PWB Failure
Convert EDS data into actionable insights …
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 …
- Residual solder flux absorbs moisture from the environment, creating an ionic solution under the MLCC.
- Under applied voltage, metals such as silver (Ag), tin (Sn), nickel (Ni) or copper (Cu) from electrodes or solder pads ionize.
- Metal cations migrate toward the cathode.
- At the cathode, metal ions are reduced and deposited as metallic dendrites.
- 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?
Solder Joint Fractures
These images show the various types of fractures affecting solder joints in electronic assemblies, which includes mechanical overload failures, thermal fatigue due to coefficient of thermal expansion (CTE) mismatches, gold embrittlement, creep rupture failures, and vibration fatigue fractures. Understanding these failure modes is crucial for enhancing the reliability and performance of electronic components.
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 …
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:
- 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.
- 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.
- 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:
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.