Although nitrogen has been used in electronics manufacturing since the early 1970s, it was not until the introduction of no-clean technology, which requires welding in an inert gas environment, that its use gained widespread acceptance. When the first inert gas experiments were conducted in 1968, the wave soldering equipment was open. There are neither operator safety and health regulations nor sealing requirements. Initially, the use of nitrogen in wave soldering is only to reduce costs, reduce or eliminate oxidation slag, reduce machine maintenance and improve the performance of no-clean welding.
Equipment developed in the early 1990s has adopted a tunnel structure to form a nitrogen protective layer. A protective layer surrounds the wave soldering conveyor belt, preventing air from entering and leaving the inlet and outlet. The vertical height of the tunnel cavity should be as low as possible, and there are Windows on the sealing frame to facilitate the observation of the welding process. You can also remove the window and touch the inside of the machine to maintain the machine and adjust the process flow. During the process of entering and exiting the printed board, nitrogen injected into the welding system prevents air from entering through the opening. Therefore, nitrogen must maintain positive pressure. Some light hanging trapdoors are hinged along the length of the tunnel to reduce air intrusion. These suspension doors can flip upward when circuit components are near. All tunnel designs with end openings have some means of discharging nitrogen as it flows out of the tunnel inlet and outlet. This "exhaust" usually needs to be balanced in order to send the air from the room to the exhaust pipe, which helps prevent the exhaust gas from drawing excessive nitrogen from the tunnel. Note that the key here is to reduce the temperature and nitrogen loss. The length of the tunnel can be very short, only covering the preheating zone and welding groove; It can also be very long, from the feeding end to the unloading end. Therefore, the equipment of the long tunnel actually covers the flux dispensing device, the preheating zone and the wave crest welding zone. The difference between short and long tunnels is in the amount of nitrogen required: When low-temperature nitrogen with an impurity content of 1ppm to 2ppm is injected into the system, the oxygen impurity around the welding crest should be less than 10ppm. Compared to long tunnels, short tunnels consume more nitrogen and are more sensitive to the air flow in the workshop. High sensitivity to air flow often leads to instability in the purity measured in the wave crest. In any case, this device has always been used at impurity levels from 100ppm to 200ppm, and it brings clear benefits to the welding process. You can retrofit your existing equipment so that it can use nitrogen, but this will be an expensive, time-consuming process.
There is another method of wave soldering in an inert gas environment, which is to use a shield made of a shield design around the welding nozzle until the welding crest falls back to the position of the welding groove. The "sprayer" is located at the bottom of the shield and supplies nitrogen. The main advantage of this method is that the system can be contacted directly. In a sealed system, it is possible to bring the surfaces of the surface-bonded parts to reflow temperatures, resulting in solder reflow. This possibility increases if the printed board warps or the "curtain" at the exit of the tunnel touches the SMD above the printed board. On the other hand, the use of this "shielding" technology completely eliminates the temperature problem in the surrounding area after wave soldering. Electrovert and Soltec have made welding systems that use nitrogen in open crests and found that the reduction of oxidizing slag is as good as the tunnel welding system. The results of "shielding" can be compared to those obtained by using electroplating, hot coating or hot air smoothing of welded components of printed boards. Another advantage of using this new technology is that the nitrogen consumption is the same or even lower than that of the most expensive closed wave soldering systems. In dual-crest systems for surface adhesion welding, individual shielding and nitrogen supply control can be used for each crest. When there are no welded components in the system, the system can enter a waiting mode, setting the welding crest at a lower height to reduce the formation of oxidation slag and stop or reduce the flow rate of nitrogen. When the system detects the printed board, it is able to reactivate the normal job control Settings. This control mechanism further reduces nitrogen consumption. If only one wave crest can be used for welding, more nitrogen can be saved. In a tunnel system, the nozzle is required to extend above the edge of the solder tank to reach the tunnel. In a shield system, the nozzles stick to the lower part of the system, allowing for a quick and complete shutdown of nitrogen for components that are well soldered without the need for nitrogen. In addition, the short distance of solder back into the weld tank is also shielded, and the chance of solder sputtering is reduced. Compared with the tunnel design, the shielded type is easy to modify and consumes less time. The welding results are the same for most components, and the operating costs are the lowest of all inert gas wave soldering. However, these systems require more maintenance than the tunneled ones.
Nitrogen in reflow welding Nitrogen has been used in reflow welding since before inert gases were used in the wave soldering process. Part of the reason is that the hybrid IC industry has long used nitrogen in reflow soldering of surface-bonded ceramic hybrid circuits, and when other companies saw the benefits of hybrid IC manufacturing, they applied this principle to PCB welding. In this welding, nitrogen also replaces oxygen in the system. Nitrogen can be introduced into every area, not only in the return zone, but also for cooling in the process. Most reflow soldering systems are now ready for nitrogen; Some systems can be easily upgraded to incorporate gas injection.

