A PCBA was provided to SEM Lab, Inc. for evaluation of some QFN solder joints.  This is a BSE SEM image of a microsection of the QFN.

The solder joint geometry appeared to be acceptable in this section. There was a uniform layer of Cu-Sn intermetallic at both the solder/pad and lead-frame/solder interfaces suggesting that the reflow profile was acceptable.  The interfacial intermetallic layers also suggest that there was no solderability problem associated with the QFN or PWB pads.

This is a 2nd section plane that included the thermal pad under the QFN.  The level of solder fill in the PTH-vias under the thermal pad was variable.

The “toe” of the solder joints shows an excess-solder condition possibly because of the balance of forces caused by solder surface tension between the signal solder joints and the thermal pad, which determines the solder joint height.

One possible solution for mitigating these issues is to fill the PTH-vias as illustrated in the image below.

SEM/EDS analysis is a useful technique for characterizing contamination on contact surfaces.  

The figures below show EDS spectra of contamination on a gold-plated connector contact.  The contamination was causing intermittent high contact resistance.

The EDS spectrum in this case shows C, O, Na, Mg, Al, Si, Au, S, K, Ca, Ni, & Cu.  Au is the only element that should appear in the spectrum.  The contamination appears to be a combination of “dirt”, ionic compounds, and corrosion product (i.e. of Ni & Cu).

The EDS spectrum in this case shows C, O, Zn, Mg, Al, Si, Au, S, K, Ca, & Fe.  Between these two spectra we see 10%+ of the elements on the periodic table of elements, where we should only detect gold.

  • Oxygen (O) – mostly as corrosion product, i.e. oxidation 
  • Iron (Fe) – unknown source, possible corrosion product 
  • Magnesium (Mg) – likely as oxide mineral or glass constituent 
  • Aluminum (Al) – likely as oxide mineral or glass constituent 
  • Silicone (Si) – likely as oxide mineral or glass constituent 
  • Chlorine (Cl) – corrosive, likely a chemical contaminant 
  • Sulfur (S) – corrosive, likely a chemical contaminant 
  • Potassium (K) – likely as oxide mineral or glass constituent 
  • Calcium (Ca) – likely as oxide mineral or glass constituent 
  • Copper (Cu) – possible corrosion product 
  • Nickel (Ni) – possible corrosion product 
  • Zinc (Zn) – possible corrosion product

Dirt, ionic compounds, and corrosion product are likely to cause high contact resistance for a connector like this one.   The EDS spectra provide a good basis for understanding the nature of the contact resistance problem.

 

It can be challenging for failure analysts to know if device damage caused electrical breakdown or did electrical breakdown cause the damage.  Here’s an example where a failed Power MOSFET device shorted source-to-drain.  The image below shows the device geometry and pin descriptions.  The device package is a small-outline-no-leads (SON) with 26-mil pad pitch.

The solder joints were suspected to be a potential failure cause by our client, so microsection analysis was chosen as the appropriate analysis technique (versus chemical decapsulation of the device and SEM analysis of the device die).  The figure below is a BSE SEM image of the microsection of the device as-mounted on the PCBA.

This is one of the source solder joints, which has a void and shows an excess solder condition at the toe.  Neither of these issues likely contributed to the failure.

The thermal pad also shows solder voids, but they are not so severe as to significantly increase the thermal impedance of the device.

The source-to-drain short was found at the front edge of the source lead frame.

The die attachment had failed on the left end of the die as shown below.

The molding compound separated from the die surface on the right end of the die (see below).  This damage raises a question about whether thermal stresses might have caused the separations (e.g. popcorn damage) causing the die to to overheat and break down electrically?  Or, did the device break down causing the thermal damage due to the heat generated by the short?

The failure current at breakdown can be estimated from the radius of the alloyed region (e.g. 1-mil radius =~ 1 ampere) [1] .  The short site (melt pipe) diameter was ~ 265 microns (or 10.3 mils), suggesting a failure current of ~ 5.2 amps, which is within the acceptable operating range at 25C (i.e. 13 amps max continuous operating current) per the device specification.  

The safe operating conditions diagram for this device is shown below.  The brown line at 5.2 amps is the current estimated from the radius of the melt pipe.  The diagram suggests that the device likely failed due either to a voltage spike above 30 volts or an intermediate voltage exceeding the pulse width indicated by the colored-dashed-lines.

It seems likely that thermal damage was caused by the failure event rather than being the cause of the failure.  The rapid thermal expansion in the region of the melt pipe likely failed the interfaces that were found to have separations.

In summary, this device most likely failed due to an over-voltage condition on the source signal.

REFERENCES

[1] Microelectronic Failure Analysis – Desk Reference 3rd Edition, ASM Press, ISBN 0 -87170-479-X, p.327.

 A shorted PWB was submitted for microsection analysis.  The client reported that there was a known issue involving break out of the board from the panel (i.e. misalignment of the V-score).  The client attempted to mitigate the break out damage through application of a bead of epoxy on the edge of the board (see optical image below).

