Pb-Free Assembly - Process

Nitrogen is used for reflowing the PCB’s mostly for Pb-Free Assembly through Nitrogen tank plant which consists of Cryo Cylinder followed by a vaporizer. The purpose of vaporizer is to convert the cold liquid Nitrogen in the Cryo cylinder into gas Nitrogen. The tanks are normally filled by the supplier of Nitrogen by bringing the truck to the plant location and then pumping cold fluid Nitrogen into the cryo-tank as and when needed.

When Pb-Free Assembly is done while Nitrogen is being used, the nitrogen vaporizes at minus 196 degree Celsius inside the vaporizer and becomes a gas. Then this nitrogen gas is transported in the special copper tubing to the reflow oven.

Usually the cold nitrogen is applied directly into the reflow zone where the temperature could reach as high as 250 – 270 degrees Celsius. So the delta T is quite high, when the nitrogen temperature is as low as 10 -15 degrees Celsius. This can cause some temperature regulation problems and finally result in large variation in soldering temperature which can be avoided by running nitrogen through pre heating section in the reflow oven before the actual reflow process itself. Other way to reduce the temperature variation could be to run the nitrogen from the plant to the reflow oven through supply tubes that are at least 50 meters or more in length.

The main advantages of using Nitrogen for Pb-Free Assembly in the reflow process v/s atmospheric air are as follows:

• Improved soldering quality: Higher surface tension as well as less reoxidation of IC terminals will increase the wetted surface by 20% to 30%, resulting in increasing solder joint strength. Higher surface tension minimizes solder balling when using fine pitch solder paste. Less oxidation also improves the wetting on NiAu and bare copper solder lands.
• Enlarged Process Window: The soldering process will be less sensitive to poor solderability and solder lands and component terminals, thereby improving the soldering BGA and CSP devices. This is more applicable for Pb-Free Assemblies that would be RoHS compliant.
• Other Benefits: Visual appearance of the solder joint surface will be smoother and shinier, especially for Pb-Free Assembly making it easy to inspect and rework. Larger surface tension can hold larger and heavier components on bottom side when performing double reflow.

Disadvantages/Cost Involved

The main disadvantages of using nitrogen are as follows:

• 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 solderibility.