A proposed root cause for the "Withering Filters" phenomenon

Fig. 1 Photo by Jerzy,
SP6FPY.  (Click to enlarge)

After having examined the failure mechanism of more than 100 ceramic filters that have succumbed to electromigration, I think I have collected enough data to propose a root cause for the phenomenon that has caused failure of a very large numbers of filters. I should gratefully thank the many fellow radio amateurs that have sent photos of their filters to me, adding to my database on the subject. Manufacturers might want to consider changing their production methods to avoid the chain of events that I propose that leads to this phenomenon.

So, let's look at the data I have collected and a proposed theory that explains the facts:

Fig. 2. Note the water droplets.

1) In all of the ceramic filters I have examined, there was a surprising quantity of water inside the plastic case (take a look at Figs 1, 2 and observe the signs of water presence on the elements and inside the black case, also the whitish flux residue on the case of the adjacent filter in Fig. 1, which shows that a quantity of water evaporated from that surface, leaving it behind). The water inside the case causes the filter to fail soon after the transceiver circuit applies DC voltage to the input, and mainly the output pins of the filter. The quantity of water is such, that can't be attributed to a filter manufacturing flaw: it's just too much. I can't imagine that the filter manufacturers would have ignored it. Also, I haven't found water inside some new filters I have "dissected". So, there must be another reason for this quantity of water inside the filter.

2) Where does the water come from, if not from a filter manufacturing flaw? I think the answer lies with the newer technologies and materials used for the fabrication of populated printed circuit boards (PCBs). Briefly, the new methods of wave soldering with non-Pb solders extensively use organic acid (OA) fluxes. Those fluxes are removed from the PCBs after wave soldering by pressure rinsing with hot water. The rinsing process takes some time to complete.

3) I think that the water ingress can be explained thus. The rinsing process uses hot water (about 70 degrees Celsius), sometimes with ionic additives. This (as well as the previous soldering process) causes the (so far dry) air inside the plastic filter case to expand, creating positive pressure inside the filter case. This results in air escaping from inside the filter case, as the temperature softens the plastic case and compromises the (not so great to begin with) sealing at the bottom of the filter case.

When the PCB exits the rinsing process, water is held by affinity under the filter case, between the filter and the PCB surface. The filter starts cooling down, and this causes the pressure inside the filter to drop under the atmospheric pressure. This negative relative pressure tends to draw a small quantity of the water under the filter inside the filter case, through the very fine cracks at the filter case seals that the positive air pressure (when the filter was hot) created. The quantity of the water is some micrograms, but it's more than enough to cause trouble. The board is then dried, but the accident has already happened. The water is already inside the filter.

4) Once inside the filter, the water can't escape and remains trapped inside the case. Some of it condenses or moves on the filter plates, starting the electromigration process where the electric field (due to the externally applied voltage) reaches sufficient value. This most often occurs at the corners of the thin output ceramic plate (element) at the output of the filter, which is only about 0.35 mm thick. This leads to the failure of the filter after some time.

5) What might be a solution? A slight modification of the rinsing - drying process might help in avoiding this phenomenon.

If the PCB is dried IMMEDIATELY after exiting the rinsing chamber with pressurised HOT air, then the water might not get inside the filter case, as the pressure difference doesn't occur this way before the water has been removed from under the filter.

I have no further knowledge of the exact conditions used in any particular case, so the above proposal may not always apply. But each manufacturer could certainly review the method used and modify it accordingly, so as to avoid the conditions that drive water inside the filter case (and possibly inside other "sealed" components, such as miniature relays).

If someone with more knowledge of the PCB processing methods wishes to add (or correct!) something in the above, please send an e-mail to sv8ym@raag.org.

73 DE SV8YM

Addendum: The above suggest that if you repair (clean and dry thoroughly with hot air) or replace a filter that has gone bad through water ingress during the rinsing procedure with a brand new one that hasn't come in contact with water, you may not have problems in the long run, even if you don't also install DC-blocking capacitors. That is so because you need BOTH water AND a voltage gradient to start electromigration. A completely dry filter probably can withstand 7-8 V indefinitely (as experience with older rigs shows - the PCBs were washed with organic solvents a long time ago, in the paleolithic Pb era!).

 

Does your IC-E92D have any loose screws?

"It's the loose screws in my head!"

The IC-E92D is a very well constructed handheld transceiver, with a really solid feel of quality. I bought mine in 2008, in order to wet my feet in the new D-Star ocean. I also have the external speaker - microphone adaptor, which I think makes the rig a lot more pleasant to use.

