|Ask the Experts|
August 17, 2020
No-Clean Flux Residue After Selective Soldering
After selective soldering connectors there is a small amount of no-clean flux residue left behind. This minimal amount of residue is still gooey, but is only visible under a microscope. It cannot be seen with the naked eye.
Is there any objective quality standard I can use to judge how much flux residue can be left behind?
Is there any evidence supporting the claim that no-clean flux residue degrades reliability in electronics?
|Expert Panel Responses|
You need to determine the active species left in these residues in order to begin to make a decision on long term reliability. You will need to have Ion chromatography performed on the connector area and determine limits of anions and cations present.
There are industry guidelines relative to the more active species of anions such as Cl and Br. Once you have these values then you need to evaluate end use environment and if these residues will be exposed to moisture, from the atmosphere or any other pollutants that could facilitate electrolytic corrosion cells setting up that will result in ultimately the demise of the finished product.
The fact that the residues are still "gooey" is also a concern relative to picking up detritus that could short out between pins. There are no free lunches in this one and it might be less expensive to change fluxes whereby the residues are not sticky and totally benign.
S T and S Testing and Analysis
If the No-Clean flux has been fully activated (experienced a temperature cycle commensurate with the flux or paste manufacturer's solder profile recommendation), then the residue should be benign. Make sure that the outer extremities of the flux residue have indeed met the minimum required temperature to fully activate the flux.
If it is sticky after proper soldering and without the addition of a "cleaning" agent, I would choose another flux or paste that dries to a thin, brittle, transparent film. Gooey residues will gum up test probes and attract/adhere dust particles and may be inadvertently transferred to connectors or gold fingers through handling.
It is important to know that the No-Clean chosen has been tested by the flux manufacturer to be relatively free of ionic contaminants after reflow and passes industry-standard surface insulation resistance and electrochemical migration tests.
We have to go back to the SIR and ECM of the flux residue. Liquid/wave fluxes are tested pattern up and pattern down. If a flux passes the test both pattern up and pattern down we should be able to presume that the residue is electrically safe to leave on the circuit board, regardless of the quantity.
Both pattern up and pattern down are necessary to ensure that the action of the wave and the absence of it do not degrade surface insulation resistance.
The issue is not "quantity" of flux at all, but the fluxes "qualities". If the flux residue is a dielectric material then it is a dielectric under all quantities.
Technical Support Engineer
There is no industry standard that details how much flux residue can be left behind. The amount of no clean flux residue left after selective soldering is dependent upon the amount of raw flux applied and the time and temperature of preheat and soldering. Most flux manufacturers recommend applying a certain mass of flux per unit area, and this is fairly easy to measure.
If the correct amount is applied and the flux is heated properly through soldering then the residue should not affect the reliability of the circuit board. Copper corrosion, SIR and electrochemical migration testing are done per J-STD-004 to validate this.
Selective soldering operations are susceptible to incomplete heating of the flux due to the localized nature of the soldering. Flux that has been incompletely heated may still contain active ingredients that can corrode metals or cause dendritic growth.
Some flux manufacturers have tested fluxes in raw (unheated) state and have declared that the residues are safe to leave on the circuit board. These type of fluxes do not need to be heated to pass corrosion, SIR and Electrochemical migration testing. Your flux supplier should be able to tell you if the flux residue is "safe" in it's raw state.
Failure analysis labs have data suggesting that no clean fluxes can cause electronic failures. NTS (Trace Labs) and Foresite are two such laboratories which could provide information on this.
If your no-clean flux residues are not visible to the naked eye after selective soldering, congratulations you are running a tight process. Sections 10.6.1. & 10.6.4 of IPC-610 provide you with the standard criteria to determine when you have too much residue. There is also evidence and a number of studies stating that no-clean flux residues alter the performance of RF circuits. Other than that, I do not know of any other reliability issues.
Senior Manufacturing Engineer
There are published standards that dictate the amount of ionic residue can be left on an assembly. Many of these standards were established years (or even decades) ago and may or may not be relevant today. It is difficult to state an acceptable contamination level without knowing details about your assembly, its intended climatic environment, the voltage and current traveling through areas with residue, and the cost of assembly failure.
One thing to keep in mind... In prior years, this assembly would be cleaned as a matter of "best practice". The introduction of low-residue fluxes (also called no-clean) caused many assemblers to eliminate a cleaning process all together. Back when cleaning was a normal and required part of the assembly process, all process residues were removed, not just flux.
When much of the industry switched to low-residue fluxes, cleaning stopped altogether. This allowed all residues to remain on the assembly, not just flux residue. As assemblies (and connectors) miniaturized over the years, so did the tolerance for residues. Electrochemical Migration (ECM) failures now occur as a result of less contamination than ever before.
Will your assembly fail? Maybe yes, maybe no. What we do know is without conductive residues (flux and other process residues), ECM cannot occur.
If the no clean SMT paste flux residue is still gooey and connecting pads or pat to via it may very well be moisture absorbing and create a parasitic leakage path even under controlled humidity conditions over time.
I suggest that you run the assemblies powered in 40C 90%RH conditions for a couple days and functionally assess the residues effect on the circuit performance.
President/Senior Technical Consultant
No-clean residues on PCBA after wave solder process primarily consists of resin and weak organic acids and are engineered to be left on the board in place. These residues are tested as per IPC-004B and are deemed electrically safe and do not pose a corrosion concern. Attempting to remove these residues in a bench top operation will not remove the residues completely under most circumstances. This poses several significant reliability risks
a) Most labs cannot/do not test residues in a partially cleaned state. Therefore it's difficult to guarantee they remain electrically and chemically 'safe' once they have been altered by exposure to unknown chemicals
b) Most cleaners (IPA being the most common) will actually reduce SIR values by degrading the resin component of the residue. Resin consists of very large and stable molecules and altering them can liberate unreacted activators that can have unanticipated consequences.
c) Bench top cleaning operations are often poorly monitored and highly subjective. They usually spread contaminates from dirty brushes from assembly to assembly and simply dilute the residues and do not remove them. Insoluble white residues are a common outcome from inadequate cleaning attempts.
To avoid spread of contamination and spreading of flux residues, it is advised to leave the residues unaltered, as tested., and not to be cleaned by any chemicals or brushing.
Based on the product that you are using, you will have more or less no clean flux residue on the board. this is not a defect though so I would not worry about that.
From the inspection perspective, align your magnification with the IPC610 criteria. Just because you are using higher magnification does not mean that what you have found is a problem. following the inspection criteria for magnification will help you be objective and not looking for problems where there are none.
On the other hand the no clean flux residue looks like you've described it after reflow. Again, the presence and quantity depends on the type of the product you are using and also the reflow profile used on the machine.
Engineering and Operations Management
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