When Risk to Life Rides on a Single Component
Versatile tools are givng components intended for medical devices and systems a thorough check-up to make sure their health doesn’t hurt ours.
When medical equipment fails electronically, the immediate cause is often the failure of a single component such as a plastic-packaged Integrated Circuit (IC), an Integrated Gate Bipolar Transistor (IGBT), a ceramic chip capacitor, an LED, or another layered component. Within the component itself, the local failure is typically either the breaking of a connection, the overheating of the die or, in the case of ceramic chip capacitors, the forming of a leakage path.
…used to regulate the high levels of voltage in CT scanners, X-ray machines, the laser employed in LASIK surgery and the gradient coils in MRI machines.
The root cause of the failure is typically a structural anomaly that was created within the component package during its fabrication. Structural defects within the circuitry on the die itself are relatively rare because electrical testing catches these defects. A structural anomaly within the component package may be any of these:
- A gap within a single material. A gap within a single homogeneous material is typically a void (a trapped air bubble), less commonly a crack. Voids are trapped in materials while in a fluid or powder state—the fluid or powder, which is then cured, solidified or fused. Voids are often seen, for example, in the mold compound of a plastic-encapsulated microcircuit (PEM).
- A gap between two materials that should be bonded. A separation between two solid materials is typically very thin. An example: separation of the base plate from the solder in an IGBT. In service, thermal cycling and exposure to contaminants can cause gap-type defects to expand and break connections.
- Mechanical stress within one or more materials. Mechanical stress may ultimately result in a broken connection or a broken die. It may also cause telltale distortion of the surface of a component package. Component packages affected are usually BGA packages or others having relatively large areas.
- Tilting or displacement of an element within the package. Tilting can affect (among others) the die in a PEM, or the ceramic raft in an IGBT module. Displacement of the die in a PEM can put stress on wires.
These four classes of structural anomalies are all good reasons for internal inspection of components before they are used in production of a medical system or device. Because the anomalies may be small in their x-y dimensions and vanishingly small in their z dimensions, an acoustic micro imaging tool is typically used to inspect the components before use.
Tools are available which can detect and image gaps as thin as 200 angstroms (Å). Such tools can also operate nondestructively and use a fast-scanning ultrasonic transducer. Sonoscan’s C-SAM® series of tools can image and analyze anomalies as small as five microns in width, report the depth of an anomaly, and create a 3-D acoustic image of a component. The C-SAM tools can also create, nondestructively, cross-sectional views through a component in as many vertical planes as desired. Numerous other imaging modes have been developed for C-SAM tools. Laboratory tools are used for development work and for small production quantities. Automated tools are used for imaging of larger numbers of components to remove rejects.
Keeping Pace with Reliability
In many medical appliances, the potential impact of component failure on the patient makes the need for long-term component reliability very high—much higher than, for example, the desired component reliability in many consumer products. There are of course exceptions. The reliability of a single ceramic chip capacitor in which a defect could slightly dim the brightness of the display on a bedside patient monitor is not critical. The monitor still functions, and the patient has suffered no harm. But in the same monitor a component whose failure could cut the power supply or distort incoming data would be a threat.
The most critical component applications are in implantable devices such as pacemakers. A PEM used in a pacemaker may be required to be free from any internal defect that could cause electrical failure, even if the chance of failure is remote. High-risk locations for gap-type defects in PEMs are between the mold compound and the die face, in the die attach material, along the top side of the lead fingers, and between the mold compound and the die paddle.
Where true high reliability is needed, even small structural defects in any of these locations would probably cause a PEM to be removed from production. In this regard, medical standards are as rigorous as, or nearly as rigorous as, military and aerospace standards applied to systems that absolutely must work when required or that, once launched, cannot be repaired. The most harmless structural defect in a PEM is probably a small void in the mold compound in a location far from the die face, wires, lead fingers or other critical elements. Even though the risk posed by such a defect is extremely small, in truly critical mil/aero and medical applications a PEM having such a defect would be scrapped.
