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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