As a former technician specializing in Nuclear Instrumentation equipment
I have seen numerous examples of flux wicking causing embrittelment,
also causes moisture to intrude sometimes causing corrosion. Most of
the equipment is 1970 vintage so it has passed most electronics
practical lifetime, however for the nuclear industry, it is a standard
to have instrumentation that old. We religiously clean and inspect
every connection every 5 years and replace a few out of thousands.
There is a tool that can be used to stop the solder and flux from
wicking up the insulation. a pair of self closing tweezers with a little
cup on the end to fit over the insulation and around the wire, but it
doubles the soldering time. I have found that tinning carefully, and
tinning the connection point, cleaning both with 200 proof and then
adding only enough flux to allow clean reflow works best, again cleaning
with 200 proof. I have found in my 40++ years of working on
electronics that nothing cleans better with no residue then 200 proof
grain alcohol, added benefit it dries the connection as it will absorb
any residual moisture.
pat goodyear
Control Technician Retired.
On Fri, Jun 6, 2014 at 6:20 AM, Carl VanWormer wrote:
> 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
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