In order to block the flow of air into the reflow furnace to prevent the oxidation of the element feet in reflow welding. The use of nitrogen reflow welding is mainly to enhance the quality of welding, so that welding occurs in an environment with very little oxygen content (less than 100PPM), which can avoid the oxidation problem of components. Therefore, the main problem of nitrogen reflow welding is to ensure that the oxygen content is as low as possible.

Nitrogen reflow is the filling of the reflow furnace with nitrogen.

With the increase of assembly density and the appearance of Finepitch assembly technology, nitrogen filled reflow welding process and equipment are produced, and the quality and yield of reflow welding are improved, which has become the development direction of reflow welding. SMT nitrogen maker nitrogen reflow has the following advantages:

1 Prevent reduced oxidation

2 Improve the welding wetting force and speed up the wetting speed

3 Reduce the occurrence of tin balls, prevent bridging, and obtain better welding quality

It is particularly important to use less active flux solder paste, but also improve the performance of the solder joint, reduce the discoloration of the substrate, but the disadvantage is the obvious increase in cost, this increase in 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 line oxygen content analyzer, The principle of oxygen content testing is that the oxygen content analyzer is first connected to the collection point through nitrogen reflow welding, and then the gas is collected, and the oxygen content value is analyzed by the oxygen content analyzer to obtain the purity range of nitrogen content.

There is at least one nitrogen reflow gas collection point, and there are more than three high-end nitrogen reflow gas collection points, and the requirements for welding products are different. Now solder paste manufacturers are committed to developing no-wash solder paste that can perform good welding in a high oxygen atmosphere, so that nitrogen consumption can be reduced.

For the introduction of nitrogen in reflow welding, it is necessary to carry out cost benefit analysis, income includes product yield, quality improvement, rework or maintenance cost reduction, etc., complete and correct analysis often reveals that the introduction of nitrogen does not increase the final cost, on the contrary, it can benefit from, rare liquid nitrogen, as well as nitrogen machine, nitrogen selection is more flexible.

How much oxygen content in the nitrogen furnace is appropriate in PPM?

Checked the relevant literature below 1000PPM infiltration will be very good, indicated that 1000-2000PPM is the most commonly used but most of the actual use of 99.99%, or 100PPM nitrogen, and even 99.999%, or 10PPM, and some customers actually use 98% nitrogen, or 20000PPM

Another argument is that the OSP process, double-sided welding, should be below 500PPM when there is PTH, and it means that the number of steles is increased and the printing accuracy is not high, which is caused.

Most of the furnaces in use today are forced hot air circulators in which controlling nitrogen consumption is not an easy task. There are several ways to reduce the consumption of nitrogen and reduce the opening area of the furnace inlet and outlet. It is important to use a partition, curtain or similar device to block the space that does not use the partial inlet and outlet. Another way is to use the principle that the hot nitrogen layer is lighter than the air and does not mix easily. In this way, a natural nitrogen layer is formed in the heating chamber, reducing the make-up amount of nitrogen and maintaining the required purity.

Double-sided reflow double-sided PCB has been quite popular, and gradually becoming more and more popular, the main reason is to provide designers with very good elastic space, so as to design more compact, low-cost products. Until today, dual panels have been reflow welded above (component face) and then wave welded below (pin face).

A current trend is toward double-sided reflow soldering, but there are still some problems with this process. The bottom element of the large plate may fall off during the second reflow welding process, or the partial melting of the bottom weld may cause solder joint reliability problems. Several methods have been found to achieve double-sided reflow soldering: One is to glue the first side of the component, so that when it is turned over for a second time into the reflow, the component will stay in place and not fall off. This method is commonly used, but requires additional equipment and procedures, which increases the cost. The second is the application of different melting point of the solder alloy, the first side is to use a higher melting point of the alloy and in the second side with a low melting point of the alloy, the problem of this method is that the low melting point of the alloy selection may be limited by the working temperature of the final product, and the high melting point of the alloy is bound to increase the temperature of reflow welding, which may cause damage to the components and PCB itself.

For most components, the solder surface tension of the weld point is enough to grasp the bottom component to form a high-reliability solder joint, the ratio of the weight of the component to the pin area is used to measure whether this kind of successful welding is a standard, usually in the design will use 30g/in2 this specification, the third is the method of blowing cold air at the lower part of the furnace. This can maintain the bottom solder joint temperature of the PCB below the melting point in the second reflow welding. However, the potential problem is due to the occurrence of temperature difference between the top and the bottom, resulting in internal stress, which requires effective means and processes to eliminate stress and improve reliability. These process problems are not simple but are being successfully solved. There is no doubt that the next few years, dual panels will be a great development in number and complexity.