Figure 1. Lead fingers in this PEM (plastic-encapsulated microcircuit) design are very short. Wires are therefore close to the package exterior and susceptible to delamination-caused corrosion.
Select figure to enlarge.

Field failures in medical electronics systems often originate in structural anomalies in the plastic-encapsulated microcircuit, or PEM. The structural anomalies may be very small, but over time thermal stresses and exposure to moisture and contaminants can cause them to grow until they cause an electrical failure. The types of stress depend on environment. That is, an implanted device sees essentially no thermal stress, but may be susceptible to residual stresses originating in its construction. These field failures may be avoided by eliminating the structural anomalies from production.

Most of these anomalies consist of cracks, delaminations, or voids. An unresolved residual stress is not visible acoustically, unlike a crack resulting from residual stress. The various anomalies may be formed during construction and encapsulation of the PEM, or during assembly processes. Voids may form during application of die attach material and the die, or during injection-molding of the mold-compound. Delaminations are non-bonded regions between two materials such as the mold compound and the die face, or the mold compound and the lead fingers. Cracks are nearly always popcorn cracks, which occur much less frequently than a decade ago.

Because delaminations, cracks, and voids are gaps in solid materials, they can be imaged by acoustic micro imaging systems. The transducer of an acoustic micro imaging system scans the PEM while pulsing VHF or UHF ultrasound into it and receiving the return echoes. The echoes are returned only by material interfaces within the PEM, and not by the bulk of homogeneous materials. An acoustic image will display, for example, the bonded interface between the mold compound and the die face, but the strongest echoes come from gaps - meaning delaminations, cracks, and voids. For example, if a portion of the top surface of a die is bonded to the mold compound above it, and an adjacent region is disbonded, both regions will be imaged, but the disbonded region will return much higher amplitude signals to the transducer and will appear much brighter in the acoustic image.

The acoustic images illustrating this article were made by the SonoLab division of Sonoscan, Inc. One of the main functions of the laboratory is to screen PEMs for internal anomalies before the PEMs are used in the production of medical systems. Medical equipment manufacturers may also screen their own PEMs with in-house C-SAM acoustic microscopes. How rigorous the component screening may be depends on the extent to which system failure threatens the life of the patient. A pacemaker or diabetic pump is more critical than a system that does not directly impact patient survival, or whose impact on the patient is gradual rather than immediate.

Figure 2. Black dots (circled) in the acoustic image of this PEM are voids in the mold compound among the wire loops. Diagram shows side view.

Acoustic imaging is a direct and nondestructive way of weeding out anomalies, but in some cases anomalies, and the resulting field failures, can be avoided by careful design. For example, a lead frame that incorporates very short lead fingers and that creates a situation in which the outer wire bond is close to the exterior of the package. If a delamination occurs along the short lead finger, moisture and contamination can easily corrode the wire bond and break its connection. Not all packages having short lead fingers experience this type of failure; many operate for years without a problem. However, selecting an equivalent device with longer lead fingers when designing a medical electronics system may prevent this type of failure.

The diagrammatic view and the acoust ic image of a PEM having short lead fingers are shown in Figure 1. The red areas in the acoustic image are delaminations along several of the lead fingers. What the acoustic image actually displays is the interfaces between materials (the mold compound and the die; the mold compound and the die paddle, etc.). In this image the defect-free interfaces are gray. Red in this image is used to display the delaminations, which have the highest-amplitude reflections.