JF
What you suggest is permissible because you are not mixing flux
residues, having removed the first one before applying the second one.
If you did it without the intermediate cleaning and tried to clean
afterwards, then you would probably be in DEEP trouble because some of
the WS flux residues would be held within the matrix of the no-clean and
would not be removed.
I still maintain that adding a liquid or gel flux to the use of a cored
solder flux, even of the same make and type, is not only dangerous but
even stupid. You are changing a reasonably well controlled process with
minimal but sufficient flux, restrained to where it is needed, to a
totally uncontrolled and uncontrollable process, spread far beyond to
where it is needed and where the thermal characteristics are not maintained.
As an illustration, take an RA flux (but a similar argument applies to
all flux types). This has a rosin matrix activated by, for example, an
amine hydrochloride. At about 160 deg C, the activator starts to
decompose, releasing hydrogen chloride gas which is the reducing agent
which "eats" away metallic oxides on the parts to be joined and on the
solder wire itself. However, decomposition is not complete until the
flux reaches about 250 deg C. Within the confines of a hand-soldered
joint, using a cored wire, the solid rosin itself does not spread far
from the joint, so there is a good chance that most of the activators
are decomposed with a soldering iron temp of, say, 350 deg C. Result:
almost no chlorides left in the residues, except the metal chlorides
from the reaction with oxides, which are held, more or less harmlessly,
as a solid solution within the rosin matrix. There is almost no free
active hydrogen chloride.
Now compare this with adding extra liquid flux. Firstly, there is no
guarantee that it can be applied ONLY in the immediate vicinity of the
joint; most operators will slosh it on in excess in the belief that if a
little is good, a lot is better. Secondly, being liquid, by definition,
the activators are not held in a matrix until all the solvent and
vehicle has evaporated, which may take several minutes at room
temperature. Apply a soldering iron and the solvent will boil as soon as
it reaches about 70 deg C; it will sputter active flux around the joint
over several mm if not cm. As this flux has not reached 250 deg C, nor
will it ever reach it, it will have free unmodified hydrochloride
activators alive and kicking, ready to cause ionic leakage and
corrosion. As the temperature rises further, the extra flux, still soft
by the unevaporated components within the rosin, will spread like mad,
carrying hydrochloride activators away from the joint region. Even
worse, some of it will reach about 160 deg C and the hydrogen chloride
will be partially liberated from decomposition but not be liberated from
the flux soup. Now, hydrogen chloride is hygroscopic and will absorb
moisture from the atmosphere and do you know what happens when this
happens? It forms hydrochloric acid. Now, you may want to take the risk
of having such a compound on your assemblies: I wouldn't.
So, the choice is yours: having a small quantity of innocuous flux
residues in the immediate vicinity of the solder joint OR having a wider
zone of dangerous sputtered and spread flux residues capable of causing
corrosion, ionic conduction and conductive metallic dendrites. It's as
simple as that.
Brian
Bissonnette, Jean-Francois wrote:
> Ok... This may be very basic to a lot of you be here is the question
> anyway...
> What if non compatible fluxes are mixed? Say a board that went through wave
> process with water washable flux, then cleaned properly and finally some
> specific areas are hand soldered with no-clean (...the flux being fully
> activated
> of course). Is there any potential problem here?
>
> What are the scenarios where this could end up in a nightmare... and what
> would be that nightmare?
>
> JF
>
> -----Original Message-----
> From: FOX, Ian (York Rd) [mailto:[log in to unmask]]
> Sent: Thursday, February 27, 2003 10:34 AM
> To: [log in to unmask]
> Subject: Re: [TN] Adding liquid flux when hand soldering
>
>
> As a materials engineer by trade and having worked in the high rel
> electronics (aero) industry since the early eighties, I feel qualified to
> comment on this particular thread. Firstly, I wouldn't expect anyone to
> disagree with the statement that you don't mix flux types, that is cast in
> stone. There are however compatible liquid and plasticized fluxes sold by
> all the usual suspects and we for instance use an RMA QPL listed product
> from Alpha and so my comments reflect this.
>
> In a practical hand soldering operation on an assembly with a reasonable
> thermal demand, the plasticized flux tends to degrade pretty rapidly during
> the initial stages of the hole fill operation and once you have a solder
> plug you cannot introduce additional flux to the still to form joint area,
> you are reliant on whatever plasticizes material managed to get there during
> the initial stages of joint formation. Now with the vagaries of any hand
> soldering operation you will get the occasional little bu**er of a joint
> that won't go right first time. The addition of a drop of compatible liquid
> flux to the joint area before the application of the iron and wire does go a
> long way to overcoming the poor fillers (for want of a better description).
