Page 66
www.us-tech.com
February, 2018
Measuring Surface Flatness with Acoustic Micro Imaging Tools
By Tom Adams, Consultant, Sonoscan
ultrasound into a sample, collect the echoes and create acoustic images. These images show the desired depth without harming the sample. These tools can even create nondestructive cross-sections.
A Acoustic micro imaging can also
be used to inspect the surface of a sample, rather than its interior. The purpose may be simply to ascertain that a batch of newly-packaged devices have a surface that meets the manufacturer’s specifications. There may be surface features, such as mold marks, but the rest of the sur- face should meet flatness standards. Imaging the surface can also be
useful as an early indicator of inter- nal defects. An internal void, for example, may cause local elevation of
Using acoustic micro imaging to inspect a sample’s surface can be an early indicator of internal defects.
the surface. Internal material stress- es, even in the absence of a void or other gap, can also cause surface dis- turbances. Such stresses are espe-
Figure 1: Measuring the distance from the transducer to a single x/y position on a surface.
when scanning a tray of parts. In that second, it will perform several thousand pulse-echo functions at each of several thousand x/y loca- tions on the sample’s surface. Each pulse travels from the transducer through water at about 4,900 ft/s (1,500 m/s) to the sample. The trans-
each pulse-echo sequence is discrete. When imaging the surface, the
echoes are said to be gated on the surface — meaning that echoes from other depths may arrive at the trans- ducer later, but are ignored. Ultrasound pulsed into the sample travels through the sample until
coustic micro imaging tools, or acoustic microscopes, are well- known for their ability to pulse
cially significant, since they are like- ly to turn into field failures.
Basic Acoustic Imaging At a basic level, the laterally-
scanning transducer of the acoustic microscope may cover 3.3 ft/s (1m/s)
ducer is typically a few millimeters above the sample’s surface. If, for example, the transducer
is 0.25 in. (6 mm) above the surface, ultrasound will reach the sample surface in 4.044 µs and return at the same rate. The time is so brief that
enough of the pulse has been absorbed that no readable echo is possible, or until the pulse exits the backside of the sample. In Sonoscan’s C-SAM® tools, one of the many imag- ing modes scans the backside of sam- ples to detect those regions where no ultrasound exits. These acoustic shadows are caused by gap-type defects higher in the sample.
Measuring Surface Flatness When mapping the flatness of a
sample, what matters is the eleva- tion of each of the thousands, or mil- lions, of x/y locations that will con- tribute pixels to the acoustic image. When an ultrasonic pulse strikes a material interface, part of the ultra- sound is typically reflected as a col- lection of echoes, while the rest is transmitted across the interface. The percent of ultrasound
reflected or transmitted depends on the physical properties of the two materials at the interface. The inter- face between water and an ordinary mold compound used in a plastic- encapsulated IC reflects about 35 to 40 percent of the ultrasound and transmits the rest. The degree of reflection gives
the echo its amplitude, or signal strength. The echo also has polarity — positive if the interface was from a
Continued on page 69
Find your parts in seconds with InoAuto
Ultra Lean Material Handling System
Reduce reel storage space by 60%
Reduce material handling time by 70%
Reduce travel time by 60% Increase inventory accuracy
Store 1120 reels in .42 sq. meters Mobile
InoAuto Smart Storage System lights up required parts
ROI
in less than six months
EVALUATION UNITS AVAILABLE NOW
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116