Flip Chip package failure mechanism
04/01/1998
Flip chip package failure mechanisms
Janet E. Semmens, Sonoscan Inc., Bensenville, Illinois
Tom Adams, Consultant, Lawrenceville, New Jersey
Completely assembled flip chip packages are notoriously difficult to inspect for critical defects. Acoustic micro imaging tools with sophisticated electronic subsystems can identify the major types of hidden internal defects. Failure mechanisms can thus be determined and remedied.
Flip chip packages are compact with short conduction paths and unique failure mechanisms. Many of the packaging defects in flip chips can be imaged and analyzed by acoustic micro imaging. These defects include disbonds at the top and bottom of the solder bumps, cracks and voids within individual bumps, undersized and bridged bumps, underfill voids and delaminations, and delaminations around individual bumps.
Defects can be broadly classified into three types according to their consequences. A few defects (for example, a single small void in the underfill not near a solder bump) may never become the cause of component failure. Some defects (such as a completely disbonded solder bump) are immediately identified because they cause an electrical anomaly in the chip`s performance.
The third type of defect - the most insidious - has no immediate electrical consequences but will unexpectedly cause failures in the component. The eventual failure mechanism is often a broken interconnect, such as a disbonded solder bump or a cracked die.
This last category is by far the most destructive overall because it includes hidden internal defects that can lead to unexpected returns from the field and damage to a manufacturer`s reputation for product reliability. Hidden internal defects typically cause a failure after their dimensions have grown during repeated thermal cycling.
For example, an initially harmless underfill delamination may grow until it encounters and shears a solder bump bond. A void adjacent to a solder bump (a "halo defect") may not grow itself, but may cause damage by highly stressing the solder bump or by providing a gap into which solder can creep.
Some defects do not involve material movement. Irregular distribution of filler particles within the underfill may create a long-term thermal expansion differential that eventually results in a secondary (and lethal) defect.
The origin of many hidden internal defects in flip chips (see table) can be understood by examining two process areas: the placement and subsequent treatment of solder bumps, and the flow of liquid underfill material between the chip face and the substrate.
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Bump defects
Flip chip designs call for solder bumps that are homogeneous and all of the same size. Aberrations in any of these parameters can lead to hidden internal defects. For example, a post-reflow undersized bump in an undersized bond area to either the chip or the substrate can result in eventual failure.
Individual solder balls may also contain cracks or voids that can be viewed acoustically in the finished flip chip package (Fig. 1). The differentiation of a particular defect within an individual solder ball as a crack or a void is often a moot point; either defect creates room for solder creep that will deform and potentially break the interconnect.
Solder balls may also be grossly mis-sized, a condition that leads to poor interconnects. They can be contaminated with flux material that may contribute to two somewhat similar defects: the outright bridging of adjacent bumps, and the dendritic growth of solder between adjacent bumps. Dendritic growth generally occurs in combination with a delamination of the underfill material.
Underfill defects
As underfill flows between the chip face and the substrate, it should ideally wet all surfaces it contacts, fill the entire space, and cure properly. Lack of wetting can lead to delaminations of various types and sizes. Although it may be difficult to determine the real cause of non-wetting, flux residue is often considered a factor.
The distribution of filler particles within the epoxy is also very significant. The filler particles, which cover a defined range of sizes, should be evenly distributed throughout the three-dimensional volume of the cured underfill. When distribution is not even, the resulting particle segregation may be horizontal and/or vertical.
In horizontal segregation, some regions of the cured underfill have very high particle counts, while other regions have very low particle counts. Regional variations can form an acoustically visible pattern that is probably related to the flow pattern of the fluid underfill. In vertical segregation, some or all of the particles settle to the bottom; in extreme cases, the substrate may be covered with a continuous thin layer of particles.
Segregation of particles can lead to degraded thermal conductivity in the cured underfill. Evidently, segregation can also be the sole factor in the formation of certain voids; observations in Sonoscan`s laboratory repeatedly showed the presence of voids in areas of high particle concentration in the cured underfill.
