Nitrogen reflow soldering is to fill the reflow oven with nitrogen, in order to prevent air from entering the reflow oven to prevent oxidation of the component feet during reflow soldering. The use of nitrogen reflow soldering is mainly to enhance the quality of soldering, so that soldering occurs at the extreme oxygen content.
The oxidation problem of components can be avoided in an environment below 100PPM. Therefore, the main problem of nitrogen reflow soldering is to ensure that the lower the oxygen content, the better.
With the increase in assembly density and the emergence of fine pitch assembly technology, nitrogen-filled reflow soldering processes and equipment have been produced, which have improved the soldering quality and yield of reflow soldering, and have become the development direction of reflow soldering. Nitrogen reflow soldering has the following advantages:
(1) Prevent reducing oxidation;
(2) Improve welding wetting power and speed up wetting speed;
(3) Reduce the production of solder balls, avoid bridging, and obtain better soldering quality.
But its disadvantage is the obvious increase in cost. This increased cost increases with the amount of nitrogen. When you need to reach 1000ppm oxygen content and 50ppm oxygen content in the furnace, the general nitrogen content test is by the matching online oxygen content analysis The principle of oxygen content testing is that the oxygen content analyzer is connected to the collection point through nitrogen reflow soldering, and then the gas is collected. The oxygen content value is analyzed by the oxygen content analyzer to obtain the nitrogen content purity range. There is at least one collection point for nitrogen reflow soldering gas, and there are more than three collection points for high-end nitrogen reflow soldering gas. The requirements for soldering products are different and the demand for nitrogen is very different. For the introduction of nitrogen in reflow soldering, a cost-benefit analysis must be carried out. Its benefits include product yield, quality improvement, reduction in rework or maintenance costs, etc. A complete analysis often reveals that the introduction of nitrogen does not increase the final cost. On the contrary, we can benefit from it. At present, liquid nitrogen and nitrogen generators are common. The choice of nitrogen is more flexible.
What is the appropriate PPM for the oxygen content in the nitrogen furnace?
Relevant literature demonstrates that the wet below 1000PPM will be very good, 1000-2000PPM is the most commonly used, but most of the actual use process uses 99.99% or 100PPM nitrogen, even 99.999% or 10PPM, and some customers even use 98% Nitrogen is 20000PPM. Another saying is that in OSP process, double-sided welding, when there is PTH, it should be below 500PPM. At the same time, the increase in tombstones is caused by low printing accuracy. Most of the furnaces currently in use are of the forced hot air circulation type, and it is not easy to control the consumption of nitrogen with such furnaces. There are several ways to reduce the consumption of nitrogen: One is to reduce the opening area of the furnace inlet and outlet. It is very important to use partitions, roller blinds or similar devices to block the unused part of the inlet and outlet space. The other is to use the principle that the hot nitrogen layer is lighter than air and not easy to mix. When the furnace is designed, the heating chamber is higher than the inlet and outlet, so that a natural nitrogen layer is formed in the heating chamber, reducing the amount of nitrogen compensation and Maintain the required purity.
