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physical law of Dewetting

ianchan

#18053

physical law of Dewetting | 1 November, 2001

I refer to IPC-A-610C, section 12.4.5, the defect term "dewetting".

Can the experts pls help define the actual stages that constitute the formation of a dewetting defect?

I do understand the common concept teachings that there may be contamination of the PCB pad in question, or may be due to poor quality solder paste. What I really would like to learn, is how/what really goes on during the reflow zones, that make dewetting "happen"?

what common SOP tatics are employed to rectify such dewetting defects?

Everyone's advise and shared experienced is appreciated, greatly. Thanks.

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#18056

physical law of Dewetting | 1 November, 2001

This is not about the physical laws but besides contamination, which will actually IMO cause NO WETTING, it is quite vital that the time-limit the flux is active is not exceeded during your reflow process. Depending on your paste/flux you will have a certain time the flux remains active and "removes" your oxydes. Once this feature of the flux stops and you�re still not ready with your soldering reoxydation, which is taking place at a much higher rate at these elevated temperatures, can�t be encountered anymore and dewetting takes place. So be sure that you profile in accordance with the paste specs or look for a paste that fits your profile requirements. Nitrogen might help prevent reoxydation but that may be an expensive "crutch" when you have other chances to beat dewetting.

good luck

Wolfgang

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SMT-Engineer

#18061

physical law of Dewetting | 1 November, 2001

I guess this clarifies the defect more then looks at the process, but I�ll just through it out there for the sake of learning something new. If anyone would comment on this please feel free�.

There are many different varieties of dewetting defects. A true case of �dewetting� is when the solder initially wetted to the land and then pulled back leaving some areas that were not wetted. You can see if this is the case through investigation under a tool microscope. The other possibility (although not formally defined) is what I have heard referred to as partial nonwetting? This is when the land was never completely initially wetted but parts of it remain completely nonwetted. Finally, of course, there is nonwetting which is when the base metal was not wetted at all, but this is very rare.

If the condition is truly a dewetting type of defect then your focus should be more on your process and less on your materials. This is because your PCB has already proven its solderability by initially wetting the entire surface of the land, but then your process caused the solder to retreat. If, on the other hand, you have a case of �partial nonwetting?� then you could still be dealing with solderability issues as well as process issues. From first glance both defects appear the same.

Remember in most no-clean and all RMA (Rosin Mildly Active) pastes the rosin/flux is by design not as able to wett the lands as well as the older pastes were.

As Wolfgang had said you need to watch the times on your profile so as not to cook away your flux before it has finished doing its job. Another thing you could look at doing as an alternative to adjusting the times is reducing your soak temperature to achieve the same result.

Hope this helps,

Jonathan

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ianchan

#18076

physical law of Dewetting | 1 November, 2001

Thank you 4 the clarification and classification :)

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ianchan

#18077

physical law of Dewetting | 1 November, 2001

Thanks for the awareness, will look into my profile, and try analyze which type of dewetting r we actually encountering...

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