Conformal Coating Testing

“Conformal Coating Evaluation for Use in Harsh Environments Utilizing a Modified Sir Test Protocol”
Author: Casey H. Cooper; This e-mail address is being protected from spambots. You need JavaScript enabled to view it .
Abstract: The deadly combination of moisture (humidity), electrical potential (voltage bias), and ionic contamination (residue) is enough to create electrochemical failures such as dielectric failure and current leakage – both of which result in degradation in performance, if not complete failure, of the electronics hardware. The surface insulation resistance (SIR) test is a common standard used to evaluate effects of assembly materials and manufacturing processes (i.e., residual materials) for hardware operating in hot, humid environments. Although many test specifications and protocols exist, a more aggressive test protocol is required for high-reliability electronics to determine if the materials and processes used to manufacture the assembly will result in current leakage or dielectric breakdown between conductors. This paper studies a custom test protocol designed to environmentally stress screen conformal coatings for their ability to withstand moisture ingression and protect the conductors from moisture-related failures such as corrosion and dendritic growth formations. Analysis of data captured from continuous voltage monitoring (for dendritic growth/shorts), in addition to traditional SIR testing, is sufficient to evaluate and identify the optimal material set and process parameters to manufacture electronics for operation in harsh environments. (SMTA Pan Pac Symposium, January 2011)

Connectors

“Assembly and Rework of Large Surface Mount Connectors with Wafers”
Authors: Phil Isaacs, Sven Peng, Seow Wah Sng, Wai Mun Lee and Alex Chen; This e-mail address is being protected from spambots. You need JavaScript enabled to view it .
Abstract: The migration to more complex surface mount connectors creates new challenges in assembly and rework. The difficulty manifests itself through a number of factors: The increase in connector body size; the increase in mass; a significant increase in the number of contacts, or leads; the increase in lead density, and the number of rows of leads that are now hidden by the connector housing from inspection or from touch-up all make the assembly and rework of the connector much more challenging. The use of advanced vapor phase reflow equipment is a key element to the success of this process. This paper discusses how to assemble this type of connector, how to rework the connectors and how to ensure compliance to all connector requirements without sacrificing solder joint reliability and performance. (SMTA Pan Pac Symposium, January 2011)

Signal Integrity

“Methods for Discovering and Mitigating Switching Regulator Noise Coupling”
Authors: Amy Luoh, Xiaoning Ye, Jiangqi He, Muhammed Elgousi and Yuan-Liang Li
Abstract: Our server board design flow has been required to address increasing occurrences of reduced signal margin, memory failures, unreliable booting and system hangs. This paper describes multiple voltage regulator (VR) noise coupling mechanisms observed in server board designs and the application of various analysis methods to discover the root cause and mitigate VR noise coupling issues. Comparisons for multiple designs of analysis versus lab measurements and also near field scanning measurements, which have discovered further potential coupling mechanisms, are shared. (DesignCon, February 2011)
Solder Joint Reliability

“Damage Accumulation in Pb-Free Solder Joints for Complex Loading Histories”
Authors: Peter Borgesen, et al; This e-mail address is being protected from spambots. You need JavaScript enabled to view it .
Abstract: The long-term life of solder joints in service is commonly assessed by extrapolation of the results of accelerated tests such as thermal cycling or vibration, according to some model or analytical expression. What is commonly ignored is that realistic service conditions are almost never well-represented by cycling with a fixed amplitude, even if much lower. A limited number of efforts have shown life under alternating or simultaneous thermal cycling and vibration to not obey Miner’s rule of linear damage accumulation; i.e., it could not easily be predicted based on measurements of life in the two types of cycling individually. This was ascribed to differences between crack initiation under the two types of loading, but in the case of Pb-free solder joints, significant effects must also be associated with differences in recrystallization. Strong deviations from linear damage accumulation were also observed for sequences of a single type of loading with different amplitudes. Worse, preconditioning by cycling with a particular amplitude did not only lead to a very different life than predicted by Miner’s rule in subsequent cycling with other amplitudes, it also affected subsequent acceleration factors. We show how predictions may overestimate life in service by orders of magnitude. Based on comprehensive research into the nature and behavior of realistic SAC solder joints, when and how current damage accumulation concepts must fail to account for life under various combinations of loading is discussed. This includes predictions of consequences for assessments of life under realistic service conditions. (SMTA Pan Pac Symposium, January 2011)

This column provides abstracts from recent industry conferences and company white papers. Our goal is to provide an added opportunity for readers to keep abreast of technology and business trends.