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August 12, 2014

Cause of Incomplete Solder Reflow

Cause of Incomplete Solder Reflow
We have some PCB assemblies that show incomplete solder flow due to signs of flux exhaustion. Is incomplete reflow of solder paste as per Section 5.2.3 of IPC-A-610E a defect and would require rework even if the fillet appears to meet IPC standards? See image.

R.S.

Experts Comments

Iím not sure the exact meaning of flux exhaustion, however my interpretation is, there wasnít enough flux in the solder paste to prepare the pad/land area and have the molten solder flow over the complete pad or land area itself. This would be related to percentage flux to metal ratio content of the paste rheology.

As for the solder paste not reflowing and appearing as the solder balls grain structure on the surface of the solder paste, this is due to not enough applied heat which will prevent the solder paste to become totally molten and coalesce forming a homogeneous molten solder volume. If the solder paste appears to not have reflowed and is as identified in IPC-A-610 section 5, the issue is that the proper intermetallic has not formed at both the component and pad/land surface interface and this is a defective condition.  

The reasoning of why it may happen on only a few pads of a particular component is most likely due to the heat sinking characteristics of the pad/land area. It may be connected to inner layers of the circuit board or it may have a buried via within the pad, which is heat sinking the heat away from the solder paste to make a complete solder joint. 

I would suggest two issues, one review the artwork to determine the metal load on each pad and review the thermal profile for the particular product being processed to verify the proper temperatures were met during the reflow cycle.
image
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.
It's very hard to tell from the tiny photo, but it looks like the roughness is confined to the solder on the land, and not so much on the joint. The key here is whether the issue will affect the mechanical reliability of the joint. If the joint is properly wetted and formed, that means there was sufficient flux activation to achieve the desired solder joint formation. If the shape and extent of the fillet meets the requirements, I would regard the roughness on the solder remaining on the land as a process indicator.

All that said, if you are seeing this, obviously some process correction needs to take place. The first thing to do is to determine if this is really flux exhaustion, or if it is due to insufficient temperature. A profile should tell you right away, since flux exhaustion is due to a "too hot, too long" condition, the opposite of a too-cool condition.
image
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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