Question: We are having an issue with the cleanliness of flux. The excess no-clean flux is located in an Ethernet port connector. The units pass all tests with no issues, however; we have recently had a couple of field returns where it was indicated that the units were having intermittent connections. We have made a decision to remove all flux from the hand soldering operation and inspect all finish goods. IPC-A-610 Section 10.4 is very vague about this issue. Can you comment on this?
Answer: We have to find out how where the additional flux is being added. Is it coming from a spray flux application, like a wave solder operation? Is it additional flux which was added during a manual soldering operation?
I found the paragraph you were referencing in Rev D of the 610 document. In Rev E of the document it is in section 10.6.4 Cleanliness – Flux Residues – No-Clean Process – Appearance, which is in section 10.4.4 of Rev D, but it has been updated in Rev E.
The last bullet Defect Class 2, 3 has been changed to Class 1, 2, 3, in the Rev E book, therefore; your condition is a defect condition and needs to be rejected and dispositioned.
Why did this happen? Low-solid content flux is designed so the activators volatilize off when exposed to soldering temperatures, which in this case are either at the wave soldering system or the soldering iron. When additional flux is added or the existing flux has not reached soldering temperatures, it is still active and corrosive. What is happening is the flux is causing a film condition within the connector and preventing a good contact, thereby creating an intermittent connection.
Take a look at the process. Look at where the flux may be coming from and make the appropriate changes to prevent the flux from getting into the connector. Secondly, do not apply additional flux to make the solder flow better, as this flux will never reach soldering temperatures and be problematic when the product is shipped to its operational life.