Hi all, So we are using the following inductor in a series RLC circuit.
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(C = 0.01 uF ceramic (npo) f~6 kHz, Q >= 30.) The inductor must be a little ?on the edge?. When we got the first bat ch in there were 3 out of 100 that had terrible Q and low inductance (as me asured on a SRS lcr meter.) (Q measured at 10 kHz.) In the process of fi nding the bad eggs all the inductors were tested. Now another batch of bo ards has come for testing and for the first 3 all the inductors are bad. I nductors are hand soldered and then go into an ultrasonic cleaner. (I?m not sure about exact times and temperatures, but maybe 65 C for maybe 1/2 hour?) Could the solvent, ultra-sonics or temperature cause the failure? Anyway to bring the inductors back to life?
Thanks,
George H. (I?m going to try and kill a few in the bath this morning.)
When water or solvent get in under the sleve it get sucked in because of the capillary effect of the pores between the windings. If it is water you use, you might have shorted water windings or just water with bad conductivity.
The inductor ought to be moisture open so the moisture or solvent can escape during the drying fase - or the windings must be soaked in lacquer or enameled so a another fluid can not get in any more.
The ultrasonic treatment might damage the isolation? I assume the ultrasonic treatment produce vacuum/air/solvent gas bubbles, that implode - and in this process produce very hot spots on the surface. The are treating the electronics with cavitation implosions! No surface (isolation) can endure that?:
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Quote: "... Such cavitation is often employed in ultrasonic cleaning baths and can also be observed in pumps, propellers, etc. ... However, it is sometimes useful and does not cause damage when the bubbles collapse away from machinery, such as in supercavitation. ..."
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Quote: "... The agitation produces high forces on contaminants adhering to substrates like metals, plastics, glass, rubber, and ceramics. This action also penetrates blind holes, cracks, and recesses. The intention is to thoroughly remove all traces of contamination tightly adhering or embedded onto solid surfaces. ... Most hard, non-absorbent materials (metals, plastics, etc.) not chemically attacked by the cleaning fluid are suitable for ultrasonic cleaning. Ideal candidates for ultrasonic cleaning include small electronic parts [!?], cables, rods, wires and detailed items, as well as objects made of glass, plastic, aluminum or ceramic.[8] ..."
Check a good and a bad one carefuly under a microscope. Cut and gently peel off the yellow sleeve, then unwind the cooper. Look for hair cracks in the core. Also check the wire and look for embrittlement of the enamel because that could cause two turns to short.
batch in there were 3 out of 100 that had terrible Q and low inductance (as measured on a SRS lcr meter.) (Q measured at 10 kHz.) In the process of finding the bad eggs all the inductors were tested. Now another batch of boards has come for testing and for the first 3 all the inductors are bad. Inductors are hand soldered and then go into an ultrasonic cleaner. (I ?m not sure about exact times and temperatures, but maybe 65 C for maybe 1/2 hour?) Could the solvent, ultra-sonics or temperature cause the failu re? Anyway to bring the inductors back to life?
Geesh, now you tell me :^) We now have to do this Rhos stuff to ship to Europe, and we recently switch ed solders and added this ultrasonic cleaning step to all pcb's.
batch in there were 3 out of 100 that had terrible Q and low inductance (a s measured on a SRS lcr meter.) (Q measured at 10 kHz.) In the process o f finding the bad eggs all the inductors were tested. Now another batch o f boards has come for testing and for the first 3 all the inductors are bad . Inductors are hand soldered and then go into an ultrasonic cleaner. (I ?m not sure about exact times and temperatures, but maybe 65 C for maybe 1/2 hour?) Could the solvent, ultra-sonics or temperature cause the failu re? Anyway to bring the inductors back to life?
Thanks Joerg, (see my reply to Glenn) It appears the solvent is doing somet hing.. I'll try peeling back the yellow cover and see what that does.
Short answer is not more of this solvent for inductors!
I've seen data sheets that warn about ultrasonic cleaning, stuff like crystal oscillators mostly.
We do both leaded and ROHS, with rosin flux. We have a Baron Blakeslee cleaning machine, one of the constant-distilling solvent things. One side has a tank with boiling solvent and a deflux detergent, and the other side is pure distilled solvent with a spray wand. We submerge a board in the boiling stuff for a minute then spray it good with clean solvent. Works beautifully.
I sneak in sometimes and dunk my hand-built prototypes.
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Hmm, I don't see my reply to Glenn either. So let me recap. My first test was just to stick the inductor into the solvent. It failed w ith one dunk. I also made a mistake in my first post, the inductance *incr eases* and the Q goes down. (Resonant freq from 6.13kHz to 5.43 kHz so L~8
6 mH.)
I also found that some of the 'bad' inductors that I pulled several weeks a go are now OK.
So I peeled the yellow heat shrink off of a bad one. No change. Then wash ed with hot water. a bit better.. then dried with heat gun. Good as new.
We've cleaned boards for years in an ultrasonic cleaner, in a solvent bath. But, they are ALL hermetic package parts, or at least as hermetic as the typical plastic chip packages are. I suspect these inductors may be toroids with open windings, and the ultrasonic agitation, plus heat and solvent may be wrecking the wire enamel. I wouldn't be surprised that some solvent might just wash the enamel off the wire.
If that has happened, the inductors are trashed, no way to rejuvenate shorted windings. You (Klaus) may have to wash the boards first, THEN install the inductors, if this cleaning procedure is required. If it is just the solvent, then a change to a less aggressive solvent is in order.
I'll try to find the article, but yes Ultrasonic cleaning is bad for components because of the stresses involved. It can weaken bonding wires and cause stress fractures. You dont see the failures right away but over time they become apparent.
De-ionized washing or a good old vapor degreasing is better. For the DIY ers Ensolve works good.
Water has a dielectric constant of about 80. So, there should be some shift in Fo due to the increase in stray C. However, I can't justify your change with just the dielectric constant.
I believe you may be misapplying the part. It's not intended for High-Q use - it's a loose tolerance hash choke employing an iron dust core. The lossy-er the better, in that application. The Q measurement itself is a fairly good indication of functional integrity, however.
The low inductance can be caused by internally shorted turns and shorts through the core - a product of vibration, bumps, abnormal winding tension, poor layering technique, poor core surface finishing, heat cycling and lousy magnet wire.
If bad parts were found in the first batch, prior to processing, this was/is a sign to reject the whole batch. There are plenty of other sources that won't exhibit this failure mode.
ed with one dunk. I also made a mistake in my first post, the inductance * increases* and the Q goes down. (Resonant freq from 6.13kHz to 5.43 kHz so L~86 mH.)
ks ago are now OK.
washed with hot water. a bit better.. then dried with heat gun. Good as n ew.
Thanks for all the responses, (Jon, legg, Martin, John S.)
John I think you nailed it. The problem was the water. (Apparently the so lvent is water with some special 'soap' added.) So the water gets between the turns and increases the turn to turn capacitance. (At least that's my working hypothesis.) There's a piece of data I didn't really notice and t hat is with a 'wet' inductor the resonance not only shifts to lower frequen cy and amplitude, but the peak of the resonance happens at a phase shift th at is less than 90 degrees from the applied voltage.
legg, I didn't have any part in selecting the inductor. (My only contribut ion was to suggest a nice npo ceramic cap rather than a film one.)
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