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June 2014

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From:
"Wenger, George M. [Contractor]" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Wenger, George M. [Contractor]
Date:
Mon, 9 Jun 2014 13:21:17 -0500
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As someone who sent 33 years working in the "Bell System" were solder assembly quality/reliability was the primary concern,  I'll throw my two cents into this discussion.  I've never like the name "No-Clean".  Any flux used for soldering that doesn't go through a cleaning process would be a "No-Clean" if the meaning is it wasn't cleaned off.  The old BellCore (now Telcordia) GR-78 gives criteria for determining if a soldering flux residue needs to be cleaned off for reliability reasons.  If you make a solder joint with a flux and don't clean it off and it passes the BellCore reliability testing then we would consider that flux a reliable "Leave-Behind" flux meaning if the flux residue wasn't cleaned off there were no reliability risks.  In general most of the old fluxes classified as R & RMA passed the "leave-Behind" requirement.  In fact, our experience was that especially for RMA fluxes the reliability was much better if you did leave them behind rather than tried to clean them.  If you take an RMA flux and try cleaning it with alcohol (which isn't a very good solvent) what you wind up doing is dissolving and removing most of the rosin in the flux residue, which is what was encapsulating the ionic activators, which in turn allowed any ionic activators to be mobile and the first time the humidity increased you had the worse case for corrosion (moisture, activators, and electrical potential).

I can see that with a low-solids type "No-Clean" flux that the flux would wick up the braided wires and yes when a soldering iron was placed on where you wanted to make the solder joint the flux activators would be heated and reduce oxides and allow a good solder joint to form but the flux that wicked up the wires may not have gotten to a high enough temperature to de-activate the activators and then the humidity goes up in use you could have a problem.  If I were pre-tinning braided wires I would only us an RMA flux qualified according to BellCore GR-78 to be a "Leave-Behind".

Regards,
George
George M. Wenger
Failure Signature & Characterization Lab LLC
609 Cokesbury Road, High Bridge, NJ 08829
(908) 638-8771 Home  (732) 309-8964 Mobile
E-mail [log in to unmask]
[log in to unmask]


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Carl VanWormer
Sent: Monday, June 09, 2014 12:59 PM
To: [log in to unmask]
Subject: Re: [TN] tinning wires - flux entrapment and long term reliability

I've heard stories of no-clean and hand soldering causing problems.  In an IR-reflow oven, all of the no-clean flux achieves the passivation temperature, becoming inert.  With hand soldering, the flux melts, runs away from the heat source, and penetrates any tiny crevice it can find.  There is an area at the perimeter of the heated area that is hot enough to cause the flux to flow, but not hot enough to cause it to become active.  If this is true, than this is the problem I'm worried about.  Comments, please?

Thanks,
Carl



Carl B. Van Wormer, P.E., AE7GD
Senior Hardware Engineer
Cipher Engineering LLC
    21195 NW Evergreen Pkwy Ste 209
    Hillsboro, OR  97124-7167
    503-617-7447x303
    [log in to unmask]     http://cipherengineering.com


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Monday, June 09, 2014 9:49 AM
To: [log in to unmask]
Subject: Re: [TN] tinning wires - flux entrapment and long term reliability

It's been some years back, but we were building a cabinet along with all the boards and cables. The cables had silver plated braided shielding over them and we would have to solder the shield wires to either a lug or contact. We were using manufacture XXX no-clean flux. It had been a rainy spring that year, and rained about every other day for at least a month, so you know the humidity was high. 

One of the supervisors came to me after that month and said; "Steve, we got a problem, all the cables in the cabinets are turning green..." so I went and looked at them. Sure enough the shield wires were turning green, and it was down close to the end sections of the cables where they had been soldered:

http://stevezeva.homestead.com/files/Green_Junk_1.jpg

http://stevezeva.homestead.com/files/Green_Junk_2.jpg

http://stevezeva.homestead.com/files/Green_Junk_3.jpg

http://stevezeva.homestead.com/files/Green_Junk_Close.jpg

I had a hard time figuring out how this was happening, because right at the ends where the soldered connection it was fine. The connections were soldered and cleaned with alcohol. Best I could figure was that the operators had flux on their fingers when they handled the cables and got the flux up on the cable where it wasn't reacted with heat, and wasn't cleaned, and with the high humidity that we had that month turned the shield wire green.

