2016 Volume 57 Issue 10 Pages 1685-1690
Sn-0.7Cu and Sn-0.7Cu-xGe solder alloys were prepared to investigate the influence of trace Ge on liquid Sn-0.7Cu lead-free solder at high temperature. The spreadability and the wetting force of solders were tested, and the oxidation-resistance was also evaluated by eye observation and skimming at 250℃～370℃. The results have shown that trace Ge can improve the spreading rate of Sn-0.7Cu, but have a few effect on the wettability. The oxide slag quantity of Sn-0.7Cu was three times more than the Sn-0.7Cu-0.012Ge at the same temperature and period, the optimal content of Ge to improve the oxidation resistance of Sn-0.7Cu was 0.012 mass%. The growth factor of oxide film on the surface of liquid Sn-0.7Cu solder (k250℃ = 1.59 × 10−6, k370℃ = 3.03 × 10−6) were both twice higher than the Sn-0.7Cu-0.012Ge (k250℃ = 0.56 × 10−6, k370℃ = 1.04 × 10−6) at 250℃ and 370℃ respectively.
With the miniaturized and lead-free development of electronic products, people put higher request forward the solder. Researches had been shown that the best composition of alternative Sn-Pb alloy solder were Sn-Ag, Sn-Cu and Sn-Ag-Cu eutectic alloy at present1). However, manufacturing costs of electronic products will increase due to the expensive price of the high-silver Sn-Ag and Sn-Ag-Cu eutectic solders which was difficult to accept for electronics manufacturers. Sn-Cu alloy was considered to be a cost-effective lead-free solder for soldering because of its relatively cheap and extensive source of raw materials2).
Sn-Cu based lead-free solder mainly referred to the Sn-0.7Cu eutectic alloy whose melting temperature was 227℃ and soldering temperature was generally higher than 260℃. However, the wettability and the spreadability of Sn-0.7Cu lead-free solder were poor, and Sn-0.7Cu alloy was very prone to be oxidized at high temperature. Therefore, it will not only produce a lot of wastes, but also increase the defects probability of solder joint for wave soldering applications which would seriously affect the reliability of soldering joints3). Thus, it is expected to improve its wettability and oxidation resistance performance by adding trace amount of elements such as; P4,5), Ag3,6), and Al7). However, these studies generally are centered on low temperature around 260℃. Electronics manufacturers sometimes need to use the same solder in a wider temperature range in the actual production process, such as 250℃～400℃. So the research on wettability and oxidation resistance of solder at high temperature is particularly important. Ge element can not only improve the density of the interfacial IMC to increase the strength of soldering joints, but can also improve the oxidation-resistance of solders at high temperature8–11). But the research about Ge element is insufficient, so the influence of adding trace Ge on the wettability and the oxidation resistance of Sn-0.7Cu lead-free solder at high temperature were researched to solve this problem in the paper.
Tin with 99.95% (in mass, similarly hereinafter) and copper with 99.99% were weighed as the mass ratio of 99.3: 0.7, and melted at 400℃ with the protective atmosphere. The Sn-2.5Ge master alloy was melted into the Sn-0.7Cu solder according to the ratio. Finally, Sn-0.7Cu-xGe solders were obtained by pouring into a mold after removed the surface oxide slag at 400℃ for 30 min.
The surface of Cu board was polished bright with 600# Carbide sandpaper, then cleaned with alcohol and aged at 150℃ for 60 min. The solder balls of Sn-0.7Cu-xGe alloy with the weight of 0.2 g, rosin-based flux (it was composed of 75 g isopropanol, 25 g foral and 0.4 g diethylamine hydrochloride) and Cu board with the size of 30 mm × 30 mm × 0.3 mm were used for the spreading experiments at 250℃, 300℃ and 350℃ respectively (Fig. 1). The height of the spread solder were measured by micrometer, the solder ball was defined as the spherical form to calculate the diameter.
The spreading process of solders.
