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46 SMT Magazine • January 2017 by Rich Heimsch SUPER DRY-TOTECH EU Controlling Oxidation and Intermetallics in Moisture-Sensitive Devices MORE THAN JUST DRY AIR To avoid the damage of micro-cracks and de- lamination during the processing of electronic components, appropriate environmental stor- age is essential. The introduction of lead-free soldering and the associated higher processing temperatures involved makes moisture man- agement even more important. Lead-free reflow increases the consequent saturated vapor pres- sure within components considerably (up to 30 bars). The same component that could be safe- ly processed before lead-free becomes a mois- ture sensitive device with limited floor life. The difference is often two sensitivity levels higher classification (MSL) and shorter allowable expo- sure time ("floor life"). Component suppliers should deliver these moisture sensitive components in effective pro- tective packaging to avoid absorption of humid- ity during transport and storage. These mois- ture barrier bags (MBB) are made from multi- ple layers of plastic and aluminum. Properly prepared and sealed, they are also a protective packaging that can prevent oxidation. ESD bags or zippered plastic bags do not protect against moisture. After opening the package, the time begins during which the components absorb humidity. Depending upon ambient humidity and temperature, the components can be safe- ly used only within a limited time period. This time period is classified by the IPC/JEDEC J-Std 033C. When a component has exceeded the al- lowed exposure time the component can be dried and made safe again through a baking process, traditionally done at 125°C. The com- ponent should be processed especially careful- ly after that. A repeated absorption of humidi- ty must be avoided because the baking process should not be repeated. Even one exposure to baking at these tem- peratures induces oxidation and inter-metallic growth, which reduces the wetting ability of the connection surfaces. Intermetallic thickness has been shown to increase by approximately 50% when baking at 125°C for four days. Thicker in- ter-metallic layers can lead to a reduction in sol- der joint integrity and in extreme cases reduce solderability. To fight this well-known effect, many sup- pliers of baking ovens provide an additional re- duction of oxygen by means of a nitrogen at- mosphere or vacuum during the drying process. Setting the clock back to zero for the compo- nent can take in excess of 72 hours, inevitably Figure 1: Saturated vapor pressure. Figure 2: Wetting times.