Cleanliness and splattering are the two main properties of AgCu28 vacuum solder, and the oxygen content of the raw Ag and Cu directly affects the splattering of AgCu28 vacuum solder. And the copper raw material is usually oxygen free copper. According to the national standard GB/T5231-1985, its oxygen content is ≤ 0.0020% (mass fraction), which can meet the production requirements of AgCu28 vacuum solder. For silver raw materials, silver powder made from silver ingots sold on the market or recycled by production enterprises has a significant difference in oxygen content, with some ranging from 0.0010% to 0.0020% and others even exceeding 0.0100%; However, the silver powder made from recycled materials by production enterprises has a higher oxygen content, usually ranging from 0.0250% to 0.0300%. Therefore, it is necessary to carry out degassing treatment on the silver raw material. This article studies the silver degassing process of AgCu28 vacuum solder.
Test results and analysis
1.1 Degassing of silver ingots
1.1.1 Degassing by vacuum melting method
Using a 25kg vacuum induction melting furnace, and three corundum crucible melting furnaces, each furnace 14kg, the vacuum degree is 1.2Pa, insulation and degassing time are 10min, 15min, 20min, cooling and casting, then analyzing the ingot oxygen content. The
silver ingots processed by the above process are mixed with oxygen free copper to make AgCu28 vacuum solder. Taking some samples and analyzing the cleanliness and splattering according to the national standard GB/T 4907-1985. The results are shown in Table 1.
Table 1 Degassing effect of vacuum melting method and its influence on the performance of AgCu28
| No. |
Insulation and degassing time/min |
Oxygen content/mass fraction% |
The cleanliness of AgCu28 |
The splattering of AgCu28 |
| 0 |
(silver ingots) |
0.0160 |
— |
— |
| 1 |
10 |
0.0100 |
Ⅰ |
Unqualified |
| 2 |
15 |
0.0060 |
Ⅰ |
Unqualified |
| 3 |
20 |
0.0040 |
Ⅰ |
B |
From Table 1, vacuum melting method can be used for deoxidation. When the insulation and degassing time is less than 20 mins, its splattering does not meet the standard requirements. As the insulation and degassing time increases, the oxygen content of the silver ingot gradually decreases. According to this trend, if the insulation and degassing time is further extended, the oxygen content of the silver ingot may decrease to below 0.0020%, which can meet the production requirements of vacuum solder AgCu28. Theoretically, during melting, the dissolved oxygen in the metallic solution can enter the gas from the liquid surface, and the oxygen in the gas phase is extracted from the furnace. Ideally, this process should continue indefinitely, but due to factors such as vacuum degree and temperature, it can only reach a dynamic equilibrium state. Therefore, maintaining a high vacuum degree and a long insulation and degassing time, the oxygen content of the silver ingot can be kept below 0.0020%. However, considering the actual production efficiency and raw material consumption, this method is not only time-consuming but also increases the volatilization of silver under high vacuum degree, which increases the consumption of raw materials and pollutes the vacuum equipment. Therefore, this method is not advisable in practical production.
1.1.2 Degassing by activated carbon method
Using a 25kg vacuum induction melting furnace and a corundum crucible, add activated carbon (granular activated carbon and powdered activated carbon in three furnaces), with a vacuum degree of 1.2Pa, and insulation and degassing times of 10min, 15min, and 20min, respectively before cooling and casting. Then analyzing the oxygen content of the ingot. The silver ingot processed by the above process are mixed with oxygen free copper to make AgCu28 vacuum solder. Taking samples from the alloy ingot and analyzing the cleanliness and splattering according to the provisions of the national standard GB/T 4907-1985. The results are shown in Table 2.
