There are many variables here so a definitive answer is not possible.

You should first consider the following points:

1. Is the flux used for rework no clean ?
2. Does the ALL of the flux used reach the correct temperature to both activate and then become inert ?

Organic flux needs to reach a certain temperature to firstly activate and then become inert.

The main problem with using flux in a rework situation is that it is difficult to achieve the correct temperature for all of the flux used and given that operators vary in their techniques you cannot have a repeatable process.

Potentially active organic flux residues left on a circuit board will probably cause reliability issues, especially if moisture is involved.

The potential problems will be related to conductive paths (dendrites) created between pads or tracks where a DC voltage differential exists.

Chris Palin European Manager, HumiSeal Chris Palin is currently managing European sales and support for HumiSeal Conformal Coatings. His expertise is in test & reliability, solder technology, power die attach and conformal coating.

It depends on the nature of the "organic flux residues."

Pure rosin can be described as "organic" but its residues would not be expected to create any problems.

On the other hand, the term "organic flux" is usually understood torefer tofluxesbased on organic acids. The residues of these fluxes are potentially corrosive and, if not completely removed,could create current leakage problems in the short term and catastrophic failures in the long term.

Consultation with the supplier of the flux should establish where between these limits the flux in questionlies, together with the need toremove the residuesand the recommended method.

Harold Hyman Consultant, VJ Electronix Harold Hyman has been involved in metallurgical aspects of the electronics industry since the 1950's, and in semiconductor development and engineering for STL, Ediswan & RCA. He later joined HTC, a pioneer of vapor phase soldering and continued industry experience Dynapert, GenRad, Teradyne, SRT and VJ Electronics.

The big issue here is that the organic flux residues you describe are likely to be at least partially water-soluble and of course acidic (and maybe even contain halides - really bad news!).

Add in dissimilar metals, ambient temperature and humidityand you havethe perfect pre-requisite conditions for galvanic/dendritic corrosion to occur with susceptible metals.

The use of a conformal coating will delay the onset of this corrosion mechanism for an indefinite period of time, related to the adhesion between the coating and the board substrate, by requiring that the swelling of the residue as it absorbs moisture (whilst the coating will significantly reduce moisture reaching the residue, it won't completely eliminate it over a long period of time, nothing will... even metal and glass are permeable over a long enough period of time)causes a blister that delaminates the coating from the substrate).

Make sure you use the absolute minimum amount of the least active, halide-freeflux that will enable you to solder your parts. Good technique from your operators and components that wet well (excellent solderability) will facilitate this, to try and mitigate the effects as much as you can.

Phil Kinner European Sales Manager, PVA Inc Phil Kinner is European Sales Manager of PVA, Inc.

No-clean rosin-based rework fluxes should not need any cleaning as long as the flux gets completely activated. Typically a flux gets activated with heat.

In a rework operation, the person could "pour" a lot of flux thinking that this helps soldering. Also, when using a solder tip and wire for soldering, flux surrounding the solder joint may not see enough heat and therefore may not activate completely.

This is a potential reliability concern as unactivated flux could cause dendritic growth and corrosion. Using no-clean rework fluxes that pass SIR unactivated ensures that even if the flux does not activated during soldering, there are no concerns with dendritic growth & corrosion.

If a water-wash rework flux is used, it has to be cleaned off after the soldering operation.

Karthik Vijay Technical Manager, European Operations, Indium Corp. Karthik Vijay is the Technical Manager for Indium Corporation's European Operations. He is based in the UK and is responsible for technology programs and the technical support function for Indium Corporation's customers in Europe. His expertise is focused on solder paste, engineered solders, thermal interface materials, and semiconductor-grade electronics materials.

You have to ask, what is left behind if I do not clean the board after rework. If the flux is a no clean, then there is a good chance that the residues will not damage the board. Any other type of flux may leave a reactive component that combined with moisture will attack the copper traces on the board over time.

There are other considerations, is the board going to be conformal coated? Will coating stick to the board or will the flux stop the coating adhering to the board. Does the board have to pass any form of ionic testing for cleanliness?

Think about what is left behind and what the next step is for the board and work with your flux supplier to understand the implications and this will guide you.

Steven Adamson Market Specialist, Nordson ASYMTEK Market Specialist for Nordson ASYMTEK. Mr. Adamson worked for Kodak, Motorola and Plessey, ICL in the UK with 5 US and 2 UK patents. He was awarded a HNC in electrical engineering and was 2008 President of IMAPS. Mr. Adamson was a respected mentor in the electronics industry. He passed away October, 2011. Learn about the Steve Adamson Memorial Annual Scholarship Fund.

The answer lies in what type of organic residue is left.

My recommendation is to have an ion chromtagraghy analysis performed to determine what ionics are left on the board and if they are active.

Three types to test for anion - weak organic acids - cations

Each category has elements that promote corrosion

Last question is the flux type - no clean - water soluble - rma type flux systems - also what activation level - ROLO or greater.

Ion chromotagraphy will determine the active ions present and answer your question.

Mark McMeen VP Engineering Services, STI Electronics Inc. Mark T. McMeen is STI Electronics Inc.ʼs Vice President of Engineering Services. He oversees the daily operations of the Engineering Services division of STI. He has over 18 years experience in the manufacturing and engineering of PCBs.

The greatest concern to leaving the flux residue on the board is corrosion and the board failing during operation. Another concern that has been voiced in the electronics assembly community is that flux residue and humidity can combine to induced tin whisker formation if you are using a lead-free solder.

The best way to reduce the rework is to investigate ways of improving your first pass yields, such as tighter control on the reflow or wave process from screen printing to component placement and the possible use of a low residue flux system with an inert atmosphere in the reflow or wave process.

Here is our response, in a short and sweet way:

The two most common failures are:

1. Electrochemical Migration which results in dendrite growth and failure of the assemblies
2. Contamination Induced Leakage Current.
   
   

Umut Tosun Application Technology Manager, Zestron America Mr. Tosun has published numerous technical articles. As an active member of the SMTA and IPC organizations, Mr. Tosun has presented a variety of papers and studies on topics such as "Lead-Free Cleaning" and "Climatic Reliability".

My answer begins with "It depends..."

Flux residues can contain relatively benign carbon-based materials( i.e. organics left behind or resulting decomposition from thermal effects), or not so benign (i.e. traces of chloride or other halogen salts which may may or may not interact with the organics residues, to cause corrosion over the long term).

Perhaps it's my grandmother talking, but "cleanliness is best" ...  I would urge cleaning, cleaning, cleaning!

Of all of the flux types, OA (Organic Acid) flux is the absolute worst to leave on an assembly. OA flux residues remain conductive and corrosive after reflow. OA fluxes are among the most aggressive of the common flux types.

In fact, in most assembly environments, OA flux residue is normally removed within one hour of reflow.

Bottom line ... Remove the flux residue shortly after reflow.

Mike Konrad President, Aqueous Technologies Mr. Konrad has been in the electronic assembly equipment industry since 1985. He is founder and CEO of Aqueous Technologies Corporation, a manufacturer of automatic de-fluxing equipment, chemicals, and cleanliness testing systems.