Book Excerpt: The Printed Circuit Assembler’s Guide to Process Validation


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It is assumed that readers are familiar with manufacturing electronic circuit assemblies in accordance with IPC-J-STD-001 or IEC 61189-1; both are titled “Requirements for Soldered Electrical and Electronic Assemblies.” In October 2018, IPC-J-STD-001 Revision G was amended and released with a new Section 8 of the document titled “Cleaning.” The change was primarily the removal of a “cleanliness” level of 1.56 mg/square cm of NaCl equivalence. This small change has great significance in relation to how companies ensure the electrochemical reliability of their products because many have employed this measure as the sole arbiter of their entire production process.

Given the current trends towards smart infrastructures, connected vehicles, electric vehicles, and their need for ubiquitous charging points, industrial IoT (IIoT), and others, electronic circuit assemblies are deployed into humid and potentially corrosive environments arguably to a greater extent than ever before; hence, they are at greater risk of failures due to electrochemical migration (ECM) in the form of dendrites (Fig. 0.1). On the other hand, the operation of electronic devices with self-heating, the designed housing concepts, and the impact by a large thermal mass, like in automotive applications, are already known and applied countermeasures for lowering the risk of ECM as they are influencing the local humidity on an electronic level.

There are two very distinct processes going on: electrophoretic and electrochemical. Dendrites have been described as a mini plating bath. Electroplating requires a conductive electrolyte, whereas electrophoretic processes tend to be used in a dielectric fluid (although some processes add an electrolyte as well). Initially, the condensate is neither a great electrolyte nor dielectric; therefore, both processes contribute to the corrosion process.

Gen3_book_fig01.jpg
Figure 0.1: Dendrites are formed from different processes.

The electrophoretic separates and migrates ionic particles toward the cathode, a distance between the electric charge depending on the electrostatic force and retardation force of the particles (think DNA). In most cases, the material appears first like white cigarette smoke, centrally between the polarities. Typically, this white material is flux residues dispersed with tin, and the more contaminated the board, the more pronounced this material becomes. When hydrogen gas bubbles are observed at the cathode, this indicates the current flowing and rising pH levels forming a conductive electrolyte; the process is now electrochemical, and dendrites begin to form from the cathode.

The withdrawal of the IPC cleanliness level now encourages the industry to adopt a better and more rigorous approach to ensuring electrochemical reliability. Beginning by examining the original IPC requirement and the reasons for its removal, this book provides a guide regarding electrochemical reliability testing as to surface insulation resistance (SIR) and conductive anodic filament (CAF) testing, and presents the new IEC 61189-5-504 “Process Ionic Contamination Test (PICT)” as a practicable methodology for monitoring process control in volume production.

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