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MUF and wafers get new acoustic tools

Modifications of production processes and changes in device dimensions require new non-destructive techniques to inspect for reliability. Tom Adams, consultant, Sonoscan, Inc. discusses recent advances in the acoustic microscope imaging of molded underfill flip chips and 200mm and 300mm silicon wafers.

BEFORE THE INTRODUCTION OF MOLDED UNDERFILL, the attachment of a flip chip to its substrate was imaged acoustically by a transducer that scanned the exposed silicon back side of the chip. The transducer pulsed ultrasound into the chip thousands of times a second as it scanned, and received the return echoes. Silicon is an excellent propagator of ultrasound; very little of the ultrasound is absorbed by the silicon during its fast passage to the underfill material and solder bumps that are the objects of interest.

Ultrasound is reflected at material interfaces - chip face to underfill, chip face to solder bump, and others. The most highly reflective interfaces are those between a solid (silicon, solder, underfill)

and the air or vacuum in a gap. Some common gaps in flip chips are cracks within bumps, voids in the underfill, and non-bonding of bumps to their pads. These are the items that will determine whether a flip chip passes or fails.

When scanning the silicon back side of a flip chip, Sonoscan’s personnel could use C-SAM® acoustic microscope systems equipped with very high frequency transducers because of the high acoustic transparency of silicon, and because some of the die were quite thin. Transducer frequencies could be 230, 300 or even 400 MHz. These transducers are all designed and manufactured by Sonoscan. Higher frequencies give better spatial resolution in the acoustic image. Higher frequencies are also absorbed more rapidly when traveling through materials, but silicon absorbs so little silicon that penetration of the pulse is only mildly affected.

Figure 1. Ultrasound (red) pulsed into a molded underfill flip chip must travel through the attenuating underfill (blue) to image the solder bumps

36 Issue IV 2013

The introduction of molded underfill gave manufacturers the ability to simplify production processes: instead of first underfilling the flip chip, and then later perhaps overmolding it, they could accomplish both functions in a single operation. But it also meant that overmolded flip chips could no longer be imaged at the high frequencies used on bare chips.

The problem is in the encapsulant material. The materials used in molded underfill are more absorbing of ultrasound than the overmold that is applied to bare-silicon flip chips as a separate process after acoustic imaging. The

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