homologous temperature is different for these two solders.

**Figure 10.** Tensile strength of test alloys as-cast and after thermal aging at 150°C for 500 h.

104 Recent Progress in Soldering Materials

Besides solder alloys, the industries are also working actively to develop sintering material to replace high Pb solder. The driving force to develop sintering material is not only to replace the existing high Pb solder but also to prepare a solution for future application. Based on the power IC roadmap, the IC operating temperature can go up to as high as 600°C especially for the silicon carbide (SiC) die. With such high operating temperature, it will be a challenge for keep high thermal conductivity throughout the entire product life. In some cases, the joint has degraded thermal conductivity due to the thermal fatigue crack within the joint especially in hot and cycling atmosphere. Therefore, in developing the Alconano®, Nihon Superior has compared the performance of Alconano® versus the high Pb solder and conventional SAC solder after exposure to thermal cycling reliability testing, −40°C/+200°C with dwell time of 30 min. The comparison results are shown in **Figure 12**.

Pb-free liquid-phase diffusion bonding (LPDB) material is another emerging material, which can potentially be used as high-temperature Pb-free die attach material. The bonding process of this LPDB is very similar to the modified BiAg paste mentioned above. Similar to the modified BiAg paste, the LPDB paste consists of two types of powder, high melting point and low

**Figure 11.** Porosity check after pressurized sintering on Ag sintering joint.

**Figure 12.** Change in thermal resistance of high temperature die attach material in a function of thermal cycling. 10×10 mm silicon die on Al<sup>2</sup> O3 substrate with Cu plate heat sink was used as test vehicle. The sintering condition was 300°C, 40MPa pressure assisted and 3 min sintering duration.

melting point. Unlike the modified BiAg paste where both types of powder will melt during the bonding process, the high melting point powder of LPDB will never melt during the bonding process but only the low melting point powder will melt and react with soldering pads and simultaneously react with the high melting powder to form a new IMC. During the bonding process, homogenization of the molten occurs. IMC will be formed because of the reaction between the molten low melting point powder and high melting point powder. And, this newly formed IMC has much higher melting point than the low melting point powder. Therefore, the molten will start to solidify even before the cooling starts due to the change of melting point. After the bonding process, the joint will only remelt at a temperature higher than the bonding temperature. This is why this material is also called transient liquid-phase diffusion bonding material. Nihon Superior Co., Ltd. has participated a project run by Ames lab from Iowa State University in developing this LPDB material. Ames lab is mixing the high melting point Cu-10Ni powder into the commercial SN100C® fine powder to make this LPDB material. The bonding process will transform all SN100C® powder into high melting point (Cu, Ni)6 Sn5 IMC which will only remelt at 525°C [8]. This is a very important characteristic to make it a potential high temperature Pb-free die attach material. Moreover, the Ni addition into Cu6 Sn5 has significantly improved the properties of this IMC. It has made it a more robust joint because the Ni has inhibited the polymorphic transformation of the allotropic Cu6 Sn5 as discussed earlier in this chapter. According to Choquette and Iver [8], the Ni addition into Cu6 Sn5 should improve the ductility and strength of the joint as well. This LPDB material can be a potential drop-in solution in replacing the high Pb solder because it can complete the bonding at conventional reflow temperature (240–260°C), but it will only re-melt at 525°C.
