BGAs are susceptible to damage from thermal-mechanical warpage stresses as well as mechanical bending stress.

The damage tends to be subtle and isn’t visible at low magnifications such as the image above of a microsectioned device.

At higher magnification a fracture became apparent in the upper layers of the die.

In the image above, the die attachment appears to have failed and there is a void in the molding compound due to a processing flaw at molding.

The bottom of the die separated from the molding compound and a fracture radiates away through the molding compound.

However, the PWB laminate under the BGA pads also showed fractures suggesting that the assembled PCBA was bent out of plane. It doesn’t take a great deal of bending to induce this type of damage, so the problem may not be apparent at the moment it is caused.

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This is an optical image of the capacitor showing the part number.

This is the section after final polish. Note the lack of a solder fillet between the right side termination and the lead.

This is a BSE SEM image of the section. These anomalies are parallel in the plane of the capacitor plates (i.e. knit line defects) raising a question about the “as-sintered” strength of the structure.

This fracture shows characteristics of thermal shock damage, possibly during attachment of the device leads to the capacitor end caps.

This fracture traverses capacitor plates of opposite polarity, which typically results in a shorted capacitor. The fracture appears to propagate from a knit line defect suggesting the root cause is related to the original firing of the multilayer ceramic element.

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SEM/EDS is useful for detecting the use of red phosphorus in IC molding compounds.

At one time (~1990s), red phosphorus was introduced as an environmentally-friendly flame retardant in molding compounds for semiconductor devices.

This turned out to be a mistake.

A rash of electrical leakage related failures ensued.

There are now reports of red phosphorus use in wire insulation for cabling and power cords, suggesting that the industry may not have learned its lesson.

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Chlorine residue on this ENIG land caused corrosion of the underlying nickel and copper to form copper and nickel oxides on the gold surface.

Apparently the 3 – 7 micro-inches thick immersion gold layer is not always impervious to chloride migration.

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This is a SE SEM image of the device die. The source bond wire (left) was fused open. The gate bond wire (right) was intact.

This is the source bond pad. This material (arrow) is fused gold-copper-silica-silicon.

The source bond wire is fused open suggesting excessive source current caused the failure.

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This diode failed due to electrical overstress, likely reverse biasing greater than 1000V. The die fractured when it was de-processed. The melt-through site is indicated by the arrow. Quality issues may have been a factor including post and die attach solder voids, and potting voids, which would be expected to impact the maximum power capability of the diode causing it to fail at lower than rated conditions.

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This is a BSE SEM image of the microsection of the tantalum capacitor.

The breakdown site is indicated by the arrow and is associated with the edge of the slug near the lead attachment at the cathode.

This is an EDS dot map that shows the amount of alloying and inter- diffusion associated with the failure event.

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Fracture of this MLCC was most likely caused by PWB bending, and it resulted in an internal short. This can occur during breakout from a panel, or during ICT, or due to PCBA warpage after reflow.

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This is a BSE SEM image of a parallel microsection showing an electrical breakdown site on an inner laminate layer of the PWB. The short developed due to copper electromigration through damaged areas of laminate from a previous operation where excessively warped boards were laminated to heat sinks.

Check out SEM Lab, Inc.  to learn more.

Check out SEM Lab, Inc.  to learn more.