We switched the no-clean flux to manufacture XXX and the problem went away.

Steve 


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Nutting, Phil
Sent: Monday, June 9, 2014 8:25 AM
To: [log in to unmask]
Subject: Re: [TN] tinning wires - flux entrapment and long term reliability

So for many years I have followed the discussion about flux cleaning and alcohol always get mentioned as a great way of dissolving the flux and depositing it on a much wider area.  There has to be a better way to "clean" the wire entrapped flux if it really must be cleaned.  I agree that OA flux is not a good solution.  My current choice is to use "no-clean" flux cored solder when tinning wires and then leave it alone.
Soldering the wire into the board can then be done with "no-clean" or other flux cored solder.  Where we do not make anything that is designed as mission critical this process seems to work for us.

Phil Nutting

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Monday, June 09, 2014 9:30 AM
To: [log in to unmask]
Subject: Re: [TN] tinning wires - flux entrapment and long term reliability

You do not tell us what flux you are using.
But as a general rule, one must never tin insulated wires using OA flux.
Only RMA or no-clean should be used, and that followed by dipping the tinned ends in alcohol.


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Carl VanWormer
Sent: Friday, June 06, 2014 8:20 AM
To: [log in to unmask]
Subject: [TN] tinning wires - flux entrapment and long term reliability

My understanding:
While tinning wires with conventional soldering methods, the wire is heated and the flux-cored solder is applied to the wire.  As the flux is melted, it wicks up between the wires and the solder flows in, wetting the wires.  The liquefied flux flows up the wires farther than the solder, and some of the un-passivated flux is trapped inside the insulation, around the Copper strands where the solder stopped flowing.

My experience:
One of our control modules had failed in an automotive "road splash"
environment.  Our connector terminals had been soldered to the wires that came out of the "waterproof" strain-relief assembly.
Troubleshooting let me to cable harness with an open circuit between a wired connector pin and the other end of the wire.  The wire and pin looked good, but a gentle tug on the pin popped the 5mm length of soldered wire out of the wire's insulation, revealing a discoloration at the end of the solder-flow where the Copper wire had been "disappeared".
A few mm inside the wire insulation, there was another discolored blob at the end of the wire's total length of good Copper wire.  Our conclusions of "not quite waterproof" and "chemistry experiment" led me to be concerned about the problem.

Current worry:
We have a product with a "requirement" that some 16-gauge stranded Copper wires be soldered to our PC board.  The plan is to have the cable assembly arrive with pre-tinned wires, and then the wires will be soldered to the board with "no-clean" flux.  The product is not expected to be in the water, but may be "near" a wet environment, maybe mounted in a pouch on some motorcycle gear.  I'm worried about the tinning process forcing un-passivated flux up, inside the insulation, to wait for a "humid" condition to start another "chemistry experiment."

Questions:

1.       Assuming we must solder wires to my PC board, is there any
guidance on how to keep "chemistry experiments" from happening on my product?

2.       Are there any other "very small" connection methods for 15 Amp
wires that I should consider that I might be able to fit on my tiny PC board that would eliminate my worry?

3.       Am I just being overly paranoid?

Thanks,
Carl





Carl B. Van Wormer, P.E., AE7GD
Senior Hardware Engineer
Cipher Engineering LLC
    21195 NW Evergreen Pkwy Ste 209
    Hillsboro, OR  97124-7167
    503-617-7447x303
    [log in to unmask]<mailto:[log in to unmask]>
http://cipherengineering.com<http://cipherengineering.com/>

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