Basic FA Flows; Die-Level FA Flow

Basic Failure Analysis (FA) Flows

Every experienced failure analyst knows that every FA is unique. Nobody can truly say that he or she has developed a standard failure analysis flow for every FA request that will come his or her way. FA's have a tendency of directing themselves, with each subsequent step depending on the outcome of the previous step.

The flow of failure analysis is influenced by a multitude of factors: the device itself, the application in which it failed, the stresses that the device has undergone prior to failure, the point of failure, the failure rate, the failure mode, the failure attributes, and of course, the failure mechanism. Nonetheless, FA is FA, so it is indeed possible to define to a certain degree a 'standard' FA flow for every failure mechanism.

This article aims to give the reader a basic idea of how the FA flow for a given failure mechanism may be standardized. 'Standardization' in this context does not mean defining a step-by-step FA procedure to follow, but rather what to look for when analyzing failures depending on what the observed or suspected failure mechanism is.

Basic Die-level FA Flow

1) Failure Information Review. Understand thoroughly the customer's description of the failure. Determine: a) the specific electrical failure mode that the customer is experiencing; b) the point of failure or where the failure was encountered (field or manufacturing line and at which step?); c) what conditions the samples have already gone through or been subjected to; and d) the failure rate observed by the customer.

2) Failure Verification. Verify the customer's failure mode by electrical testing. Check the datalog results for consistency with what the customer is reporting.

3) External Visual Inspection. Perform a thorough external visual inspection on the sample. Note all markings on the package and look for external anomalies, i.e., missing/bent leads, package discolorations, package cracks/chip-outs/scratches, contamination, lead oxidation/corrosion, illegible marks, non-standard fonts, etc.

4) Bench Testing. Verify the electrical test results by bench testing to ensure that all ATE failures are not due to contact issues only. The ideal case is for the customer's reported failure mode, ATE results, and bench test results to be consistent with each other.

5) Curve Tracing. Perform curve tracing to identify which pins exhibit current/voltage (I/V) anomalies. The objective of curve tracing is to look for open or shorted pins and pins with abnormal I/V characteristics (excessive leakage, abnormal breakdown voltages, etc.). FA may then be focused on circuits involving these anomalous pins. Dynamic curve tracing, wherein the unit is powered up while undergoing curve tracing, may be performed if static curve tracing does not reveal any anomalies.

6) X-ray Inspection. Perform x-ray inspection to look for internal package anomalies such as broken wires, missing wires, incorrect or missing die, excessive die attach voids, etc, without having to open the package. Xray inspection results must be consistent with curve trace results, e.g., if x-ray inspection revealed a broken wire at a pin, then curve tracing should reveal that pin to be open.

7) CSAM. Perform CSAM on plastic packages to determine if the samples have any internal delaminations that may lead to other failure attributes such as corrosion, broken wires, and lifted bonds.

8) Decapsulation. Once all the non-destructive steps such as those above have been completed, the samples may be subjected to decapsulation to expose the die and other internal features of the device for further FA.

9) Internal Visual Inspection. Perform internal visual inspection after decap. This is usually done using a low-power microscope and a high-power microscope, proceeding from low magnification to higher ones. Look for wire/bond anomalies, die cracks, wire and die corrosion, die scratches, EOS/ESD sites, fab defects, and the like. SEM inspection may be needed in some instances.

10) Hot Spot Detection. If curve trace results indicate some major discrepancies between the I/V characteristics (especially with regard to power dissipation) of the samples and known good units, then the samples may have localized heating on the die. For example, an abnormally large current flowing between an input pin and GND may mean a short circuit from this input pin to GND. Shorts such as this will emit heat that can be located by hot spot detection techniques.

11) Light Emission Microscopy. If the device does not exhibit abnormalities in power dissipation that may indicate hot spots, light emission microscopy may be performed to look for defects that emit light. Note that an emission site does not mean that it is the failure site.

12) Microprobing. Microprobing becomes necessary if no hot spots nor abnormal photoemissions were seen from the samples. Microprobing may entail extensive circuit analysis wherein the failure site is pinpointed by analyzing the die circuit stage by stage or section by section. The thought process used when troubleshooting a full-size circuit also applies to die circuit troubleshooting.

13) Die Deprocessing. Perform die deprocessing to look for subsurface damage or defects if the above FA steps were not successful in locating the failure site.

Basic Ball Lifting FA Flow

1) Failure Information Review. Check the customer's description of the failure for telltale signs of ball lifting, i.e., a) functional or catastrophic failures that may indicate an open bond; b) pins that become intermittently open when pressure is applied to the package or if the device is subjected to elevated or extremely low temperature; or c) high-resistance or permanently open pins.

2) Device/Lot History Review. Check the FA history of the device to determine if it has exhibited ball lifting returns previously. Check the assembly and test history of the lot to determine if the lot has exhibited any yield or process issues potentially related to ball lifting. Sad to say, most ball lifting issues have assignable causes and are non-random in nature, so containment or bounding of the problem must be meticulously pursued.

