Good news: YAESU has added the DC blocking capacitors at the IF filters of the FT-857D!

Fig. 1. The new FT-857D PCB.

Sometimes I think it's mothing short of a miracle that we can buy new technology products (like our transceivers) at VERY reasonable prices (not exactly so when I got into the hobby, 30 years ago - a handheld transceiver was a very expensive item in my country, it cost more than the monthly wages of a public employee).

In sharp (and delightful) contrast, recently I bought an Agilent E4406A Vector Signal Analyser,  an HP3586A Selective Level Meter and a Hagenuk Digiflex LAN Time Domain Reflectometer from the surplus market, at what must be a tiny fraction of their original cost! Mind you, the E4406A was about 45000 euros less than a decade ago! Long live companies like NOKIA (that's where my wonderful E4406A came from) and the ever-faster changing industry standards!

Fig. 2. The Serial Number.

But I digress... It seems that the nasty filter problem has caught the attention of the manufacturers! Tomi from Romania has sent me a photo of the PCB of the FT-857D he bought recently (Fig. 1 - click on the photos to enlarge), in which we can see that the DC blocking capacitors have been added! Thumbs up for Mama Yaesu! 

Maybe they do that for all their products now. I am sure other manufacturers have also taken steps to cure the nasty filter plague, but I haven't seen any concrete proof yet.

Tomi also sent me the serial number of his rig (Fig.2) - to save those that would like to add the capacitors in their recently-built rigs from the trouble! Thanks, Tomi!

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!).

 

Removing the filter case without desoldering it from the PCB

Desoldering the ceramic filters in order to fix them can be a tough proposition in many cases. So, I came up with a different way of removing the plastic case - without removing the filter from the PCB..

Take a look at the picture (it's from the repair of an TM-D710E). Using long nosed pliers, grab the filter case exactly as shown and, gently but firmly, pull the case by - very carefully! - rocking the pliers in the direction of the red double arrow (perpendicularly to the longitudinal axis of the filter). The case usually comes off relatively easily and the offending elements can then be easily inspected, removed and cleaned, restoring proper filter function. Be careful not to lose the bronze tensioner spring plate that (usually) comes off with the case. This method of case removal has worked every time for me (I have fixed more than 50 filters up to now), saving me considerable time and work. Of course, after repairing the filters, don't forget to add the DC blocking capacitor at the output side of each filter, so that you won't have to remove the filter case again!

***ADDENDUM: Carefully heating around the base of the filter case for a few seconds with a hot-air soldering gun set at about 200 degrees Celsius usually makes the case come off much more easily. I said "Carefully", please take note!

The withering filters - adding a blocking capacitor when space is tight

Adding the DC blocking capacitors to protect the ceramic filters in your  rig may sometimes prove a bit difficult with the newer, extra small SMT components. Sometimes, there just isn't enough space or PCB trace length to accommodate the new part. In such cases, alternative methods might help, like the one in the photo (click to enlarge).

Here, the capacitor has been added only at the output pin of the filter (top right in the photo), because the output side of the filter is the most vulnerable (owing to the thinner ceramic resonator there). Due to the very small dimensions of the cicuit (the distance between the input and output pins of the filter in the photo is 8.3mm) there wasn't enough PCB trace length to comfortably add the capacitor by cutting the trace. So the solder was sucked out of the pin's plated-through hole, which left the pin standing in the center, unconnected. Then, the size 0603 capacitor was soldered at an angle, bridging the distance between the top of the filter's pin and the PCB trace. Problem solved, without PCB cutting (which may also sometimes prove dangerous to nearby SMT components)...Be sure to check whether the filter's pin actually stops making contact to the plated-rhrough hole, after you suck out the solder!

(The photo is from a President brand marine portable VHF tranceiver, that had gone completely "deaf" due to filter deterioration - after reviving the filter and adding the cap, it's back singing and dancing. The manufacturer [what a surprise!!] had omitted the DC blocking capacitors between the IF IC and the 2nd IF ceramic filter.)

The Withering Filters: A microscopic view

In a previous post I discussed the deleterious effects of electromigration and corrosion on ceramic intermediate frequency filters, when DC potentials are applied to their pins, especially the output pin. I have collected some more data that may be of interest to those following the subject. Take a look at the photos, click on them to see an enlarged version.

