Question: We have some PCB assemblies that show incomplete solder flow due to signs of flux exhaustion.
Is the incomplete reflow of solder paste as per Section 5.2.3 of IPC-A-610 a defect requiring rework even if the fillet appears to meet IPC standards?
Answer: I’m not exactly sure of what you mean by flux exhaustion, however; my interpretation is, there wasn’t 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 a solder ball grain structure on the surface of the solder paste, this is due lack of enough applied heat, which will prevent the solder paste becoming 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 this may be happening 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 items to consider. One is to review the artwork to determine the metal load on each pad. Secondly, review the thermal profile for the particular product being processed to verify the proper temperatures were met during the reflow cycle.