**2. Evolution from high silver to low silver solder**

In 2000s, SAC system had been identified as replacement for Sn-Pb eutectic solders in fulfilling the RoHS compliance. Different countries or organizations had chosen different SAC composition as their preferred solder composition. They are Sn3.0Ag0.5Cu, SAC305 (Japan), Sn3.5Ag0.9Cu, SAC3509 (European Union), and Sn3.9Ag0.6Cu, SAC3906 (USA) [3].

In 2000, it was the era of consumer product, which had incorporated many electronic components in its system such as personal computer, television, radio, rice cooker, and windowmounted air conditioner. These products were mostly using through-hole components and/ or big surface mount technology (SMT) components. It implies that the consumption of solder per unit device was huge during that period of time. It explains the reason of mass adoption of SAC305 instead of other Pb-free solutions. It is because the lower silver content of SAC305 as compared to other SAC alloys mentioned in former paragraph. Silver is a precious metal and its inclusion in solder alloy does increase the cost of solder significantly. For example, to produce 1 kg of SAC305, 1 ounce of silver needs to be added. It depends on the market price of silver, assuming USD20 per ounce, the cost of silver in 1 kg of SAC305 is almost 50% of the selling price of 1 kg of SAC305 solder bar. When the silver price surges, for example, in April 2011, when the silver price was USD 40 per ounce, the cost of silver of a SAC305 solder bar was more than 50% of its selling price. For those compositions with higher silver content, the ratio of silver cost against solder bar selling price is usually more than 50% depending on the tin price. It becomes a big burden to solder users. Solder bar is mainly used in wave soldering process. It was a very common process in printed circuit board assembly (PCBA) for those consumer products mentioned above. During the peak of this era, the consumption of solder bar could exceed 2000 kg per month for a single customer who has more than 10 wave soldering lines in production. Another reason of mass adoption of SAC305 instead of other higher Ag bearing SAC alloy is more a political reason. During that time, Japanese consumer product was very popular and common. *National* brand electric rice cooker should be one of the must have electrical appliances in every home. SAC305 was the Japan Electronics and Information Technology Industries Association, JEITA recommended Pb-free alloy too.

Nevertheless, the industries were looking for alternatives for SAC305 very quickly after its introduction to industries, even though with its acceptance as Pb-free alloy in replacing SnPb-6337 solder. In general, there are two major reasons in justifying this direction, cost and drop impact resistance of SAC305 alloy. The users were looking for alternative to mitigate the high cost of SAC305 as mentioned above. This was the time when low SAC alloy started to join the supply chain of solder. **Table 2** lists some of the low SAC alloys available in market offered by different solder suppliers. The introduction of these low SAC alloys was as soon as 2 years after the mass adoption of SAC305 alloy as Pb-free solder in the industries. Nevertheless, SAC305 is still the major adopted alloy in the industries after 10 years of RoHS enactment. It is worth to note that Nihon Superior has introduced Ag-free Pb-free solution since 1999. It has become a main solution in soldering especially in wave soldering and Pb-free hot air solder leveling (HASL) process.

existing alloys to close the gap between the user and supplier of solder material. In the chapters that follow the evolution of Pb-free alloy and the techniques used in enhancing solder

In 2000s, SAC system had been identified as replacement for Sn-Pb eutectic solders in fulfilling the RoHS compliance. Different countries or organizations had chosen different SAC composition as their preferred solder composition. They are Sn3.0Ag0.5Cu, SAC305 (Japan),

In 2000, it was the era of consumer product, which had incorporated many electronic components in its system such as personal computer, television, radio, rice cooker, and windowmounted air conditioner. These products were mostly using through-hole components and/ or big surface mount technology (SMT) components. It implies that the consumption of solder per unit device was huge during that period of time. It explains the reason of mass adoption of SAC305 instead of other Pb-free solutions. It is because the lower silver content of SAC305 as compared to other SAC alloys mentioned in former paragraph. Silver is a precious metal and its inclusion in solder alloy does increase the cost of solder significantly. For example, to produce 1 kg of SAC305, 1 ounce of silver needs to be added. It depends on the market price of silver, assuming USD20 per ounce, the cost of silver in 1 kg of SAC305 is almost 50% of the selling price of 1 kg of SAC305 solder bar. When the silver price surges, for example, in April 2011, when the silver price was USD 40 per ounce, the cost of silver of a SAC305 solder bar was more than 50% of its selling price. For those compositions with higher silver content, the ratio of silver cost against solder bar selling price is usually more than 50% depending on the tin price. It becomes a big burden to solder users. Solder bar is mainly used in wave soldering process. It was a very common process in printed circuit board assembly (PCBA) for those consumer products mentioned above. During the peak of this era, the consumption of solder bar could exceed 2000 kg per month for a single customer who has more than 10 wave soldering lines in production. Another reason of mass adoption of SAC305 instead of other higher Ag bearing SAC alloy is more a political reason. During that time, Japanese consumer product was very popular and common. *National* brand electric rice cooker should be one of the must have electrical appliances in every home. SAC305 was the Japan Electronics and Information Technology Industries Association, JEITA recommended Pb-free alloy too.

Nevertheless, the industries were looking for alternatives for SAC305 very quickly after its introduction to industries, even though with its acceptance as Pb-free alloy in replacing SnPb-6337 solder. In general, there are two major reasons in justifying this direction, cost and drop impact resistance of SAC305 alloy. The users were looking for alternative to mitigate the high cost of SAC305 as mentioned above. This was the time when low SAC alloy started to join the supply chain of solder. **Table 2** lists some of the low SAC alloys available in market offered by different solder suppliers. The introduction of these low SAC alloys was as soon as 2 years after the mass adoption of SAC305 alloy as Pb-free solder in the industries. Nevertheless,

Sn3.5Ag0.9Cu, SAC3509 (European Union), and Sn3.9Ag0.6Cu, SAC3906 (USA) [3].

alloy strength are discussed.