The wettability of Sn-0.7Cu alloy and Sn-0.7Cu-0.012Ge alloy were tested by SAT-5100 solderability tester at 250～350℃ respectively, rosin-based flux and pure copper with the size of 30 mm × 5 mm × 0.03 mm were used for the dipping tests. The experiment conditions are as follows: speed, depth and time of its immersion are 2 mm/s, 3.0 mm, 5 s respectively. Wetting time tw refers to the time when the wetting angle is 90°, as shown the point B in wetting curve of Fig. 2. The shorter the tw is, the better wettability.
The schematic of wetting curve.
Lead-free solders of 200 g were melted in the tin furnace with the diameter of 33.5 mm. The changes of the surface color of the molten solders were observed for 30 min at 250℃, 310℃ and 370℃ respectively. The oxide slag of 30 times (30 min) were collected and weighed at a 30℃ interval from 250℃ to 370℃, and the skimming period is 1 min each time. The relationship between the oxide slag quantity and the skimming period were also researched and the skimming period were 2 and 4 min, 6 and 8 min respectively.
The overflowing ability is that the solder melted to wet the base-metal, and then spread out. The overflowing ability mainly depended on the surface tension and the liquidity of solder (viscosity), which is an important indicator to reflect the wettability of solder. The overflowing ability can be measured in terms of the spreading rate.
The heights of different solders spreading on the copper surface are measured and the solder spreading rate can be calculated as eq. (1)12):
| \[S_R = (D - H)/D \times 100\%\] | (1) |
Where SR is the spreading rate (%); D is the diameter of solder ball, mm; H is the height of the solder after spreaded out, mm.
The spreading rate of each solder increases with rising temperature, as shown in listing Table 1. The spreading rate of Sn-0.7Cu solder increases by 10.6% as the temperature rised from 250℃ to 350℃, and the spreading rate of Sn-0.7Cu-0.009Ge, Sn-0.7Cu-0.012Ge and Sn-0.7Cu-0.015Ge solder also increase by 9.7%, 7.9% and 6.5% respectively. It is showed that the effect of temperature on the spreading rate of Sn-0.7Cu and Sn-0.7Cu-xGe is significant.
| Temperaturents | 250℃ | 300℃ | 350℃ | |
|---|---|---|---|---|
| Solders | ||||
| Sn-0.7Cu | 68.41% | 72.81% | 75.63% | |
| Sn-0.7Cu-0.009Ge | 72.37% | 77.14% | 79.44% | |
| Sn-0.7Cu-0.012Ge | 73.84% | 77.97% | 79.67% | |
| Sn-0.7Cu-0.015Ge | 75.03% | 78.43% | 79.87% | |
It also can be seen that the spreading rate of solder increase with the increasing of Ge content at the same temperature from Table 1. Such as the spreading rate of Sn-0.7Cu-0.009Ge, Sn-0.7Cu-0.012Ge and Sn-0.7Cu-0.015Ge solder are improved by 6.0%, 7.1% and 7.7% than Sn-0.7Cu respectively at 300℃, and improved by 5.0%, 5.3% and 5.6% than Sn-0.7Cu respectively at 350℃. We know that liquid Sn-0.7Cu alloy is composed of liquid Sn and solid coarse Cu6Sn5. Ge elements maybe can refine coarse Cu6Sn5 of alloys, so it can improve the liquidity after adding Ge element; the accumulation of Ge element on the surface (Fig. 7) can effectively decrease the surface tension of solder, so the spreading rate increases with the increasing of Ge content.
The effect on the wetting curve is not significant because the addition amount of Ge is very trace, so choose the middle Ge content for the dipping test. Figure 3 shows the relationship between the wetting time and the temperature of Sn-0.7Cu and Sn-0.7Cu-0.012Ge at 250℃～350℃. The wetting time of two kinds of solders are sharply declined from 250℃ to 275℃, which has shown that the wetting rate rapidly accelerated with rising temperature. But the wetting time is not significantly decreased when the temperature is exactly over 275℃.
