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Posted on 02 July 2019

TO and D-PAK Components Evaluated Acoustically

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Plastic-encapsulated component packages with the TO and D-PAK designations are often used as transistors that regulate voltage in power modules. TOs are used in through-hole mounting, while D-PAKs are used in surface mounting. Often the packages have 3 external leads, although some types have as many as 15.

By Tom Adams, consultant, Sonoscan, Inc.

The die is usually small in comparison to the area of the die paddle, which also serves as a heat sink. Their function as voltage regulators requires that these components dissipate considerable heat. The package is designed to remove heat: the large heat sink may be extended externally as a flange having a hole that permits the TO or D-PAK to be bolted to a heat sink that may be part of the printed circuit board. The heat-dissipation capability of these packages means that they can survive in thermal environments where another package design would fail.

But TOs and D-PAKs can still fail electrically. The electrical failure is often caused by one or more structural failures that were already present within the component package when it was placed on the board. The structural failure is nearly always a material separation of some type. The delamination of the die from its heat sink is one example: the gap will block heat from being removed, and the die will overheat and fail. A delamination between the mold compound surrounding the die and the heat sink can also reduce heat removal to some extent, but more importantly it can expand after repeated thermal cycling and extend under the die, causing the die to overheat.

Delaminations can also occur on the tops of the relatively tall posts to which wires from the die are bonded. Both TO and D-PAK packages are relatively thick compared to other plastic packages. It is the height of these posts that makes them thick. Delaminations or other gaps at the top of the posts can, like gaps anywhere in the package, expand with thermal cycling and break an electrical connection. They are also natural collection points for moisture and contaminants that percolate into the plastic package. The various gaps - delaminations, cracks and voids - that occur in these packages usually occur during manufacture of the component. Some may occur during assembly and reflow.

When components are tested electrically before assembly, only the components that are electrically flawed will fail. A component having one or more internal structural gaps will often pass electrical tests and give no hint that it harbors an anomaly that can unexpectedly cause the TO or D-PAK to fail. Engineers who perform moisture sensitivity level testing, during which parts pass through reflow three times, are accustomed to plastic encapsulated components that pass electrical tests, but when imaged by acoustic micro imaging (AMI) turn out to be “train wrecks.”

Acoustic image showing the internal

To ferret out the components having not-yetlethal structural anomalies, AMI is often used to screen plastic TO and D-PAK packages before assembly. The very high frequency ultrasound used by the Sonoscan C-SAM® system (or by one of its automated systems) is pulsed by a scanning transducer into the top surface of the component and propagates downward. Where the pulse strikes the bond between two solids, part of the pulse is reflected back to the transducer as a medium-amplitude echo, and the remainder crosses the bond and travels deeper. The medium-amplitude echo will become a gray pixel in the acoustic image; the higher the amplitude, the lighter the gray. This is amplitude- mode acoustic imaging, the most frequently used mode.

Should the pulse encounter a gap of any kind, >99.99% of the pulse is reflected back to the transducer, where it will become a bright white pixel. The delamination or other gap is filled with air or another gas. At the top of the gap, the material interface that the ultrasonic pulse encounters is between a solid material and air. The acoustic properties of these two materials are so vastly different that the reflection of the pulse is virtually total.

In the completed acoustic image, made up of pixels from thousands or millions of coordinates, black regions occur where the pulse encountered only a single homogeneous material (and thus no material interfaces), gray regions are solid-to-solid bonds, and white features are defects. The bright white features are often pseudo-colored red to make their riskiness unmistakable.

Side view diagram showing the six gates

Figure 1 is the amplitude-mode acoustic image of a TO-220 (the D-PAK in Figures 3, 4 and 5 is a different component). It was made using a single relatively wide gate to encompass features from the wire bonds at the top of the posts to the die and heat sink. Gaps have a unique echo signature and have been colored red (more serious) and yellow. At top center is the gray, square die, with two wires attached to its face. The mold compound is delaminated along three sides of the heat sink. These narrow delaminations would have to expand laterally a considerable distance to reach and travel under the die, but there is already at least one tiny yellow delamination feature near the left top of the die.

