Mike

Yes and no. I agree they are less active, as a generalisation, than
liquid fluxes, but they are, by definition, ionic and the residues are
highly hygroscopic. The reason that the joints become matt is not
corrosion of the metal but because the flux residues remove the
refractory oxide on the surface of the metal and it is this that
preserves the brilliance of the joint. As, hopefully, the powder in the
jar is not oxidised and is in an anaerobic environment, the metal is
only very slowly "chewed up" (lifetime of the paste).

However, my other points (polymerisation and hydrogen bonding to the
substrate) are MORE valid with paste residues than with liquid flux
residues, so I still recommend either immediate washing or the use of a
hold tank. And a second heating will only make the situation worse.

Brian

Mike Fenner wrote:
> This is good advise for highly active [low pH] liquid fluxes, but Joel is
> asking about lead free solder paste.
> The activity level in WS solder pastes is much lower, closer to ordinary NC
> types.
> [If the flux in paste were active enough to attack solder it would chew up
> the powder in the jar, making the paste useless].
> Well formulated water wash pastes are able to take a delayed wash cycle
> permitting cleaning to take place after a couple of reflows (one each side)
> or post reflow component attach and repair, thus allowing a single wash
> after all assembly.
>
>
>
> Regards
>
> Mike Fenner
> Indium Corporation
>
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>
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
> Sent: 19 January 2006 08:50
> To: [log in to unmask]
> Subject: Re: [TN] Time before Cleaning WS Flux
>
> As a general rule, water-soluble flux residues are very dangerous. They are
> usually very acid and can attack metals on the assembly. At the best they
> will make the solder joints matt. At the worst, increasing quantity of the
> residues will form adsorption bonds with the substrate and components within
> the residues may polymerise, making it quasi-impossible to clean the
> residues after, say, an hour or so. It therefore is **very** necessary that
> cleaning be started within minutes of soldering. However, there is an
> excellent work-around that has been proven and that can extend the hold time
> before cleaning to a few hours, the use of a chelating neutraliser. Many
> flux manufacturers offer these under trade names such as Neutralizer 2051,
> Rinse-Aid and so on. They are mildly alkaline, buffered, room-temperature,
> dilute baths (pH typically 8.5 to 9), which must NOT be confused with
> saponifiers, which are unsuitable for this job. They serve many functions,
> but the assemblies can stay therein, immediately after soldering/reflow,
> from 2 minutes up to, say, 4+ hours. They will actually improve the cleaning
> quality at almost no cost. The assemblies should be cleaned within minutes
> of their removal from the holding tank. The makers usually recommend a 5%
> solution of the concentrate, but a 2% solution has been proved equally
> effective.
>
> You can see this technique being used on the old video at
> http://www.protonique.com/video/ in the context of wave soldering.
>
> Brian
>
> Joel Mearig wrote:
>
>>I am currently using a water-soluble, lead-free solder paste from
>>Kester. I am trying to minimize the cleaning cycles so I leave flux on
>>the product for an extended period of time. Do any of you have any
>>recommendations on the amount of time flux can be left on the product
>>without affecting the cleaning process? Thanks in advance for your input.
>>
>>
>>
>>Joel Mearig
>>
>>Delta Tau Data Systems, Inc.
>>
>>
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