[time-nuts] GPS Timing Antenna Failure - Long
Van Horn, David
david.vanhorn at backcountryaccess.com
Mon May 14 12:56:26 EDT 2018
There's a great article out there on the web. It takes a bit of digging, but the title is "Low voltage, the incompetent ignition source".
They discuss fires on PCBs caused by trace contaminants and dendrite growth. A PCB with sustained flame is shown, powered by a lithium coin cell.
From: time-nuts <time-nuts-bounces at febo.com> On Behalf Of Larry McDavid
Sent: Monday, May 14, 2018 9:19 AM
To: Time-Nuts Mail List <time-nuts at febo.com>
Subject: Re: [time-nuts] GPS Timing Antenna Failure - Long
Usually, in electronics, when we hear about "whiskers" we think of tin whisker growth. That is surely real but not all whiskers are tin. In fact, some whiskers are organic rather than metallic; sometimes these are even somewhat semi-conducting. Such conductance paths lead to very confusing troubleshooting results!
Rosin solder flux, RA (Rosin Activated) or RMA (Rosin Mildly Activated), contains organics that aggressively clean oxides from PWB pads and metal leads of components. This activated flux is actually corrosive and will often do strange things if left in place. It used to be that this flux was removed by vapor-phase TCE cleaning but that chemical got into ground water and is no longer used for flux cleaning. But, there are aqueous saponifiers that can clean rosin flux effectively.
Often, the final water rinse from assembled PWB cleaning is checked for conductance, even high ohmic conductance, and cleaning not deemed complete until this test is passed. Leaving activated rosin solder flux on an assembled PWB is a really bad thing to do.
I don't know what was in the dark brown flux residue I found on my Symmetricom antenna board, but it should not have been there. It would not have produced tin whisker growth but it could easily have produced other conductance paths across the soldered coax pads. Those could have been RF paths and not have caused higher dc current draw by the antenna.
Again, I can't be certain the flux residue caused my GPS antenna failure, but I believe it very likely and that is supported by the end result of fixing the antenna.
Unless someone has something new to add, surely we have beaten this topic to death. I was only trying to help others who might have a similar GPS antenna failure.
On 5/13/2018 6:42 PM, Hal Murray wrote:
>> I can't count the number of times flux and whisker growth has
>> caused problems in circuitry and connectors.
> A whisker might explain things. Would that also show up as over-current?
Larry McDavid W6FUB
Anaheim, California (SE of Los Angeles, near Disneyland) _______________________________________________
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