94 Recent Progress in Soldering Materials

**2. Evolution from high silver to low silver solder**

With lower silver content in SAC alloy, some users were hoping the hot tear or shrinkage cavity could be mitigated. This phenomenon which was first believed as a process defect was found when users migrating from SnPb-6337 to SAC305. To date, many users are still confused by this phenomenon even though IPC has classified it as an acceptable phenomenon. **Figure 1** shows the comparison of few low SAC alloys and a eutectic Tin Copper Nickel alloy.

Besides the appearance of solder whose difference is easily noticeable, the users were also concerned about the reliability of low SAC especially the intermetallic compound (IMC) formed in between the solder and soldering pad. **Figures 2** and **3** show the IMC of SAC 305, low SAC, SAC1205, and Ag-free Pb-free solder (SnCuNi) on Copper (Cu) pad and Nickel Gold (NiAu) pad, respectively. Both SAC alloys regardless of Ag content were showing columnar growth of IMC and the IMC continued to grow during the heat treatment at 125°C for 500 h. The SnCuNi IMC appeared to be flat and stable even after the heat treatment. Besides the columnar IMC, crack was observed at the Cu pad after heat treatment within the IMC of SAC305 but not at the SAC1205 and SnCuNi alloys. This observation matched the explanation proposed by Nogita et al. in his journal [4]. The Ni addition in Pb-free alloy has successfully suppressed the polymorphic transformation of Cu6 Sn5 , which is the culprit of this IMC crack. There is transformation of hexagonal to monoclinic Cu6 Sn5 and vice versa at temperature of 186°C due to the allotropic attribute of Cu6 Sn5 . This transformation involves volume change of the Cu6 Sn5 IMC, which causes high stress at this layer. The stress has exceeded the strength of this IMC and caused the crack within this layer. Micro crack might have formed within the SAC305 IMC during the solidification process of joint formation and the cracks propagated during the heat treatment due to IMC growth. This has explained why the crack still formed even though the heat treatment temperature was just 125°C, which was far below the polymorphic transformation temperature.


**Table 2.** List of low SAC solder and no Ag solder as alternatives of SAC305.


**Figure 1.** Appearance comparison of different SAC alloys and a eutectic SnCuNi alloy.


**Figure 2.** IMC formed on Cu pad with solder alloy of SnCuNi (SN100C®), SAC305 and SAC1205 (LF35).

Besides the cost concern mentioned earlier, another driving force which triggered the industries to consider low SAC alloy was the drop impact resistance. It is widely accepted that silver addition can effectively reduce the liquidus and increase the yield strength and modulus of SAC alloy; also because of the high strength and high modulus which make the alloy

**Figure 3.** IMC formed on NiAu pad with solder alloy of SnCuNi (SN100C®), SAC305, and SAC1205 (LF35).

readily transfer the stress to the solder joining interface which is the region of IMC [5]. Due to this mechanism, most of the failures found in drop impact resistance test with conventional SAC alloys mostly locate at the IMC region, and most of these failures are early failures. By reducing the Ag content of SAC alloy, the modulus of alloy decreases. It has significantly increased the resistance to the drop impact. The urge of looking for an alloy with high drop impact resistance increased further due to the popularity of portable devices in mid-2000 to late-2000. During that period of time, portable music player such as MP3 player and iPod and handheld game console were very common. The robustness against drop shock was crucial to keep a reasonable reliability of such devices. For example, during the initial implementation of Pb-free soldering, most of the solder ball used in ball grid array, BGA package is SAC305 composition. Many mobile integrated circuits, ICs had migrated to LF35 or SAC1205 BGA solder ball in order to increase the drop test performance of the ICs. However, by only lowering the Ag content of SAC alloy did not give a total solution to meet the expectation of users. Many other microalloying had been deployed by solder manufacturers in order to increase the alloy performance of low SAC alloy such as adding Nickel, Ni, Zinc, Zn and Manganese, Mn into the alloy system. As shown in **Figure 2**, the addition of Ni into LF35 composition has suppressed the polymorphic transformation of Cu6 Sn5 during the solidification. It cannot be achieved by only reducing the Ag content of SAC alloy.

The acceptance of low SAC or SnCuNi as Pb-free solution is also encouraged by the readiness of other material to Pb-free soldering process. One of the major challenges in adopting low SAC or SnCuNi alloy is the high melting point of this alloy. It has approximately 44 and 10°C higher melting point comparing to eutectic SnPb alloy and SAC alloy, respectively. However, the market is now more ready for this Pb-free soldering. The reflow and wave soldering machines are having higher heat capacity to accommodate the high melting point of Pb-free solder, PCB, and electronic components are using higher glass transition (Tg) temperature materials and soldering flux can survive longer in high-temperature reflow. All these

Besides the cost concern mentioned earlier, another driving force which triggered the industries to consider low SAC alloy was the drop impact resistance. It is widely accepted that silver addition can effectively reduce the liquidus and increase the yield strength and modulus of SAC alloy; also because of the high strength and high modulus which make the alloy

**Figure 2.** IMC formed on Cu pad with solder alloy of SnCuNi (SN100C®), SAC305 and SAC1205 (LF35).

**Figure 1.** Appearance comparison of different SAC alloys and a eutectic SnCuNi alloy.

96 Recent Progress in Soldering Materials

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 IoT era.

The following section discusses the development of Pb-free alloy into third generation alloy for high reliability application.

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