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March 2015

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From:
Guy Ramsey <[log in to unmask]>
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Date:
Fri, 20 Mar 2015 08:49:59 -0400
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New meaning for the term "rings true".

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Carl VanWormer
Sent: Thursday, March 19, 2015 4:46 PM
To: [log in to unmask]
Subject: [TN] Sealed relays fail after water wash - story #2

Here is the second of two stories about relay failures caused by our
assembly process:

We have another product with mostly surface mount parts, along with one
through-hole relay and some 48-pin through-hole connectors.  After the SMT
parts are reflowed, the boards go through a wave soldering operation to
attach the four connectors and the relay.  Our first batch of 50 boards had
6 fallouts that were traced to "bad" relays.  Since I had recently been
burned by water in "sealed" relays (another style, from another
manufacturer), my paranoia kicked into high gear.

The datasheet shows these relays as "Fully sealed", with a note to "Contact
your OMRON representative for the ratings on fully sealed models."
Searching the relay manufacturer's website turned up a few more documents.
One document, titled Electromechanical Relays Technical Information shows
that for Fully Sealed relays:
Boiling cleaning and immersion cleaning are possible. Ultrasonic cleaning
will have an adverse effect on the performance of relays not specifically
manufactured for ultrasonic cleaning. The washing temperature is 40°C max.

My guess is that our wash process (and probably yours, too) is above 40°C.
This is probably how I screwed this one up . . .

Opening up the 6 failed relays showed significant water inside.  Since these
boards were produced last month, they had been sitting on the shelf for
several weeks, with the water and other chemicals causing problems,
including mechanical binding.  My real fear was that other boards that
tested "good" might have water inside and would soon go bad.  The failed
boards showed several different problems, but a common feature attracted my
attention.  During the initial board testing, as I ramped up the DC power
supply (that activated the 24V rated relay coil), all of the good boards
made a little "tick" sound as the relay armature took up slack in the
system, but didn't have enough force to overcome the return-spring force.
This sound occurred at about 8V, except for the "bad" boards.  I touched a
9V battery to the coil drive pins while holding a bad relay against my ear.
I could only hear a slight "tick" sound.  Holding a good (new, never been
washed) relay up to my ear, then touching the battery to the pins gave a
louder, stronger "tick" that had a little ringing character that was not
present in the bad relays.  The difference was subtle, but similar to the
difference in sound from dropping an old (Copper) penny and a new (Zinc)
penny.  I cut the top from a good relay, listened to the sound as I touched
the coil terminals to a 9V battery (reference, good).  I ran water over the
exposed (good) relay, and shook off the excess.   Activating the dampened
"good" relay gave a sound that was more muffled, lacking the "ring" of the
little vibrating parts as the water film damped the vibrations.  With this
test, I was able to check all of the "good" boards, finding that they all
had the "ring" of goodness (hopefully with no significant internal water).

I'll send some pictures to Steve for his wonderful picture hosting service.

Once again, our planned solution to the problem is to add the relays after
the final board cleaning, using a no-clean flux solder.


Once again, our conclusion is: don't wash relays, regardless of the
datasheet information.



Later,

Carl



p.s.  there will be no follow-up water-in-relay stories from me because I'll
never allow a relay to run through a wash system. This goes for
potentiometers, too, since I've had "sealed" pots fail after washing.





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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