QFN failures are increasingly important as more devices are offered in this package style, sometimes exclusively. There are some notable manufacturing issues related to assembly processes for QFNs, and this post discusses two of the more common ones that cause failures.

Trapped conductive residue under the device

The capillary gap under assembled QFNs is typically very small and difficult to clean, so solder flux residue and moisture lead to corrosion and electrical leakage causing what initially looks like a device failure. The example below shows this type of condition and resulted in failure.

Another example shows several QFN signals bridged by conductive residue.

CCE mismatch induced warpage and subsequent failure of solder joints

The QFN package typically contains a relatively large volume fraction of silicon relative to other package styles, which creates a significant CTE mismatch between the QFN and the board. In addition, the solder joint height is minimal resulting in increased stiffness of the solder joints. These factors can lead to substantial residual stress in the solder joints. Any additional mechanical stress due to depaneling or ICT probes can overload the solder joints resulting in fractures. The example below shows a fractured joint that popped up indicating there was residual elastic strain in the QFN and PWB after assembly.

The examples of manufacturing issues related to assembly processes for QFNs discussed above suggest a need for rigorous process validation for both solder reflow and cleaning processes.

 
SEM Lab, Inc. recently announced a new capability for measurement of thin film layer thickness using EDS data. This post shows an example where the aluminum bond pad thickness and the underlying Ti-W barrier metal layer thickness are measured simultaneously on a silicon IC device. Fig. A shows the EDS spectra of three bond pads on the device. Fig. B shows the layer thickness values for two different devices (A & B) at three different bond pads (1, 2, & 3).

Fig. A – EDS spectra of three bond pads on a device die.

Fig. B – Layer thickness values for two different devices (A & B) at three different bond pads (1, 2, & 3).

The ability to measure the aluminum bond pad thickness and the underlying Ti-W barrier metal layer thickness simultaneously makes this a very efficient method for characterizing IC devices.

Thin Film Thickness Measurements Using EDS

SEM Lab, Inc. has developed a method for measuring thickness of thin film deposits on various substrates, including multiple layers of different materials, using EDS data.  The method is extremely efficient and is therefore a cost effect alternative to conventional and FIB microsection techniques (approximately half the cost).

One application for this method is measurement of aluminum deposits on silicon die, particularly wire bond pads. The example below (Fig. A) shows a BSE SEM image and raster scan EDS spectrum of an aluminum bond pad on silicon, and the calculated thickness of the aluminum layer.

A second example (Fig. B) is gold plating thickness over nickel or nickel-phosphorus alloy.  The thickness of the gold layer can be determined for electroless-nickel immersion-gold (ENIG) PWB finishes and thin electro-plated gold (< 15 microns) on boards or component leads.

There are numerous other applications for this measurement method, which has significant advantages over conventional direct measurements on cross-sections including,

• Measurement requires only one EDS spectrum versus multiple direct measurements from cross-sections to achieve a meaningful average
• Non-destructive to the location being examined
• Simultaneous acquisition of image to document surface morphology
• Approximately half the cost of conventional and FIB microsection techniques

In summary, SEM Lab, Inc. can provide thickness measurements of thin film deposits on various substrates, including multiple layers of different materials, using EDS data for approximately half the cost of conventional and FIB microsection techniques.  Please contact us to discuss your specific requirements for measurements of thin film deposit thickness.

Check out SEM Lab, Inc.  to learn more.

EOS damage on this chemically decapsulated operational amplifier was more subtle than most cases that we have analyzed. This suggests that the stress condition was likely just above the maximum operating conditions for the device. In this case the damage was associated with an output pin likely overloaded by another device.

Other examples of EOS damage can be found here …

Example 1  Example 2

SEM Lab, Inc. has seen a number of knit line related failures in multilayer ceramic capacitors since 2002 when three different cases appeared (i.e. different clients & different MLCC manufacturers). Here are some examples of what this defect looks like …

August 2002

August 2002

February 2008

July 2009

August 2019

September 2019

A plot of the cumulative analysis tasks performed by SEM Lab, Inc. that involved knit line related failures is shown below. It is not clear if the up tick in these occurrences is real or not because of the limited data set. However, this may be something to be concerned with because this can be considered a latent defect that is rather difficult to screen out.  This condition does not significantly affect electrical parameters of the capacitors and is not typically visible in non-destructive techniques such as x-ray imaging.  The knit line fractures tend to cause damage in the adjoining dielectric layers and often cause secondary fracture in the dielectric between opposing electrodes resulting eventually in internal short circuits.

Knit line fractures are considered a capacitor manufacturing defect caused by lamination problems such as poor adhesion between the metal and dielectric layers.  As such, it should be routinely monitored using microsection analysis on a suitable sample of each production lot of capacitors.  However, the rate of occurrence may be too small to be detected in lot samples.  That might explain why we continue to see knit line fractures in our laboratory.

A client provided a failed CMOS Flip-Flop IC to SEM Lab, Inc. for failure analysis.  An output signal reportedly failed when the circuit was connected to an electronic counter.

This is a BSE SEM image of the device after grinding through the alumina ceramic top of the CerDip package into the die cavity. The bond wires on the device were found to be intact.

SEM examination of the die area associated with the output signal revealed that the metal run between the wire bond pad and the first contact on the die was fused open.

This is a higher magnification image of the metal run between the wire bond pad and the contact that fused open.

The dimensions of the fused section of metallization and the bond wire were estimated and input into a model for calculation of the electrical overstress (EOS) event pulse width.  The pulse width was estimated as in the range of 1-100 microseconds, which is consistent with a high voltage transient EOS event.

This image shows the same output area on a known good device for comparison.

The analysis results suggested that the electronic counter associated with the failure should be investigated for proper grounding.