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February 2013

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Inge Hernefjord <[log in to unmask]>, TechNet E-Mail Forum <[log in to unmask]>
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Wed, 6 Feb 2013 15:35:24 +0000
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TechNet E-Mail Forum <[log in to unmask]>, "Stadem, Richard D." <[log in to unmask]>
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"Stadem, Richard D." <[log in to unmask]>
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Totally agree. Any gun in the hands of the wrong person....
Plasma cleaners work wonders when all else fails. But you are correct, the operator must be skilled and totally understand the process, correct selection and ratio of gas, flow rate, dwell time, etc. That is why I think it is usually better and cheaper to have the supplier strip and replate, if necessary.

From: Inge Hernefjord [mailto:[log in to unmask]]
Sent: Wednesday, February 06, 2013 9:11 AM
To: TechNet E-Mail Forum; Stadem, Richard D.
Subject: Re: [TN] Silver Sulfide & Plasma


........Plasma will take the finish right off....?????   Are you sure you talk plasma cleaning and not  reversed sputtering? THAT takes the finish, because of very heavy ion bombardment (etching) in opposition to cleaning (reaction) that use much less energy .  I had trouble with the operators, because they did not follow the instructions, but performed plasma operation as they thought was best.  Then it may happen that the finish goes. i have heard about similar problem with other industries, they  buy the machine, install it, but don't send the operators on adequate courses. And later tell the world that plasma cleaners are crap.

Inge


On 6 February 2013 14:53, Stadem, Richard D. <[log in to unmask]<mailto:[log in to unmask]>> wrote:
Hi, Rich
If you have mildly tarnished IAg-finished PWBs, I recommend you print paste on part of one of the PWBs or a complete solder sample coupon and send it through the reflow as a solderability test. They will usually solder out to the edges of the pads just fine. You can then wick off any solder and still use it as a production PWB.

Wayne is correct. Plasma will take the finish right off.

If there are solderability issues, go back to the PWB vendor. There are processes where they can strip off the IAg finish and re-plate the IAg, and I have done this two or three times with no issues. If the PWB supplier does not have this capability, there are others who can do this for you, even if they did not fabricate the PWBs in the first place. Typically the PWB supplier outsources this type of plating, and they can tell you who can re-plate it for you.
I have used IAg PWBs that sat around in a warehouse for 5 years in a previous life, completely exposed to the elements, and had turned a darker shade of pale, so to speak. But they soldered up with a better DPMO than the ones less than 6 months old!
dean

-----Original Message-----
From: TechNet [mailto:[log in to unmask]<mailto:[log in to unmask]>] On Behalf Of Wayne Thayer
Sent: Tuesday, February 05, 2013 5:31 PM
To: [log in to unmask]<mailto:[log in to unmask]>
Subject: Re: [TN] Silver Sulfide & Plasma

Hi Rich-

Mildly oxidized ImAg usually solders fine, even if it doesn't look good.  So try a bit.  If you really need to do something, then I don't think a plasma will help.  There is only a very tiny amount of silver on the surface, so if you remove the ugly stuff, you'll most likely expose a bunch of copper.

So IF anything is needed, then the common approach is to REDUCE the silver oxides and sulfides back to elemental silver.  This is done the same way as advertised frequently on late night TV (only they use snake oil and do voodoo dances):

Heat some water up to 70C and add 1TBS/quart of baking soda.  Dissolve the baking soda.  Then take some new shiny aluminum foil and crumple it up, then iron it back out.  Put this in the hot water bath with the circuit board.  It doesn't have to actually make contact with the circuit board, and you really don't need a whole lot of aluminum foil.  Within about 2 minutes, the freshly exposed aluminum will pull the oxygen and sulfur away from the silver and you will be back close to the original finish.

Rinse THOROUGHLY in DI water, then dry the board in an oven until it is completely baked out.

Wayne Thayer

-----Original Message-----
From: TechNet [mailto:[log in to unmask]<mailto:[log in to unmask]>] On Behalf Of Richard Kraszewski
Sent: Tuesday, February 05, 2013 5:54 PM
To: [log in to unmask]<mailto:[log in to unmask]>
Subject: [TN] Silver Sulfide & Plasma

Any of you ever have luck cleaning silver sulphide off of a PCB enough to make it solderable with NC flux, by using plasma?

If so , can you please provide me with the gas mix you used?

Thanks

Rich  Kraszewski
PLEXUS


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