Table 2 Degassing effect of activated carbon method and its influence on the performance of AgCu28
| No. |
Insulation and degassing time/min |
Oxygen content/mass fraction% |
The cleanliness of AgCu28 |
The splattering of AgCu28 |
| Granular activated carbon |
Powdered activated carbon |
Granular activated carbon |
Powdered activated carbon |
Granular activated carbon |
Powdered activated carbon |
| 0 |
(silver ingots) |
160 |
— |
— |
| 1 |
5 |
50 |
35 |
Ⅰ |
Ⅰ |
A |
A |
| 2 |
10 |
30 |
15 |
Ⅱ |
Ⅰ |
A |
A |
| 3 |
15 |
20 |
8 |
Ⅰ |
Ⅱ |
A |
A |
From Table 2, degassing with activated carbon, granular and powdered activated carbon has better degassing effect than vacuum melting, and powdered activated carbon is better than granular activated carbon.
Theoretically, after adding activated carbon, the following chemical reactions occur:
C+O2↑=CO2↑
2C+O2↑=2CO↑
The solubility of CO and CO2 in silver is lower than that of oxygen, which is more conducive to being extracted from the furnace under vacuum condition, so as to has better degassing effect. In addition, due to the much smaller surface area of granular activated carbon compared to powdered activated carbon, thus the surface area in contact with oxygen is smaller; In the same time, the reaction between granular activated carbon and oxygen is insufficient, which cannot achieve good degassing effect. If the insulation and degassing time is extended, granular activated carbon and powdered activated carbon should have equivalent deoxidation effect.
1.1.3 Degassing by vacuum graphite crucible smelting method
Using a 25kg vacuum induction melting furnace, graphite crucible (high purity, high strength, high density), vacuum degree of 1.2Pa, insulation and degassing time of 10min, 15min, 20min respectively, cooling and casting, then analyzing the oxygen content of the ingot. The silver ingots processed by the above process are mixed with oxygen free copper to make AgCu28 vacuum solder. Taking samples from the alloy ingots and analyzing the cleanliness and splatter according to the provisions of the national standard GB/T 4907-1985. The results are shown in Table 3.
Table 3 The degassing effect of vacuum graphite crucible melting method and its influence on the properties of AgCu28
| No. |
Insulation and degassing time/min |
Oxygen content/mass fraction% |
The cleanliness of AgCu28 |
The splattering of AgCu28 |
| 0 |
(silver ingots) |
160 |
— |
— |
| 1 |
10 |
35 |
Ⅰ |
A |
| 2 |
15 |
16 |
Ⅱ |
A |
| 3 |
20 |
8 |
Ⅰ |
A |
From Table 3, using a graphite crucible (high purity, high strength, high density) to degas silver ingots, the effect is the same as adding activated carbon, but the carbon volatilization of the graphite crucible is small during the experiment, and there is almost no pollution to the vacuum equipment, ensuring the cleanliness of AgCu28 vacuum solder. This method not only achieves good degassing effect without pollution, but also improves production efficiency and reduces costs, is definitely the most suitable method in practical production.
1.2 Degassing of silver powder
1.2.1 Vacuum degassing of silver powder ingots
Put
silver powder ingots (three ingots with dimensions of φ80mmx200mm) in a vacuum annealing furnace (temperature of 500 ℃, vacuum degree of 2x10-3Pa) for 1 hour, 3 hours, and 6 hours respectively for degassing. Then analyzing oxygen content of the degassing ingot. Mixing oxygen free cooper with degassing ingots to make AgCu28 vacuum solder (three furnaces). Taking samples from the alloy ingots and analyzing the cleanliness and splattering according to the national standard GB/T4907-1985, and the results are shown in Table 4.
Table 4 Degassing effect of vacuum degassing method for silver powder Ingots and its influence on the performance of AgCu28
| No. |
Insulation and degassing time/min |
Oxygen content/mass fraction % |
The cleanliness of AgCu28 |
The splattering of AgCu28 |
| 0 |
（silver powder） |
260 |
— |
— |
| 1 |
1 |
120 |
Ⅰ |
unqualified |
| 2 |
3 |
90 |
Ⅰ |
unqualified |
| 3 |
6 |
70 |
Ⅱ |
unqualified |
From Table 4, the vacuum degassing using silver powder ingots has no obvious effect and time-consuming. As the silver powder is pressed into the ingot, oxygen is squeezed inside the ingot, meanwhile, a dense film is formed on the surface of the silver powder ingot, which makes it difficult for gas to escape during degassing, resulting in poor degassing effect.
