Basic FA Flows (Page 2) - Ball Lifting FA Flow

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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.