Why reflow Printed Circuit Boards in a Nitrogen atmosphere?

Before going into the details of why companies solder their Printed Circuit Board (PCB) assemblies in a Nitrogen atmosphere, its important to point out that most assemblies are generally reflow soldered in an air atmosphere.  Soldering in air is perfectly fine for vast majority of products.  Assuming the process engineer in charge properly configured a reflow profile for the particular board design, solder joints will have good wetting and finish in compliance with IPC quality standards.

Circuit boards have many variables in terms of design complexity, material of the laminate, and end user product requirements.  The boards that go into entry level consumer products do not need to meet the same quality standards as high value and high reliability products.  Think of aerospace, medical, military, industrial, and telecommunications products.  For the latter, reflow soldering with Nitrogen cover gas to have low levels of Oxygen is common and preferred in many cases. There are a few main reasons for this.

First, Nitrogen is an inert gas that is purged in the reflow chamber to displace Oxygen. Low levels of Oxygen (below 500 PPM) mean there is less oxidation at the solder joints when the alloy is in a molten state. This yields metallurgically stronger solder joints with less voiding and in turn higher quality product that is able to withstand longer thermal cycles and pass the required reliability testing. For most applications, Oxygen PPM level below 1,000 may be required however, some specify O2 under 10 PPM which is essentially an Oxygen free environment.

Second main reason for using Nitrogen during reflow soldering is that it allows for a wider process window than traditional air reflow. PCB assemblies that are thermally heavy due to their mass or materials may take too long to get to reflow temperatures. In an air reflow oven, this will cause flux burn off and by the time solder paste reaches melting temperature there may not be enough flux left to remove the oxide film from the board pads to allow for proper wetting of solder. If reflowed in a Nitrogen atmosphere, even if most of the flux is burned off, low concentration of oxygen means there is very little oxides at the pads so oxidation will not interfere with quality of the solder joint formation. This means a PCB may be reflowed with a profile that takes considerably more time than is recommended by the solder paste manufacturer and still have high quality solder joints.

Third reason is related to the specific reflow equipment used for soldering.  Some reflow ovens can only run in either air or Nitrogen atmosphere, not both.  If a company has multiple product designs, ranging from “simple” / low mass PCBs to complex high density assemblies, and limited manufacturing capability – as most companies do.  They tend to configure their reflow oven as an all-in-one solution, being able to handle the most complex board assembly they plan to process through.  Hence, the reflow oven will have a Nitrogen hooked up and it will be used for all assemblies.  Electronic Manufacturing Service (EMS) providers who are setup for high mix – low volume production also face the same scenario.  Unless they are one of the top tier companies with a much larger manufacturing capacity and variety of equipment at their disposal.  Not all reflow ovens can only be setup for either air or Nitrogen.  To plug ATCO’s own product, the PRO 1600 SMT Reflow Oven can have a permanent Nitrogen connection and still run in either air or Nitrogen.  The end user is able to tailor whether or not Nitrogen is supplied into the heating chamber specific to the profile selected within the software.  So the same oven can be used for traditional air reflow as well as for processing with Nitrogen gas when necessary.

In some cases, cost may also be a factor in deciding whether or not to use Nitrogen for reflow.  If a customer is running a high-volume / throughput production type reflow oven, they will either obtain a Nitrogen generator to be stationed nearby or have a large tank outside the facility used to supply the whole building, as part of the overall  facility equipment.  Nitrogen cost varies significantly based on the production scale.  Obtaining a Nitrogen generator may be worth it for high volume environments but typically does not make sense for low volumes as they can cost over $25,000.  For low volume occasional reflow, obtaining a portable nitrogen tank is the most practical solution. Pictured below is a 230 liter liquid Nitrogen tank positioned next to ATCO’s PRO 1600 SMT Reflow Oven.   It’s ideal for R&D / lab volume reflow and generates 5,000 CF of Nitrogen gas which is enough for about 150 reflow cycles.  Cost to have Airgas or Praxair fill up the tank is only around $140 USD (a tank can be rented for less than $50 per month).  This translates to material cost per reflow cycle of approximately $1.  Additionally, multiple circuit boards or panels can be reflowed in a single reflow cycle so the cost per individual PCB may only be as low as a few cents!                          

SMT Reflow Oven with Nitrogen

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