What is nitrogen reflow soldering? What are the characteristics of nitrogen reflow welding?

With the increase of assembly density and the emergence of fine pitch assembly technology, nitrogen filled reflow soldering technology and equipment have emerged, improving the welding quality and yield of reflow soldering, and have become the development direction of reflow soldering. Nitrogen reflow soldering is the process of filling the reflow soldering furnace with nitrogen gas to prevent air from entering the reflow soldering furnace and prevent oxidation of component feet during reflow soldering. The use of nitrogen reflow welding is mainly to enhance welding quality, so that welding occurs in environments with very low oxygen content (100PPM), which can avoid oxidation problems of components. Therefore, the main problem with nitrogen reflow welding is to ensure that the lower the oxygen content, the better.

Nitrogen reflow soldering has the following advantages:

(1) Prevent and reduce oxidation;

(2) Improve welding wetting force and accelerate wetting speed;

(3) Reduce the generation of solder balls, avoid bridging, and achieve better welding quality.

However, its disadvantage is that the cost increases significantly, which increases with the amount of nitrogen used. When you need to reach 1000ppm and 50ppm oxygen content in the furnace, the nitrogen content test is usually carried out by a matching online oxygen content analyzer. The principle of oxygen content testing is to connect the oxygen content analyzer to the collection point through nitrogen reflow welding, then collect gas, and analyze the oxygen content value through the oxygen content analyzer to determine the purity range of nitrogen content. There should be at least one gas collection point for nitrogen reflow soldering, and at least three gas collection points for high-end nitrogen reflow soldering. The requirements for welding products vary, and the demand for nitrogen varies greatly.

For the introduction of nitrogen in reflow soldering, a cost-benefit analysis must be conducted, which includes product yield, quality improvement, reduction of rework or maintenance costs, and so on. A complete and accurate analysis often reveals that nitrogen introduction does not increase the final cost. On the contrary, we can benefit from it. Currently, common methods include liquid nitrogen and nitrogen generators, and nitrogen selection is also more flexible.

What is the appropriate oxygen content for PPM in a nitrogen furnace?

The relevant literature argues that wettability is good below 1000PPM, and 1000-2000PPM is the most commonly used. However, in actual use, most of them use 99.99% or 100PPM of nitrogen, and even 99.999% or 10PPM. Some customers even use 98% or 20000PPM of nitrogen. Another theory is that in the OSP process, double-sided welding is required, and when there is PTH, it should be below 500PPM. At the same time, the increase in standing stones is due to the low printing accuracy.

Most of the furnaces currently used are forced hot air circulation type, and controlling the consumption of nitrogen in these furnaces is not an easy task. There are several methods to reduce nitrogen consumption: one is to reduce the opening area of the furnace inlet and outlet, and it is important to use partitions, rolling shutters, or similar devices to block the unused space of the inlet and outlet. The other is to use the principle that the hot nitrogen layer is lighter than air and not easy to mix. When designing the furnace, the heating chamber is made higher than the inlet and outlet, forming a natural nitrogen layer in the heating chamber, reducing the compensation amount of nitrogen and maintaining the required purity.