Life on the edge. The first photo (25X magnification) shows the corroded edges of a filter element (the shunt output element, which due to its small thickness gives the most trouble). Observe the damage to the plating at the corner and along the upper edge, also the metal / oxide deposits that have short-circuited the element, rendering the filter inoperative. Measuring with an ohm-meter, the resistance was about 30 ohms between the plated surfaces (where it should have been a very large value, in the tens of megohms or more). However, after cleaning the element's edges in the way I have described, the filter (amazingly) returned to normal! I have repaired numerous such filters this way, and, after the addition of DC blocking capacitors in the circuit, they seem to stay healthy and happy.

 A rough corner. The corner of the element in the previous photo (now in 200X magnification). Electromigration and corrosion have admittedly done a great job of stripping away the metal plating and short-circuiting the element. Check out the dendritic growths at the edge of the remaining plating. The small magnification factor doesn't do them justice. Anyone with a spare electron scanning microscope? I could accept a hand-me-down, you know, I am not that snubbish...

Filter-pox. The effects of moisture inside the filter, on the other elements. Although rather spooky-looking, the elements actually checked out all right. The leftmost thick element is the one at the input. Owing to its thickness, the input element is a lot more tolerant of the DC bias abuse. In fact, I have yet to find a troublesome input element, the trouble is always at the thin (~0.35mm) output element (seen to be missing at the far right). The elements between the input and output elements of the filter don't "see" the DC bias, so they don't suffer the dire consequenses. They get their spots, nevertheless. Moral: It doesn't pay being in the middle of any mess.

An inside job. The spring plate compressing the elements in the filter case is oxidised, too. Yes, those shiny sparkles are indeed tiny droplets of water. Some of the filters I've examined contained a surprising quantity of water. The damage was roughly proportional to the quantity of water, which suggests that the manufacturing could be improved, so as to prevent water from entering the filter's case. The problem is, we're talking about filters that cost a couple of euros retail, and it's always true you get what you pay for. I have yet to see a good crystal filter deteriorate due to moisture ingress, they're truly "hermetically sealed".

A very green face. The end plate at the input side, also oxidised. Although certainly a disturbing view, the oxidation there doesn't do much harm to the functionality of the filter (well, up to a point, I guess!)

It's aliiiive!! To test a little theory of mine, I experimented by applying about 60V DC through a 33KΩ resistor to the input and output pins of a new TOKO ceramic filter. The high voltage was intended to speed things up (things = the deterioration of the filter). This went on for two months, after which the output element was short-circuited (as expected), but the thicker input element was mostly unaffected. So, in conclusion, one may take a calculated risk and add a DC blocking capacitor only at the output pin of the ceramic filters of this type, as the input seems a lot more forgiving to abuse (and most transceivers apply only about 8V to the filter). The output pin is the one closer and opposite to the arrangement of the three grounding pins (in the TOKO brand filters I have found in Kenwood transceivers). Check the manufacturer's data sheets to find out about the other brands and types.

Radio amateurs (and every sensitive person) should establish a movement for the rights of those wonderful, innocent, so unjustly suffering components, the ceramic filters. Don't just sit there! You could write to your manufacturer of ham radio transceivers and ask for the continuing torment of those poor beasts to end at last, or something like that!...

***ADDENDUM: For those who want to learn more about the phenomenon that I propose that causes DC-biased filter failure, please take a look here: http://www.ami.ac.uk/courses/topics/0158_emgr/index.html
There you can admire two great photos of the results of electromigration across tracks and solder resist on printed circuit boards, plus lots of interesting relevant information. Clearly, humidity and voltage gradients at small distances are a bad combination!!

