The influences of Ag nanoparticles on voids growth and solderability about Sn3.0Ag0.5Cu/Cu solder joint

Although Sn3.0Ag0.5Cu solder (SAC305) has higher reliability, there are a large number of harmful voids in solder joints. Larger voids can reduce thermal conductivity of solder joints. However, as a microstructure growth inhibitor, the influences of Ag nanoparticles on voids growth are not clear at present. Herein, we prove that Ag nanoparticles can increase SAC305 solderability, but promote voids growth. Ag nanoparticles and SAC305 solder paste were mixed by mechanical stirring for 0.5 h. Next, SAC305-xAgP (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5 wt%) was obtained. The results indicated that solder solderability was improved increasingly with Ag nanoparticles addition. The lower the amount of nanoparticles added, the greater the final loss. After being aged at 100 °C for 150 h, the voids stopped growing. Due to the violent reaction between Ag nanoparticles and flux, the final voids growth rate became faster, and the average voids size changed from 14.34% to 24.91%.


Introduction
With the advancement of electronic technology, the requirements for various properties of solder are getting higher and higher.Sn-Ag-Cu alloy is recognized as the most potential alloy system that can replace Sn-Pb solder due to its excellent comprehensive properties [1][2][3].However, compared with traditional Sn-Pb solder, Sn-Ag-Cu solder still has some disadvantages [4][5][6][7].For nanocomposite solders, nanoparticles can fix the grain boundaries, prevent grain boundaries from sliding, and increase the diffusion energy of grain boundaries, thereby improving the creep resistance of the alloy and improving the reliability of the solder [8][9][10][11][12].With the continuous maturity of nanotechnology, the modification of solder by adding nanoparticles may be an important research direction in the future.
Mukherjee S indicated that SAC105 microstructure was refined by micro-alloying element [13].The study revealed the influences of interfacial intermetallic compounds (IMC) on SnAg solder creep resistance [14].The literature indicated how Sn-based solder microstructure was refined by nanoparticles addition [15][16][17].As known, Sn can react with Ag, Cu, Ni elements and so on.The literature showed the alloying between Sn and reactive nanoparticles [18].The above studies focused on the solder microstructure and IMC.However, due to poor fluidity and high viscosity, flux and impurities cannot be discharged from the solder joint in a timely and effective manner during the soldering process.Therefore, there are voids appeared in the solder joint [19].Although SAC305 solder has high reliability, SAC305/Cu solder joint contains a large number of voids.Electrical conductivity, thermal conductivity and mechanical properties of the solder joint can be weakened due to voids.As a result, the heat of the solder joint is promoted indirectly, and then the reliability of the solder joint is reduced actually [20][21][22].However, there are not enough researches on the solderability improvement and voids problems of SAC305 with Ag nanoparticles.
To further improve the solderability of SAC305 and reduce the porosity inside the solder joint, SAC305-xAg (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5 wt%) composite solder paste obtained by mechanically mixing Ag nanoparticles and SAC305 solder.The experimental results indicate that the addition of Ag nanoparticles reduces the thickness of the solder joint and increases the spreading of the brazing material on the Cu substrate.When 0.5 wt% of Ag particles were added, the wetting time and wetting force increased by 4.3% and 4.9% respectively.The average volume of pores increased linearly when the aging time was 0 ∼ 150 h.After 150 h of aging, the average volume of pores stopped changing.

Procedures and analysis
SAC305-xAgP (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5 wt%) composite solder paste was prepared by mixing mechanically with Ag nanoparticles and SAC305 solder for 0.5 h.Firstly, to prevent agglomeration of the nanoparticles, the nanopowders were dried at a constant temperature of 120 °C for one hour and then the nanoparticles were well ground using an agate grinding bowl and ultrasonically shaken to disperse them.Subsequently, the Ag nanoparticles were homogeneously mixed into the SAC305 solder paste by mechanical stirring using a precision force multiplier electric stirrer.The entire preparation of the SAC305-xAg composite solder paste was carried out under air conditions with a stirring time of 30 min to ensure homogeneous mixing of the solder paste and nanoparticles.A Cu substrate without an oxide layer was coated with 150 mg of solder paste, which was consistently applied in a conical shape.Solder paste was heated at 280 °C for 15 s in the heating bath, and then cooled in air.The remaining paste was then heated to separate the alloy from it.The solder joints were aged in an environmental box at 100 °C for 50 h, 150 h and 250 h respectively.The environmental box was full of air.The relative humidity had been 30 %RH all the time.
The thickness of the solder joints after cooling was measured by a micrometer, and the average value was taken.In the experiment we measured 10 data points and obtained the thickness of the solder joint by calculating the average.According to formula (1), the spreading ratio of solder joint was calculated [23]: Where S R is the spreading ratio of solder on Cu substrate, and H is the height of solder joint in mm.Where D is the radius of alloy as a perfect sphere in mm.According to the formula [24] (2), the alloy theoretical density (D) was calculated: Where m n are percent mass fractions of constituent metals, and d n are standard densities of constituent metals.As known, standard densities of Sn, Ag, and Cu metals are 7.28 g cm −3 , 10.49 g cm −3 , and 8.96 g cm −3 , respectively.
To measure the solderability of solder, the measured densities of solder alloys were tested by density balance (SQP, Sartorius Co.) firstly.To minimize measurement error in the experiment, seven data points were collected for each alloy composite material's density measurement.The average of these data points was then used to determine the density of the alloy composite material.In the next, for analyzing the effect of Ag nanoparticles on SAC305 solderability, the solderability of solder joints was tested by a solderability tester (MUST SYSTEM III, England), and the test temperature was 280 °C.In the end, to observe the voids, the solder joints were observed by nondestructive flaw detection (XD7500VR Jade FP, Nordson Dage Co.).

