September, 2013
www.us-tech.com
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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 cir- cuits and their packages during SMT production which could form unwanted films. Nitrogen can be applied as part of SMT man-
ufacturing for reflow soldering in an enclosed oven or for wave or vacuum soldering. Nitrogen is the most abundant gas in the atmosphere, which typi- cally consists of about 78 percent nitrogen, about 20.9 percent oxygen, about 0.9 per-
Nitrogen Helps Boost SMT Quality N
By Jeffrey Paulownia
itrogen plays an important part in surface- mount-technology (SMT) packaging. A growing number of reflow-oven suppliers
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), gen- erally 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 oxy- gen, 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 recog- nized by China’s electronics manufacturing indus- try with the 6th SMT China Vision Award. The award was decided by a panel of independent judges for how the Inert Wave Soldering technolo- gy has helped reduce solder defect rates and con- tributed to improved, lower-cost SMT production. The award noted that the firm’s efficient use of nitrogen in their wave soldering technology helps
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Gas-generating systems such as this model NGM2 from Atlas-Copco can convert compressed air to nitrogen gas for SMT manufacturing.
cent 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 dur- ing reflow heating/soldering can be minimized. Most importantly, nitro-
gen or N2 gas is inert, and does not prompt unwanted reactions during the soldering and SMT manufactur- ing 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-produc- ing 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 varia- tions. In effect, the use of nitrogen during assembly and packaging helps to widen the process window, result- ing in a boost in manufacturing yield. Forming gases for SMT manu- facturing processes and other circuit
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