Acoustic micro imaging
In the past few years, acoustic micro imaging has provided much information on the condition of post-cure flip chip packages. The very high frequency ultrasound - used nondestructively - creates images of internal features in the packages. The transducers alternately pulse ultrasound into a part and then, a few microseconds later, receive the return echo, which contains both amplitude and phase information.
On its round-trip to and from the depths of the part, the ultrasound is variably reflected by interfaces such as the bond between a solder bump and the die face, or between the solder bump and the substrate. The degree of reflection depends on the acoustic impedance of the two materials on either side of the interface. The acoustic impedance is the product of a material`s density and the speed of ultrasound in the material.
For example, an interface between the silicon of the die and the epoxy of the underfill creates a moderate difference in acoustic impedance, resulting in a distinct image of the interface. The greatest differences in acoustic impedance, however, are between normal production materials and gap-type defects. Gap-type defects behave like air, through which very high-frequency ultrasound does not propagate. The acoustic impedance of air and of gap-type defects is zero, which is why voids, disbonds, delaminations, and the like are so easily imaged acoustically.
The return echoes from both normal features and defect features arrive back at the transducer at slightly different times, depending on their locations. The return echoes can be electronically gated to exclude all echoes except those from a very specific depth, such as the interface between the solder bump and the substrate. The resulting acoustic image will display features and defects at this interface, but will not display features from other levels within the device.
By gating at successively deeper levels, a sequence of acoustic images can be made that displays features at various levels within the device. Broad gating can also be used to include most or all of the thickness of the device. In addition to gating, the ultrasound is focused on the internal level of interest.
Gating can allow for imaging of an internal interface (the technique is called an interface scan) or the bulk of material such as the solder bumps (bulk scan). A third approach electronically gates on a level that is below the level of interest. The returning echoes will image the lower level, but a defect in any of the levels above will cast a characteristic acoustic "shadow" which shows that a defect is present above the gated level (loss of back echo). A scan of the return echoes from the x-y location of the shadow will then show the depth of the defect and permit it to be gated and imaged.
The relatively small diameter of solder bumps - currently 200-300 ?m - requires high frequencies to yield high resolution. Until recently, flip chips were typically imaged with a 100 MHz transducer. Higher frequency transducers greatly improved image quality. At 230 MHz, multiple voids or cracks within a single solder bump are imaged distinctly; other very small features such as the halo defect are also more clearly seen.
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Figure 1. Schematic side view of a flip chip package shows some defects that can be seen acoustically: a) solder bump disbond; b) delamination at chip face; c) filler particle segregation; d) undersized solder ball; e) delamination at substrate; f) void in underfill; g) halo delamination; and h) void or crack in solder ball.
Flip chips are usually imaged from the top of the packaged device, i.e., through the backside of the silicon die. If the presence of an anomaly is suspected at an unknown level within the device, broad gating or the loss of back echo technique may be used initially. Broad gating may be the preferred method if an anomaly encompasses more than one level - if, for example, the absence of underfill in one region has caused the die to crack.
Narrower gating is used to image underfill delaminations from the chip or the substrate. To image the distribution of particles in the underfill voids and bridged solder bumps, the full thickness of the underfill is typically gated.
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Figure 2. Acoustic image shows segregation of filler particles in underfill. Dark areas are concentrations of particles; such areas are often the site for formation of voids such as the two shown here.
The bonds of a solder bump to both the die and the substrate are very small, but critical, features. A bond may be normal in size, partially present, or entirely absent. It is sometimes difficult to resolve a clear acoustic image of the bump bonds to the substrate because of the number of layers of material through which the ultrasound must pass. Resolution can sometimes be improved by turning the device over and imaging through the relatively fewer layers in the substrate, providing they are acoustically transmissive.
