The excellent IPC-TR-587 technical report, ‘Conformal Coating Material and Application ‘‘State of the Industry’’ Assessment’ delivers the results of a recent major study on conformal coating, outlining an IPC study of major conformal coating types, coating application techniques, and coating cure technologies, characterizing the final film thickness on common component surfaces. It highlights that the nominal thickness applied is not particularly representative of the thickness or coverage of conformal coating materials on the various metallic surfaces. In many cases, the film thickness, although visually not zero, was below the limit of measurement.

In most manufacturing specifications, conformal coating thickness is specified as the thickness of the final polymer film on a flat, unencumbered surface of the assembly, however conformal coating thickness on other assembly and component surfaces is usually not characterized.

In this study, we compare the condensation resistance performance of assemblies coated with state of the art materials to those used in the state of the industry materials. The ‘no-clean’ SMT assemblies were subjected to controlled condensing environments using the National Physical Laboratory’s (NPL) static chamber method, relying on suppressing the temperature of the test board below the dew point, whilst simultaneously measuring Surface Insulation Resistance. Six different coatings were applied and cured, using a variety of common application methods – the same methods used in the state of the industry report, at normal nominal thicknesses, as measured on witness coupons. The coatings are assigned a Coating Protective Index (CPI) score based on their ability to maintain the SIR value of each test site above the widely used 100MΩ pass/fail criterion. A board from each coating set was extensively cross-sectioned after the condensation testing and examined for coating thickness and coverage, to understand how thickness, coverage and the inherent physical material properties combine to determine the Coating Protective Index.

The same 6 coatings were applied to SIR test coupons NPL TB33A (400μm lines, with 200μm spacings) at the same dry thickness. Coupons were tested with and without the same reflowed no-clean paste solder used in the condensation assembly. The SIR was measured throughout a 1000 hour experiment at 85°C/85%RH, a common compatibility test often performed in the automotive industry, to understand the influence of the solder paste / coating compatibility on the coating protective index achieved during the condensation testing.

In the final part of this experiment, the same six coatings were applied to conventional two-dimensional test boards (with and without solder-paste) at the same dry-film thickness, and subjected to immersion in deionised (DI) water at 50V, whilst the coatings resistance was calculated by measuring the leakage current. This allows the comparison of SIR values from condensation experiments with immersion experiments, to better understand whether immersion testing could be a faster & more predictive indicator of coating performance in harsh environments.

A surface insulation resistance study is presented on coated B52-like test boards where a large number of QFN components of different package designs are coated with non-optimal conformal coating process parameters creating bubbles at the QFN leads. Optical inspection showed that bubble sizes have been larger than the lead distance of the QFN, therefore failed according IPC-A-610 acceptance criteria. The bridging situation of the bubbles has been further investigated by means of cross section analysis but lacking a definite result. The electrochemical performance has been tested up to 1000h at elevated humidity condition of 40°C/92%rH at 30V and 65°C/93rH at 20V based on IPC-9202 after thermal pre-aging of the test assemblies. No failures related to electrochemical migration have been found. Further test at cyclic humidity conditions were also passed without any sign of microclimatic issues caused by the bubbles in the coating layer. The material combinations in this study combining conformal coating, solder paste, PCB material and components are found to be robust against microclimate risk from bubbles and voids in conformal coating. Large numbers of QFN show a sufficient electrochemical reliability despite failed in optical inspection. Consequently, the risk of reducing electrochemical performance by bubbles in conformal coatings is overestimated.

A supplier was experiencing laminate cracks in an FR4 dielectric constructed material product after successfully producing this Printed Circuit Board (PCB) for a number of years. The issue was noted during conformance coupon inspection following 3X thermal stress testing, and the cracks were isolated to outer layer dielectrics, layers 1-2 and n to n-1 only. These PCBs have microvias in the outer layers and buried vias from layers 2 to n-1. There are no plated through holes. This PCB had previously gone through a rigid qualification in accordance with MIL-PRF-31032/1 (2020), thermal shock requirements and there were no known significant process changes or material changes since qualification. All materials and critical manufacturing procedures cannot be changed without design agent approval (and potentially some instances requalification). The PCBs are a “hybrid” design, using two unrelated FR4 materials for the outer layers and layers 2 to n-1.

The reasons for the laminate cracks were not clearly evident, but certainly had a thermal component as a factor. The design agent, working with our independent testing agent, developed a finite element analysis (FEA) computer-aided model in an attempt to predict laminate crack initiation and propagation. The model utilized material characterization testing data, which had been performed by the design agent as well as detailed design characteristics of the PCB itself to build a robust model.  This model predicted high stress concentrations from surface pads to buried via locations.