The use of nitrogen in reflow welding has the following advantages:

1. The terminal and pad are wetted quickly. 2. Less weldability change. 3. Improve the appearance of flux residue and solder joint surface. 4. Fast cooling without copper oxidation

With the lead-free process has been put on the agenda, how to smoothly import lead-free has become the most concerned issue for SMT users. How to choose the most suitable and self-produced nitrogen source? How to determine the specific parameters of the nitrogen atmosphere? How much is the cost going up?

In fact, there are several ways to supply nitrogen sources, you can have a gas fractionator, buy bottled nitrogen from a gas company, buy liquid nitrogen from a gas company and on-site nitrogen (N2 generator) to choose from. A gas company or one with a large use of N2 may have a gas fractionator (N2Distillation) which will work with compressed air to liquify, and then with a fractional distillation of nitrogen and oxygen at different boiling points. This equipment covers a large area, and the cost is expensive, not suitable for general enterprises. Small volume users can purchase cylinder nitrogen from the gas company. Nitrogen is stored in high-pressure cylinders and transported directly to the gas point for use. Bottled nitrogen has the advantages of being used on demand, flexible and convenient. However, it has the disadvantages of high risk, high cost, troublesome transportation and storage. If bottled nitrogen is no longer sufficient for current production, you should purchase liquid nitrogen from a gas company or choose on-site nitrogen to obtain the required nitrogen. The liquid nitrogen storage tank or dewar tank is used to store liquid nitrogen, and the liquid nitrogen is gasified into gaseous nitrogen when it is needed, and can be used after decompression and heating. Liquid nitrogen has the characteristics of convenience and convenience, but the liquid nitrogen in the storage tank needs to be replenished frequently, which also brings trouble and pressure to the procurement and transportation. At the same time, the long-term use of liquid nitrogen has high cost, troublesome transportation, and is greatly affected by the supply source. On-site nitrogen production and Membrane separation nitrogen (Membrane) and Pressure Swing Adsorption (Pressure Swing Adsorption) nitrogen production machine. Membrane separation nitrogen machine is a high-tech technology that arose in the 1980s. The device uses air as the raw material, and the hollow fiber membrane uses oxygen and nitrogen to penetrate the membrane tissue at different rates - water and oxygen can pass through and nitrogen cannot, so as to achieve oxygen and nitrogen separation. The purity of nitrogen produced by the membrane separation nitrogen production mechanism is low, generally 95-99.9%. Moreover, the membrane separation nitrogen machine has a large energy consumption, and its core component - hollow fiber membrane is mainly dependent on imports, with high prices, long delivery cycle, and troublesome equipment follow-up maintenance.

PSA nitrogen machine mainly uses carbon molecular sieve as adsorbent, compressed air as the main raw material, the adsorption rate of oxygen and nitrogen is different, carbon molecular sieve preferentially adsorbed oxygen, and most of the nitrogen is enriched in the non-adsorbed phase, to achieve the separation of oxygen and nitrogen, get the gas we need. Based on the principle and process of pressure swing adsorption, two adsorption towers were used to adsorb alternately in parallel, one tower working and one tower regenerating, and nitrogen was produced continuously. One time may take the purity of 98-99.99% qualified product gas (Dongguan City Troy Gas Equipment Co., Ltd. produced Troy brand nitrogen machine one-time extraction purity of up to 98-99.995%). If the nitrogen produced by the PSA nitrogen production mechanism can be further deoxygenated by the nitrogen purification device, 99.9999% can be obtained, that is, high-purity and high-quality nitrogen with oxygen content below 1ppm.

