To avoid extra paste, temper the ball depth.

Figure 1 shows the base of a package-on-package device covered in dip paste. Clearly the package has been dipped to an excessive depth in the paste dip unit. This will be due to incorrect setup of the unit. The bottom of the dip unit, the dip plate, should define the depth of the paste on the balls, and normally should be set to achieve 40% to 50% of the ball depth. There should be suitable process control checks on the dipping unit during production, and the depth of the paste should be measured manually or automatically.

• Other possible causes for excess paste include:
• Component not parallel with dip plate.
• Movement of the component on pickup nozzle during dipping.
• Incorrect depth of paste in dipping unit.
• Package warpage.
• Dipping plate not parallel with component balls.
• Uneven or excess paste in dipping unit.

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); chris.hunt@npl.co.uk. His column appears monthly.

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