August, 2014 Continued from previous page
more effective. One plastic-packaged IC may be subject to delamination of the mold compound from die paddle in the area around the die. By itself, this may not be a high-risk defect. But in a given part type, delamination at this device loca- tion may follow a history of expanding under the die, blocking heat flow from the die, and causing overheating that leads to die failure. Knowing the structure of the component can
also be helpful in defining simple but effective accept/reject criteria. Very complex criteria mean that analysis of the results will be slower and take longer, whether performed by a technician or by computer controlled with analysis software. Another component may frequently form
delaminations between the mold compound and the top of the die. Delaminations at this spot are rarely, if ever, innocuous because they can expand and break lead bonds on the die. Successful screening of electron-
ic components also involves careful gating. The gate is the depth, set by the operator, from which echo signals
Careful gating depth, set by the operator, provides
echo signals that are used to make the acoustic
images that will show the locations and the sizes of the defects.
are used to make the acoustic images that will show the locations and the sizes of the defects. Only the echoes whose arrival time means that they are from the gated depth are used to make the acoustic image. Gating on the whole thickness of a component may not give good results because the viewer of the acoustic image can’t tell the depth of a defect; it might be in the mold compound far above the die, or it might be on top of the die. In most components, critical
defects tend to occur in specific loca- tions at specific depths, and gating includes those depths. New software from Sonoscan makes it possible to image multiple (to 100 or more) depths separately during the same scan by means of gating. In scanning devices in DPAK (TO-252) surface- mount-technology (SMT) packages, for example, the output could consist of one image for the die and one for the top of the post. In flip-chip pack- ages, significant defects occur at the depth of the mold compound and sol- der bumps, but even this narrow depth may be imaged with multiple gates. In plastic ball-grid-array (BGA) packaged components, the sig- nificant defects may be at the die depth or in the substrate.
Delaminations at the top of the die are rarely, if ever, innocuous because they
can expand and break lead bonds on the die.
Plastic BGA components are
somewhat different from other com- ponents because significant defects are often caused by delaminations within the substrate. A BGA device has multiple layers that may be beyond the range of ordinary reflec- tion-mode imaging. Such substrate- level delaminations can be found, however, through the use of a C-SAM acoustic microscopy system in its Thru-Scan mode. In this mode, ultra- sound energy is pulsed into the top side of the component and detected by a sensor below the component.
These components were rejected due to excessive lead finger delaminations (shown in red).
www.us-
tech.com Risky Components Removed by Ultrasound
Since substrate delaminations block ultrasound transmitted from above, the delaminations appear at the sensor as black acoustic shadows. Using a pixel size appropriate to the size of a
defect of interest can boost the effectiveness of a acoustic microscopy system. The C-SAM acoustic microscopes are capable of detecting defects with diameters of 5µm or smaller, although crucial defects in most electronic components are much larger than this. Larger pixel sizes means that fewer x-y coor-
dinates have ultrasound energy pulsed into them, and scanning can take place much more rapidly than when using smaller pixel sizes and a greater number of x-y coordinates. Since acoustic micro- scope pixel sizes can now be adjusted to meet pre- cise requirements, a user does not need to overscan a component and slow down a scan by using a too- small pixel size, or miss defects by underscanning and using too large a pixel size for a particular component to be tested. Contact: Sonoscan, Inc., 2149 E. Pratt Blvd.,
Elk Grove Village IL 60007 % 847-437-6400 fax: 847-437-1550 E-mail:
info@sonoscan.com Web:
www.sonoscan.com r
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