In recent years, with the continuous improvement of welding product quality and environmental protection requirements, lead-free welding has been widely used. Inert nitrogen welding is popular in improving product quality. Although the nitrogen atmosphere is purchased at a cost, it is much cheaper than the environmental requirements and maintenance costs due to welding defects. Therefore, the role of inert nitrogen atmosphere in SMT welding is paid more and more attention, and will be gradually accepted and widely used. However, the cost of nitrogen use largely depends on the flow rate, operation time, refluxing furnace structure and production process, among which the choice of nitrogen source is more important.
01 There are two main sources of nitrogen: liquid canned nitrogen (cryogenic air separation), which is produced by nitrogen generators (nitrogen is extracted directly from the air rather than cryogenic air separation).
Liquid nitrogen in the tank (one ton of liquid nitrogen is equivalent to nitrogen 799m3 at room temperature and pressure, with a general loss of 5%):
Gaseous nitrogen is produced by large nitrogen making equipment and converted to liquid nitrogen after ultra-high pressure and ultra-low temperature treatment (usually 500Mpa and 1800℃). We use gaseous nitrogen (that is, nitrogen is delivered to the equipment). Therefore, before the use of liquid nitrogen, a gasification process is required, which is achieved through the carburetor: reduced pressure temperature rise. In general, nitrogen can be evaporated at room temperature. Liquid nitrogen tanks need to be purchased or leased separately, which is a big investment and high cost to use.
02 Membrane separation nitrogen making machine, PSA nitrogen making machine:
Membrane separation nitrogen machine: the production of nitrogen purity up to 99.9% or oxygen content ≤=1000ppm. The advantage is that qualified nitrogen can be produced within two to three minutes of starting and can be started and stopped at any time. The system is stable and reliable, nitrogen purity fluctuation is small, maintenance is small, maintenance is simple; Disadvantages: Higher investment than PSA nitrogen machine.
PSA nitrogen making machine: Carbon molecular sieve as adsorbent, through pressure swing adsorption process directly produce nitrogen purity of 99.99% or oxygen content <=100ppm nitrogen filling equipment.
03Application of field nitrogen machine in SMT industry
Gas consumption and nitrogen purity should be considered first when selecting the on-site PSA nitrogen machine:
1. Air consumption per unit time (usually calculated as cubic meter/hour): The air consumption of different brands and models of furnaces varies with the input size of PCB and the speed of the chain. Therefore, accurate gas consumption should be based on field tests. The nitrogen consumption of each furnace of Foxconn Group is 20m3/h.
2. Purity of nitrogen (how many 9, or ppm of oxygen content); Firstly, the purity of nitrogen in the furnace was determined, and then the purity of nitrogen at the outlet of the generator was determined. The presence of oxygen molecules is a necessary and sufficient condition for oxidation. Under the same conditions, the higher the oxygen content, the stronger the oxidation reaction; On the contrary, the lower the oxygen content, the weaker the oxidation reaction. Of course, the more pure the nitrogen, the better. However, it is necessary to consider the balance between the investment cost and the defect rate and rework volume.
3. At present, most electronic manufacturers choose the oxygen content of 99.99% or 100ppm, some choose 99.9% or 1000ppm oxygen content, and a few choose 99.999% or 10ppm oxygen content. Therefore, the exact purity should be determined based on factors such as product grade, allowable defect rate, company policy, and product requirements for wettability. After determining the purity of nitrogen in the furnace, then determine the purity of nitrogen at the generator outlet; Normally, nitrogen machines and SMT production lines are not placed in the workshop, but on the roof of the workshop or outside the workshop. The nitrogen machine and the furnace are connected by multiple pipes, which may reduce the purity of nitrogen. Therefore, the outlet purity of the nitrogen generator should also have a balance. The nitrogen machines used by Foxconn are 99.99% pure and contain less than 100ppm of oxygen for export.