 

 The BSE SEM image below shows the location of the short (dashed oval) and a laminate fracture that propagated from the edge of the board (arrows).

 The higher magnification image below of the short site showed carbonized epoxy resin and melted glass bundles.

 A region just outside of the short site showed conductive anodic filaments (CAF) in epoxy fractures and de-bonded glass/resin interfaces.

 EDS analysis confirmed that the filaments were copper.

So, why didn’t the epoxy bead on the edge of the PWB prevent the CAF short?  The likely explanation is either one or both of these reasons, (1) epoxy is non-hermetic so moisture can ingress in spite of an epoxy bead or (2) moisture and perhaps process chemistry were trapped in the capillary spaces created by the break out damage before the epoxy bead was applied.

Some edge card connector solder joints were reportedly failing open.  This is an optical image of the connector that showed fractures (red arrows) in the epoxy staking material apparently used to attempt to mitigate the problem.

This is an optical image at intermediate stage of micro-sectioning into the suspect solder joints. The displacements (yellow arrows) are very large relative to what the solder joints could be expected to survive.  Note also that the back face of the connector body is not normal to the PWB as it should be, which is the likely cause of the mechanical loading on the solder joints.  When the subject connector is mated with its mating connector, the loads on the solder joints are exactly as indicated by the displacement vectors shown in this image.

These are SEM images of solder fractures as viewed in a parallel and traverse section of the solder joints.  The evidence that these are creep rupture failures includes (1) the large upward displacement of the connector leads and (2) the fact that the fracture propagated through the bulk solder joint rather than through an intermetallic compound layer or at an interface.

So, why didn’t the epoxy staking material work to prevent these failures?  Because epoxy has a very low mechanical stiffness relative to the connector leads, so in spite of the addition of staking material the majority of the applied load is still carried through the leads to the solder joints.

 

These are chip resistors that were reported to be failing with high resistance.

This is a BSE SEM image of a microsection through the approximate centerline of the resistor.

This solder joint appeared to have failed in a brittle failure mode at the interface between the solder and the nickel barrier plating (i.e. brittle interfacial fracture).

The same issue was found on multiple PCBAs suggesting that this was some sort of systemic failure.

  The analysis results suggested that the resistor failures were most likely caused by mechanical stress (likely in bending) at high strain rate.  Another factor in the failure might have been the PWB cut-out nearby the resistors.

  Other factors to consider are  (1) degree of warpage of the PCBA after reflow and (2) relatively low PWB stiffness which could be improved by increasing the PWB thickness or adding stiffeners for example.


Response to Kurt Larson’s comment … ” It is also likely the reflow process held the solder in liquidous for too long and a too high peak temperature. This would dissolve the solderable layer on the resistor base plating into the greater bulk of the solder fillet. This is a common problem when the temperature of the reflow process is increased to speed up the process, and the process is not profiled for assemblies of varying thermal mass.”

These images suggest that in this case the reflow process was nominal based on the thickness of the nickel diffusion barrier plating.

The image below shows a row of solder joints on a large BGA near the center of the row.

The corner balls on the BGA were elongated into an “hour glass” shape due to the warpage of the package (e.g. image below). 

The chart below shows the solder joint height versus the distance from the corner ball for three different assemblies (Ax, BX, & CX).

The amount of warpage in BGA assembly is driven by a number a factors including the BGA design, the board design, and the reflow process parameters (e.g. top versus bottom-side temperature and cool down rate).  In the present case, it appeared that the BGA design (i.e. CTE constrained by large die) was the primary factor.

This is the solder-side of a PCBA that failed shorted.  The cause of the short was determined to be electrochemical migration (ECM), but there were secondary effects as shown in the next image.

These are vapor deposited copper & tin crystals that most likely formed as a result of the failure where vaporized copper & tin condensed as these feathery structures.

The EDS spectrum suggest the composition of the crystals includes copper & tin.

This is BSE SEM image of a microsection of a flex circuit showing two internal copper conductor layers.

This is an area where the copper conductor is corroded.  The corrosion initially attacks the grain boundaries.

This image shows that the corrosion completely consumed the copper layer at this location(top-right segment).

This is a through-hole solder joint where the internal copper conductors are almost completely gone due to corrosion as well as the nickel surface lands.

This is a higher magnification image that shows the missing material more clearly.

This is an elemental map that also shows the missing copper layers and nickel lands more clearly.

Some tips to avoid this type of corrosion include …

  1.  chose compatible materials (adhesives and other polymers can contain chlorides, sulfur, etc. … add moisture and voltage bias and corrosion  can accelerate dramatically)
  2.  clean thoroughly &  DRY at each appropriate process step (this includes bare flex fabrication and assembly processes)
  3.  thermal damage can create paths for ionic contamination and moisture to migrate into the internal conductors, so minimize thermal exposure