Cold-joint defect has been critically discussed in flip chip interconnections, especially for lead-free solder and Cu pillar bumps in fcFBGA (flip chip fine-pitch BGA). In the case of Cu pillar bump, the only known effective method for cold-joint detection is package Open/Short Test. This study introduces an effective alternative method of Cold-joint detection while during assembly setup stage, which employs the moderate filler segregation phenomenon among Cu pillar bumps, especially for Cu pillar bump without solder cap. For joint connections with Cu-pillar bumps, a voltage separates the underfill material's filler and resin due to the electrode potential difference between Cu and Sn. The filler migrates towards the Sn, and given this behavior of the underfill material, we can easily detect the cold-joint through SAT (Scanning Acoustic Tomography) because of the difference of color distribution reflected in the C-SAM (C-mode scanning acoustic microscopy) image. If the joint interconnection is good, there is filler segregation — a moderate level filler segregation, which can be reflected as grayish-colored area in C-SAM images. In the contrary, the whiter area signifies the absence of filler segregation, and further considered to be due to no contact of Cu pillar and the substrate's pre-solder; thus, signifying Cold-joint defect in this area. With the use of this method, the cold-joint defect can now be detected by SAT during assembly process. However, the separation of resin and filler potentially affects the local properties of the underfill material and may cause the uneven stress distribution. Therefore, temperature cycling test was also performed to confirm that the moderate filler segregation does not have any concern during reliability test.
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