Packing MSDs correctly can reduce popcorning during reflow.

Moisture-sensitive devices must be correctly packaged to prevent mechanical damage during handling or cracking as they pass through reflow. Figure 1 shows components packed in waffle trays tethered with a thick plastic band, which would be inadequate to maintain the tray’s position, if dropped. In the case of fine-pitch gullwing-leaded components, this would probably lead to bent leads on the four corners of each device.

Although the package when opened does have a desiccant bag, there is no evidence of a relative humidity indicator strip, which should also be present to show RH level. High levels of humidity can lead to popcorning or cracking of the component body or the internal wirebonds during reflow. (Examples of cracking are also illustrated in the Defects Database.)

These are typical defects shown in the National Physical Laboratory’s interactive assembly and soldering defects database. The database (http://defectsdatabase.npl.co.uk), available to all this publication’s readers, allows engineers to search and view countless defects and solutions, or to submit defects online. To complement the defect of the month, NPL features the “Defect Video of the Month,” presented online by Bob Willis. This describes over 20 different failure modes, many with video examples of the defect occurring in real time.

Chris Hunt is with the National Physical Laboratory Industry and Innovation division (npl.co.uk); This e-mail address is being protected from spambots. You need JavaScript enabled to view it . His column appears monthly.