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48 SMT Magazine • January 2017 bringing along considerable costs for nitrogen, and only a low rest-oxygen content of less than 13 ppm stops the oxidation. Lead-Free Soldering Alloys Because of the considerably higher content of tin in lead-free soldering alloys, the need to consider oxidation protection during storage has increased in importance. This is caused by higher oxidation tendencies of these alloys and the generally more difficult wet ability and flow properties of lead-free soldering alloys. The Oxidation Process The oxygen causing the oxidation originates from two different sources. The first is the oxy- gen molecule, found world-wide in our atmo- sphere. However, because of its strong atomic bond it only occurs at temperatures higher than 40°C. The second and in fact more aggressive bearer of oxygen is the water molecule. Here, the oxygen atom weakly connected, and con- siderable oxidation can be observed at low tem- peratures. This means that not the content of oxygen, far more the content of humidity is decisive for the oxidation percentage in stored components. Technically, it is possible to solve both problems at the same time. However, it is important to avoid heating above 40°C there- by eliminating the air-oxygen as a reaction part- ner, and to provide a strong dehumidification of the air at the same time. To achieve this, dry storage systems have been designed that can produce internal atmospheres of below 1% RH. With this extremely low content of humidity it is possible to protect the components against the additional absorption of moisture and also to remove the moisture already absorbed. As the diagram below shows, even storage in very clean nitrogen does not provide actual dehu- midification of components as levels under 0.1 Wt % are not possible. Modern Desiccant Technology Ultra-low humidity desiccant technology is now available that can sustain a low rest-hu- midity of <0.5% RH (0.05 grams H20/m3) ef- fectively a "moisture vacuum." The latest tech- nology also provides recovery times (after door openings) of less than three minutes. This pro- vides practical working access throughout the day without raising the average RH above the J- Std-033C specified safe storage level. Unlike clay or silica, these storage areas (which can be thousands of cubic feet in size) use a crystal known as zeolite. It is a molecu- Figure 3: Drying efficiency. CONTROLLING OXIDATION AND INTERMETALLICS IN MOISTURE-SENSITIVE DEVICES

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