Figure 3. Side view diagram shows some of the design concerns and structural anomalies that can lead to field failures. 1: very short lead finger. 2: voids in mold compound. 3: small die on large die paddle. 4: voids in die attach material. 5: delamination between die and mold compound. 6: delamination between die paddle and mold compound. 7: delamination on lead finger.
Select figure to enlarge.

The growth of anomal ies, and their ability to cause electrical failures, can in part be explained by the ways in which PEMs respond to moisture and contaminants in the atmosphere. Moisture settles on the outer surface of the PEM, along with a variety of contaminants. Molecules of both moisture and contaminants gradually migrate into the mold compound. In the interior of the PEM, the molecules tend to settle on sur faces at material interfaces - meaning the lead fingers, the die paddle, and the die face. A layer of water and contaminants only several molecules thick can become an electrolytic cell, which begins corrosion of nearby materials. If there is an existing delamination on a lead finger, it may become an electrolytic cell, or the water and contaminants may form a delamination. Eventually the resulting corrosion may, for example, break a wire bond and cause an opening. Alternately, it may destroy the mold compound between two lead fingers and cause a short.

There are many other possible scenarios. Figure 2 is the acoustic image of the wire loops in the mold compound, just above the top of the die. The groups of fine dark lines are the wire loops. At the same depth in the mold compound are small voids (one group is circled). The white area in the diagram highlights the critical depth in the component. Where a void is too close to the wires, the void may begin the corrosion process and ultimately break one or more wires.

Another potentially troublesome design is a PEM in which a relatively small die is mounted on a relatively large die paddle. Since moisture and contaminants tend to collect on surfaces, and since the die paddle is often the largest internal surface, this combination tends to result in disbonded die. Packaging reliability specialist Paul Melville at NXP Semiconductors, San Jose CA points out that a small die on a large paddle means that the wires will be very long, and therefore susceptible to wire sweep during molding.

An effective way to prevent field failures is to image PEMs acoustically before they are used in production in order to remove those having internal defects. PEMs having internal defects that move through production are likely to pass end-of-line electrical testing. Melville routinely images a great variety of PEMs to determine their moisture sensitivity level for IPC/JEDEC standards. He has often observed PEMs whose acoustic images reveal extensive internal damage, yet pass electrical tests because the cracks and other flaws have not yet broken a wire or, in extreme cases, cracked the die. Thermal stresses, moisture and corrosion will later cause a field failure. Figure 3 shows some of the internal anomalies and design problems that can result in electrical field failures.

Figure 4 is the acoustic image of one-half of a PEM that has a frequently observed defect: the silver-plated inner ends of the lead fingers -- just where the wires from the chip are bonded -- are delaminated (red areas) from the mold compound. The absence of a bond between the mold compound and the lead finger in this area means that thermal changes can cause movement, and that moisture and contaminants can gather here. Either of these events can break wires. A likely cause of this defect is the choice of a mold compound that does not adhere well to silver. In a sense, this is a design problem, but the design and material choice are in the hands of the component manufacturer, and not the assembler of the medical system.

Figure 4. Delaminations (red features) at inner ends of lead fingers, perhaps caused by using a mold compound that does not adhere well to silver.

Since the delaminations are not likely to be immediately lethal, this is a type of defect that can slip through electrical inspection. Where high reliability is imperative, manual or automated acoustic imaging is used to screen PEMs before assembly. For large-volume automated imaging, PEMs are conveyed in JEDEC- style trays. Smaller lots can be placed on flat trays and imaged by laboratory-type acoustic micro-imaging systems. In both methods, software tools can identify and analyze specific features in specific locations in an acoustic image. The accept/reject criteria are determined by the user of the PEMs. The multiple delaminations in the PEM shown here are obviously in a critical location and would be a cause for rejection, but a single small delamination that is far from either end of a lead finger might be acceptable because it is not near wire bonds and because it would presumably take a very long time to expand to the point where it creates an open path to the exterior.

Once a lot of PEMs have been screened acoustically and moves into production, they may still be damaged by handling, reflow, or other processes. For this reason, mounted PEMs are sometimes imaged acoustically after reflow. Imaging at this point may be more difficult. For example, a tall component may have to be removed to provide access for the ultrasonic transducer.

Still, if a hard-to-pinpoint process flaw is suspected, acoustic imaging remains a nondestructive and fairly fast method of analysis. Even more important, the troubleshooting clues it gains during the production process could be the difference between life and death in the field.