Results
The acoustic imaging of a flip chip underfill with strong segregation of the filler particles (dark regions) shows that two voids formed in the areas of particle concentration (Fig. 2). The formation of voids where filler particles are overly dense is such a frequent phenomenon that acoustic micro imaging users automatically look for voids when particle segregation is present. Voids probably form because of the low presence of epoxy adhesive in the area. Regardless of multiple root causes, voids are the most frequently seen internal defects in flip chip packages.
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Figure 3. Gating at the chip-to-bump interface provided this acoustic image showing delamination (pink) between chip and underfill at one corner and along one edge.
Flip chip delamination between the underfill material and the die face can occur along both edges and corners (Fig. 3). The most likely cause of delamination is flux residue that prevents the fluid underfill from thoroughly wetting the face of the die. Similar delaminations are likely to grow after thermal cycling until a solder bond is broken. In Fig. 3, a single solder bump at the very corner of the die in the larger delamination appears to be missing. Since the narrow acoustic gating used to image this level does not extend downward into the body of the solder bump, this bump may be present but disbonded (i.e., the delamination may already have broken the bond).
Figure 4 shows a flip chip device with two separate defect categories. First, there are four voids; at least two seem related to particle segregation. Second, small defects partly or completely surround many of the solder bumps; these halo defects (named for their configuration) are thin vertical delaminations that follow the contour of the solder bump. Like delaminations, they probably form when flux residue prevents the fluid underfill from wetting individual solder bumps.
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Figure 4. White halo defects partly surround solder bumps in this acoustic image. A type of vertical delamination, halo defects probably result when flux residue contaminates the bump surface. Solder will probably creep into the delamination.
Halo defects are easily seen acoustically, but are so small that they may escape detection by x-ray imaging. Because of their small size and location, and because both solder and epoxy tend to smear, they are very difficult to find by grinding or sectioning the device. They are significant defects because high stress is likely to develop in the nearest solder bump, and because they constitute available space into which solder can eventually creep. Like horizontal delaminations, they may also grow larger over time.
Acoustic micro imaging can also monitor the size of the solder bump bond (Fig. 5). Gated at the bump-to-chip interface, images can disclose undersized bonds with high relative electrical resistance during use. The solder bumps themselves may be of normal size and still exhibit undersized bonds.
Defects within individual solder bumps are shown in Fig. 5, which was made at the high-resolution frequency of 230 MHz. Each of the solder bumps has a length of 0.25 mm. One area of underfill is dark because of particle concentration; as would be expected, there is a void in this area. Very fine gating at the level between the solder balls and the chip face actually puts the underfill level slightly out of focus. Some of the voids imaged have dimensions of <100 ?m.
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Figure 5. Each solder bump has a length of 0.25 mm in this 230-MHz acoustic image. White areas are voids in the chip-to-bump bond.
The images shown here were made in failure analysis environments. Acoustic micro imaging of flip chips has, however, recently entered a new production-environment phase, in which an unattended automated system handles stacks of flip chip devices in JEDEC trays. Imaging is the same as with lab-based instruments, but the throughput is much faster. Tray images are stored, but the useful output takes the form of spreadsheets and charts that show the occurrence of both individual defects and defect trends.
Conclusion
Acoustic micro imaging is a proven technique, uniquely suited to the inspection of flip chip defects. The technique can identify the major categories of hidden internal defects. Higher-frequency transducers and optimized signal filters can achieve resolution below 100 ?m, and detectability well into the submicron range.
Voids are the most commonly observed hidden defect, resulting from problems in either the solder bumps or the underfill epoxy. The single most likely cause of all hidden defects is flux residue.
TOM ADAMS is a consultant and writer based in Lawrenceville, New Jersey. He has written extensively on semiconductor topics. Phone 609/883-5040.
JANET SEMMENS is Chief Applications Scientist at Sonoscan, Inc. She has authored more than 40 technical publications on acoustic microscopy. Sonoscan Inc., 530 East Green St., Bensenville, IL 60106; ph 630/766-7088, fax 630/766-4603, e-mail [email protected]