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PCBs Are Moisture-Sensitive Devices
September 7, 2018 | Richard Heimsch, SUPER DRYEstimated reading time: 3 minutes
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.
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