MLCC Flux Entrapment

MLCC short circuit failures on PCBAs are often caused by bending fractures, where internal electromigration shorts develop between opposite electrodes along the fracture.  Short circuit failures are also caused sometimes by capacitor manufacturing defects such as knit line failure, firing cracks, or dielectric porosity.  But we see more frequently in recent years cases where the capacitor short is external, growing due to electrochemical migration through flux entrapped under the MLCC from terminal to terminal.

Fortunately, our primary method of examination of MLCCs is a microsection of the capacitor as mounted on the PCBA (Fig. A).  This orientation is ideal for evaluation of bending fractures.  It also affords us a look under the capacitor.

An EDS map for Sn (Fig. B) shows a large concentration of tin below the capacitor and above the solder mask.  This indicates that an electrochemical cell existed under the capacitor allowing tin to migrate from the anode to the cathode.  The PCBA was operating in a moist environment, which is a contributing factor.

A second way we can look at the problem is to mechanically excavate the solder joints and remove the capacitor from the PCBA so that residue and corrosion damage can be observed on the board surface under the capacitor (Fig. C) and on the bottom surface of the capacitor (Fig. D).

Electrochemical corrosion and electromigration can involve more than corrosion of tin, as we also see corrosion of silver, copper, and nickel plating associated with metal end caps and PWB mounting pads or from SAC305 solder.

Contamination by chlorine and bromine flux activators is common and accelerates the rate of corrosion.

This is how it works …

1. Residual solder flux absorbs moisture from the environment, creating an ionic solution under the MLCC.
2. Under applied voltage, metals such as silver (Ag), tin (Sn), nickel (Ni) or copper (Cu) from electrodes or solder pads ionize.
3. Metal cations migrate toward the cathode.
4. At the cathode, metal ions are reduced and deposited as metallic dendrites.
5. Dendrites grow until they bridge the gap between electrodes, creating a short circuit.

Even if the corrosion process doesn’t create a hard metallic short, the generation of ionic species under the capacitor can cause large enough leakage currents to generate circuit faults.

Time to failure is a function of many parameters including quantity and type of solder flux residues, operating temperature, moisture level, and electric field magnitude.  The electric field becomes a significant factor as capacitor size decreases.  As MLCC sizes decrease, the gap between electrodes (spacing under the component) also shrinks so that the electric field is higher.

So my question to circuit designers is …

How do you incorporate these considerations into your design rules?

The relationship between PCBA bending and failures is critical, as bending stresses introduced during manufacturing, assembly, or operation can lead to a wide range of failures, including cracks, fractures, and deformations. These failures often occur in solder joints, component leads, and critical connection points, compromising the reliability and durability of the assemblies. Key mitigation strategies include robust design practices such as incorporating stiffeners, careful material selection to withstand mechanical stresses, controlled manufacturing processes to minimize stress introduction, and careful handling procedures. Understanding and addressing these factors through targeted improvements can significantly reduce bending-related failures, ensuring more reliable electronic assemblies.

see …

MLCC Bending Fracture

MLCC bending fracture

Mechanical Overstress of Resistor Solder Joints

Damage in BGA Device

Why is failure analysis not testing?

“Failure analysis” and “testing” are related but distinct processes in the context of engineering, quality assurance, and various scientific fields. Here’s a breakdown of how they differ:

1. Purpose:
   • Testing: This is generally conducted to ensure that a product, component, or system meets specified requirements before it is put into service. Tests are performed under controlled conditions and aim to evaluate performance, durability, and safety.
   • Failure Analysis: This comes into play after a product or system has already failed. The primary goal is to determine why the failure occurred, which can involve identifying the root causes of failure and the conditions that led to it.
2. Process:
   • Testing: Involves applying pre-determined stresses or operational loads to a product to simulate actual or accelerated operating conditions. This could include stress tests, performance tests, and endurance tests.
   • Failure Analysis: Involves a detailed examination and analysis of the failed component. This might include visual inspections, microscopic examination, chemical analysis, and mechanical testing to understand the failure mechanism.
3. Outcome:
   • Testing: Aims to confirm that the design and manufacturing meet the expected standards. It’s predictive and preventive, helping to catch potential failures before products reach the market or are used in critical applications.
   • Failure Analysis: Aims to learn from a failure after it has occurred. The insights gained can lead to design improvements, changes in manufacturing processes, or updates in maintenance procedures to prevent future failures.

Thus, while testing is about prevention and verification before failure, failure analysis is about diagnosis and correction after a failure has occurred. Both are crucial but serve different phases of a product’s life cycle and quality management.

Connectors

This is a summary of SEM Lab, Inc.’s data on connectors analyzed over the past twenty-seven years.  The most frequent cause of connector problems is shown in descending order.

Contamination

The most frequently occurring issue affecting connectors is contamination (Fig. A), which suggests that clean manufacturing environments and strict contamination control measures are a necessity.  However, the use environment is just as important and much more difficult to control.

