December, 2016


www.us- tech.com


Page 49


Post-Reflow Acoustic Micro Imaging B


By Tom Adams, Consultant, Sonoscan


efore plastic-packaged ICs and other compo- nent types are mounted on a board, they are frequently inspected by a variety of methods


including acoustic micro imaging tools to ensure quality. Ultrasound is able to reveal structural defects in ICs, flip chips, ceramic chip capacitors and other components whose materials and geom- etry (typically, with at least one flat surface) make acoustic imaging possible. Acoustic imaging is usually carried out on prod-


ucts for military, aerospace, medical and high-end consumer applications. Imaging systems such as those found in Sonoscan’s C-SAM® series image and analyze internal structural defects so that the offending components can be removed from production. Pre-screening acoustic imaging,


usually in large quantities of a single component type, finds the delamina- tions and other gap-type defects that form during component fabrication or handling. Internal gaps in compo- nents present two dangers: they may expand through thermal cycling and mechanical shock until they break a connection, and they are natural col- lection points for water and other chemicals that percolate into pack- ages, where they form corrosive cells that may break connections.


Spotting Trouble Occasionally, defects can occur


after acoustic screening. The first hint of trouble may come when an unusually large number of boards fail electrical tests. If optical, visual inspection of the board and of the components shows no obvious cause, the mounted components can be inspected again acoustically. The assumption is that something went wrong between initial acoustic inspection and end-of-line electrical testing. In the past, the culprit was often local overheating during reflow, but today, the defects are more likely to also be caused by improper handling. One possibility is that the auto placement/insertion may have damaged the components. In some failures, the problem is sim- ply that no acoustic inspection was performed before mounting. Scanning a populated board dif-


fers substantially from scanning a JEDEC-style tray of loose compo- nents. In a tray, all parts are identical and on the same plane. The ultra- sound-pulsing transducer can remain at the same height throughout the scan. On a board, however, the com- ponents and other structures have varying heights. Gating of the return echoes may also differ. In a tray, the user may gate on (accept for imaging) only those echoes from material inter- faces within a depth of interest — the interface between the mold compound and the die face, for example, in a package type that has a history of defects at that interface. But in scan- ning a populated board, gates are likely to be wider to encompass com- ponents of different thicknesses.


Acoustic Imaging When a pulse of ultrasound


leaves the transducer, it does its work in a few millionths of a second. It enters the layer of water that couples the transducer to the sample; at this water-to-solid interface, a portion of the ultrasound is reflected back to the transducer and a portion crosses the interface into the mold compound (to use a plastic-packaged IC as an exam- ple). As the ultrasound passes through


the mold compound, it is absorbed to some degree, but there are no large material interfaces. The mold compound also sends back smaller echoes from par- ticles and voids in the material, and these echoes are then used to help characterize the mold material. As the ultrasonic pulse approaches the die


face, one of two things may happen. First, if at a given x/y scanning coordinate there is a delamina- tion or other air-filled gap on top of the die, the ultrasound will never reach the die itself. Instead, it will almost all be reflected by the interface between the mold compound and the air, even if the air-filled gap is only 20 nm thick. Second, if the mold compound is firmly bonded to the top of the


die, a portion of the ultrasound will be reflected back to the transducer, while another portion will cross this material interface and travel deeper into the package, sending back echoes from deeper material interfaces. These two events, reflection and transmis-


sion, occur at thousands of x/y coordinates per sec- ond as the transducer scans a component. Each echo from within the gated depth of interest becomes a pixel in the completed acoustic image. As the transducer scans, it will collect high-ampli- tude echoes from the mold-compound-to-air inter- face, and medium-amplitude echoes from mold- Continued on page 51


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