Acoustic surface mapping detects IC package warpage

Plastic packaged ICs such as BGAs and PQFPs sometimes contain potentially lethal construction defects, such as non-bonds, voids and cracks. Before board mounting, you can remove these flawed components from production using an acoustic micro imaging tool. Scanning anywhere from a few components to hundreds of thousands of components, the tool images and analyses gap-type defects that can expand in various ways and cause field failures.

You can using several modes of acoustic imaging depending on the interaction of ultrasound and gap-type defects within the IC package. Gap-type defects within an IC package have the property of reflecting VHF or UHF ultrasound very strongly—far more strongly than solid materials properly bonded together. Gap-type defects therefore show up brightly in images made by acoustic micro imaging tools.

The same acoustic micro imaging tools, but operating in a different mode, are used to identify plastic IC packages where no internal defects exist but where strong mechanical stresses are present. These stresses are chiefly caused by curing of the epoxy mould compound. When the IC packages experience thermal cycling in service, additional stresses may cause internal cracks and other defects that can in turn cause sudden electrical failures.

Mechanical stresses, unlike internal gap-type defects, are revealed by scanning only the top surface of the IC package where there is distortion to the package geometry. In the normal operation of the tool, ultrasound is pulsed at the package surface, just as for internal imaging. The pulse penetrates and is reflected from internal interfaces; in addition, the reflection from the top surface is used for surface mapping. There are likely to be local variations in the height of the surface, and these variations are typically the result of internal stresses.

Measuring acoustic surface flatness

To detect and image these variations in height, Sonoscan has developed and patented a method that can be performed by its C-SAM micro imaging tools. Called Acoustic Surface Flatness (ASF), the method measures the arrival time of each of the millions of echoes coming from X,Y coordinates on the sample surface.

BGAs are very common package types that are sensitive to warpage that is easy to measure with the ASF method. Many other package types are also subject to internal stresses introduced during fabrication of the component. When a warped BGA package is mounted on a board, additional warping from thermal stresses can lift the corners of the package and break the connections of the solder balls beneath the corners.

Warping can also affect package-on-package assemblies (PoPs), and particularly those where one or more memory chips are placed on top of a processing chip.

Echoes from the package interior

The interaction between a typical plastic IC package and a pulse of ultrasound is shown in Figure 1. The focused ultrasonic transducer raster-scans the surface of the package at speeds that can exceed 1m/s. Several thousand times a second, the transducer launches a pulse of ultrasound into the top surface and receives the return echoes.