An acoustic micro imaging tool uses an ultrasonic transducer that performs a horizontal scan above a component or a tray of components. While moving laterally at a speed that may exceed 1 m/s, the transducer sends thousands of pulses of VHF or UHF ultrasound each second into the sample or samples. After a pulse is launched into the sample, it is reflected by internal material interfaces at one or more depths. Solid-to-solid interfaces reflect part of the ultrasound and transmit the balance, but solid-to-air interfaces reflect all of the ultrasound. The process is extremely fast: after a pulse at one x-y location has been inserted and has returned its echoes, the transducer has moved slightly, and the next pulse is launched—and this sequence takes place thousands of times per second. The complete data for a single component may consist of thousands or even millions of echoes.
Figure 1 is the acoustic image of one portion of a JEDEC-style tray of PEMs scanned by an automated acoustic micro imaging tool. The regions that reflect all of the ultrasound – i.e., gaps – have been converted from bright white in the monochromatic original image to red. The die is the pale rectangle at the centerof each PEM; around and under it lies the die paddle. All of the gap-type defects seen in this image are delaminations.
In the PEM at the center of the image, the red area represents delamination of the mold compound from the die paddle. Several other devices also have delaminations of the die paddle. Delaminations of this type are worrisome because they can expand under the die and block heat transfer from the die.
In the PEM at lower left several of the lead fingers are red because their top sides are delaminated from the mold compound. J-STD-020 recommends that a lead finger that is delaminated along two-thirds or more of its length be a cause for rejection. The risk is that the delamination may expand its length until it creates an open path from the outside to the die. Several of the lead fingers in this PEM are delaminated along their entire length. A few lead fingers on other devices have partial delaminations.
For a medical application requiring long-term reliability, PEMs like these are not good candidates. Most have delaminations that would probably lead quickly to failure; some may already be electrical failures. The PEM at upper right has a barely visible delamination on one of its longer lead fingers, and a delamination along one tie bar. It might be suitable for some medical applications.
Figure 2 is the acoustic image, made with a laboratory tool, of a multi-layer ceramic chip capacitor. The red areas are delaminations along the dielectric between electrode layers. Delaminations in ceramic chip capacitors tend to form conductive pathways between electrodes and result in failure. The delaminations in this capacitor are so extensive that its life expectancy—if any—would likely be brief.
Figure 3 is the acoustic image made by scanning the base plate of an IGBT module. IGBT modules were developed in the early 1990s and are used to regulate the high levels of voltage in CT scanners, X-ray machines, the laser employed in LASIK surgery and the gradient coils in MRI machines. They made possible the development of portable defibrillators, where they control the biphasic voltage waveform applied to the cardiac arrest victim.
To make this image, the transducer scanned the horizontal base plate and collected echoes from the ceramic raft above the solder layer that joins the base plate and the raft. The ceramic raft is tilted. The colors used in this image do not identify defects; instead, the arrival time of each echo, rather than its amplitude, was used to determine the pixel color. The distance of the raft from the transducer (and from the horizontal base plate) is greatest in the white and pink areas at the right edge of the image. The solder therefore is thickest (and the raft surface most distant) in this region. The solder becomes thinner (and the raft surface moves closer) as you move to the left. The curved black area at far left is likely very thin. The smaller variously shaped features are voids (trapped air bubbles) in the solder; their color depends on their depth. The two large red voids at upper left are at about the same distance from the transducer as the raft (also red) is near the left edge of the image.
IGBTs generate large amounts of heat that must be dissipated through the base plate and heat sink. Voids in the solder block heat flow. Tilting of the raft may also interfere with heat dissipation. One advantage of acoustic imaging of IGBT modules is that rafts that fail before encapsulation can often be successfully reworked.
Structural defects in component packages can take many other forms —for example, voids that are in contact with solder bumps in flip chips. Identifying those components having structural defects and removing them from production prevents failures that may pose risks to patients.
Tom Adams is a consultant to Sonoscan, Inc.