> It is true to say that most joints can be adequately soldered with just the
> original flux core, but it is so much easier with the addition of a drop of
> liquid flux. Provided the two fluxes are compatible, provided the assemblies
> are properly cleaned and verified as such then the use of a small amount of
> liquid flux should not be an issue, although there are always occasions
> where it isn't recommended and the aforementioned switches is one such
> instance.
>
> Ian Fox
> Goodrich Engine Control Systems
>
> -----Original Message-----
> From: Brian Ellis [mailto:[log in to unmask]]
> Sent: 27 February 2003 14:13
> To: [log in to unmask]
> Subject: Re: [TN] Adding liquid flux when hand soldering
>
>
> What do you mean by less heat? A lower temperature? A shorter time? Both?
> All that any of these means is that the conditions to form the optimal joint
> no longer exist. Please do not forget that the formation of a solder joint
> is a chemical reaction whereby (say) tin and copper react to form
> intermetallic compounds. The optimal conditions require a given combination
> of temperature of the tin AND copper over a given time. Reduce either, and
> the joint is no longer optimal. The functions of the flux are numerous, but
> its acidic activity is mainly to reduce metallic oxides so that the tin and
> the copper are clean enough for the metallurgical reaction to take place.
> This oxide reduction occurs mainly during the preheating phase, before the
> copper reaches soldering temperature, so the type or quantity of flux has
> relatively little influence on the speed of the reaction, although a highly
> reactive flux, such as a water-soluble organic hydrochloride type, may just
> reduce the overall time by a tiny fraction of a second.
>
> "In a quick poll of some of my fellow old-timers who worked on military
> stuff all agreed." I guess your "old-timers" must be quite old, from the
> days when soldering was a "seat-of-the-pants" art and not a science, like it
> is today. Any person mixing two flux types for a military application would
> be shot down in flames by the inspectors from the UK Ministry of Defence
> and, presumably, the US DoD or sent out to Iraq to make sure Saddam's famous
> WMD would fail from faulty electronics. What you are proposing is an
> antithesis of reliability. Please quote me a single MIL Spec since Julius
> Caesar's days that said, "Thou art permitted to make whatever mixture of
> fluxes you please." or words to that effect. In all my 50 years of
> experience in this industry, I have never, ever, seen it done: the only
> times I've seen liquid flux used for hand soldering, it was with coreless,
> solid solder wire.
>
> In the very unlikely event that there is insufficient flux in a cored solder
> wire, then I very seriously suggest that, before you jeopardise the
> reliability of your products, you consult the manufacturer of the wire and
> have it exchanged for one with a slightly greater percentage of flux, or you
> change suppliers.
>
> Brian
>
> Phil Nutting wrote:
>
>>Brian,
>>
>>I can't argue with your statements. I can only speak from personal
>>experience (including a test last night) that adding the liquid flux
>>makes a much better solder joint with less heat applied to the joint.
>>In a quick poll of some of my fellow old-timers who worked on military
>>stuff all agreed.
>>
>>Gee, what would Moonman say to this one?
>>
>>Phil
>>
>>-----Original Message-----
>>From: Brian Ellis [mailto:[log in to unmask]]
>>Sent: Thursday, February 27, 2003 3:17 AM
>>To: TechNet E-Mail Forum.; Phil Nutting
>>Subject: Re: [TN] Adding liquid flux when hand soldering
>>
>>
>>If you are using cored solder, you should never add flux. Because:
>>a) liquid flux will spread beyond where you need it and add to
>>contamination
>>b) the chances are the two fluxes will be incompatible with each other
>>c) there is a distinct possibility the residues will be impossible to
>>clean off correctly or, if they are both "no-clean", they will form a
>>dangerous flux porridge which could be ionically incompatible with
>>reliability
>>d) you are insulting the wire manufacturer who has determined the
>>quantity and type of flux, through scientific lab tests, to optimise
>>what you are trying to do.
>>
>>IOW, you will cause far more harm than good.
>>
>>Brian
>>
>>Phil Nutting wrote:
>>
>>
>>>Is it a common practice to add liquid flux when soldering by hand or
>>>do you rely on the flux core of the solder? My 30+ years says add the
>>>liquid flux for the best solder joint, but that was back when I was
>>>involved with rocket science.
>>>
>>>Phil Nutting
>>>
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