To develop acoustic techniques and establish the limits of detection of these techniques for the various types of PCBs and the various types of defects defined in the defect catalogue. To design, develop/engineer the ultrasonic generation hardware including the man-machine interface with an algorithm for processing the ultrasonically acquired data to classify the presence of defects/flaws.

Background
Scanning acoustic microscopy (SAM) of integrated circuits (IC's) is an established technique in Quality Control of the IC manufacturing processes. It is especially useful for detection of laminar type defects, such as delamination, popcorn defects etc. It is a complimentary technique to X ray inspection.

SAM inspection is a relatively slow technique because it requires mechanical scanning of a transducer over the part to be tested. Ultrasound frequencies of 15 - 50MHZ are normally used for inspection of plastic encapsulated IC's - this gives a reasonable balance between image resolution (which requires high frequencies) and penetration (which requires lower frequencies). The inspection must be done with a water coupling medium because high frequency ultrasound cannot travel through air.

Image Resolution
The image resolution depends mainly on the focal spot size of the transducer and the mechanical scanning index. The focal spot size is determined by the transducer design - generally the higher the frequency the smaller the focal spot, which gives a better resolution. With a high frequency and small focal spot however, the mechanical indexing has to be done on a fine pitch - this means that more scan lines are required, which takes a long time.

Establishing the fastest potential scanning speed
One important factor that is being established during this project is the relationship between the frequency (or focal spot size), the scanning index and the image resolution. This will be done using the "Phase 1 SAM" by setting up scans on standard PCB's with assembled IC components and known defects.

For example, combinations of the following conditions are typical.

Transducer Frequency	15MHz	50MHz
Scan index	0.2mm	0.05mm

The scanning time for each IC can be varied by changing the scan index and the number of lines per second scanned.

Images may be compared under all these conditions, noting the total scan time and the image resolution (this will be a largely subjective judgement initially).

This will show whether important defects can be detected within reasonable scanning times for an in-line system.

Initial scan speed trial results
The specific defect that was being investigated is delamination of the materials with within the BGA packaging - see irregular shaped white area in fig. 3. This defect can arise when vapour pressure of moisture inside a non hermetic package increases greatly when the package is exposed to the high temperature of solder reflow. Under certain conditions, this pressure can cause internal delamination of the packaging materials from the die and/or substrate. In the most severe case, the stress can result in external package cracks. Taking the situation further The Joint Industry Standard document IPC/JEDEC J-STD-020C cites that in extreme cases the internal stress and resulting bulging of the package can crack giving an audible "pop". This phenomenon is called the Pop Corn effect and is visualised here.



Results show that despite less resolution, the defect is still highly visible.

Irrigated water nozzle
The design and manufacture of an irrigated water nozzle as an alternative to total water immersion is currently being undertaken.

In-line SAM Initial Concept
The development of the SAM phase 2 will incorporate the handling of the PCB so as not to sacrifice speed. Also a suitable method for drying the PCBs using air knifes arranged across the conveyor is being investigated. Finally, defect detection algorithms will be integrated with the SAM application software.