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.

SEM Lab, Inc. supports printed wiring board failure analysis and construction analysis.  This presentation documents some of the most common failure causes seen in this laboratory.  These include Hi-Pot failures, lamination failures, inner layer separation, corrosion failures, PTH Cu-plating failures, and conductive anodic filament failures.  These failures can be mitigated to a significant degree by utilizing construction analysis to verify PWB quality.  Construction analysis allows assessment of conductor layer thickness, dielectric layer thickness, PTH copper thickness, layer-to-layer registration, standard via & micro-via quality, PTH fill quality, and drilled-hole quality to name a few critical feature attributes. Download >>>  Failure Analysis of PWBs

These are some representative examples of corrosion failure of electronic components and assemblies characterized in this laboratory.

Example 1: This is a BSE SEM image of a contact tine from a telecommunications jack. The tine is gold-plated over a nickel-underplate over copper alloy. The primary corrosion product was copper oxide that indicated corrosion of the tine base metal. Corrosive agents were found on the surface including sodium and potassium hydroxides, which in the presence of moisture can result in galvanic attack of the base metal through minute defects (pores and seams) in the plating resulting in the type of damage observed here.

Example 2: This is an electro-mechanical relay that failed to function due to internal corrosion where the corrosion product (rust) prevented the armature from fully actuating. Bromine was found in the corrosion product, which likely accelerated the corrosion damage.

Example 3: This is an example of severely corroded PTH-vias as viewed from the PWB surface and in cross-section. The corrosion appears to have been caused by residual chloride from a highly activated soldering flux. The un-filled vias easily trap process chemicals including solder flux. Another example of a corroded PTH-via is shown here.

Example 4: This is an aluminum bond pad on a failed plastic-molded op amp that was chemically decapsulated. The aluminum bond pad had been nearly completely etched away due to internal corrosion, which resulted in failures due to increased electrical resistance. Some aluminum remained in the corners of the pad (arrows). CSAM imaging showed internal separation of the molding compound from the die surface and lead frame. In addition, fractures were found in the plastic molding compound suggesting that the parts were damaged by “popcorning” during solder reflow.

Corrosion related electronics failures are common in this laboratory’s experience.

MICROSECTIONS

The terms microsections and cross-sections and metallography all refer to essentially the same process, which has been in use for a very long time.  This metallurgist was first involved in preparation of metallurgical sections in 1979 while working as a Project Engineer for Northwest Technical Industries, which is now PA&E’s Bonded Metals Division.  Metallographic cross-sections such as the one shown in Fig. A were prepared using standard metallographic practices to document the “wave pattern” and other features of the explosively bonded material.

The same standard metallographic practices are used to microsection electronic components, printed wiring boards, solder joints, etc., which are typically combinations of metallic, ceramic, and polymeric materials.  Some examples are shown below.