Yes, that is partly the theory, but what of the practice? Actually, the
change is physical, rather than chemical: at soldering temperature, say
~250°C, most (but never all) of the activators sublimate into a vapour
phase (incidentally, as do the organic hydrochlorides in WS flux). But
sublimation is certainly nowhere near complete at lower temperatures.
What of the flux that penetrates through the holes and up the leads,
then spreads out somewhat over the top side of the board? Does that
reach a quasi-complete sublimation temperature? No! Then, as the board
moves through the sublimated vapour, can you guarantee that none of this
vapour will not condense on cooler components? No, I thought not. Then,
if some flux or vapour is trapped under a component with low clearance,
what action is there to ensure that it is evacuated? Then there is the
possibility of hydrogen bonding between activator molecules and the
surface of the epoxy substrate. If this occurs, then the bond will be
stronger than the forces causing sublimation.
Take any board soldered with such fluxes and make an extract from it and
analyse the extract: you will find considerable quantities of activators
present (the quantity will diminish with time, as the sublimation
process will continue slowly, even at room temperature).
Can these quantities be dangerous? Yes, sometimes. There is a well-known
but undocumented (for obvious reasons) case of a European telecoms
manufacturer making street-box multiplexer assemblies on large
four-layer boards, in the late 1980s. After a few months, some boards
started to trickle back with corrosion and malfunctioning. Then the
flood started. Under the conditions of street boxes, the flux residues
started reacting. Incidentally, this manufacturer happened to be using
the same "no-clean" flux as the anecdote I recounted in the previous
post. This error cost the manufacturer something in the seven figures.
As it happens, I was slightly involved because he changed to WS and I
supplied the ionic contamination tester for his subsequent process
control. OK, this was possibly an extreme example of the manufacturer
not properly qualifying his process and materials (I can assure you he
did it well, second time round, even introducing conformal coating for
street box electronics which are, of course, subject to condensation
conditions, on occasion).
Another case I was involved with was a manufacturer of high power
electronics, running on 380 V 3 phase circuits with currents in the
hundreds of amps. He had been using a "no-clean" flux (known make) for
over 2 years, without problems. He then decided to contract the
assembly/soldering to an outside company, also using a "no-clean"
(unknown make) flux. The results were catastrophic (and spectacular). I
was asked to find out why, in the first batch produced outside, whole
conductors on 105 µm thick copper, several millimetres wide, between the
power in connectors and the fuses, had literally volatilised, while the
fuses were intact, after a few days in real service. It transpired that
flux residues under and around the massive fuse-holders had started to
conduct, heated up, sublimated and condensed on the top of the
fuse-holders, where the clearance between them was too small, causing a
flash-over. This illustrates that not all "no-clean" fluxes and their
residues are created equal, even if some are more equal than others!
Brian
Blair K. Hogg wrote:
> Brian,
>
> Interesting story. However, isn't the principle behind NC flux that the flux changes chemically with the heat of soldering and then becomes non-corrsive?
>
> Blair
>
>
>>>>[log in to unmask] 05/04/04 11:08AM >>>
>
> Phil
>
> In all the many tens of installations I have done, never once have I
> used any parts that contact the flux that were not in PVC, PP or
> titanium (notably fingers), other than the sintered plastic foaming
> "stone". Big mistake to use brass or stainless steel, both of which are
> attacked like gangbusters by the flux. I agree that a PVC air knife is
> very useful for adjusting the quantity of flux, before the pre-heat.
>
> Actually, the worst case of corrosion I have seen of a soldering machine
> was not with a WS flux but with a "no-clean" of a particular brand made
> in N. Europe. I was visiting a factory which had been using it for
> several years for telecoms equipment. I "naively" asked the engineer
> whether he was not worried about his boards corroding in service. He
> replied that the flux in question was non-corrosive, bringing forth the
> manufacturer's data sheet as "proof". I then asked him, equally naively,
> how he explained that every bit of iron/steel was practically rusted
> through in a 50 cm radius round his fluxer. He had a good look, turned
> pale and said, "Mon Dieu!"!!! :-( I then asked him what qualification
> tests he had done and he replied that their sole criterion was the
> "no-clean" flux that soldered best. It simply hadn't occurred to him
> that high-activity = high risks. I knew what to expect because I had
> seen other cases with that one, particular, flux. I just checked and
> this same flux is still manufactured!
>
> Brian
>
> Phil Nutting wrote:
>
>
>>Ramon,
>>
>>Brass fittings in the Flux tank were eaten through, rivets on the fluxer assembly were dissolved to the point where they fell out, the stainless steel enclosure of the flux reservoir and fluxer now look rusty and the excess flux dripping off boards into the pre-heat elements have caused flash over arcs that trip circuit breakers.
>>
>>The solutions to date have been to change the brass fittings to stainless steel, the rivets have not been replaced, the stainless enclosures are cleaned more frequently and the air knife has been adjusted for air velocity and angle to reduce the amount of excess flux.
>>
>>These items sound like we have a lot a trouble with OA (or WS), but it yields better boards as Brian Ellis has outlined. I'll take the problems to get better yield.
>>
>>Phil
>>
>>-----Original Message-----
>>From: Dehoyos, Ramon [mailto:[log in to unmask]]
>>Sent: Tuesday, May 04, 2004 8:27 AM
>>To: Phil Nutting; [log in to unmask]
>>Subject: RE: [TN] OA / RMA
>>
>>
>>
>> Thanks Phil:
>> What parts of the machine have been eaten or corroded and what steps have you made to slow down the corrosion?
>> Regards,
>> Ramon
>>
>>
>>
>>>-----Original Message-----
>>>From: Phil Nutting [SMTP:[log in to unmask]]
>>>Sent: Monday, May 03, 2004 2:25 PM
>>>To: [log in to unmask]
>>>Subject: Re: [TN] OA / RMA
>>>
>>>Ramon,
>>>
>>>About 3 years ago we switched from a no clean flux to an OA flux. This change was driven by a change in board washing equipment. We had been using a Terpene & Alcohol based batch system that no one liked and switched to a closed loop DI batch cleaner.
>>>The plus side has been better solder joints out of the wave and cleaner looking boards after cleaning.
>>>The minus side has been that the OA flux really eats up everything in sight in the wave soldering machine. We have accepted this as a cost of business when using the OA flux.
>>>
>>>The good out-weighs the bad.
>>>
>>>Phil
>>>
>>>-----Original Message-----
>>>From: TechNet [mailto:[log in to unmask]]On Behalf Of Dehoyos, Ramon
>>>Sent: Monday, May 03, 2004 2:19 PM
>>>To: [log in to unmask]
>>>Subject: [TN] OA / RMA
>>>
>>>
>>> Hi Technetters:
>>> Has anybody done a comparison study of RMA/OA fluxes for wavesolder? Or any personal experiences in this regard will be appreciated..
>>> Thanks in Advance
>>> Ramon
>>>
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