Qualification of the Assembly Process of Flip-Chip BGA Packages ...

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Texas Tech University, Nivetha Shivan, May 2012 Usually, water is used as a medium for the IC during analysis as air is a poor transmitter of ultrasonic signals. The frequencies of the ultrasonic pulses from the transducer vary between 10MHz to 400MHz. Low frequency ultrasound penetrates deeper into the package but gives poor resolution images. On the contrary, high frequency ultrasound penetrates less, but produces images with more resolution. The decision about the ultrasonic frequency is usually made based on the geometry of the part and the materials in the package. For flip chip units in this project, 230MHz was the frequency that was used to analyze the package acoustically. 4.4.1.1 Failure Modes � White area in the underfill region in the die to bump CSAM image could mean there is delamination in the underfill material. � Dark bumps in the die to bump CSAM image could mean bump cracks or open bumps (delamination of the bump from the die). The interference of the ultrasound waves between the interface and the cracked bumps could appear as dark bumps in the image. � White spot in place of bumps in the image could mean bump delamination. A device is suspected to have a failure if any abnormality is seen while comparing the CSAM image of the device post stress test to the CSAM image of the same device prior stress test. This is illustrated in Figure 4.14. 41
Texas Tech University, Nivetha Shivan, May 2012 Image of a device prior stress Image of the same device post stress showing white spot in place of bumps Image of a device prior stress Image of the same device post stress showing dark bumps Figure 4.14: Examples of 1:1 comparison of CSAM images before and after stress test Although CSAM provides us images of the interface of interest and helps us to identify anomalies in the image post stress, it is a technique which is used for learning purpose and there is no specific criterion to decide that a device will fail because it has one, two or fifty dark/white bumps. The unit could still pass an electrical test as the connections could still be intact. The white/dark bumps in those cases needs to be analyzed further by doing the cross-section which is the only way to find out if there was a real physical failure mode associated with the anomaly. 42
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Page 15 and 16: 2.1 What is Qualification? [7] Texa
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Qualification of the Assembly Process of Flip-Chip BGA Packages ...

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