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) 0.80 0.91 0.95 0.98 1.10
 Solder joint condition No reflow Blocked paste stencil aperture Insufficient paste deposit Good reflow Dishing

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.

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