Hi Carl,
I have ran into the same problem a few years ago and we had realized the wave operator used to put the boards in the in-line washer shortly after they were coming out of the wave. So the relays used to be quite thermally shocked, which was damaging the seal.
We've also seen different wash behaviour between 2 different relay brands. Omron was in the equation, but I don't remember whether they were the better or the worse.
Regards,
Ioan Tempea, P. Eng.
Manufacturing Engineer, Satellite Systems
MacDonald, Dettwiler and Associates Corporation, 21025 Trans-Canada Highway, Ste-Anne-de-Bellevue, QC, Canada H9X 3R2
Tel: +1-514-457-2150 x3556
www.mdacorporation.com
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-----Message d'origine-----
De : TechNet [mailto:[log in to unmask]] De la part de Carl VanWormer
Envoyé : Thursday, March 19, 2015 2:40 PM
À : [log in to unmask]
Objet : [TN] Sealed relays fail after water wash
Here is the first of two stories about recent relay failures caused by our assembly process:
We have been selling an assembly that has 16 through-hole relays on 4-layer FR-4 boards. Our CM runs these boards through a wave soldering process, using water-clean flux, and then washes the boards in a dishwasher using the local water (quite pure). We changed to a different (pin compatible, customer approved) relay source for the last batch of 100 boards, and then suffered some production test fallouts. Upon analysis of the failed boards, the relays seemed to be the cause. With a little more work, I discovered water inside the sealed relay packages.
We are using a subminiature power relay that states "Epoxy sealed for automatic wave soldering" as one of the bulleted features on the datasheet. I found another document on the vendor's website titled "Relay Technical Notes", which covered cleaning, mentioning "Sealed relays can be safely cleaned by immersion." We sent several failed relays to the vendor for analysis. From their response, we learned more about the relay manufacturing process, which includes a vent hole that allows for outgassing. The last step in the relay assembly process fills the vent hole depression with a clear UV cure epoxy, followed by UV exposure to cure the epoxy, sealing the relay. Their failure analysis report indicated that the relay seals had been compromised.
We are assuming that the epoxy seals (on the bottom of the relays) failed during our product assembly process (in either the wave soldering, or the washing processes), compromising the integrity of the seal. Our guess is that the epoxy didn't bond well to the relay vent hole (pick any reason you like). If the broken seal allows the internal heated air to escape while the wash water is under the relay, when the air inside the relay cools, water may be sucked into the relay. This sequence of events would explain the water discovered inside of relays that had failed our final assembly testing.
To avoid field failures on the current batch of units, we decided to remove all potentially damaged relays, replacing them with new parts (without a final wash). Analysis of the removed relays showed about half of the parts had visibly defective seals, with several showing the vent plugs almost all the way out of the vent. (pictures available if there is interest and Steve will host them)
Since the vendor says we are doing something wrong to break the vent seals, we have decided to avoid the problem for any future units. We now add the relays after the initial board wash, using hand soldering with no-clean solder.
Our conclusion: don't wash relays, regardless of the datasheet information.
Later,
Carl
p.s. another relay story will follow . . .
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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