I did say ~250°C, not a categorical 250°C. Most 63/37 waves operate
between 240 and 250°C. There are several mechanisms to help in reducing
the activity of "no-clean" flux residues. The most popular is
sublimation. Another is the encapsulation of activators in a resin or
rosin matrix. A third is the chemical decomposition, although I think
this is less commonly encountered nowadays. A fourth is to polymerise
the activator either into a relatively harmless chain or to use them for
cross-linking a prepolymer into a resinous mass. There may even be
combinations of them. Notwithstanding, there is no sich animal as a
"no-residue" flux. Even the proponents of "no-flux" soldering under a
reducing atmosphere, such as hydrogen/methane, will find metallic
compounds where the reduction occurs (for ex., complete reduction of
copper oxide under hydrogen requires a much higher temperature than
soldering temperature).
Conclusion: no matter the type of flux, there will always be some metal
compound residues and some raw and modified flux residues. The aim of
the manufacturer is to make this residue porridge as harmless as
possible. Some succeed better than others.
Brian
Guy Ramsey wrote:
> 250°C ! Only the flux directly in contact with the soldering tools would
> see that temperature. Heck our wave temp isn't 250°C. So, what is the
> physical change? Are you saying then: Low-residue fluxes may not require
> cleaning because the low level of activators remaining are trapped in a
> residue that may contain them safely, in some end use environments?
>
>
>>-----Original Message-----
>>From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
>>Sent: Friday, May 07, 2004 3:57 AM
>>To: [log in to unmask]
>>Subject: Re: [TN] OA / RMA
>>
>>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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