Acoustic microscope verifies material bonds

Bonds between molded plastic and other materials can be critical to product performance. We usually check them by cutting a cross-section, but looking at ultrasound pictures could save a lot of time.

The quality and long-term reliability of injection molded products frequently depend on the bond interface between the plastic and a second material, which may be another plastic. The quality of the bonding often relates to the electrical, mechanical, and thermal stability of the end product.

This is certainly the case in many overmolded devices such as medical implants and a variety of electronic modules. For example, a hermetic seal may be required to ensure a sterile environment as well as the overall functionality and reliability of the device. It is typically this bond interface that is subject to the environmental, thermal, and mechanical stresses that the product is likely to encounter in its service lifetime.

Cut in, or listen?

Determining whether the bond is intact, or whether it contains delaminations or voids, can be done by physically sectioning the part, but destructive analysis has drawbacks. First and foremost, the part is destroyed in the process. Second, the cut or grinding area must intersect any defects at the bond zone. Third, it is often difficult to apply data from a small number of parts to the entire production process, and to ascertain what production steps need to be modified and in what way. It’s also time-intensive.Alternatively, acoustic microscopes (AMs) have the advantage of being nondestructive and relatively fast (scanning can take less than 1 second to several minutes per part, depending on size and resolution). In addition, the data or images they produce are a road map of the interface, which can guide improvements to product and process control. The technology of an AM is based on an ultrasonic transducer that scans the part, pulses ultrasound (ultrasonic sound waves) into the part, and receives the return echo signals. Two important considerations are the frequency of ultrasound being pulsed and the characteristics of the materials that make up the part.

As the transducer scans the part and performs its pulse-echo function several thousand times a second, it collects the signal data that make up the pixels in the completed acoustic image. Some materials absorb much of the pulsed ultrasound; these materials (lead is one example) are said to be “lossy,” and are typically imaged with a low-frequency transducer because low-frequency ultrasound penetrates farther into the material.

Low frequencies range from about 5-30 mHz. Such low frequencies produce relatively low-resolution acoustic images—that is, the individual pixels are relatively large. Higher-frequency transducers range up to 300 mHz and yield higher-resolution images. Their ability to penetrate a given material diminishes as the frequency increases.

The ultrasound pulsed into a part sends back return echo signals only at material interfaces (Figure 1). With a homogeneous material such as plastic, there are no return echoes. An interface sends back an echo because the two materials at the interface differ in acoustic velocity (the speed at which ultrasound travels through the material, measured in meters per second) and density (measured in grams per cubic centimeter).

Multiplying acoustic velocity by density gives the acoustic impedance of the material. The bigger the difference in acoustic impedance at an interface, the higher the amplitude of the return echo.

The image taken of a plastic-metal or plastic-ceramic bond usually generates a moderate acoustic impedance difference. The echo from this interface is adequate to create pixels for the acoustic image.

Seeing multiple interfaces

Some of the ultrasound crosses this interface and travels deeper into the part. If there is a second, deeper interface, echoes from that interface also can be imaged. The operator usually knows which interface is of interest and “gates” the acoustic microscope by focusing the transducer on the target depth and electronically excluding all echoes whose arrival times place them at higher or lower depths within the part. If there are two or more depths of interest, the operator can make a separate image for each depth.

The concept of a moderate difference in acoustic impedance at a material interface goes out the window when a gap-type defect such as a delamination or a void is present. Gaps, voids, and cracks typically contain air or another gas. The density of the gas is far less than the density of the adjacent solid materials. At the same time, the speed at which ultrasound travels through the gas is also far less than the speed at which it travels through solid materials. (A rough parallel: Audible sound travels through air at around 500 ft/sec, but through steel at around 5000 ft/sec).

A gap between two materials creates an additional interface: Instead of a single interface between materials A and B, each material now has an interface with the gas-filled gap. But the only interface that matters in acoustic microscopy is the interface between the top of the gap and the material above it. The difference in acoustic impedance between the solid material above and the gas below is so profound that virtually all of the ultrasound is reflected from this interface as an echo signal.

This means, first, that any gap-type defect will appear very bright in the acoustic image and will be in high contrast to other features imaged acoustically at the same depth. Second, the near-total reflection means that almost no ultrasound is propagated across the gap, and features directly below the gap cannot be imaged.

Sound paints sharp pictures

The acoustic microscope needs access to only one surface of the part, and that surface needs to be flat—or, more precisely, needs to appear flat to the transducer. Injection molded parts are sometimes cylindrical. These parts are mounted horizontally in a fixture that rotates the part minutely after each horizontal scan by the transducer. The transducer thus collects pixel data as though the surface were planar; the resulting acoustic image shows the depth of interest in an “unrolled” format.

Figure 2 is the unrolled acoustic image of the interface between molded plastic and an internal metal layer. In this part, the circumference of the cylindrical metal layer is greater than the width of the bond; the acoustic image is therefore long and narrow. The gray/white regions of the interface indicate a good bond, where the amplitude of the echo is determined solely by the acoustic properties of the plastic and the metal.

The red areas represent delamination of the plastic from the metal. It has become a convention among acoustic microscope operators to color code the highest-amplitude regions (i.e., internal defects) in red. The edges of the delaminations are shown in yellow, the color of the next-highest amplitude. In this sample, the peculiar, somewhat symmetrical shape of the delamination may give production engineers hints about its root cause.

The actual thickness of the delamination shown here is relatively unimportant for acoustic imaging. Recent investigations carried out for acoustic microscope supplier Sonoscan have shown that delaminations and other gap-type defects reflect virtually all of the pulsed ultrasound, even when their thickness is only on the order of 100-1000Ã….

After the delamination in this part has been imaged nondestructively through the plastic layer, the part itself is still available for destructive physical analysis. One advantage of performing acoustic microscopy first is that the acoustic image shows exactly where to cut and grind in order to find the features of greatest interest.

Works on plastic alone, too

Figure 3 is the acoustic image of a flat (noncylindrical) part. The depth of interest in this case was not the bond of the molded plastic to another material, but the bulk of the plastic itself. Ultrasound was therefore gated and focused on the bulk of the plastic, ignoring both the top surface of the plastic and the plastic’s bond to a second material. This technique is known as a “bulk scan.”

If the plastic were truly homogeneous, the bulk scan would result in a perfectly featureless acoustic image, of the same color throughout. What appears instead are hundreds of red (if large) and black (if small) anomalies. These are air vesicles, introduced into the plastic during the injection process. They indicate a serious process control problem.

Tom Adams is a consultant with Sonoscan Inc. (Elk Grove Village, IL). You can reach him at (847) 437-6400 or visit www.sonoscan.com.