Study On Defect Inspection Of Flip Chip Based On Ultrasonic Excitation

Defect of flip chip (FC) is always hidden between die and substrate, which brings difficulty to inspect defect of FC. In order to solve the problem, the vibration model is proposed for explaining the theory of defect inspection. Natural frequency of FC and vibration velocity of solder bumps are also researched with a defect inspection method based on air-coupled ultrasonic excitation. The main content is as following:1. Tension springs with mass were used to simplify solder bumps of FC, and then natural frequency equation of vibration model of FC was derived. The theory shows that its natural frequency can be decreased if there were defective solder bumps for FC and vibration velocity of defective solder bumps can be increased.2. A defect inspection method based on air-coupled ultrasonic excitation is proposed. In which, FC is excited with air-coupled ultrasound, and then the vibration signal is measured by using a laser vibrometer to inspect defect of solder bumps.3. Natural frequencies of FCs were calculated, simulated and measured. Research shows that the more the solder bumps missing are, the lower the natural frequencies are. For FC with peripheral solder bumps, if changing rates of the 3rd ~5th natural frequencies were all higher than 9%, there are solder bumps missing for it. For FC with area solder bumps, changing rates of the first 7 natural frequencies are sensitive to corner and peripheral solder bumps, and thus it can be used to approximately locate solder bump missing.4. Vibration velocity of solder bumps was also simulated and measured. Research shows that vibration velocity of a defective solder bump and its deviation factor are both greater compared with a good solder bump. For a peripheral solder bump, if its deviation factor was greater than 0.45, it has been missing. For an area solder bump, if its deviation factor measured was lower than 0.3, it is good; if the deviation factor higher than 0.35, it has been missing. And this inspection method can also locate defective solder bumps.Therefore, the defect inspection method based on air-coupled ultrasonic excitation is effective to measure natural frequency of FC and vibration velocity of solder bumps for defect inspection.