The relationship between temperature and soldering time.
The relationship between wetting force and temperature of Sn-0.7Cu and Sn-0.7Cu-0.012Ge at 250℃～350℃ is shown in Fig. 4. It can be seen that the wetting force is gradually increased, and the wetting force of Sn-0.7Cu-0.012Ge was slightly higher than Sn-0.7Cu when the temperature is below 300℃. However, the wetting force of Sn-0.7Cu-0.012Ge is slightly lower than Sn-0.7Cu when the temperature is exactly over 300℃. The accumulation of Ge element on the surface (Fig. 7) can effectively decrease the surface tension of solder, but the activity of rosin-based flux decreases after 300℃, so the change of the wettability with the temperature are not very noticeable after 300℃.
The relationship between temperature and soldering force.
Addition of trace Ge can improve the wettability and shorten the wetting time of molten Sn-0.7Cu solder on the copper surface, but its effect is not significant when the content of Ge is too small (The maximum content of Ge element is 0.015% (150 ppm) in the paper).
3.2 Influence of trace Ge on the oxidation resistance of Sn-0.7Cu solders(1) The color change occurring on the surface of solders
According to the physical principles of film color, the oxide film exhibits a specific color when the wavelengths of the incident light have a definite multiple relationships with the thickness of the metal oxide film. The color of oxide film changes regularly with the thickness of the oxide film. The metallic luster disappears then the bright mirror surface becomes blurred, consequently it shows a different color when the oxide film becomes thick.
Table 2 shows the pictures of the oxide film on the surface of solders at 250℃, 310℃ and 370℃ for 30 min relatively. It can be seen that the oxide film on the surface of Sn-0.7Cu solder turns to gold and Sn-0.7Cu-xGe molten solder remains bright mirror surface after holding for 30 min at 250℃, except the oxide film of Sn-0.7Cu-0.009Ge is slightly yellowish color. After holding for 30 min at 310℃, the surface oxidation of Sn-0.7Cu is further intensified and the color of oxide film changes to light purple. Interestingly, the surface oxide film of Sn-0.7Cu-xGe turns to azure at the same time, and the oxide film of Sn-0.7Cu-0.009Ge are darker, while the oxide film of Sn-0.7Cu-0.012Ge and Sn-0.7Cu-0.015Ge are relatively shallow. The surface oxide film of Sn-0.7Cu-xGe fades azure and turns to gray after holding for 30 min at 370℃, and the oxide film of Sn-0.7Cu-0.009Ge turns to white-gray and completely lose luster while the oxide film of Sn-0.7Cu-0.012Ge and Sn-0.7Cu-0.015Ge retains weak sheen.
| Solders | Sn-0.7Cu | Sn-0.7Cu-0.009Ge | Sn-0.7Cu-0.012Ge | Sn-0.7Cu-0.015Ge | |
|---|---|---|---|---|---|
| Temperature | |||||
| 250℃ |  |
 |
 |
 |
|
| 310℃ |  |
 |
 |
 |
|
| 370℃ |  |
 |
 |
 |
|
(2) The change of oxide slag quantity with skimming period and temperature
Figure 5 shows the relationship between the oxide slag quantity and the temperature of Sn-0.7Cu-xGe and Sn-0.7Cu alloy at atmosphere. It can be seen that the oxide slag growth rate of Sn-0.7Cu is significantly higher than Sn-0.7Cu-xGe, although the oxide slag quantity on the surface of four kinds of lead-free solder increases with rising temperature. The total quantity of oxide slag of Sn-0.7Cu is about 2～3 times more than Sn-0.7Cu-xGe at 250℃, while the total quantity of oxide slag of Sn-0.7Cu is about 3～4 times more than Sn-0.7Cu-xGe at 370℃. It can be seen that the oxide slag quantity of Sn-0.7Cu-0.009Ge is more than Sn-0.7Cu-0.012Ge and Sn-0.7Cu-0.015Ge, but less than Sn-0.7Cu solders.
