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From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Tue, 12 Jul 2016 15:29:13 +0000
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I have been using unfluxed copper braid for years with absolutely no evidence of any solder wetting issues afterwards. But the new parts are almost always put on and soldered during the same shift.



When used properly I don't believe there is any chance of CuSn IMC forming on the pad.	



The wrong method is to lay down a dry braid directly onto the pad and try to melt and wick the residual solder through the braid, applying pressure with the iron tip. I can see how this improper method would allow a lot of copper to dissolve into the molten solder initially, and be left behind on the pad surface.

The right method is to pre-flux a small portion of the end of the unfluxed wick with the program-approved tacky flux, cut the braid at a slight angle and "sweep" the molten solder away with just the end of the braid. The braid is applied to the side of the solder iron tip  to transfer the heat. The iron tip is applied directly to the pad and only the very end of the wick sweeps (wicks) the molten solder away.



I never allow the use of solder braid purchased pre-fluxed for three reasons;



1) The prefluxed braid is often the wrong chemistry, and mixing chemistries is an absolute no-no, ie, using a no-clean pre-fluxed braid on a CCA that is soldered with water-soluble flux and where an aqueous wash is performed afterwards. The no-clean residues will remain on the CCA. 

2) Vice-versa. It is very difficult to control solder braid prefluxed with RMA, OA, or NC fluxes. They WILL be used improperly, either out of ignorance or the operator's personal preference, or lack of control.

3) Over time the prefluxed braid oxidizes, and typically displays a green or white tint. When used, those oxides transfer onto the pad and affect solderability.



As for solder suckers, well, let's just say they suck and leave it at that.



Odin



-----Original Message-----

From: TechNet [mailto:[log in to unmask]] On Behalf Of David Hillman

Sent: Tuesday, July 12, 2016 8:55 AM

To: [log in to unmask]

Subject: Re: [TN] Using high temp solder after removing standard solder



Hi folks - I agree with George's assessment. The critical parameter is not letting the IMC oxidize. Once the original solder is removed as George detailed, then re-tinning with the 95/5 alloy right away is key process step.



Dave Hillman

Rockwell Collins

[log in to unmask]



On Tue, Jul 12, 2016 at 8:50 AM, George Wenger <[log in to unmask]>

wrote:



> Hi Mary Grace,

>

> If your Sn95/Ag5 solder wets the wiring and components after SN63/Pb37 

> removal I wouldn't be concerned about the integrity of the solder joint.

>

> However, the concern I would have is the ability for the Sn95/Ag5 

> solder after wicking away the standard solder. I'm not a real fan of 

> using solder wick. When you solder to a component that has copper 

> terminations the "glue" that keeps the solder attached to the copper 

> is copper-tin intermetallic compound (IMC) that forms between the copper and solder.

> Solder wick is a very good means of removing all of the molten solder 

> from a copper termination and it leaves behind the copper-tin IMC that 

> doesn't melt at soldering temperature. So if the reason you are 

> removing the

> SN63/Pb37 solder and re-soldering with Sn95/Ag5 solder is to make the 

> solder joint Pb-Free then solder wick is the method you would want to use.

> The other method to remove the old solder would be to use a "solder 

> sucker". This method sucks away a lot of the molten solder (i.e., 

> takes off most of but not all of the molten solder) and leaves a small 

> amount of old solder on top of the copper-tin IMC. When you re-solder 

> the Sn95/Ag5 should wet the component termination very well. If you 

> removed the Sn63/Pb37 solder with a solder wick the copper-tin IMC is 

> then on the outside surface of the component termination and is 

> exposed to the environment and it can readily oxidize. The problem 

> with oxidized copper-tin IMC is that the oxidized IMC can't be reduced 

> with standard soldering fluxes and the surface will not wet well with 

> solder. It may be that you can wick off the old solder and re-solder 

> immediately with the new solder and get good wetting because the IMC 

> hasn't had time to oxidize. If you do soder wicking at one station and 

> then at some later time do re-soldering at another station you will probably find solderability issues.

>

>

> Regards,

>

> George Michael Wenger

>

> George M. Wenger

> Failure Signature & Characterization Lab LLC

> 609 Cokesbury Road, High Bridge, NJ 08829

> (908) 638-8771 (Home) (732)-309-8964 (Cell) [log in to unmask]

>

> ----- Original Message -----

>

> From: "Mary Grace Keenan" <[log in to unmask]>

> To: [log in to unmask]

> Sent: Tuesday, July 12, 2016 8:15:08 AM

> Subject: [TN] Using high temp solder after removing standard solder

>

> I have a question regarding using the high temp solder after removing 

> standard solder Sn63/Pb37. After wicking away the standard solder, we 

> need to re-solder the wiring and components using the high temp solder 

> (Sn95/Ag5). We are concerned that the integrity of the solder joint 

> and the temperature rating may be negatively affected. Is this a 

> legitimate concern? Any suggestions?

>

>

> Regards,

> Mary Grace Keenan, Quality Assurance Manager Taber Industries

> 455 Bryant St.

> North Tonawanda, N.Y. 14120

> [log in to unmask]

> Phone (716) 694-4000 Ext.134

> Fax (716) 694-1450

>

>

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