Acoustic imaging reveals die stack layers.
By Ann R. Thryft, Contributing Technical Editor -- Test & Measurement World, 2/1/2010 2:00:00 AM
 Ray Thomas SonoLab Manager Sonoscan |
Acoustic microscope imaging is commonly used along with x-ray inspection during semiconductor production and failure analysis to reveal internal flaws such as cracks and voids. Until recently, however, acoustic microscopy was not good at finding and analyzing defects on specific layers of a stacked die assembly. Ray Thomas, manager of Sonoscan’s SonoLab division, described new software algorithms that his company developed to improve the use of this imaging technique for nondestructive, offline examination of stacked die.
Q: What are the main challenges that engineers face when inspecting stacked die assemblies using acoustic microscopic imaging?
A: The process engineer needs to know where defects are located in a die stack, along with their size, extent, and type, such as a crack on the third die layer, or a delamination between the fifth and sixth layers. In a semiconductor package containing a single die, acoustic imaging technology propagates ultrasound into the package to make an image of its internal features, such as cracks, delaminations, or voids, which can then be inspected. While ultrasound penetrates solid materials, it is reflected by the interface of two different materials, such as mould compound and the die. These reflections produce an image of the interfaces.
The additional die layers and die attaches in a stacked die configuration create complexity. Reflections from the interfaces on each layer merge with the reflections from interfaces at other levels in the stack and can’t be separated, creating a composite image that’s difficult to interpret. Sorting out this mess is nontrivial, but it must be done to assess stack reliability.
Q: What other technologies can be used for stacked die inspection?
A: The alternatives are destructive physical analysis techniques. No other nondestructive evaluation method is used to separate out the layers so you know where defects are located. The fact that ultrasound can bounce off of the layers is a strength of that technology. The complication of reflections from so many layers merging together in an image is the difficulty.
Q: What advancements have you made in acoustic imaging and how do they help solve the inspection problem?
A: Our new proprietary software capability gives us a level of detail where you can see individual layers with some accuracy and assign defects to specific layers. What we’re changing is the ability to correctly interpret the signals. There was a great deal to learn about how the reflections from multiple interfaces interact with each other. Also, not every part configuration is inspectable. For example, when die in the stack are roughly the same size, you tend to have better access throughout the stack’s thickness, but when one die overhangs another, access becomes limited.
Materials also affect accuracy. For example, mould compound is a composite material so it scatters ultrasound. If we can inspect a stack of bare die that has not yet been overmoulded, we can use a higher frequency to get better accuracy between layers. Typically, we’re looking for cracked die, or delaminations and voids between layers. But some of the other things we detect along the way include nonuniformities in the die-attach material, which may be caused by incomplete curing or incomplete mixing.
No related content found.
- 0 rated items found.
Datasheets.com Electronic Parts & Inventory Search
185 million searchable parts
- Part Number
- Description
- Inventory
- Products
- Manufacturers
View All Webcasts
