A number of similar Printed Wiring Assemblies (PWAs) had been subjected to final electrical tests on automated test equipment when a common test failure was recognized. All PWAs were failing for high leakage resistance; the occurrences were not on the same electrical nets, but all PWAs had similarities in that the failed signals in question had traces on layers 2 and n-1. Extensive analysis ensued and determined there was significant oxide treatment residue on layers 2 and n-1 of a sub compositefor this board design. This is a sequentially laminated Printed Circuit Board (PCB) where layers 2 thru n-1 are laminated initially, followed by a second and final lamination of layers 1 and n.

The first objective of this paper is to describe theoxide residue case history discovered with numerous PWAs. The second objective is to provide a summary of methods and techniques engaged in when 1) determining the oxide residue condition and 2) determining whether the functional performance of the printed wiring assemblies (PWAs) would be impacted by the condition.

Initially, conformance coupons from the affected lot of PCBs were reviewed. It was discovered that bright field inspection techniques did not effectively detect any anomalies, so dark field techniques were employed during the failure analysis investigation. During this review, small white areas were seen at the interface of layers 1 and 2, as well as n-1 and n. In an effort to better observe the anomaly, horizontal grinds were conducted down to the dielectric layer interface where the crystalline structure residue was prolific. During this investigation, other PCB lots (already built into PWAs that passed electrical tests) were found to have been affected, but to a less severe degree. A variety of techniques were utilized to evaluate the issue and determine the viability of using oxide-residue contaminated PWAs. These techniques included, but were not limited to: visual examinations, PCB cross-section analysis, horizontal grinds, scanning electron microscope (SEM) evaluation with Energy Dispersive Spectroscopy (EDS), electrical simulations, and Conductive Anodic Filament (CAF) testing.

This paper provides a structured, methodical approach to failure analysis for this type of failure, as well as to determining the impact to functional performance. It may be utilized as a guideline for others facing a similar predicament.

SMT-flux residues are feared to be a potential risk factor for quality issues in electronic assembly and interconnect technology. This work shows how the amount of no clean flux residues can be reduced by adapting the reflow conditions and shows that even highly viscous flux residues are still humidity robust. Especially, no clean solder pastes are believed to cause numerous quality issues such as reduced humidity robustness, particles sticking in viscous flux residues, reduced adhesion of coatings and other issues during the following assembly and interconnect (AIT) processes. Those issues can be tracked down to both the physical and chemical properties of the flux residues namely the chemical composition, the viscosity, the mass, the covering and the distribution on the PCB and the components. To assess the risks and to control the flux residue properties a deep understanding of the mechanisms during and after reflow soldering is crucial. This understanding can only be reached with suitable measurement methods to analyze the different physical and chemical properties of the flux residues. This work presents a gravimetrical approach to measure the amount of flux residues remaining on the PCBA after reflow soldering. The results show that the outgassing and reaction behavior of the flux residues are caused by 3 main impact factors: the thermal load (reflow temperature and time), the local partial pressures of the outgassing organic gases (solder joint design, e.g., covered/open solder joints) and the global partial pressures (reflow atmosphere, e.g., gas circulation speed, oxygen partial pressure). No clean solder fluxes consist mainly of resin (encapsulates ionic contaminations), solvents (make paste well printable) and activating acids (remove metal oxide layers). Viscous flux residues where the resin matrix is not hard are often believed to be a source for ionic contaminations which could lead to electrochemical migration. Here, SIR (surface insulation resistance) measurements of PCBAs with highly viscous no clean flux residues prove a high humidity robustness of no clean solder pastes. The knowledge about the main impact factors on the physical and chemical properties of flux residues allow to optimize the no clean SMT reflow soldering and could be a solution for the issues, which are encountered with highly reactive clean solder pastes e.g., electrochemical migration.

In a lead-free reflow process, temperatures are higher, and materials use outgasses more than in a leaded reflow process. The trends toward higher density populated boards and more pin-in-paste technology also increase solder paste use. More components and more solder paste result in more outgassing of chemistry during the reflow process. Some assemblies report condensation of vapors when the cold printed circuit board enters the oven. Little is known about the interaction between these condensed materials in terms of the interaction between these condensed materials and the reliability of the assembly. Apart from the question of reliability, a printed circuit board contaminated with a small film of residues after reflow soldering is not desirable.