Contamination appears frequently as a cause of failure. It can occur on contact surfaces, socket pins, and within connectors. Contaminants are typically from metallic debris due to wear, or chemicals from processing or the use environment, or polymer contamination from processing or material outgassing. Contamination can lead to increased resistance, corrosion, and eventual failure of electronic connectors.

Wear and Fretting Corrosion

Wear on connector and socket pins is the next most frequent cause of failure. Fretting corrosion, which is a type of wear that occurs at the contact interfaces due to slight oscillatory movement (i.e. vibration), can lead to poor electrical contact and failure (Fig. B).

Wear and fretting corrosion are common problems at contact interfaces, highlighting the need for durable materials and design strategies that mitigate fretting.

Material Defects and Plating Issues

Finally, defects in base materials, like defects in Kovar pin base material under the nickel-plating layer (Fig. C), or inadequate nickel under-plating thickness, or thin gold contact plating, suggest a need for stringent quality control in material selection and plating processes.

Conclusion

The percentage of analysis task in this laboratory broken down by cause is as shown below …

In conclusion, addressing contamination, wear, and material integrity should be a priority in addressing reliability issues for electronic connectors.

LEDs fail for a variety of reasons ….

Vias are important features in printed circuit boards as they distribute signals from one layer to another.  Occasionally, faulty vias are responsible for electrical failure of PCBAs.  This post illustrates a variety of printed wiring board vias and some of the defect conditions we have observed that are associated with vias.

These are thermal PTH-vias associated with a QFN device.  The vias are intended to be solder filled, but the solder is nearly absent on one of these two vias, which defeats the purpose of heat conduction away from the device.

This is a polymer filled PTH-via that exhibited a plating problem.  The printed circuit board is a QFN device substrate, where an open circuit condition was attributed to the defective plating.

This is a high aspect ratio polymer filled and capped PTH-via that exhibited excellent fill density and copper thickness in the hole.

This is a high aspect ratio polymer filled and capped PTH-via that exhibited excellent fill density but showed insufficient copper thickness in the central region of the holes.

This is a high aspect ratio polymer filled and capped PTH-via that exhibited excellent fill density and copper thickness in the hole (left).  However, there was a weakness associated with copper adhesion of the cap to the plated edge of the hole that cause open circuits post reflow soldering (right).

More examples can be found here:

Via Corrosion

Corroded PTH Via

PTH Via Barrel Crack

Flux Cleaning is Essential

PCBA cleaning (called flux cleaning or defluxing) is extremely effective in solving or preventing problems caused by flux residue.

Use of high-density mounting of components on PCBAs is accelerating, and in these cases even a small amount of flux residue may cause electrical isolation losses affecting signal integrity.  In these cases, flux cleaning is required.

Cleaning, specifically flux cleaning or defluxing, is highly effective in addressing and preventing issues caused by flux residue. Flux residue can lead to various problems such as increased electrical conductivity on the board surface, increased risk of corrosion, and compromised reliability of electronic components.

When low-residue or no-clean flux is used, there are cases where the flux cleaning process is omitted. However, in fields where high reliability is required, such as automotive modules and space-related equipment, the flux residue cleaning process is essential to avoid the problems described above.

Ionic cleanliness refers to the measure of ionic contamination present on the surface of a material or component, typically in the field of electronics manufacturing. It quantifies the amount of electrically conductive ions, such as chloride, sulfate, and other residues, that are left behind after manufacturing processes like soldering, flux application, or cleaning.

Ionic contamination can occur due to various factors, including the use of fluxes, soldering materials, handling processes, and environmental conditions. These ionic residues, if not properly controlled or removed, can have detrimental effects on the reliability and performance of electronic devices.

To assess ionic cleanliness, a common method is to measure the electrical conductivity of an extracted solution from the material or component. This is typically done using techniques like ion chromatography (IC) or resistivity of solvent extract (ROSE) testing. The results are expressed as the concentration of ionic species, typically in units of parts per million (ppm) or micrograms per square centimeter (μg/cm²).

Standards such as the IPC-TM-650 and MIL-STD-2000 provide guidelines and specifications for acceptable levels of ionic contamination in different electronic assemblies or components. These standards help manufacturers ensure that the ionic cleanliness requirements are met to maintain the reliability and functionality of the final products.

Overall, the goal is to ensure optimal performance, reliability, and longevity of electronic devices by addressing flux residue, maintaining adequate ionic cleanliness levels, and mitigating the impact of moisture. Adhering to industry standards and manufacturer guidelines is essential in implementing effective cleaning processes to meet the specific requirements of different applications and sectors.

Introduction

Our failure analysis laboratory has seen several cases of the same WLED failure mechanism in the past two years, where the lens material is Ce:YAG phosphor in silicone elastomer and the failures appear to be caused by degradation of the lens material.  This paper uses three cases to illustrate this failure mechanism and the failure modes associated with it.

The WLED technology for these cases utilize cerium-doped yttrium alumina garnet (Ce:YAG ) phosphor particles suspended in optically clear silicone elastomer.  This material appears to be yellow in color because the phosphor particles convert blue light to yellow light.  The phosphor-converted WLED is viable because the combination of blue and yellow appears to the eye as white light.

