Nitrogen in Reflow soldering in the PCBA process consideration is becomes a common during reflow soldering. The aim of every organization to establish a process that reduces Costs, increases productivity, and improves customer satisfaction.
In reflow soldering process when paste is heated in presence of oxygen will form oxides which are non solderable surfaces and results poor wetting of molten alloy on printed circuit boards.
Why Nitrogen in Reflow soldering used
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 advantage of nitrogen has become more significant in defect reduction when using fine pitch component or Ball Grid Arrays (BGA, s).
A study says that if Nitrogen in Reflow soldering is used process the yield lowered down up 6 to 7%. In other words, we can say the 60 to 70% defects can be reduced if nitrogen is adopted in process
Let us talk on the major variable factors that impacting for oxidation and discuss their effects on solder joint formation.
Temperature:Oxidation is directly related to temperature higher temperatures mean faster rates of Oxidation. 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 which can result in soldering defects.
Surface Area: As pitch of fine pitch components are reducing the solder paste particle size is became smaller to meet good solder print requirements. As solder paste particle become smaller the ratio of surface area to volume increases. That can be explained by below formula-
So if we are using finer size solder particles means more providing more surface of oxidation that increase soldering defects.
Oxygen Levels: For solid joint formation require that oven will run with low level oxygen level as possible to reduce oxidation. The oxygen level measure in parts per million PPM.The normal environment contain 21% oxygen which is almost 210,000 ppm. But the most of oven having option to at less than 100 ppm oxygen Environment.
Benefits of N2 in a 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.
- Cost of Nitrogen: The single most important aspect of Nitrogen usage is the cost of installing the Nitrogen plant and renting the tank for the usage and consumption as well as constantly replenishing the tank with liquid nitrogen.
- Precise solder paste print required: A higher surface tension can result in a higher number of solder bridges, at very fine pitch device levels, if the solder paste print is in-accurate.
- Possibility of tomb stoning: This is especially true if the components used are very fine pitch devices or small chip with poor solderability
The usage of nitrogen highly on depends upon the application and 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 nitrogen atmosphere will help to contribute wider process window.
However, the usage of nitrogen adds some cost but considering the long-term saving due to high quality and the sound solder joint with less repair/rework it can be managed and ultimately leads to the profitability of any organization.