The relationship between the temperature and the oxide slag quantity.
The relationship between the oxide slag quantity (g) and the skimming period (T) of Sn-0.7Cu and Sn-0.7Cu-xGe at different temperature are shown in Fig. 6. It can be seen that the generation rate of the oxide slag of four kinds of lead-free solder increase with the decreasing of skimming period, and the oxide slag quantity with the skimming period of 4 min is about 2/3 times more than the skimming period of 2 min. The oxide slag quantity of Sn-0.7Cu alloy is significantly higher than Sn-0.7Cu-xGe alloy at the same skimming period and temperature. The oxide slag quantity of Sn-0.7Cu-0.009Ge is more than Sn-0.7Cu-0.012Ge and Sn-0.7Cu-0.015Ge, but less than Sn-0.7Cu solders at the same temperature. The influence of Ge on the oxide slag has little changes when the Ge content is over 0.012 mass%. Therefore, the optimal content of Ge to improve the oxidation resistance of Sn-0.7Cu is 0.012 mass%.
The relationship between the oxide slag quantity with the skimming period at 250℃ (a), 310℃ (b) and 370℃ (c).
It appears that the amount of slag produced at 250℃ is greater than at 310℃, because the oxide slag contains a small amount of pure tin at 250℃ due to the poor liquidity of solder at low temperature. The relationship between oxide slag quantity and time obeys a parabolic rate law at a certain temperature, and the oxidation rate is as follows13).
| \[\Delta M = Akt^{\frac{1}{2}}\] | (2) |
Where ΔM is the increasing mass; A is the surface area; t is the heating time; k is oxide layer growth factor, which is as follows.
| \[k = k_0 \exp (-B/T)\] | (3) |
Where T is heating temperature; k0 and B are constants. It can be calculated from the formula above that the oxide layer growth factor of Sn-0.7Cu lead-free solder (k = 1.59 × 10−6) is about twice higher than the Sn-0.7Cu-0.012Ge (k = 0.56 × 10−6) at 250℃; and the oxide layer growth factor of Sn-0.7Cu lead-free solder (k = 3.03 × 10−6) is about twice higher than the Sn-0.7Cu-0.012Ge (k = 1.04 × 10−6) at 370℃.
The oxidation of Sn-0.7Cu lead-free solder is the chief concerned matters at present during the wave soldering process and the high-temperature dip soldering process. The oxidation resistance of Sn-0.7Cu-0.012Ge is the best, and the spreading rate is in the middle, but it has a few effects on the wettability. Figure 7 is the XPS of the oxide film on the Sn-0.7Cu-0.012Ge alloy surface. The atomic percent of the oxide film at different temperatures are shown in Table 3. It can be seen that the oxide film on the surface is mainly SnO at low temperature, and are SnO2 and GeO2 composite oxide film at high temperature. The weight ratio of Sn: Ge is calculated to 1: 0.00012 in the alloy and is 1:0.055 on the surface at high temperature. The results described that Ge is enriched on the surface, 74 times higher than the alloy.
The XPS of the oxide film on the Sn-0.7Cu-0.012Ge alloy surface. (a) The XPS test of Sn, (b) The XPS test of Ge.
| Temperatures (℃) | Sn3d5 (At%) | O1s (At%) | Ge (At%) |
|---|---|---|---|
| 250℃ | 16.18% | 83.82% | Not detected |
| 370℃ | 16.34% | 82.76% | 0.9% |
The more negative of the oxide Gibbs free energy is, the easier for the metal oxidized according to the principles of thermodynamics13). The relevant metal oxides Gibbs free energy is shown in Table 4.
