The singing detective

Acoustic imaging provides a non-invasive method of scrutinising the innards of a computer chip. Sonoscan consultant Tom Adams explains the value of the technology in detecting potentially dangerous flaws in plastic ball grid array packages.

Field failures in plastic IC packages are often the consequence of internal packaging flaws that have grown since the time of component manufacture and board assembly. These flaws are typically delaminations, cracks or voids. If a particular flaw breaks a wire or otherwise causes an electrical failure, its existence, or at least its impact, will likely be discovered during final electrical test. The threat to reliability lies in internal flaws that cause electrical failure after electrical testing.

Because delaminations, cracks and voids are all gap-type defects, their response to normal thermal cycling in service is predictable. In the words of one engineer, "they open and close, open and close, and get bigger until they find a wire or another susceptible element".

The least invasive method for finding these internal flaws is acoustic micro imaging. The use of acoustic imaging after reflow has grown since the introduction of lead-free processing, not only because higher reflow temperatures are more likely to introduce flaws, but also because the new combinations of materials may encourage gap-type defects.

In plastic ball grid array (BGA) packages, as in other package types, gap-type defects may exist at various depths. Defects within the bulk of the moulding compound (an isolated void, for example) may be relatively harmless. Defects where the moulding compound meets the substrate (delaminations are most frequent here) might also have little impact on reliability if they are far from wire bonds and bond pads. But delaminations where the moulding compound meets the die face are always serious, as are flaws in the die attach and in the substrate itself.

Because of the way ultrasound interacts with material interfaces, an acoustic image can capture a very specific depth with a BGA package. Ultrasound pulsed into a plastic BGA by the scanning transducer is reflected only from material interfaces, and not by homogeneous bulk materials. (An isolated void in the moulding compound is imaged because of the interface between the moulding compound and the air within the void.) Echoes reflected from different depths arrive back at the transducer at slightly different times, and by defining a time window the operator can choose to image, for example, only the depth at which the moulding compound interfaces with the substrate.

A plastic BGA imaged acoustically at this depth is shown in the planar image in Figure 1. In this image, ultrasound has been pulsed through the moulding compound and reflected back from the interface with the substrate. The die, the traces at the surface of the substrate, and the bond pads are visible. In addition, there are numerous small irregular white features. These are delaminations between the moulding compound and the substrate.

The delaminations are bright white because echo signals from the interface between the moulding compound and the air in the delaminations have very high amplitude. The precise degree of reflection depends on the difference in acoustic impedance (density multiplied by acoustic velocity) of the two materials at the interface. Most of the delaminations are small and many are in relatively innocuous locations, away from critical elements. None is near the die, but several are adjacent to or actually on top of bond pads. Additionally, there is the suggestion of a defect in the area of the die - the slightly brighter area near the centre of the die, which is itself slightly outside of the gated depth.

A method developed at Sonoscan uses the data from multiple planar scans at increasing depths to create what is termed an acoustic solid, a three-dimensional image that can be tilted electronically and then sectioned in various ways to provide a three-dimensional internal view of the sample.

The BGA shown in Figure 1 has been imaged in this manner in Figure 2. The moulding compound - but not the delaminations at the substrate depth - has been removed. In addition, a rectangular segment of the die has been removed electronically to investigate the apparent defect associated with the die.

The delaminations at the substrate depth are red in this colour image, because a colour map was used in which red identifies areas having the highest return signal amplitude. The defect on the die is also red, and is a delamination between the moulding compound and the die face. Defects at the die face, because of the likelihood that they will expand and break wires, are usually considered far more serious than delaminations at the substrate.

Just to the left of the red die face delamination, a rectangular area of the die has been removed from the acoustic solid. The right wall actually cuts through the delamination. The blue circular feature is an acoustic shadow cast onto the die attach by the delamination above it. There are, however, no defects in the visible portion of the die attach itself. If desired, the rest of the die attach could be exposed by sectioning the acoustic solid again. One of the advantages of a three-dimensional acoustic solid is that it can be tilted and sectioned in an unlimited number of ways.

The die face of a different plastic BGA is shown in Figure 3. The damage here is catastrophic: some 70% of the die face is delaminated (red areas) from the moulding compound, and the likelihood of damage to wire bonds is very high. Only the grey area is still bonded. In the acoustic colour map at the left, red is at the bottom of the scale, indicating the strongest possible negative reflection - negative meaning that the ultrasound has passed from a material of high acoustic impedance (the moulding compound) to a material of very low acoustic impedance (the air or other gas within the delamination).

The thickness of the delamination itself is of little importance acoustically, since tests have shown that reflection from gap-type defects is near total even if the thickness of the gap is in the range from 100 to 1,000. Parts of the edge of the delamination are yellow, indicating slightly lower reflection amplitude. These may be regions into which the delamination has recently expanded. The wires surrounding the die are also yellow, not because of any defect but because of their curved profile and their very small diameter.

In BGAs, internal mechanical defects are not confined to the depths of the die and the moulding compound. Figure 4 is an acoustic image gated within the substrate itself. To make this image, ultrasound travelled from the transducer through the moulding compound, through the die and the die attach and into the substrate, from which it was reflected. Because of the bulk of materials that the ultrasound must pass through, a lower acoustic frequency (30MHz) was selected, because lower frequencies are better at penetrating deeply.

The two large circular bright areas within the substrate are delaminations between the substrate layers. Probably they were caused by thermal stresses that were relieved by separation of the layers. As they expand laterally, these delaminations are likely to encounter - and break - vias. Like defects at other depths, the real electrical damage may not occur until after final electrical testing has taken place.

Imaging plastic BGAs for internal defects has two advantages: it removes from production those components that have reliability-threatening defects, and it provides feedback data that can be used to modify processes. In many assembly plants, acoustic imaging is being used intensively as a quick evaluator of lead-free processes. In lines where true high reliability is critical, it may be used to screen components before assembly to remove those having internal defects.