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Risky Components Removed by Ultrasound
By Tom Adams, Consultant, Sonoscan, Inc., Elk Grove Village IL M
any potential failures of electronic systems in the field are avoided each year because components likely to fail were removed
from the production stream before being mounted on a printed-circuit board (PCB). These ill-fated components were identified by acoustic microimag- ing, which can locate and screen out components with internal structural defects, such as delamina- tions, voids, cracks, and incomplete bonds, that can lead directly to electrical failures. Failures may occur after a delamination grows due to nor- mal thermal cycling and breaks an electrical con- nection, or because contaminants collect in a crack of initiate corrosion that can form an electrical con- nection that was not intended. Components that are inspected acoustically
are typically plastic-encapsulated integrated cir- cuits (ICs) or ceramic chip capacitors, although ceramic IC packages and some other component types can also be inspected by acoustic microimag- ing. Engineers performing moisture sensitivity level (MSL) testing on new package designs fre- quently come across components with extensive internal structural damage that nonetheless pass required electrical tests in the manner of fully functional components. Some heavily damaged components fail electrical testing, but some soon- to-fail components may pass simply because no connection has as of yet been broken.
A pulse of ultrasound energy
will return the strongest echoes from gap-like defects such as delaminations and cracks.
Acoustic microscopes, such as the Sonoscan C-
SAM® models from Sonoscan, are extremely sensi- tive to material interfaces. When the transducer of
such a microscope sends a pulse of ultrasound ener- gy into a component under test and its material interfaces (which it does thousands of time per sec- ond while scanning), the strongest echoes are received from gap-like defects such as delamina- tions and cracks. Even a gap with z-dimension of only 100Ångstroms reflects more than 99.99 percent of the ultrasound energy. Well-bonded material
Acoustic screening of electronic components
can be performed in a number of different ways. When the test volume is low, such as less than 1,000 components, screening will likely be per- formed manually on a laboratory-type acoustic microscope. For larger test volumes, a fully auto- mated in-line test system is more likely to be used, where the handling of components, the imaging process, and identification of rejects are fully auto- mated. A semi-automated acoustic microscope sys- tem may be more appropriate for applications that fall between small and large volumes. To perform acoustic microscopy, components
to be scanned are placed in JEDEC-style trays or, in the case of ceramic chip capacitors, arranged on a flat plate. Typically, the whole tray or plate is scanned at once. Recently, advanced acoustic microscopy systems have been developed that increase throughput by scanning only the critical region of each component, without scanning other areas of the component or the tray. Successful mass screening is achieved when
This C-SAM systems is set up for a scan of two trays of components.
interfaces, on the other hand, reflect a much lower percentage of the ultrasound energy — 20 or 50 per- cent perhaps, but much less than 99.99 percent.
Non-Critical Components Some inexpensive, noncritical electronic prod-
ucts incorporate components that have not been screened acoustically because the product is inexpen- sive and the consequences of failure are minor. As the consequences of failure grow in importance, however, so too does the need for acoustic screening.
the greatest number of possible rejects is identified and removed in a reasonable time and for reason- able cost. For a given component type, accept/ reject criteria depend on a user’s requirements. Screening for a plastic-encapsulated IC being used in a moderately priced consumer product might simply look for relatively large defects, those with a high probability of causing electrical failure. In a military application, however, the same compo- nent type might be rejected for relatively innocu- ous defects such as isolated small voids in the mold compound.
Knowledge Aids Screening Knowledge of the structure and failure histo-
ry of different components can make screening Continued on next page
August, 2014
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