Electronics Assembly Knowledge, Vision & Wisdom
Test Methods for Electrochemical Consistency in PCB Assembly Processes
Test Methods for Electrochemical Consistency in PCB Assembly Processes
This paper will review traditional and emerging methods to characterize flux residues and cleanliness of finished assemblies.
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Authored By:
Brook Sandy-Smith
Indium Corporation

Summary
There are several industry-accepted methods to determine the electrochemical reliability of electronic assemblies. These methods are typically designed to either simulate humid environments in accelerated lifetime testing, or assess the species of ionic residues present on surfaces. Some can be used to test prototypes or test boards, while some are more applicable for quality and consistency testing in a production environment. The increasing complexity of high-density assemblies, along with low standoff components, imposes greater associated challenges related to assessing electrochemical reliability. This has led to the development and adoption of new methods for testing ionic residues that can lead to electrochemical migration.

This paper will review traditional and emerging methods to characterize flux residues and cleanliness of finished assemblies, including surface insulation resistance, electrochemical migration, ROSE extraction, and other emerging test methods. Data will be shared to reveal how different methods can detect process variations in different ways, and compare the results.

With new revisions to IPC J-standard-001 and proposed changes to Section 8, the philosophy for ensuring assemblies are clean after a cleaning process has evolved. The way we look at the consistency of no-clean process residues for process control may also be evolving.

Conclusions
Process qualification is critical to ensure a reliable end product. Since the testing includes SIR on a test board with components, along with process control methods and IC to characterize any suspicious ionic species, it is an efficient way to tie reliability testing to an everyday process control plan. The inclusion of all material selections including solder mask, surface finish, and solder materials, along with processing aligned with a true production environment yields results that mimic true assemblies as closely as possible.

In order to be successful in characterizing a new process and design, the test method details are critical. Board fabrication, test coupon preparation, solder application, and reflow profiling are all critical to achieving accurate results.

Although the experiment discussed in this paper did not yield passing results, meaningful lessons can be taken away. Ongoing testing will be conducted with a high level of attention to detail to procedures and incoming board inspections. Most importantly, additional work will be done to identify board materials that will standardize the test, enabling use for future solder material performance comparisons and process window characterization.

Initially Published in the SMTA Proceedings

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