The transceiver performed flawlessly, no problems at all, up to a few days ago, when I noticed that when I keyed the transceiver in medium and high power output on UHF using the provided rubber antenna, the display backlight would consistently and magically turn itself on and strange beeps and noises would emanate from the external speaker. The phenomenon was absent in the low and super-low power settings, so I reasoned that some kind of RF intrusion into the logic circuits was to be blamed. Removing the external speaker/mic cables changed things a bit (only full power produced the strange effects), but the goblin was surely still there. A quick search on the Internet revealed that other users have also reported similar problems with the display going blank or even getting inverted (!), so I was not alone out there. 

But what had changed to produce those effects? I hadn't changed anyhthing, the rig functioned with exactly the same accessories from day one. So, it was logical to think that the reason was internal.

Before opening the case, I studied the service manual a bit. I observed that the screws holding the transceiver together also had another very important mission: (especially the top two) pressed the main printed circuit board grounding spring contacts on the chassis. So these (and of course the other internal screws) are cleverly forming RF-tight enclosures within the transceiver. Could it be that some of those screws had gone loose, compromising this important function?

So, to take a first shot at it, I proceeded to carefully tighten the six screws that hold the transceiver together (they are at the back side, two of them near the top and four of them under the battery - you have to remove the battery to see them). Indeed, they took about 1/4 turn to feel nicely tight again (BUT DON'T OVERDO IT WITH THAT SCREWDRIVER, PLEASE!).

I am happy to report that the problem vanished immediately. I loosened the screws a bit again to see if it would come back, but it didn't, obviously because the contacts were not disturbed enough this way. My theory is that the screws gradually become a bit loose with normal use and thermal cycling, making the shielding / grounding contacts unreliable and "leaky". A bit of tightening and everything is back to normal again. Perhaps some day I will also have to open the case and tighten the internal screws, too. If the problem doesn't go away by tightening the externally accessible screws (especially the two ones at the top), that's the next logical step. This reasoning may also hold for other transceivers with the same shielding method. I must say I have certainly seen a lot of strange problems in other modern VHF/UHF transceivers go away when I tighten the screws that hold the printed circuit boards on the cast aluminum chassis!! They have a tendency of coming loose, mainly due to thermal cycling action.

Enjoy your IC-E92D and see you on D-Star!

Some more mods for the IC-7000 amplifier chain

During the efforts to cure the self-destructive tendencies of the DRIVER unit of the IC-7000, I noticed that the pre-driver transistor, Q102 (RD01MUS1), which also works in class-A, gets too hot to touch (the dependable index finger test never fails - although my "probe" seems to suffer a bit with each test!). The copper surface around the transistor  serves as a heat sink, but it's obviously not enough. Elevated temperatures almost certainly lead to serious problems with semiconductors in the long run, so I decided to add an improvised heat sink to both the pre-driver and the pre-pre-driver Q101, 2SK2854. The photo tells the story (click to ENLARGE): A small heat sink is made with a piece of solid copper wire (of 1.5 mm diameter) and then soldered to the (grounded) source tabs of Q101 and Q102. Just make sure the heat sink doesn't touch anything when you put the PCB back in its place. The transistor operates without losing its cool now. One could even  slightly lower the idle current of the pre-driver Q102, by paralleling R112 (3.9 kΩ) with another resistor of suitable value (around 10 kΩ or so), but I haven't done that, as the transistor operates at a quite acceptable temperature with the new heat sink. The next photo shows how the new heat sink fits into the available space. Be careful! Accidents can cause much woe...

Good luck and take care (and your time)  in performing the mod!

IC-775 DSP overheated voltage regulators

Recently I had the opportunity to service a good friend's  IC-775DSP (an excellent quality rig, by the way). After completing my work, I noticed that two SMD voltage regulators in the middle of the PLL unit (at the underside of the rig) were getting very hot, to the point of discoloring the PCB around them. The venerable and extremely accurate index finger test confirmed the situation, and I had to let out a muffled cry, as my bold "probe" suffered the dire effects of the alarmingly elevated temperature of the ICs. I measured their output voltage and saw that it had dropped a bit from the nominal value - a common symptom with chronically overheated three-terminal regulators. So, I proceeded to install an improvised heat sink (see the photo, click on it to enlarge).

The heat sink is made from solid copper wire with a diameter of 1.5 - 2 mm. The wire is bent to a shallow "Π" shape (that's the greek letter "pi") with the proper dimensions and soldered to the tabs of the regulators and the adjacent shield cans, which thus become part of the heat sinking arrangement (don't worry, there aren't any heat - sensitive circuits in them). The poor regulators work at a far lower temperature now, which is good for their health and longevity - as the IC-775 DSP even in our digital era is definitely a keeper, with a high spec receiver that's a pleasure to listen to and a lot of conveniences for the operator - including a very effective DSP system.