|Gas-generating systems such as this model NGM2 from Atlas-Copco can convert compressed air to nitrogen gas for SMT manufacturing. |
Nitrogen plays an important part in surface-mount-technology (SMT) packaging. A growing number of reflow-oven suppliers now offer their systems with oxygen as well as optional nitrogen environments for operators who wish to assemble their devices with an inert gas. Quite simply, nitrogen is a low-cost, inert gas that does not react with the hot metal surfaces of circuits and their packages during SMT production which could form unwanted films.
Nitrogen can be applied as part of SMT manufacturing for reflow soldering in an enclosed oven or for wave or vacuum soldering. Nitrogen is the most abundant gas in the atmosphere, which typically consists of about 78 percent nitrogen, about 20.9 percent oxygen, about 0.9 percent argon gas, and small amounts of water vapor and other gases. By increasing the percentage of nitrogen in the manufacturing environment for SMT devices relative to the amount of oxygen, the oxygen and moisture levels can be reduced and the oxidation of metal surfaces during reflow heating/soldering can be minimized. Most importantly, nitrogen or N2 gas is inert, and does not prompt unwanted reactions during the soldering and SMT manufacturing processes. Because it is inert, many manufacturers store finished SMT products in cabinets maintained at low humidity by purging with dry air and/or by pumping in a nitrogen atmosphere.
Beneficial Effects of Nitrogen
Applying N2 gas in reflow solder systems is sometimes debated by assembly companies, weighing the cost of investing in nitrogen-producing equipment or buying stored gas. But few assemblers will argue that the use of an inert atmosphere such as N2 can not only improve solder joint quality and manufacturing yields but also reduce sensitivity to SMT manufacturing process variations. In effect, the use of nitrogen during assembly and packaging helps to widen the process window, resulting in a boost in manufacturing yield.
|Pillarhouse's Pillargen 40 Nitrogen Generation system's design technology (www.pillarhouseusa.com) offers compact size coupled with ultra quiet operation. |
Forming gases for SMT manufacturing processes and other circuit assembly functions are often a combination of hydrogen gas and an inert gas such as N2. These mixtures are usually intended to maintain the amount of hydrogen below a certain percentage by volume so that it is below a point at which the hydrogen can spontaneously combust. Commercial suppliers of forming gases for SMT manufacturers, such as Air Products (www.airproducts.com), generally consider a mixture of gases well below the combustion point to be inert gas mixtures. In fact, in the process of injecting a forming gas into a reflow solder oven, no matter what the effort to minimize the level of a mixture gas, such as oxygen, into the input gas flow, some small amount of oxygen will find its way into the forming gas. A reflow solder oven with gas inlet simply cannot be made hermetic to totally seal out oxygen from the forming gas mixture.
Just over a year ago, Air Products and its use of nitrogen in wave soldering applications (as part of its Inert Wave Soldering technology) was recognized by China's electronics manufacturing industry with the 6th SMT China Vision Award. The award was decided by a panel of independent judges for how the Inert Wave Soldering technology has helped reduce solder defect rates and contributed to improved, lower-cost SMT production. The award noted that the firm's efficient use of nitrogen in their wave soldering technology helps reduce nitrogen flow rates, increase solder joint quality, and ease the transition to lead-free solder technology.
Because N2 is not an oxidizing gas, it prevents or slows oxidation in circuits and packages, although different reactions may occur with a forming gas, depending upon its percentages of hydrogen and nitrogen. Numerous studies have examined the effects of solder cooling speeds, the types of solder pastes, and the types of solder fluxes on the reliability of lead-free solder joints, while also considering the impact of the reflow atmosphere on the quality of the solder joints. While the studies indicate that performing reflow solder in a highly nitrogen atmosphere typically provided better quality lead-free solder joints, the cooling rate of the solder during the reflow process also affected the quality of the solder joints.
Of course, controlling every solder variable in an SMT manufacturing process can be near to impossible. In a white paper, "Developing a Reliable Lead-Free SMT Assembly Process," from Kester (www.kester.com), a list of 15 process variables is detailed, including the melting temperature of the solder alloy, the flux chemistry, the impact of solder voids on the SMT products, and the impact of nitrogen versus air reflow processes. An application note from device supplier Altera Corp. (www.altera.com), "SMT Board Assembly Process Recommendations," points out the importance of printed-circuit-board (PCB) surface finish and PCB materials on the type and temperatures of the soldering process used and the expected reliability of the final products, since material factors can greatly impact an assembly process.
|Production reflow ovens such as this BTU Pyramax have zones specifically designed to accept inert Nitrogen gas during reflow. |
But adjusting and monitoring the flow of nitrogen for an in-line reflow solder process can be accomplished without unusual modifications to a manufacturing process. In fact, some manufacturers of reflow solder systems, such as Manncorp. (www.manncorp.com), offer their equipment with options for nitrogen compatible configurations. The firm's CR-6000 can be equipped with a nitrogen-compatible heating chamber designed for efficient transfer of heat, so that nitrogen consumption is minimized while maintaining low levels of oxygen (O2) gas in the forming gas.
To assist manufacturers seeking to shift their SMT processes to nitrogen environments, companies such as Atlas-Copco (www.atlascopco.com) offer industrial gas generators for N2 and O2. These systems are capable of on-site production of required gases at any time and for 24 hours per day, 7 days per week. The company points to the improved safety of using its gas generators versus storing and handling high-pressure cylinders of gas, with significant economies of scale when investing in the gas generators. Its gas generators are available in a variety of different sizes and flow rates, for applications in SMT manufacturing through food and beverage storage and even in fire prevention. Gas-generating systems such as the NGM2 are constructed with all necessary filters, pressure gauges, and flow meters for accurate system monitoring at all times.
Also, nitrogen gas generators from South-Tek Systems (www.southteksystems.com) include tank-mounted and cabinet-based systems for flexibility. The systems apply pressure swing adsorption (PSA) technology to achieve high-purity nitrogen from a compressed-air source. Similarly, laboratory nitrogen gas generators from Cole-Palmer (www.coleparmer.com) can work with a dedicated compressed-air source to produce nitrogen at amounts to 500 mL/minute with purity of 99.9995 percent, and industrial nitrogen gas generators from Parker (www.parker.com) can produce as much as 99.9995 percent pure compressed nitrogen gas from nearly any compressed air supply.
Still, some SMT manufacturers may prefer to receive inert gas, such as nitrogen, in canisters, and gas suppliers such as Praxair (www.praxair.com) offer 99.9999 percent grade nitrogen in a number of different cylinder types, including AS, T, and AQ cylinders. The gas content (in cubic feet per cubic meter) varies according to cylinder as does the product grade and quality assay specification (amounts of other gases contained in the cylinder). For example, a product grade specified as "oxygen free" contains less than 0.5 ppm oxygen gas.
For SMT manufacturers considering whether a shift to a nitrogen atmosphere can be a benefit, perhaps the most essential consideration is whether high reliability or low cost is more important. Obviously, producing and supporting a nitrogen atmosphere in production, whether from gas generators or from stored compressed gas canisters, is an added cost compared to a production process without nitrogen. While manufacturing SMT products in a nitrogen environment can improve reliability, it will also result in higher manufacturing cost. Nitrogen can also benefit a new manufacturing process and one that tends to be low in volume, compared to not using nitrogen in a more mature manufacturing process and one that has already been proven for high volumes.