X-ray inspection is generally used to detect shorts under area array components. But x-ray images can also provide a valuable insight into the quality of joint formation of the arrays. We have been able to show that the relative sizes of the balls in x-ray images before and after reflow can reliably detect the following defects :

  • No reflow / insufficient reflow
  • Blocked stencil aperture
  • Insufficient paste deposit

The actual diameter of the ellipsoid formed after reflow under an area array component provides  valuable information for solder joint quality assessment. A typical example :

Zone“-3” zone“-2” zone“-1” zone“0” Target zone“+1” zone
Ellipsoid diameter (mm)0.800.910.950.981.10
 Solder joint conditionNo reflowBlocked paste stencil apertureInsufficient paste depositGood reflowDishing

In conjunction with visual inspection methods and other X-ray signature identifiers , the method developed in this investigation  provides indicators for process development and serve as a tool for solder joint quality evaluation during production. The proposed process can be implemented using an entry level X-ray machine.

Read the full article here