This study investigates the evaporation of chemicals during the reflow process from preheating until cooling. Different solder pastes are compared with respect to outgassing. Residues collected in the reflow process were submitted to Thermal Desorption-Gas Chromatography-Mass Spectrometry (TDGMS) to determine which volatile and semi-volatile substances the residues contained. A test vehicle was made to investigate if these gasses could be eliminated using a catalyst. After a successful investigation, these catalysts were installed in reflow ovens on individual zones and in those areas where the most outgassing was determined. Different catalysts and solder pastes, chain lubricants, and other materials were part of the study.

ENIG, electroless nickel immersion gold is now a well-regarded finish used to enhance and preserve the solder-ability of copper circuits. EPIG, electroless palladium immersion gold, is a new surface finish also for enhancing and preserving solder-ability but with the advantage of eliminating Electroless Nickel from the deposit layer. This feature has become increasingly important with the increasing use of high frequency PWB designs whereby nickel’s magnetic properties are detrimental. We examine these two finishes and their respective soldering characteristics as plated and after steam aging and offer an explanation for the performance deviation.

Comparing the results of steam age test data shows a clear benefit of EPIG over ENIG. After even a short duration exposure to steam, ENIG finishes failed to solder. Much longer steam exposures produced little to no effect on EPIG plated samples. Rapid oxidation of the electroless nickel phosphorous layer when stressed with heat and moisture explains the superior EPIG result.

The ability to demonstrate excellent solder-ability following steam exposure represents increased fabrication reliability under non-ideal storage conditions.

The latest highest reliability requirements demand a high performance electroless nickel and immersion gold (HP ENIG). The new IPC specification 4552A has refocused the industry with reference to nickel corrosion. The interpretation of the existing specification, that judges corrosion on 3 levels, is complex and if misinterpreted can lead to phantom failures. An obvious way to avoid any potential misinterpretation is to eradicate any evidence of corrosion completely. Solderability and environmental corrosion resistance are also highly significant prerequisites when discussing Highest Reliability. These will be evaluated by High Speed Shear testing(HSS)and gas chamber testing respectively.  In addition to the more demanding requirements of the HP ENIG to satisfy High Reliability requirements, the system needs to exhibit good basic layer characteristics. It is also the intention of this paper to evaluate whether there is any ‘value added’ or unintended benefits to a HP ENIG. An example of this would be superior gold distribution and associated gold saving potential. Data generated by Design of Experiment (DOE) will be used to evaluate the impact of electroless nickel variables in combination with traditional and cyanide free immersion gold on recognized quality expectations. The results are expected to be practical and production applicable solution underpinned by solid data and it is hoped that they may dispel myths or misunderstandings within the printed circuit board (PCB)manufacturing environment.

Key words: Highest reliability, Nickel corrosion, Corrosion resistance, Solderability, Black pad

This paper details the realization of a multi-layered, printed, conformable hybrid circuit. The circuit was created using silver(Ag) flake ink as a conductive layer and UV acrylic-based ink as a dielectric layer. Both layers were printed on medical grade thermoplastic polyurethane(TPU) substrates. Traditional surface mount electronic components were attached onto the printed pads to create the multilayered hybrid circuit. Functionality of the hybrid circuit was demonstrated by integrating an embedded microcontroller, a near-field communication(NFC) chip, and a temperature sensing chip. A printed NFC antenna transmitted the measured data from the sensor circuit to a custom mobile device application. The details of the circuit design, fabrication and temperature sensor data are presented in this paper.

Key words: Additive Manufacturing, Component Attachment, Flexible Electronics, Flexible PCB, Flexible Hybrid Electronics, Printed Electronics, Printed NFC antenna, Screen Printing.

As is the case with many other markets where faster, highly capable technologies have resulted in more intelligent processes and products, the medical device sector is also undergoing a “smart” transformation. This has driven the development of medical devices that provide greater access to in-home care and monitoring and faster results for medical professionals, with the overarching benefit of better patient outcomes. Devices applied to the human body that continuously sense and report vital signs in real-time, moisture sensing systems that aid in patient health and comfort optimization, and skin-applied patches with timed drug measurement and release are all smart healthcare realities today. Sensor technologies that bridge the gap between standard, rigid assemblies and more flexible user interfaces are pushing the envelope toward smaller and more convenient form factors, making smart healthcare devices a mainstream reality. This paper will share details about multiple medical sensing applications and the advanced materials and processes used to assemble them for optimal functionality.

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