More on the withering filters case: The TS-2000 disappearing noise conundrum

A few months ago, a friend gave me his TS-2000 to check, because occasionally there was a crackling noise coming out of the speaker (and showing on the S-meter) during SSB weak signal reception (SW8KOU operates EME on 2m).
However, when I checked the rig, the noise just wasn't there. I checked for bad SMD components or soldering, but found nothing. I returned it, the noise eventually came back even louder after a few days, he gave it back to me to check it again, and guess what - the noise was absent again. I gave it back to him again, and he reported there were sporadic outbursts of noise now and then.
Eventually, the noise came back to stay, and loud it was!! It resembled lightning noise, heard during a storm on a low HF band and registered S2 ~ S5 on the S-meter. This time the noise was kind enough to be there when I powered the rig up on the test bench. It could be heard on SSB only, not on FM (with the proper test setup). I had already previously checked the rig thoroughly for bad SMD components or bad solder connections, and, having seen many "withered" ceramic filters in the meantime (in other rigs), it suddenly dawned on me that the noise could very well be emanating from a faulty ceramic filter! The schematic diagrams showed two such filters in the SSB signal's path, CF3 and CF4 in the TX-RX unit.
To make a long story short, the culprit was CF4. The filter had suffered the same degradation I describe in another post, but in a more gradual way, due to different conditions. The other cases I have examined had about 7V applied to the filter's pins, but the circuit around CF4 applied about only 1.5V to it. The degradation was there, but the symptoms and time scales were different. I measured about 1MΩ from the output pin to the nearest ground pin, which was far more resistance than the usual 20 ~ 100 Ω I had previously measured in other malfunctioning filters. That explained the crackling noise and the fact that the receiver didn't go totally deaf, like in the other cases. The lower voltage was slower to act on the filter, and the degradation was milder. I opened up the filter, and, there you are, I could immediately see the effects of oxidation on the phosphor bronze spring plate. I removed the thin element at the output side, and sure enough, it showed the telltale signs clearly (see the photo, click on it to enlarge). After carefully cleaning the edges in the way I have described, the resistance reading was more than 40ΜΩ. I soldered the filter back in place, and the noise was gone. My friend tells me that the receiver is very quiet now (but let's see what happens when eventually CF3 (with 7V applied to it) breaks down, too!).
 I will shortly publish the solution to this vexing problem, consisting of just adding two DC-blocking capacitors at each filter (the example mod will be performed in an ailing TM-D710). The same simple concept, however, holds for the modification of all similarly affected rigs.

The Mysterious Case of the Withering Filters

The ceramic filters found in almost all  radio communications equipment are indeed extremely useful components. Small, cheap and efficient, those "little black boxes" have found widespread use by all the manufacturers. I used to think that they're almost indestructible, because I had never seen one of them fail - but this fact has changed.

I have recently come across several cases of VHF-UHF transceivers where the receiver suddenly went deaf, faintly hearing signals only above -60dBm. The culprit was the ceramic 2nd IF filter (450 or 455 kHz) in all those cases. All those filters I have examined showed the same symptoms: the output side showed a low (a few tens of ohms) resistance to ground for DC, where it should have been almost infinite resistance.

After a while (and having replaced several such filters in my friends' transceivers), I became curious and investigated the reasons why those fairly robust components had become bad.

First, let's talk a bit about their structure (see the diagram I made, click on it to enlarge). Most of the 6-pole ceramic filters used in amateur radio transceivers have the general structure shown on the left. There are six ceramic (barium titanate, if memory serves) resonator elements, three in series and three shunt, connected as shown. The series elements are thick, the shunt elements are thin, and both have their wide surfaces plated with a metal (I guess it's a silver alloy). The narrow edge surfaces are not plated.

There are metal inserts between the ceramic elements, making contact to the plated surfaces of the elements and providing the electrical connections to the outside world. The whole structure is housed in a small plastic case, which is hermetically sealed with epoxy resin at the bottom side, where the pins come out.

So, what was the problem? While waiting for a replacement filter for a rig, I decided to try to pry open the case of the failed filter. I did so with the edge of a very sharp X-acto cutter, and I carefully removed the black case. One of the ceramic resonators fell off, along with a (phosphor bronze?) tensioner spring plate, which keeps everything pressed together when the filter is in its case.

It was immediately evident that something was wrong, because the spring plate was visibly oxidised, and there were suspicious looking spots at the edges of the thin ceramic element (you may notice one of them just below the right corner). I measured with my ohmmeter and saw that that element was the bad one, because the resistance reading was 19 ohms, the same value I had previously measured between the output pin and ground. If you take a good look at the next macro photo, you will see that the corners of the other elements also have low-resistance deposited paths short-circuiting the elements in the same way (also, look at the lower part of the diagram). To my surprise, with a magnifying glass I observed tiny droplets of a clear liquid (water, I think) at the inside walls of the case! What was the story here?

