Sonoscan Final DR 1/3/10 12:36 Page 23
Yield Management
In Search of White Bumps
With the continued thinning of flip chips to accommodate the ever shrinking world
of microelectronic manufacture there is an increased need for manufacturers to be
able to ascertain any issues that may occur at such advanced manufacturing. Tom
Adams, consultant for Sonoscan discusses how acoustic imaging is providing
Issue I 2010
solutions in ensuring integrity of the resulting devices.
square4
.com
W
hen applied to flip chips, the
purpose of acoustic micro
oasiasemiconductor
imaging is typically to examine the structure and
.eur
integrity of the solder bumps. These images
www
reveal conditions at depths of interest from the
connection pads on the chip to the pads on the
23
substrate - namely, the solder bumps and
features just above and below the solder bumps.
In the modes that are most often used, a
transducer scans the top side of the silicon while
pulsing ultrasound into the flip chip. Echoes
come back only from material interfaces,
including both bonded interfaces and defects.
At any given point there may be multiple
interfaces (chip-to-bump and bump-to-substrate
are two examples) directly beneath the
transducer. Echoes from different depths arrive
back at the transducer at different times. In any
microelectronics assembly, but especially in flip
chips which typically have several interfaces of
interest within a short vertical distance, the might be as large as 150 microns. Ultrasound Figure 1: Side-view
echoes from a narrowly defined depth are used having a frequency of 100 MHz would be diagram of a solder
to create a acoustic image of the desired adequate to image defects and other features of bump showing some
feature. Echoes from other depths are ignored. interest in these flip chips. Today a flip chip may of the specific depths
Because gaps such as cracks, delaminations be thinned down to 50 microns or even 25 of interest that can be
and voids reflect virtually all of the ultrasonic microns, and the solder bumps may have a imaged acoustically
pulse, they appear bright white in monochrome diameter as small as 20 microns. At the same
acoustic images. Thus the evaluation of flip chip time, the frequencies of ultrasonic transducer
assemblies often centres on “white bumps,” the have risen to 300 and even 400 MHz.
precise vertical location, and the cause.. The difference between a transducer of 100
During the past several years, dramatic MHz and 300 MHz is profound. As frequency
changes have taken place both in flip chip goes up, the spatial resolution in the acoustic
design and the science of acoustic micro image improves, meaning that finer details can
imaging. Most flip chips once had a silicon be imaged. But as frequency goes up, the
thickness of around 600 microns, and were ability of ultrasound to penetrate a material
joined to the substrate by solder bumps that diminishes fairly rapidly. Silicon is a very good
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