|Ask the Experts
August 28, 2023 - Updated
April 26, 2010 - Originally Posted
Organic Flux Residue Concerns
What short-term and long-term concerns should we have with product reliability if we skip cleaning after rework
Our rework processes will leave organic flux residues on our circuit boards.
|Expert Panel Responses
There are many variables here so a definitive answer is not possible.
You should first consider the following points:
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.
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 to refer to fluxes based 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 question lies, together with the need to remove the residues and the recommended method.
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 humidity and you have the 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-free flux 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.
Global Business Director conformal coatings division
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.
Technical Manager - Europe
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.
VP Engineering Services
STI Electronics Inc.
With most rework application we are directed to use a No-clean rosin-based flux. When completely activated it is safe to leave on the board and should not require cleaning. The key is that it does achieve the activation temperature through the rework process.
If excess flux is applied then there can be uncertainty as to the activation state of the flux. A safe approach to the rework process is to clean the flux from the assembly utilizing the recommending cleaning process for the specified flux to ensure a reliableprocess.
Circuit Technology Center
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.
Global Segment Manager
Air Products & Chemicals, Inc.
Here is our response, in a short and sweet way:
The two most common failures are:
Application Technology Manager
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.
Expect dendrite growth; Leakage current; and Eventual failure. If product powered by battery, premature battery failure. If board has RF section, RF section and preamp failures.William Simmons, Simmons Patents
If chips have pins that extend under the chip, board should be baked before adding battery. Time to failure is temperature dependent. Chemical reactions are generally faster as temperature increases.
I first would identify the flux residue to be cleaned, and then determine if the residue can and should be cleaned. The problem is in standardization of the process, simply because no two operators have the same technique when soldering or cleaning. In some cases, an operator may use too much flux to solder, causing more residue. If the fluxes are not activated during soldering, those residues must be removed in a thorough cleaning process. Different fluxes will require different techniques, so identification is essential. And when soldering, the amount of flux used must be precisely what the job demands, no more and no less. If soldering cannot be achieved without added flux (more than the amount contained in the cored wire solder) cleaning most likely will be required.Russell Claybrook, MicroCare, LLC
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