Baking is always a risk, expensive (energy), time consuming and can do damage to components in several ways. Far better to keep parts dry in the sealed bag or dry cabinet, unless you are dealing with MSL 6 which requires bake before use, no choice but to bake. If by the "spec" you mean j-std-033, whose purpose is to define what it takes to get the moisture out of parts, then from that point of view, baking longer will be fine, the parts will be good and dry.
As for over-baking's effect on solderability, the risk is much greater, especially at bake temp of 125°C. Post reflow solder joint quality should be monitored closely. Also, internal component damage is possible, but this depends on the components and their specifications as to what they can tolerate, storage time at the 125°C temperature.
Paul Austen
Senior Project Engineer, Electronic Controls Design Inc
Paul has been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 45 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, engineer, manufacture, and customer. His focus has been the design and application of measurement tools used to improve manufacturing thermal processes as well as moisture sensitive component storage solutions.
The answer begs the question. Are you asking because you have already baked the parts at the longer stated time & you are questioning whether they are still viable.
Obviously you have gone 20 hours past the spec. Has this caused heat fractures? Internal bond issues?
Component manufacturers spend a great deal of time & money testing & creating specs. My opinion has always been ... stick to the manufacturers specifications... that's why they publish them.
Not being a component engineer I cannot answer the question of have you caused any damage but I think your question should be directed to the manufacturer of the particular part.
Jerry Karp
President, JSK Associates
Based in. Northern California since 1971. Founded JSK Associates in 1979. Actively involved in soldering, cleaning, chemistries. 30 years experience in EOS/ESD control.
I really don't see an issue here. Moisture sensitive product can be baked out more than once, so the accumulated exposure, allowed by JEDEC, is sometimes more than 48 hours, say 96 hours in total if baked out twice. So baking the parts for 68 hours does not present excessive exposure.
Paul J. Koep
Global Product Manager, Alpha
Mr. Koep is responsible for product planning and technical marketing for the Preform Products at Alpha. He is the co-author of several patents in the areas of soldering applications focusing on reflow and alternative methods.
From a soldering perspective, you could have some issues. Depending upon the plating on the leads and thickness of the plating, extra time in the ovens will enhance the development of the tin/copper intermetallics which will impact the solderability of the leads. This will happen with solid state diffusion.
If the leads are gold plated, the porosity of the gold could allow the nickel underlayer plating to passivate and this would degrade the solderability of the lead.
I'm not sure aboutt he electrical functionality of the product itself but I'm sure there will be responses who address this issue.
Leo Lambert
Vice President, Technical Director, EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.
The biggest risk of baking too long is loss of solderability. Many parts today have pure (or nearly pure) tin finishes. At high temperature, the intermetallic (IMC) layer between the tin finish and the underlying metal grows, until finally it consumes the entire tin layer, and solderability is irreparably lost.
Before the IMC layer grows through to the surface, the surface will grow a thick oxide layer which will also impede solderability, though not as drastically as exposed IMC.
Fritz Byle
Process Engineer, Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
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