Ask the Experts
February 8, 2018
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:
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.
- Is the flux used for rework no clean ?
- Does the ALL of the flux used reach the correct temperature to both activate and then become inert ?
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 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.
Global Business Director conformal coatings division
Phil Kinner - Electrolube - 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
Currently with Indium Corporation and responsible for technology programs and technical support for customers in Europe. Over 15 yrs experience in SMT, Power, Thermal & Semiconductor Applications. Masters Degree in Industrial Engg, State University of New York-Binghamton.
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.
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.
With most rework application we are directed to use aNo-clean rosin-based flux. When completely activated it is safe to leave on the board and shouldnot require cleaning. The key is that it does achieve the activationtemperature through the rework process. If excess flux is applied then therecan be uncertainty as to the activation state of the flux. A safe approach tothe rework process is to clean the flux from the assembly utilizing therecommending cleaning process for the specified flux to ensure a reliableprocess.
Circuit Technology Center
Mr. Price has been a key member of the team at Circuit Technology Center since 1985. He has vast expertise, experience and understanding of complex circuit board rework, repair and modification operations. He is one of the most knowledgeable experts in this area across the globe.
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.
Mr. Arslanian has been involved in electronics packaging processing and equipment since 1981 including flipchip, TAB, wirebonding and die attach. Current responsiblities include R&D, applications, marketing and customer interaction.
Here is our response, in a short and sweet way:
The two most common failures are:
- Electrochemical Migration which results in dendrite growth and failure of the assemblies
- Contamination Induced Leakage Current.
Application Technology Manager
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!
Jim Willimas is a PhD Chemist in Polymers and Materials Science. He specialize in printing, cleaning, inks, and coatings used in electronics manufacturng operations. Williams has more than 30 years experience.
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.
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.
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. 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.
William Simmons, Simmons Patents