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January 2006

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Subject:
From:
Mike Fenner <[log in to unmask]>
Reply To:
Date:
Fri, 20 Jan 2006 12:02:49 -0000
Content-Type:
text/plain
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text/plain (188 lines)
Thanx Brian.
I can only now speak from experience of our own products:
If there is a difference following delayed washing or multiple reflow, it is
smaller than we can detect [Using present norms/testing criteria.]


Regards

Mike

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
Sent: 20 January 2006 10:16
To: [log in to unmask]
Subject: Re: [TN] Time before Cleaning WS Flux

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
>
> T: + 44 1908 580 400
> M: + 44 7810 526 317
> F: + 44 1908 580 411
> E: [log in to unmask]
> W: www.indium.com
> Pb-free: www.Pb-Free.com
>
>
>
> -----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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