(2) How to determine the specific parameters of nitrogen atmosphere SMT users before deciding to use nitrogen, first determine the purity of nitrogen in the furnace (several nine, or ppm of oxygen content), and then determine the purity of the nitrogen machine outlet. The sufficient and necessary condition of oxidation reaction is the existence of oxygen molecule. Under the same condition, the higher the oxygen content is, the more intense the oxidation reaction is. On the contrary, the lower the oxygen content, the weaker the oxidation reaction. Of course, the higher the purity of nitrogen, the better, but the balance between the investment cost and the defective rate and rework amount of the product should be considered. At present, most electronics factories including Hon Hai Precision Shares (Taiwan Foxconn) choose: 99.99% oxygen content is less than 100ppm, there are also options: 99.9% oxygen content is less than 1000ppm, a few options: 99.999% oxygen content is less than 10ppm. Therefore, the exact purity should be determined according to the grade of the product, the allowable defect rate, the company's policy, the product's requirements for infiltration and other factors. After determining the purity of nitrogen in the furnace, then determine the purity of the nitrogen machine outlet, usually the nitrogen machine is not placed in the car with the SMT production line, but on the roof, or outside the workshop, through the pipeline input furnace, there are many connection ports between, it is likely to cause the purity of nitrogen, so the nitrogen machine outlet purity is more than enough. And the gas consumption per unit time (usually calculated by how many cubic meters per hour) different brands, different types of furnace gas consumption is different, the size of the input PCB is different, the speed of the chain rotation of different gas consumption is not the same, so the exact gas consumption should be based on the field experiment.

Reference case: XXX OEM enterprise used purity of 99.99% nitrogen machine export oxygen content < = 100ppm> Phase I project one: used for a backwelding furnace production line. Purity is 99.99%, gas production is 20m3/h nitrogen system phase II project three: for three back welding line furnace production lines. Purity is 99.99%, gas production is 20m3/h nitrogen system. Phase III project four: for four back welding furnace production line. Purity is 99.999%, gas production is 100m3/h nitrogen system. The following is a nitrogen generator with a purity of 99.99% and a gas production of 20m3/hr used in an SMT production line, and a brief economic analysis and comparison are made as follows (the electricity bill is calculated at 0.6 yuan, and the operation time is 8000 hours a year) : 1, liquid nitrogen use cost of liquid nitrogen on the market is about 1200 yuan/ton, gas conversion purity of 99.99%, 780m3 of gaseous nitrogen, the purchase of 1m3 of nitrogen price is 1.4 yuan. If you add the rent of the liquid nitrogen storage tank, the purchase cost of the carburetor, the annual labor cost, etc., the cost of using 1m3 of nitrogen is close to 1.6 yuan. 2, the use cost of PSA nitrogen machine The use cost of PSA nitrogen machine is mainly the consumption of electric energy, the consumption of electric energy mainly comes from the following aspects: air compressor: the rated power of the air compressor is 22kw, the power consumption is about 80% of the rated power, that is, about 17.5kw freeze dryer: the rated power of the freeze dryer is 1kw. Nitrogen machine: the raw material of the nitrogen machine is compressed air, and the nitrogen machine itself basically does not consume electricity, which is mainly instrument electricity, rated power is about 0.05kw. Annual electricity total: (17.6+1+0.05) ×8000×0.6= 91,200 labor costs: no special duty is required, which can be ignored. Equipment depreciation cost: equipment investment is calculated at 200,000 yuan, and depreciation is 20,000 yuan per year. The annual cost is: (9.12+2) = 111,200 per cubic meter nitrogen cost 112000÷800÷0.6=0.7 yuan. The annual difference between the two nitrogen methods is 20×8000× (1.7-0.7) = 160,000 yuan. So the investment payback period of PSA nitrogen machine is only 10 months. (i.e. 16÷20) The pressure swing adsorption nitrogen machine avoids the inconvenience caused by the high cost of bottled nitrogen, dewar tanks, and the supply of nitrogen does not have to rely on retailers, avoiding the danger caused by high pressure bottled nitrogen, and the loss of production interruptions caused by gas supply interruptions. The long-term stable and low cost operation of the pressure swing adsorption nitrogen machine can avoid uncontrollable gas price rise, rent high low temperature storage tank costs, and simple operation, without the need for another gas storage tank, can recover the cost in a short time. No need for special maintenance, maintenance is also simple. Therefore, once the PSA nitrogen machine is installed, we can extract nitrogen from the air at will. jpg" style="width: 750px; height: 750px;"  />