Through design engineering builds of the product, which were based upon predictions of the model, and subsequent environmental testing, the nonconforming laminate cracks were reduced to a conforming condition. This was accomplished through surface pad geometry changes, and shifting of buried vias to maintain a minimum distance between buried vias and surface pads. The FEA model was validated, and is now expected to provide the design agent with a robust predictive design tool to help determine design rules for future enterprises.

This paper provides a detailed description of the process used for developing the model, and the engineering design changes made that led to the model’s validation.

This paper documents an investigation of printed circuit board (PCB) failures due to edge burn outs. Electrical shorts were observed at late-stage environmental stress screening and electrical testing. Failed boards had burn marks at edges that were absent of components or surface circuitry.

Possible causes were high voltage arcing, temperature exposure, improper material cure, stress related to board de-panel, foreign material inclusions, and growth of conductive anodic filaments.

Failure analysis narrowed the cause to PCB dielectric material fracture. Cracks initiated at the edge of the board and propagated towards the center of the board, resulting in the formation of conductive bridges and electrical shorts between layers. As corrective action, the use of a mechanical de-panel machine was eliminated and replaced by edge routing; this has prevented additional failures.

A dynamic flex test was performed on a spacecraft instrument harness composed of multiple individual flex cables, with each flex cable containing multiple copper traces. The purpose of the test was to demonstrate the capability of the harness to survive the number of expected flex cycles during the planned mission with appropriate margin. However, during testing, increased trace resistances and open circuits were observed beginning at approximately 10% of the total number of planned flex cycles. This paper discusses the various proximate causes that contributed to increased and open resistance in the flex cables and the subsequent redesign, manufacturing, reliability, and quality-related changes that were instituted. An analysis process, including failure analysis, non-destructive evaluation, digital imaging correlation, and parametric modeling, will also be discussed. The paper will cover the development of a robust dynamic flex harness design and include recommendations to extend flex harness life; make changes to the flex harness life test; and improve flex cable manufacturing, quality, and reliability.

Miniaturization and harsh environment market drivers within the electronics industry are presenting new challenges in terms of predictive reliabilities of control standards. As consumers demand longer product lifecycles, it is important for the industry to recognize the predictive reliability connections between their control standard, build design and quality control procedures in order to mitigate failures such as Ag migration, ECM and corrosion. A major criticism is the speed at which new industry standards or revisions are keeping pace with reliability requirements. One current example is IPC TM-650 2.6.3.7 (SIR) test that was developed for conventional applications BUT overlooked low and high voltage effects in fine pitch and electric vehicle type assemblies. As such the current accelerated testing can create defects that don’t align to field application failures.

This paper will review the latest “electro-chemical migration” methodologies and discuss the main factors of consideration – ranging from field strength, environmental conditions, test duration and process (interconnect and encapsulant chemistry, cleanability and component quality) that could be considered suitable for various markets such as Power Automation, Telecom, Automotive, and Industrial service equipment.

Buzz words to include Ag migration, SIR, ECM, potting material, conformal coating, through hole technology, Circuit board protection, flux resides, compatibility, voltage, test vehicle, Tg (Glass transition temperature)

In 2017, the Society of Automotive Engineers (SAE) G-24 committee approved revision activity for GEIA-STD-0005-1A, “Standard for Managing the Risks of Pb-free Solders and Finishes in ADHP Electronic Systems” (formerly known as “Performance Standard for Aerospace and High-Performance Electronic Systems Containing Lead-free Solder”). During the IPC COUNCIL PERM quarterly meeting, held in February 2020, a decision was made to re-organize the structure of the spec to align with Pb-free/Tin Whisker application risk areas, namely 1) Pb-free piece parts and mixed assembly technology, 2) Commercial-off-the-shelf (COTS) modules and assemblies, and 3) Pb-free design and assemblies for Aerospace & Defense (A&D). This paper will discuss the purpose behind the reorganization and will provide details on the risk areas and use of the document.

Electrical contacts, although critically important for a wide range of applications, are susceptible to degradation due to fretting corrosion, especially when sliding and vibrations occur. To overcome fretting corrosion and sliding wear, lubricants are often used. However, the use of lubricants can cause other detrimental issues. Lubricants usually consist of non-conductive fluids such as hydrocarbons and fluorocarbons. Due to fluid dynamics, when sliding, vibration or other excitation occurs, these fluids can cause prolonged gaps between the conducting metal surfaces. Practically, this has been observed in data centers where vibrations due to technician maintenance or even earthquakes can occur. Depending on the viscosity and roughness of the surfaces, the time it takes these connector surfaces to return to solid conductive contact can be many seconds or longer. This work uses a novel theoretical model of the coupled fluid and solid mechanics between the rough metallic surfaces to evaluate these intermittent breaks in contact due to sliding. The influence of variation in lubricant properties, roughness and solid material properties are considered by the model.