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Apparatus to Monitor Underbump Delamination (White Bumps) In Situ via Acoustic Emission Technique

IP.com Disclosure Number: IPCOM000234042D
Publication Date: 2014-Jan-08
Document File: 3 page(s) / 74K

Publishing Venue

The IP.com Prior Art Database

Abstract

The Back End of Line (BEOL) structures of Integrated Circuits (IC) in Flip Chip (FC) packages are exposed to high thermo-mechanical stresses during solder reflow assembly. These stresses are due to the mismatch in the coefficient of thermal expansion (CTE) between the silicon die and the organic substrate under the high thermal excursion from reflow temperature to room temperature during FC assembly process. During such a cool down from reflow temperature, the solidified FC bumps have to couple the different expansion of die and substrate and thus expose the underbump region of the die to high normal and shear stresses. Circular cracks under the FC solder bumps, often referred to as white bumps, can occur as a triggered failure mode. The discovery of such under bump delamination (white bumps) requires a lot of effort and is often only noticed after Sonic Acoustic Microscopy (SAM) inspection of large number of parts which is time consuming and cost intensive. It is therefore a desire in the industry, to know instantly whether a white bump occurred in a part during reflow, or not This publication is proposing to measure the structural born sound, emitted during the crack propagation in the BEOL, via an acoustic microphone, attached to the back side of the die, as a method or apparatus to achieve such an “In Situ” white bump detection possibility. It would offer the additional advantage to get insight about the time and temperature of white bump occurrence during the reflow and help to identify more effective countermeasures.

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Apparatus to Monitor Underbump Delamination (White Bumps) In Situ via Acoustic Emission Technique

Abstract

The Back End of Line (BEOL) structures of Integrated Circuits (IC) in Flip Chip (FC) packages are exposed to high thermo-mechanical stresses during solder reflow assembly. These stresses are due to the mismatch in the coefficient of thermal expansion (CTE) between the silicon die and the organic substrate under the high thermal excursion from reflow temperature to room temperature during FC assembly process. During such a cool down from reflow temperature, the solidified FC bumps have to couple the different expansion of die and substrate and thus expose the underbump region of the die to high normal and shear stresses. Circular cracks under the FC solder bumps, often referred to as white bumps, can occur as a triggered failure mode. The discovery of such under bump delamination (white bumps) requires a lot of effort and is often only noticed after Sonic Acoustic Microscopy (SAM) inspection of large number of parts which is time consuming and cost intensive. It is therefore a desire in the industry, to know instantly whether a white bump occurred in a part during reflow, or not

This publication is proposing to measure the structural born sound, emitted during the crack propagation in the BEOL, via an acoustic microphone, attached to the back side of the die, as a method or apparatus to achieve such an “In Situ” white bump detection possibility. It would offer the additional advantage to get insight about the time and temperature of white bump occurrence during the reflow and help to identify more effective countermeasures.

Detecting White Bumps In Situ via Acoustic Emission (AE) Technique

Circular delamination in the BEOL stack due to stresses from the thermo mechanical miss-match between substrate and die during solder reflow are an industry wide problem. Since no under fill is present during that process step, the flip chip solder bumps have to couple die and substrate and get exposed to high tension and shear due to the different expansion rate during cool down. One side of the problem is that the detection of those cracks requires a lot of recourses during qualification and production, since a large number of parts need to be investigated and tested with SAM after the parts are underfilled. Subsequently to that, the images need to be post processed and interpreted by engineers.

This paper therefore proposes to detect the propagation of such a crack or multiple cracks by Acoustic Emission Techniques directly during its creation. During propagation the crack will emit a structure born sound wave. This sound will travel through the silicon body and could be detected by a structure born sound microphone which is attached to the backside of the die. (Figure 1)

Down holder (1) with detector/ microphone (2) provides required contact to die (3)

 

Figure 1 Work principle of the detection process

The apparatus would need to contain...