Solderability
As shown in figure 1 and table 1, with Ag nanoparticles addition, the solder joint thickness decreased and the spreadability of solder on Cu substrate was improved.Notably, the solder joint thickness increased after adding 0.1 wt% Ag nanoparticles.As known, the difference in melting points between Ag nanoparticles and SAC305 is about 600 °C.During soldering, Ag nanoparticles were solid, but SAC305 was molten.As a result, the molten would increase with Ag nanoparticles addition.It can be seen that thickness was the highest with adding 0.1 wt% Ag nanoparticles.However, with adding above 0.1 wt% nanoparticles, the enhancement on solderability was dominant.Therefore, after adding 0.1-0.5 wt% Ag nanoparticles, the thickness was decreased continuously.
After Ag nanoparticles addition, the spreading ratio of solder on the Cu substrate increased.It indicated that the spreadability of solder on Cu substrate was improved.When the addition amount was 0.5 wt%, the spreading ratio increased from 76.80% to 79.10%, which increase was about 3%.There are two reasonable explanations: (1) during soldering, the acidic flux in the solder paste can react with the Ag nanoparticles, which can generate more gas inside the solder joint and accelerate the solder spreading on the Cu substrate.(2) Ag nanoparticles tend to gather on the surface of the molten alloy, which helps to reduce the surface energy of the melted alloy and promote the spreading of the molten alloy [25].
From table 2, it can be seen that the measured density was smaller than the theoretical density.It should be pointed out that the more Ag nanoparticles added, the more Ag nanoparticles escaped from the alloy.In other words, the higher the measured density, the smaller difference between theoretical density and measured density.
The density difference was caused by the agglomeration of nanoparticles.The larger and heavier Ag nanoparticles, the less likely they are to flow in the flux.As shown in figure 2, Ag nanoparticles agglomeration became more and more prominent with Ag nanoparticles added.Next, the most of agglomeration stayed in solder because of lower flowability.
As shown in figure 3 and table 3, it can be seen that the wetting force increased and the wetting time decreased continuously with Ag nanoparticles added.It indicated that the Ag nanoparticles addition has a positive effect on the solderability of solder.When 0.5 wt% Ag particles was added, the wetting time and the wetting force were improved by 4.3% and 4.9%, respectively.The surface tension of the molten solder was reduced by Ag nanoparticles, and the spreadability of the molten alloy was improved [25].

Solder joint voids
From figures 4 and 5, it can be seen that the voids average volume increased linearly when the aging time was 0-150 h.After 150 h aging, the void average volume stopped changing.The aging time in figure 4 was linearly fitted, and the fitting line slope represented the voids growth rate.From the data in figure 5, the voids growth was inhibited hardly by Ag nanoparticles.In the contrast, the void growth can be accelerated by Ag nanoparticles.
After being aged at 100 °C for 150 h, the average voids change of SAC305 was minimal with 14.34%, and the average voids change of SAC305-0.2Agwas largest with 24.91%, as shown in table 4. The reason of void growth was the growth of solder alloy microstructure: there were some small cracks between the grain boundary in the alloy microstructure [26,27], and the process of grain growth was also the process of grain boundary reduction.At the same time, the small voids in the grain boundary converged along the cracks and entered the larger voids.Therefore, larger voids grew with microstructure growth.Moreover, the Ag nanoparticles would react with the flux roughly during the soldering, which would splash solder paste to the nearby partly.As shown in the red circle from figure 6, this process can produce more gas in the solder joint.As a result, the voids average volume changed more greatly with Ag nanoparticles addition finally.
From figure 6, it can be found that the voids tended to distribute in the outer ring of solder joints.As we know, due to the obvious accumulation of solder paste in the inner ring, the heating rate of the inner ring was slower during the actual soldering.As a result, the actual temperature of the outer ring was higher than the actual temperature of the inner ring.As a result, the flux in the outer ring was more volatile to form voids, while the flux in the inner did opposite.

Conclusion
To investigate how voids growth and solderability about Sn3.0Ag0.5Cu/Cusolder joint is effected by Ag nanoparticles.In the experiments, SAC305-xAgP (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5 wt%) was obtained by mechanically mixing Ag nanoparticles with SAC305 solder paste for 0.5 h.When the addition amount was 0.5 wt%, the spreadability was improved by about 3.0%, and the wetting time and wetting force were improved by 4.3% and 4.9%, respectively.After being aged at 100 °C for 150 h, the average voids change of SAC305 was minimal with 14.34%, and the average voidschange of SAC305-0.2Ag was largest with 24.91%.The results show that Ag nanoparticles can effectively improve the spreadability and solderability of the solder on Cu substrate.In addition, voids growth was accelerated by Ag nanoparticles.

Figure 1 .
Figure 1.The relationship between spreadability and Ag-nanoparticle content.

Figure 2 .
Figure 2. The trend of nanoparticle during solder paste melting: (a) the heating beginning of solder paste with less nanoparticles; (b) the heating end of solder paste with less nanoparticles; (c) the heating beginning of solder paste with more nanoparticles; (d) the heating end of solder paste with more nanoparticles.

Figure 3 .
Figure 3. Influences of Ag nanoparticles on wettability of SAC solder.

Figure 4 .
Figure 4.The relationship between average voids size and aging time.

Figure 5 .
Figure 5.The growth rates of the voids.

Table 1 .
The influence of Ag-nanoparticle on solder joints thickness.

Table 2 .
The relationship between solder alloy density and Ag-nanoparticle content.

Table 3 .
The relationship between solder solderability and Agnanoparticle.

Table 4 .
The added ratios of void average size with aging.