PCBs Are Moisture-Sensitive Devices


Reading time ( words)

Guidelines for the proper storage, handling and moisture protection of electronic components can be found in the standards IPC/JEDEC J-STD-033C. Though these date back to 1999, there were no published standards for storage and moisture protection for printed boards until 2010, and their proper handling is still often overlooked. But with the correct storage control and the use of suitable drying methods, considerable manufacturing advantages can be gained: PCBs will remain solderable for much longer and damage during reflow due to moisture can be eliminated.

Historically, the printed board industry relied on military specifications and guidelines to define the packaging methods used to preserve the quality and reliability of PCBs during shipment and storage. Over time, of course, many of these documents became obsolete, were found to be incomplete, didn’t address leadfree assembly, or did not provide guidance for newer laminates or final finishes. Additionally, the proliferation of alternative final finishes has produced concerns and requirements for printed board packaging and handling to preserve the finish and assure good solderability.

For instance, IPC-1601A (2016 revision) Printed Board Handling and Storage Guidelines, states: “Baking is not recommended for OSP coatings, as it deteriorates the OSP finish. If baking is deemed necessary, the use of the lowest possible temperature and dwell time is suggested as a starting point.”

Organic solderability preservative (OSP) coatings are among the leading surface finish options in lead-free soldering because they provide an attractive combination of solderability, ease of processing and low cost. Compared to alternatives, however, they tend to be the most prone to oxidation. The cause for this lies in the pure copper surface protected only by the OSP coating layer. Under normal climatic conditions in a manufacturing process, after only a few minutes there will be a separation of a water film at the surface (3-5 atom layers). This then starts a diffusion process which leads to a vapor pressure balance through the OSP coat. Baking also accelerates solid diffusion between metals and increases intermetallic growth. This can lead to a “weak knee” or other solderability issues if the intermetallic layer reaches the surface and oxidizes. Effects upon other finishes (immersion tin, immersion silver, ENIG) are further detailed in the guidelines. 

IPC-1601A (2016 revision) also states: “If process controls are ineffective, and printed boards have absorbed excessive moisture, baking is the most practical remedy.” It goes on to state, “However, baking not only increases cost and cycle time, it can also degrade solderability of the printed board which requires extra handling and increases the likelihood of handling damage or contamination. In general, both the printed board fabricator and the user should strive to avoid baking by practicing effective handling, packaging, storage, and process controls…” 

In addition to moisture management at key steps in the fabrication process, IPC-1601A also makes clear that boards should be protectively packaged to limit their exposure to ambient humidity during processing and storage. And, importantly, packaged only after determining that their moisture content is below the maximum acceptable moisture content (MAMC) level, which is typically between 0.1% and 0.5% moisture weight to resin weight. Just as with components, 125°C baking temperatures degrade the solderability of PCBs. IPC-1601A warns that as little as 4-6 hours at that temperature can render HASL finished boards unsolderable. Over the decades that passed since the J-STD-033 standard was created, new technologies were developed and proven to safely reset component floor life using low temperatures and ultra-low humidity without requiring extensive time. These 4060°C and <1% methods were first adopted in Europe, and their recognition and use has now spread to North America.  

To read the full version of this article which originally appeared in the August 2018 issue of PCB007 Magazine, click here.

Share

Print


Suggested Items

Aurora Circuits on Ultra-Heavy Copper PCBs

08/03/2020 | Dan Beaulieu, D.B. Management Group
It’s always fun to talk with a company that can do something different—in this case, ultra-heavy copper PCBs, meaning over 20-ounce copper. Wanting to know more, Dan Beaulieu talked to Aurora Circuits Director of Business Development Thad Bartosz.

Simulation Technology in Acid Copper Pattern Plating

07/17/2020 | Pete Starkey, I-Connect007
PCB designers and CAM engineers may feel there is little they can do to achieve uniform finished copper thickness of PCB traces and holes. But in an illuminating discussion with Robrecht Belis, manager of surface finishing and e-coating with Elsyca NV in Belgium, Pete Starkey learned that nothing could be further from the truth, provided they act on time and use the right tools.

A Year in Review: Cultivate New Opportunities in Crisis, Start Fresh

07/16/2020 | I-Connect007 China Team
Recently, the China Electronic Circuit Industry Association (CPCA) invited Dr. Shiuh-Kao Chiang from Prismark to present an online video report regarding the current and future impact of the current epidemic on the global electronic circuit industry. The I-Connect007 China Team attended the presentation, and the following report summarizes some of Dr. Chiang’s remarks.



Copyright © 2020 I-Connect007. All rights reserved.