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Jos Brehler columns | February 03, 2006

Storage of Lead-Free Components

The drying of components has become even more important
Moisture Sensitivity Level Content (MSL) for components may have been revised for lead-free soldering compatibility since higher processing temperatures result in higher internal vapour pressures and greater risks of popcorning, cracking etc. Storage may consequently become more of an issue.

MSL are defined by IPC/JEDEC standard J-STD-020C (July 2004). Shipping, handling and use requirements for moisture-sensitive components are given in J-STD-033B (Oct 2005). These standards have been updated for the introduction of lead-free soldering technology.

Manufacturers deliver moisture sensitive components in effective protection packaging to avoid absorption of humidity during transport and storage. After opening the package the time begins in which the components absorb humidity. Depending on humidity and temperature the components can be used only within a limited time period, classified by IPC-J-STD 033B. When a component has exceeded the allowed time the humidity can be decreased through a baking process, after which the component should be processed immediately.

A repeated absorption of humidity must be avoided because the baking process should be applied only once. The main reason is the corrosion percentage which increases along with the temperature, and because of this an increasing non wettability of the connection surfaces.

Many suppliers of drying ovens provide an extra elimination of oxygen by forming a nitrogen atmosphere and respectively also a vacuum during the drying process. Setting the clock back to zero for the component can take a very long time, for only a low rest-oxygen content of less than 13 ppm stops the oxidation completely.

Because of the considerably higher content of tin in many lead-free compatible component coatings the need to protect from oxidation with the storage of components may be more important. This is caused by higher oxidation percentage of these alloys and the potential for decreased wettability and flow properties.

The oxygen required for the oxidation stems from two different sources. The first is the oxygen molecule O2, found world-wide in our atmosphere. However, because of its atomic bond oxidation only occurs at temperatures higher than 40°C. The second and in fact more aggressive bearer of oxygen is the water molecule H2O. The oxygen atom is only weakly connected, a considerable oxidation percentage can already be observed at low temperatures. 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 thereby eliminating the air-oxygen as reaction partner, and to provide a strong dehumidification of the air at the same time.

Jos Brehler
Totech

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