Deep analysis of the application of nitrogen in SMT welding

- May 21, 2020-

Nitrogen protective layer

The equipment developed in the early 1990s has adopted a tunnel structure to form a nitrogen envelope. The protective layer surrounds the wave soldering conveyor belt and prevents air from entering and exiting the inlet and outlet. The vertical height of the tunnel cavity should be as low as possible, and the sealing frame has a window to facilitate the observation of the welding process. You can also remove the window and touch the inside of the machine to maintain and adjust the process flow of the machine.

During the entry and exit of the printed board, the nitrogen injected into the welding system prevents air from entering the opening. Therefore, nitrogen must maintain a positive pressure. Some light suspension doors are hinged along the length of the tunnel to reduce air intrusion. When the circuit components are close, these hanging doors can be flipped up.

When nitrogen flows out of the tunnel entrance and exit, all open-ended tunnel designs have some methods of discharging nitrogen. It is often necessary to balance this "exhaust gas" in order to send the room air to the exhaust pipe, which helps prevent the exhaust gas from sucking excessive nitrogen from the tunnel. Note that the key at this time is to reduce the temperature and reduce the loss of nitrogen.

The length of the tunnel can be very short, covering only the preheating zone and the welding groove; it can also be very long, from the upper end to the lower end. Thus, the equipment of the long tunnel actually covers the fluxer, preheating zone and wave soldering zone.

The difference between the short tunnel and the long tunnel is expressed in the amount of nitrogen required: when injecting low-temperature nitrogen with an impurity content of 1ppm to 2ppm into the system, the oxygen impurities around the welding peak should be less than 10ppm. Compared with long tunnels, short tunnels consume more nitrogen and are more sensitive to the airflow in the workshop. High sensitivity to air flow often results in unstable purity measured in the crest.

In any case, this device has been used at an impurity content of 100 ppm to 200 ppm, and it has brought obvious benefits to the welding process. You can modify existing equipment to use nitrogen, but this will be an expensive and time-consuming process.

Shield crest

There is another method for wave soldering in an inert gas environment, that is, a shroud designed shield is used to surround the welding tip until the welding wave falls back to the welding groove. The "sprayer" is located at the bottom of the shield and supplies nitrogen.

The main advantage of this method is that it can directly contact the system. In a sealed system, it is possible to make the surface of the parts adhered to the surface reach the temperature of reflow soldering, resulting in solder reflow. If the printed board warps or the "curtain" at the exit of the tunnel touches the SMD above the printed board, this possibility will increase. On the other hand, using this "shielding" technology completely eliminates the temperature problem in the surrounding area after wave soldering.

Electrovert and Soltec have manufactured welding systems that use nitrogen in open wave crests, and they have found that the reduction of oxidized slag is as good as that of tunnel welding systems. The "shielding" results are comparable to those obtained by plating, hot-coating or hot-air leveling the soldered components of the printed board. Another advantage of using this new technology is that the nitrogen consumption is the same as or even lower than the most expensive closed wave soldering system.

In the dual wave system used for surface bonding welding, an independent shield and nitrogen supply control can be used for each wave. When there are no welding components in the system, the system can enter the standby mode, set the welding peak at a lower height to reduce the generation of oxidized slag, and stop or reduce the flow rate of nitrogen. When the system detects the printed board, it can reactivate normal job control settings. This control mechanism further reduces the nitrogen consumption. If only one wave can be used for welding, more nitrogen can be saved.

In the tunnel system, the nozzle is required to extend above the edge of the solder bath to reach the tunnel. In the shielding system, the nozzle is adhered to the lower part of the system. For components that can be welded without nitrogen, it is possible to turn off the nitrogen quickly and completely. In addition, the short distance that the solder returns to the solder bath is also shielded, and the chance of solder splashing is reduced.

Compared with the tunnel design, the shielded type is easy to modify and consumes less time. The welding results of most components are the same, and the operating cost is the lowest among all inert gas wave welding. However, these systems require more maintenance than the tunnel type.

Nitrogen in reflow soldering

Before inert gas was used in the wave soldering process, nitrogen has been used in reflow soldering. Part of the reason is that in the reflow soldering of surface-bonded ceramic hybrid circuits, the hybrid IC industry has long used nitrogen. When other companies saw the benefits of mixed IC manufacturing, they applied this principle to PCB soldering.

In this type of welding, nitrogen also replaces the oxygen in the system. Nitrogen can be introduced into each area, not only in the reflux zone, but also for the cooling process. Most reflow soldering systems are now ready for nitrogen; some systems can be easily upgraded to use gas injection.

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