Lead is a toxic heavy metal. Excessive absorption of lead by the human body can cause poisoning, and low intake of lead may affect human intelligence, nervous system and reproductive system. The global electronic assembly industry consumes about 60,000 tons per year. Solder is increasing year by year. The resulting industrial scum containing lead salts seriously pollutes the environment. Therefore, reducing the use of lead has become the focus of worldwide attention. Many large companies in Europe and Japan are vigorously accelerating the development of lead-free alternative alloys. , And has planned to gradually reduce the use of lead in electronic product assembly in 2002. Completely eliminated by 2004. (The traditional solder composition 63Sn/37Pb, lead is widely used in the current electronic assembly industry).
2. Requirements for lead-free substitutes:
1. Price: Many manufacturers require that the price cannot be higher than 63Sn/37Pn, but currently, the finished products of lead-free substitutes are 35% higher than 63Sn/37Pb.
2. Melting point: Most manufacturers require a minimum solid phase temperature of 150℃ to meet the working requirements of electronic equipment. The liquidus temperature depends on the specific application.
Electrodes for wave soldering: In order to successfully realize wave soldering, the liquidus temperature should be lower than 265°C.
Soldering wire for manual soldering: The liquidus temperature should be lower than the working temperature of the soldering iron 345℃.
Solder paste: The liquidus temperature should be lower than 250°C.
4. Good thermal conductivity.
5. Small solid-liquid coexistence range: Most experts recommend that this temperature range be controlled within 10°C in order to form good solder joints. If the alloy solidification range is too wide, solder joints may crack and make electronic products prematurely damage.
6. Low toxicity: The alloy components must be non-toxic.
7. It has good wettability.
8. Good physical properties (strength, tensile, fatigue): The alloy must be able to provide the strength and reliability that Sn63/Pb37 can achieve, and there will be no raised fillet welds on the passing device.
9. Repeatability of production and consistency of solder joints: Since the electronic assembly process is a mass manufacturing process, it is required to maintain a high level of repeatability and consistency. If certain alloy components cannot be used in mass conditions Repeated manufacturing, or its melting point changes significantly due to changes in composition during mass production, cannot be considered.
10. Solder joint appearance: The solder joint appearance should be close to that of tin/lead solder.
11. Supply capacity.
12. Compatibility with lead: As it will not be fully transformed into a lead-free system in the short term, lead may still be used on PCB pads and component terminals. If the solder is mixed with such a drill, it may cause the solder alloy to become weak. The melting point drops very low and the strength is greatly reduced.
3. Several lead-free alternatives commonly used at present:
1. Alloys used in solder paste: 96, 5Sn/3, 5Ag melting point is 221℃95, 5Sn/4, 0Ag/0, 5Cu melting point is 217℃
2. Alloy solders used in wave soldering and manual soldering: 99, 3Sa/0, 7Cu melting point is 227℃
(In terms of materials, several new solder pastes have been put on the market, but they cannot replace the existing lead-tin solders. All need to make corresponding process adjustments. The main difference between the new solder paste and the traditional solder paste is that the melting point is relatively high. The melting point of the commonly used lead-free solder paste is usually 217℃-225℃)
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