3) Failure Verification. Verify the customer's failure mode by electrical testing. If ball lifting is suspected but the unit is passing e-test, test the unit several times because the unit may have intermittently good bonds that allow it to pass. E-test must also be performed at elevated temperature if possible.

4) External Visual Inspection. Perform a thorough external visual inspection on the sample. Note all package anomalies that may indicate the unit having been subjected to thermo-mechanical stresses.

5) Bench Testing. Verify the electrical test results by bench testing at the temperature where the failure was seen. If e-test at high temperature did not verify the failure reported by the customer, perform the bench test at elevated temperature as well.

6) Curve Tracing. Perform curve tracing at ambient, elevated (125C-150C) and low temperature (-10C to -40C). This is the turning point of any ball lifting FA, because a lifted ball bond should be seen as an open pin at elevated, if not at ambient, temperature. Some lifted balls manifest at low temperature, although not as frequently. Note that the sample is unlikely to be a ball lifting failure if none of its pins is open, whether permanently or intermittently.

7) X-ray Inspection. Perform x-ray inspection as part of the FA routine. Don't expect to find any lifted balls in the xray image if no open pins were seen during curve tracing. On the other hand, if you see a lifted ball during xray inspection, then consider this as a gross case of ball lifting and ask yourself how this could have passed electrical testing.

8) CSAM. Perform CSAM on plastic packages to determine if the samples have any internal delaminations that may lead to ball lifting. Delaminations play an important part in aggravating, if not directly causing, lifted ball bonds. Movement of the plastic compound parallel to or away from the die surface as a result of delamination can shear ball bonds off their bond pads.

9) Decapsulation/Internal Visual Inspection. Perform internal visual inspection after decap. SEM inspection is most useful in verifying lifted ball bonds, since some lifted balls may not be visible optically due to the poor depth of field of optical microscopes. Once a lifted ball is found, perform further visual inspection on the affected bond pad, looking for signs of contaminants, deep probe marks/exposed oxide, cratering, metal lifting, corrosion, and other attributes that may lead to ball lifting.

10) Microprobing (optional). Some ball bonds will not appear to be 'lifted' visually, even under SEM inspection. In such cases, it is necessary to confirm that the ball bond has no electrical contact with the bond pad by microprobing. Of course, this works best if you've already identified which pin is anomalous during curve tracing.

11) Aspect Ratio Quantification. Use your SEM to estimate the aspect ratio of your ball bond. Ball bond aspect ratio is defined as the ratio of the ball diameter to the ball height, so flatter bonds will exhibit higher aspect ratios. Well-formed ball bonds would exhibit aspect ratios between 3 to 5. Balls are considered underbonded (AR<2.5) or overbonded (AR>5.5) if way outside this range. Poorly formed bonds mean a processing problem at wirebond that can lead to ball lifting.

12) IMC Quantification. Use your optical microscope to quantify the intermetallic coverage (IMC) of the ball bond. This is done by getting the percentage of the intermetallic formation on the ball bond surface. An IMC of at less than 50% (i.e., less than 50% of the bonded surface has intermetallics) indicate insufficient intermetallic formation. Try to correlate the amount and geometry of the IMC with whatever visual attributes are observed on the bond pad. Remember that poor IMC formation is most often due to bond pad anomalies that impede bonding.

13) EDX Analysis. Perform EDX analysis on the bond pads and ball bond surface to look for contaminants that may have impeded intermetallic formation. Note that silicon over the bond pad (unetched glass or Si saw dust) is a very common cause of ball lifting, so don't immediately presume that the silicon peak came from the wafer/substrate. Silicon is on top of the bond pad if its peak increases relative to that of aluminum when the SEM EHT is lowered.

14) Wire Pull Test/Ball Shear Test. If only one or two bonds have lifted, it may be useful to check the strengths of the other bonds of the sample(s). This will indicate whether the bonding problem is localized to a particular area of the die or it affects all the bonds. This is highly destructive, and must only be done as one of the last steps (if not the last one) of the analysis.

15) Conclusion. As may be discerned from above, the basic flow of a ball lifting FA consists of the following: a) looking for intermittent or open pins prior decap; b) visually and electrically confirming the ball lifting after decap; c) assessment of the IMC; d) identification of the physical and chemical abnormalities on the bond pad and the ball itself that correlate with the IMC observed; and e) subsequent investigations/simulations/evaluations to identify the root cause of these anomalies.

Basic Die Cracking FA Flow

1) Failure Information/Device and Lot History Review. Understand the customer's description of the failure, i.e., the failure mode, where it was encountered, what conditions the sample was subjected to, etc. Check the FA history of the device to determine if it has exhibited die cracking returns before. Check the assembly and test history of the lot to determine if the lot has exhibited any yield or process issues potentially related to die cracking.

2) Failure Verification. Verify the customer's failure mode by electrical testing.