Hello, electromigration! The "crime scene" had all the necessary elements required for electromigration to do its nasty stuff. But let's take things in order.

a) Ceramic filter manufacturers (ALL of them!) expressly warn against applying a DC voltage at the input and output pins of the filters. Why? The reason is electromigration!

b) Electromigration is a process where, under the influence of an electric field and in the slightest presence of moisture, metal (especially silver) starts migrating and forming conductive paths (called dendrites, from their tree-like appearance, δένδρον [dendron] in Greek) across insulating materials. This phenomenon is a major headache e.g. for IC manufacturers, significantly lowering the reliability of their products.

c) The final result in our case is that (especially across the thin ceramic element edges) conductive paths of metal (and oxides from the electrolytic process since moisture exists inside the filter) are formed, short-circuiting it. Good-bye, filter?

Don't fret, there is still hope! (if you're good at handling very small parts - that's the catch). I thought that if I could get the elements out of the structure one by one and clean their edges, thus eliminating the conductive path, perhaps the filter would work again. That's very easy, because they aren't soldered in place, they just get "clamped" between pairs of contacts when the filter case is in place. If you decide to do that, only get ONE element out at a time with a pair of needle-nose tweezers, clean all of its its narrow edges by wiping them lightly across very fine grit sandpaper a couple of times and then replace it exactly where and how it was - don't mix them up! Also, don't touch the resonator elements with your hands, finger oils will contaminate them and possibly change their resonant frequency! Carefully clean oxidation wherever you can spot it by scraping, always being careful not to spill the guts of the filter! If you do spill them, they can be put back in place IF you have taken notes and photos of the filter's structure. I cleaned and dried the interior of the case, too. Before putting each resonator back in its place, check with your ohm-meter, you should get an infinite resistance reading - anything else indicates you need to repeat the cleaning process - gently!

Finally, I put the filter back together, sealed it with a minute quantity of cyanoacrylate and soldered it back in the transceiver. Lo and behold, the receiver sprang back to life - and at full specified sensitivity, as my measurements showed. The pass-band response hasn't changed. I think that now, after my delicate sandpaper treatment, the filter is a lot less possible to again fall victim to the nasty electromigration, because all the edges are quite clean now, there isn't any metal there any more.Time will show!

The final word: Ceramic filter manufacturers are quite right in warning against applying DC voltages at the input / output pins of the filters. They specify the use of a DC blocking capacitor at the filter's input and especially the output. Application of DC voltage causes electromigration and corrosion to initiate (especially in humid environments where temperature variations eventually promote water vapor condensation inside the filter, which may have imperfect sealing), and after a period of time the filter fails in the way we discussed.

The funny thing is, most of the manufacturers of amateur radio (and commercial) transceivers amazingly and inexplicably DON'T use the blocking capacitors, instead they boldly apply DC potentials directly at the filter's pins. A survey of several schematic diagrams confirmed this, especially in transceivers where there are several ceramic filters switched in and out of the signal path with diodes  and DC bias (usually about 8V). Why they do so beats me, perhaps it is to save some cents for a pair of blocking capacitors for each filter, creating a huge reliability problem on the way...

The enterprising radio amateur can always add those capacitors in the circuit (0.1 μF, 50V, 0603 size SMD ceramic capacitors are great) and save her / his receiver from becoming deaf due to a ...withering filter! Admittedly, this is a bit difficult in most modern rigs, due mainly to the small dimensions of the components and layout... but it's certainly worth a try.

***ADDENDUM: For those who want to learn more about the phenomenon that I propose that causes DC-biased filter failure, please take a look here: http://www.ami.ac.uk/courses/topics/0158_emgr/index.html

There you can admire two great photos of the results of electromigration across tracks and solder resist on printed circuit boards, plus lots of interesting relevant information. Clearly, humidity and voltage gradients at small distances are a bad combination!!

Good ceramic filter reviving to all of you!!