Here we will discuss Why we Use Nitrogen in the reflow Oven? The use of Nitrogen in Reflow soldering in the PCBA process consideration becomes common during reflow soldering. Every organization aims to establish a process that Reduces Costs, increases productivity, and improves customer satisfaction.
Reflowing in nitrogen is not the issue, but rather reflowing in the absence of oxygen. Oxides, which are generally non-solderable surfaces, are created when the solder is heated in the presence of oxygen. This results in poor wetting of molten alloy on printed circuit boards.
Why Use Nitrogen in reflow oven
If you are interested in reducing costs, improving the quality of your process, and meeting your environmental challenges, you can do it with an inert soldering atmosphere. The most advantageous nitrogen has become more significant in defect reduction when using fine pitch components or Ball Grid Arrays (BGA, s).
A study says that if Nitrogen in Reflow soldering is done, the process yield is lowered by 6 to 7%. In other words, we can say that 60 to 70% of defects can be reduced if nitrogen is adopted in the process. Let us discuss the significant variable factors that impact oxidation and their effects on solder joint formation.
Several factors influence the oxidation process, including temperature, flux composition, metal composition, and oxygen levels around a solder joint. Now that we understand the variables that affect oxidation, let’s examine their impact.
High temperature during the reflow soldering process
Oxidation is directly related to temperature; higher temperatures mean faster Oxidation rates. When printed circuit boards are soldered in air, the metal alloy systems used in electronic assembly processes are subject to oxidation. Solder oxides inhibit wetting of the molten alloy to printed Circuit board lands and component leads, resulting in soldering defects.
Surface Area area of solder paste particles
As the pitch of fine pitch components reduces, the solder paste particle size becomes smaller to meet reasonable solder print requirements. As solder paste particles become smaller, the surface area to volume ratio increases. The below formula can explain that.
So if we are using more OK size solder particles, it means providing more surface of oxidation that increases soldering defects.
Metal Content and Condition
Some metals are more accessible to solder than others. The tin/lead coating on a board makes soldering easier than soldering to bare copper. Solderability is also affected by the condition of the metal to be soldered. Metal boards that have sat on the shelf unprotected for extended periods will have oxidized. The reflow job will be more challenging as a result. When reflowing the second side, double-sided reflow can also be more difficult. Possibly oxidation on the surface of the second side has resulted from having been heated already once
Solid joint formation required that the oven run with a low-level oxygen level to reduce oxidation. The oxygen level is measured in parts per million ppm. The legal environment contains 21% oxygen which is almost 210,000 ppm. But most ovens can operate at less than 100 ppm oxygen Environment.
Benefits use of N2 Reflow Soldering Process
- Flexibility – wider process window
- Use Of low solids;
- Implementation of Low Residue, No-Clean Soldering
- Reduction in Labor Costs and Increase in Production Rates
- Reduced cleaning – residues not polymerized or eliminated;
- Reduced automatic testing false fails;
- BGA voiding reduction;
- Reduced defects / increased yield;
- Increased joint reliability; and Performance.
- Aesthetic / shiny joints.
Here are a few Disadvantages/Cost Involved in reflow soldering in the inert atmosphere:
Cost of Nitrogen — One of the most significant aspects of Nitrogen utilization is the installation cost, the cost of renting the Nitrogen tank for usage and consumption, and the cost of constantly replenishing the tank with liquid nitrogen.
Precise solder paste print required — If the solder paste print is inaccurate, higher surface tension can result in more solder bridges at excellent pitch device levels. That required the costly smt pieces of equipment to accurate solder paste printing. Nitrogen in Reflow soldering will involve such costs as well.
The possibility of tombstoning is especially true if the components used are very fine pitch devices or small chips with poor solderability. This is why n2 reflow soldering requires more focused inspection.
The reflow soldering in an inert atmosphere depends upon the application. It is strongly recommended for class 3 products like automotive, Defense & Aerospace, Medical types of equipment, etc., where human life risk is associated, and high-quality products are mandatory. Soldering in the nitrogen atmosphere will help to contribute to a more comprehensive process window.
However, using Nitrogen in Reflow soldering adds some cost but considering the long-term saving due to high quality and the sound solder joint with minor repair/rework. It can be managed and ultimately leads to the profitability of any organization.