Ref. [1] states that “The unique silicone polymer utilized in pc-LEDs is stable over a wide temperature range and resistant to yellowing after ultraviolet exposure.”  If this is the case, then the failures discussed below were likely caused by some sort of abuse, which is likely related to the intensity of white light being generated by the LEDs or the quality of the thermal designs associated with these applications.  The cases discussed below involved very bright LEDs and the thermal designs appeared to be quite good.

Case Histories

Case #1: COB WLEDs

Case #1 is related to a chip-on-board (COB) WLED that reportedly failed within approximately half of its expected life due to color shift toward blue light.  EDS analysis (Fig. 1A) suggested that the failure mechanism involved carbon loss.  FTIR analysis (Fig. 1B) of the silicone coating from the “New” sample suggested that the silicone is polymethylphenylsiloxane.  Furthermore, comparison of the FTIR spectra for “New” and “Fail” samples (Fig. 1C) suggested that the degraded silicone coating on the failed LEDs correlates with the disappearance/weakening of Si-Ph absorption peaks, which implies that the phenyl rings in the compound disappear during the degradation process.  This mechanism may explain the loss of carbon in the silicone material shown by the EDS data.

Fig. 1A – EDS results for “New”, “Not-Yet-Failed”, and “Failed” WLEDs (atomic percent) where “(n%)” indicates the estimated amount of life consumed for each condition

Fig. 1B – FTIR spectrum of the silicone coating from the “New” sample (top) along with the best match from our spectral library (bottom).

Fig. 1C – FTIR spectra of the failed (top) and control (bottom) samples.

The loss of carbon in the silicone appears to cause shrinkage and fracture in the coating (Fig. 1D).  The fractures provide a path for blue light to escape without phosphor conversion, which might explain the color shift toward blue that was reported as the failure mode.

Fig. 1D – SEM micrograph of Ce:YAG-silicone coating.  Bright contrast particles are Ce:YAG phosphor.  Intermediate gray contrast material is silicone elastomer.  Dark contrast features are fractures in the silicone elastomer.

Case #3: Packaged WLED2

This case is another packaged LED as shown in Fig. 3A.  The failure mode in this case was open circuits caused by failure of either the bond wires or the wire bonds depending on the sample.  The lens material is the same type of Ce:YAG-silicone as the previous cases.  In this case, EDS data showed 58 ± 10 atomic percent carbon (N = 6) at random locations within the lens material.  This was the highest carbon percentage for failed LEDs of the three cases, and for the same reasons discussed in Case #2 it is believed that the average is skewed to higher atomic percent carbon due to nonuniformity of light intensity in the lens material.

Fig. 3A – LED showing two fractures in lens centered on each of two LED dice.

Fig. 3B shows EDS data for the Ce:YAG-silicone lens material of new LEDs and two failed LEDs.  The data suggests that there is a loss of carbon for the failed LEDs relative to the new LEDs.  Fig. 3C shows a mapping of carbon concentration in the silicone lens material versus location, which demonstrates that the degradation is most severe directly over the LED die where the fractures formed in the lens.

Fig. 3B – EDS data for composition of Ce:YAG-silicone lens of new LEDs and two failed LEDs.

Fig. 3C – Atomic percent carbon versus location relative to the die region of the LED.

Summary

The three cases illustrated above exhibited different failure modes, but they all appear to be caused by the same failure mechanism.  This mechanism is characterized by …

• Shrinkage and fracture of the Ce:YAG-silicone lens or coating
• Loss of carbon due to outgassing of VOCs (phenyl groups)
• Unexpected early wearout failure

[1]     Pradeep Lall, Hao Zhang, & Lynn Davis, “Failure Mechanisms and Color Stability in Light-Emitting Diodes during Operation in High- Temperature Environments in Presence of Contamination”, 2015 Electronic Components & Technology Conference, 1624 – 1632.

SEM Lab, Inc. uses SEM/EDS analysis to characterize ENIG finish quality on PWBs, as discussed here and here.  Examples of gold thickness values obtained in this laboratory (N = 14) are illustrated in the histogram shown below.  IPC-4552 suggests that the range of acceptable thickness is 0.0508 – 0.2032 microns, so 6 of the measurements exceeded the expected thickness.  This may be the case because some suppliers provided thin electro-plated gold in place of immersion-gold when the finish was specified as ENIG.

The phosphorus concentration in the electroless-nickel is expected to be 7 – 9 wt%, which corresponds with “medium phosphorus” electroless-nickel.  We detected one value below 7 wt% P, which can lead to corrosion of the electroless-nickel deposit during the immersion-gold plating process.  Values above 9 wt% P can lead to solderability issues during the assembly process.

Routine measurements of gold thickness and phosphorus concentration in electroless-nickel on incoming board lots is an important way to avoid costly processing problems at assembly and reliability issues in the field.  SEM Lab, Inc. has developed a cost effective approach to evaluating ENIG quality on PWBs.