When X-Ray and Ultrasound Join Forces

A pulse of ultrasound traveling through a solid is attenuated as it goes and is reflected and/or transmitted by material interfaces. An x-ray beam is also attenuated but ignores material interfaces. An ultrasonic pulse must be inserted at 90° to the surface, but x-ray can be inserted at almost any angle. The usefulness of these and other differences is described by Tom Adams, consultant, Nordson SONOSCAN

A pulse of ultrasound is speeding through a layer of copper at a speed around 4700 meters per second. As it moves, the pulse is mildly attenuated - meaning that it is gradually losing energy (amplitude). Suddenly it runs smack into a layer of cured epoxy. One portion of the pulse sails right across the interface between the two solids and keeps going into the epoxy. The other portion is reflected back, at the same speed with which it arrived, to the transducer that launched the pulse.

An x-ray beam travels through the same copper layer. Like the pulse of ultrasound, it is somewhat attenuated by the copper through which it is moving. When it runs into the wall of cured epoxy, the x-ray beam keeps right on moving; none of it is reflected. What does change is the rate of attenuation, because the beam is now traveling through a different material.

When both an x-ray system and an acoustic micro imaging tool are used on the same sample, examining both images can give information that can shed light on quality control or failure analysis questions. Neither energy form causes any physical damage to the part, and it is hard to think of a physical anomaly that can hide from both.

Because it is part of the electromagnetic spectrum, an x-ray beam’s velocity through a material is so high it requires no attention. When imaging a sample, the size of the beam fired from the x-ray tube expands as it moves. The x-y area of the image (magnification) is adjusted by changing the distance to the target.

The speed of ultrasound through a solid is many orders of magnitude less than the speed of an x-ray beam. A 100 MHz pulse, for example, moves through mold compound at about 3,000 meters per second. And ultrasound is launched not as a continuous beam, but as a pulse whose return echo represents an interface area measured in microns. The echo will become one pixel in the visible acoustic image. The transducer launches thousands of pulses per second as it scans, and, where there is an interface in the line of fire, collects thousands of echoes per second.

Figure 1: Air gaps in this PEM are red and yellow because they reflect nearly all of the ultrasound. Black regions reflect none.

Figure 1 is the acoustic image of a plastic encapsulated microcircuit (PEM) made by a Nordson SONOSCAN C-SAM acoustic micro imaging tool. The image was made by accepting for imaging echoes whose arrival times indicated that they originated in a vertical gate ranging from a short distance above the die face to the die attach level. Echoes originating above or below this gate were not accepted. The wires leading from the die to the lead fingers are within the gate, but they are very thin and have a circular cross section that scatters ultrasound. The result is that they reflect too little ultrasound at this frequency to be imaged.

Where no echoes were received (between the lead fingers, for example) the pixels are black. The color map along the left side of the image indicates that the echoes reflecting nearly all of the launched pulse are red. The circular red-yellow feature surrounding the die is a crack through the mold compound surrounding the die. The distal ends of most of the lead fingers are also red and yellow.

To the southeast of the circular crack is a curved black area that looks like an optical shadow - but this is an acoustic image. What happened is this: as the crack traveled upward, its path became more vertical. Where it is somewhat level, the crack reflects nearly all of the arriving pulse, and the crack is red. Where the crack becomes more vertical, the crack is yellow (the next color up on the color map at left). Where the path became even steeper, all of the pulse was deflected (echoed) at an angle and no echo was sent upward to the transducer, so the pixels in this area are black (no signal received).