The two bright gray objects at the bottom of the image are the tops of the posts. The two wires are gray diagonal features where the reach and are bonded to the posts. The wire at left is adjacent to a fairly large delamination, which over time could 1) expand and sever the die bond; or 2) collect moisture and contaminants that could corrode and break the wire. The wire at right is not immediately adjacent to a defect, but there is a small red delamination between the wire bond and the far side of the post.

A new method introduced by Sonoscan permits the microscope user to create more than one gate and make an image for each gate during a single scan of the part. For a D-PAK or TO package, the key gates will include the bonds at the top of the post, and the die. The simplest way to do this is to set several gates of equal thickness from just below the top surface of the part to a depth just below the top of the heat sink.

The diagram in Figure 2 shows how gates were set to obtain the acoustic images in Figures 3, 4 and 5. Six gates were set; their approximate depths are shown by the red bars in the diagram. Gate A encompassed mold compound and was essentially featureless. Gates C and D likewise encountered mostly mold compound. They showed the position of the wires at each depth, but little else.

Figure 3 is the acoustic image of a D-PAK different from the TO-220 in Figure 2, and was made at the depth defined by Gate B in the diagram. There are six wire bonds, and all six are red - completely or almost completely delaminated. (The seventh, smaller red item at center is probably a gap-type artifact of the packaging process.) It is possible that all of the interconnections shown here are still electrically intact, but their risk of failure in service is very high.

Gate B image shows that all 6 wire bond areas are delaminated and likely to fail

Figure 4 was made at the depth represented by Gate E in the diagram, and just catches the top of the die, which is the roughly square feature at center. Most of the die surface, it can be seen, is red and therefore delaminated from the mold compound. Within the square feature, the two yellow regions enclosed by red are the lower ends of wires that are attached to the die. The yellow color of the wires is caused by their rounded shape; they may well mask the delamination (red) of the mold compound from the die face beneath them. Die surface delaminations such as this are always cause for rejection because they are very likely to grow laterally and break wire bonds.

Gate E imaged the top surface of the die

Figure 5 is the acoustic image obtained at Gate F, where the mold compound should be firmly bonded to the heat sink. If it were firmly bonded, the bond would be medium gray; instead, the red and yellow appearance of the U-shaped bond area shows that the mold compound is largely delaminated from the heat sink. Although it has not yet done so, this delamination can expand as a result of repeated thermal cycling until it pushes under the die, greatly reducing heat removal and causing die failure.

Large yellow area in Gate F shows that the mold compound is also delaminated from the heat sink

The value of this method is that it creates as many gates, with an image for each gate, as might be needed - up to 200 gates, if desired. Such a number is surely overkill for a D-PAK or a TO, but the ability to make several or many images during a single scan means that features can be located not only in their x and y dimensions but also in the z dimension. The method can find defects that might otherwise be missed. Gates C and D in the diagram, as mentioned earlier, contained little of interest. But if the images from these gates showed voids in the mold compound at these depth, the risks from using the component could be avoided. The risks would of course be higher if the voids were close to the wires in the x and y dimensions, but the acoustic images would reveal this as well.

Another method of imaging these packages is shown in Figure 6. Known as the acoustic profile, it uses the color map purely to indicate the depth of the interface from which an echo was reflected. It is analogous to airport radar that uses sound to measure distances. In Figure 6, the color map at left is the guide to depth: white (at the top of the color map) represents the highest altitude; black the lowest.

The alternate Profile Mode uses color to indicate the altitude of an interface: white is highest altitude, black is lowest altitude

The white and pink diagonal line at right center is a wire that begins near the bond pads (which are pink) and loops upward until part of it is white. The white portion has an altitude of about 2.2mm; the pink bond pads (and the pink portion of the wire) have an altitude of about 1.8mm. The yellow die face, by contrast, has an altitude of only 0.6mm, and perhaps slightly less. The red (and partly black) heat sink interface has an altitude of about 0.2mm.

 

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