| ΔGθ/(J·mol−1) | 25℃ | 127℃ | 327℃ | 527℃ |
|---|---|---|---|---|
| Ge(s)→GeO2(s) | −596.75 | −602.65 | −619.61 | −641.19 |
| Sn(s)→SnO2(s) | −591.33 | −596.56 | −610.43 | −629.61 |
| Sn(s)→2SnO(s) | −302.61 | −309.06 | −325.03 | −344.30 |
| Ge(s)→2GeO(s) | −200.91 | −226.17 | −278.12 | −332.40 |
| Cu(s)→Cu2O(s) | −197.99 | −208.50 | −234.04 | −264.60 |
| Cu(s)→2CuO(s) | −168.55 | −173.58 | −186.83 | −203.61 |
It can be seen from Table 4 that the Gibbs free energy of GeO2 is minimum, and then is the SnO2. There will be more the oxidation of Ge on the surface because of the surface segregation of Ge elements11). On the basis of thermodynamic analysis, the expression of metal oxidation reaction is generally as follows13).
| \[xM + \frac{1}{2} y O_2 = Mx O_y\] | (4) |
The specific process is as follows.
| \[\begin{array}{l} O_2 \to O + O \\ O + 2e = O^{2-} \\ M - ne = M^{n+} \\ xM^{n+} + yO^{2-} = M_x O_y \end{array}\] | (5) |
$M_xO_y$ is any oxides, the forming of $M_xO_y$ can be completed immediately when the fresh liquid metal surface exposed to the air.
The theory of metals oxidation at high temperature has been considered that a layer of a monomolecular oxide film is created by the chemical reaction between the metal and the oxygen in the early oxidation stage, and then the ion transport or the electron movement through the oxide film was required for further oxidation. So, the oxidation process depends on the interfacial reaction rate and the diffusion rate of metal ions and oxygen ions after forming a dense oxide film.
SnO and SnO2 are mainly oxides because the content of Sn is over 90% in the Sn-0.7Cu lead-free solders. Oxides exist excess metal ions which exist in the forming of a gap ions in the crystal since the oxide type of SnO belongs to the metal ions excess type (n-type semiconductor)15). Part of Sn2+ in the SnO lattice is replaced by Ge4+ after adding traces Ge. The number of gap Sn2+ ions will reduce in order to maintain the electrical neutrality. The reduction of Sn2+ ions increases the cation vacancy on the metal surface, influences the electron (oxygen ions) migration and decreases the electrical conductivity. Therefore, these inevitably lead to the reduction of the oxidation rate.
(1) Trace Ge could significantly improve the spreadability of Sn-0.7Cu solder. The spreading rate of Sn-0.7Cu-0.009Ge, Sn-0.7Cu-0.012Ge and Sn-0.7Cu-0.015Ge solder also were increased by 9.7%, 7.9% and 6.5% from 250℃ to 350℃ respectively. The spreading rate of Sn-0.7Cu-0.009Ge, Sn-0.7Cu-0.012Ge and Sn-0.7Cu-0.015Ge solder were improved by 6.0%, 7.1% and 7.7% than Sn-0.7Cu respectively at 300℃, and improved by 5.0%, 5.3% and 5.6% than Sn-0.7Cu respectively at 350℃. But the wetting time and the wetting force were not significantly influenced.
(2) Trace Ge can significantly improve the oxidation resistance of Sn-0.7Cu solder at high temperature. The oxide slag quantity was significantly reduced with the rising of skimming period at the same temperature. The growth factor of oxide film on the surface of liquid Sn-0.7Cu solder (k250℃ = 1.59 × 10−6, k370℃ = 3.03 × 10−6) were both twice higher than the Sn-0.7Cu-0.012Ge (k250℃ = 0.56 × 10−6, k370℃ = 1.04 × 10−6) at 250℃ and 370℃ respectively.
This work was supported by the China Postdoctoral Science Foundation (NO.2015M582221) and the Program for Science and Technology of Guangdong provincial (NO.2013B090600031) and the Program for Science and Technology of Chongqing provincial (NO.cstc2015zdcy50003), Chongqing Municipal Engineering Research Center of Institutions of Higher Education for Special Welding Materials and Technology (NO. SWMT201502, SWMT201503 and SWMT201505) respectively.