1.2.2 Vacuum degassing of silver powder
Put silver powder in a vacuum annealing furnace (temperature 500 ℃, vacuum degree 2x10-3Pa) for 1 hour, 3 hours, and 6 hours respectively for degassing. Then analyzing oxygen content of the degassing ingot. Mixing oxygen free cooper with degassing silver powder to make AgCu28 vacuum solder (three furnaces). Taking samples from the alloy ingots and analyzing the cleanliness and splattering according to the national standard GB/T4907-1985, and the results are shown in Table 5.
Table 5 Degassing effect of vacuum degassing method for silver powder ingots and its influence on the performance of AgCu28
| No. |
Insulation and degassing time/min |
Oxygen content/mass fraction % |
The cleanliness of AgCu28 |
The splattering of AgCu28 |
| 0 |
（Silver powder） |
260 |
— |
— |
| 1 |
1 |
40 |
Ⅰ |
A |
| 2 |
3 |
20 |
Ⅱ |
A |
| 3 |
6 |
9 |
Ⅰ |
A |
From Table 5, using silver powder vacuum degassing has a better effect, and the silver powder with a holding time of 6 hours can fully meet the production requirements of AgCu28 vacuum solder. The reason is that silver powder is loose, and gas is easy to escape under vacuum.
1.2.3 Vacuum graphite crucible melting and degassing of silver powder ingots
Put the silver powder ingot in a graphite crucible of a vacuum melting furnace(vacuum degree of 1.2Pa) for 5 minutes, 10 minutes, 15 mins, 17 mins, and 20 mins respectively for degassing. Then analyzing oxygen content of the ingot. Mixing processed silver ingot and oxygen free copper to make AgCu28 vacuum solder (five furnaces). Taking samples from the alloy ingot and analyzing the cleanliness and splattering according to the national standard GB/T 4907-1985. The results are shown in Table 6.
Table 6 Degassing effect of the vacuum graphite crucible degassing method for silver powder ingots and its influence on the performance of AgCu28
| No. |
Insulation and degassing time/min |
Oxygen content/mass fraction % |
The cleanliness of AgCu28 |
The splattering of AgCu28 |
| 0 |
（silver powder） |
260 |
— |
— |
| 1 |
5 |
100 |
Ⅰ |
unqualified |
| 2 |
10 |
80 |
Ⅰ |
unqualified |
| 3 |
15 |
70 |
Ⅰ |
unqualified |
| 4 |
17 |
40 |
Ⅰ |
unqualified |
| 5 |
20 |
20 |
Ⅰ |
B |
From Table 6, the degassing effect of vacuum graphite crucible melting and degassing method of silver powder ingots is not significant and time-consuming; During the experiment, when the silver powder ingot melts, a large number of bubbles escape, causing the metal liquid to splash and causing a short circuit in the vacuum equipment. This method is not advisable in actual production.
1.3 Conclusion
（1）Both vacuum melting degassing and activated carbon degassing can achieve deoxidation effects. However, due to the severe volatilization of silver during vacuum melting degassing and carbon during activated carbon degassing, both seriously contaminate the vacuum equipment and cannot ensure the cleanliness of AgCu28 vacuum solder. Therefore, they are not suitable for use in actual production.
（2）Using a graphite crucible with "three highs" for vacuum melting to process silver ingots has good deoxidation effect and minimal pollution to vacuum equipment. The processed silver ingots can be used to make AgCu28 vacuum solder, with a splattering performance of Class A and cleanliness of Class I-II. This method fully meets the production requirements of AgCu28 vacuum solder and has low production costs.
（3）The deoxidation effects of vacuum degassing method and vacuum melting degassing method for silver powder ingots do not have obvious effects, they’re time-consuming and the vacuum melting degassing method is harmful to equipment, so both methods are not suitable in production.
（4）The silver powder vacuum degassing method has good degassing effect. The splattering of the AgCu28 vacuum solder produced can reach grade A, and the cleanliness can reach Grade I to II, which can be used to produce AgCu28 vacuum solder.