3) External Visual Inspection. Perform a thorough external visual inspection on the sample. Note all package anomalies that may indicate the unit having been subjected to thermo-mechanical stresses, i.e., package cracks/chip-outs, tool marks, bent leads, discolored/burned package, etc.

4) Bench Testing. Verify the electrical test results by bench testing.

5) Curve Tracing. Perform curve tracing at ambient, elevated (125C-150C) and low temperature (-10C to -40C). Look for open or shorted pins which may indicate gross die cracking. Note, however, that some die crack failures may only exhibit subtle I/V curve anomalies.

7) X-ray Inspection. Perform x-ray inspection on the sample. Check for die attach problems such as excessive voids, die overhang, insufficient die attach coverage, and insufficient fillet. Check also for molding compound voids and cracks. Gross die cracks may also be found using sophisticated x-ray equipment.

8) CSAM. Perform CSAM on plastic packages to determine if the samples have any internal delaminations that are indicative of the unit having been subjected to extremely high temperatures. Units with severe die attach abnormalities will exhibit die cracking upon exposure to temperature extremes.

9) Decapsulation/Internal Visual Inspection. Perform internal visual inspection after decap to confirm the die crack. The crack pattern on the die surface as well as the die edge must be fully understood through extensive optical and SEM inspection.

10) Full Decapsulation. Many die cracking issues involve die cracks that originate from the backside of the die. If SEM inspection of the die surface and die edge indicates that the cracks most likely originated from the die backside, then full decapsulation must be done. Full decapsulation consists of immersing the entire unit in acid to disintegrate the entire package, leaving behind the die only. The die backside crack pattern may then be inspected freely once full decap is completed.

11) Fractography. Fractography is the systematic and scientific process of determining the origination and propagation of the cracking mechanism by studying the attributes of the fracture surface of the die. Fractography is a complicated process and can only be done reliably through years of study and experience. Once mastered, fractography would be an indispensable tool for analyzing die crack issues.

Note that Steps 9, 10, and 11 all have one objective: to understand the crack origin and propagation pattern to determine what stresses were applied to the die.

12) Conclusion. As may be discerned from above, the basic flow of a die cracking FA consists of the following: a) taking note of all electrical and visual/mechanical attributes of the sample before decap; b) confirmation of the die crack after decap; c) determination of the point of origin and propagation pattern of the die crack; d) determination of the points of application and direction of the stresses most likely experienced by the die based on the crack origin and propagation; and e) subsequent investigations, simulations, or evaluations to identify the root cause of the stresses.

Basic Package Cracking FA Flow

1) Failure Information/Device and Lot History Review. Understand the customer's description of the package crack failure. Check the FA history of the device to determine if it has exhibited package cracking occurrences before, whether in the field or in the manufacturing line. Check the assembly and test history of the lot to determine if the lot has exhibited any yield or process issues potentially related to package cracking.

2) Failure Verification. Perform external visual inspection on the sample to confirm the package cracks reported by the customer. Note the similarities and differences between the customer's description of the package crack and the actual package crack.

3) External Visual Inspection. Perform a more thorough external visual inspection on the sample to completely characterize the package crack. Check how many distinct crack lines there are, where they originate and where they end, and how they propagated from these end points. Note also all other package anomalies that may indicate the unit having been subjected to thermo-mechanical stresses, i.e., package chip-outs, tool marks, bent/non-coplanar leads, discolored/burned package, etc.

4) Look for Origin/Propagation Patterns. Check how many distinct crack lines there are, where they originate and where they end, and how they propagated from these end points. If there are several units affected, check for specific patterns with regard to how the cracks are localized. Are they on one side of the package only? Do they affect certain pins only? Do they always occur at certain features of the package only, e.g., at the top-bottom package interface, at the tie bar, at the leads, etc.?

5) CSAM. Perform CSAM on the samples to check for any internal delaminations that are indicative of the unit having been subjected to extremely high temperatures. Check also for localized delaminations that correlate with the locations of the package cracks.

6) Stress Analysis. Analyze the package crack characteristics and internal delaminations to formulate your best hypothesis (or hypotheses) on how the unit was stressed. A good guideline to follow for this is that fractures always occur under tensile stresses. List down as many possible scenarios or conditions that can result in these cracks. Pay particular attention to the possibility that these have been caused in the manufacturing line. Be sure to enlist the help of the Back-end Assembly experts in generating the list of hypotheses.

7) Simulations. Perform simulations on good units to verify each of your hypothetical root causes. For example, if you think that debris under the package during DTF caused the problem, then perform DTF on units with debris underneath them. You know you've pinned down the actual cause if you've duplicated the exact package crack pattern.

FA Techniques: Failure Verification; Optical Inspection; Xray Radiography;

Curve Tracing; Decapsulation; Sectioning; Microthermography; LEM;

Microprobing; Die Deprocessing; Focused Ion Beam; SEM/TEM; Acoustic Microscopy;

Other FA Techniques

Package Crack FA Flow; Package Failures; Die Failures; Reliability Engineering;

Reliability Modeling

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