23 July 2011

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.)

23 June 2011

Dendrites, more dendrites!

The photos show another case of dendrite formation, although this time it's not in a mild, low voltage scenario (like in the infamous ceramic filter case).
That dendrite formed on one of my car's spark plugs when the ceramic insulation cracked and leakage occured, making the engine misfire and hiccup.
Observe the fractal nature of the dendrite, reminding of a lightning (which is exactly the same phenomenon at a much larger scale).
After I cleaned the ceramic surface with an abrasive disk and applied a suitable silicone oil with very high dielectric strength to the tiny crack, the spark plug worked very well for a few days until it was replaced.

Moral:     When dendrites grow
             like a thorny bush,
             your car starts going slow,
             you may well have to push...

(Click on the photos to see a larger version, the initial magnification was 25X and 200X, respectively).

10 February 2011

Addendum - The leaking Electrolytic Capacitor Plague and my TDS-460

The nagging thought that maybe, just maybe, there was more electrolyte hidden under one of the SMD ICs on the board gradually rose to an obsession. I had to see with my own eyes!
So I proceeded to remove U82, one of the ICs that had been affected, to take a look under the hood.The photo (click to enlarge) shows I had missed a spot, or rather three spots, where the electrolyte could potentially create trouble.
Fortunately, there weren't any signs of the electrolyte under the IC. The row of pins and soldered connections at the sides act somewhat as a barrier, and had prevented the electrolyte from creeping under the IC, holding it among the pins by affinity. Unfortunately, the electrolyte doesn't mind and does its frightful tricks there, too. There is no conformal coating immediately around the copper pads where the pins get soldered, and this facilitates the electrolyte getting under the coating exactly where it upsets things the most, creating parasitic conductance where there shouldn't be any. Scraping off and cleaning with IPA does the trick, a good measure of drying with hot air from an SMD soldering station and coating with a small quantity of acrylic varnish, applied with the tip of a fine painting brush (after the IC is back in place) ensures the best possible result.

And a word to the wise: Searching for information on the Web, I have seen that a lot of  fellows have had the same unpleasant experience with the Plague. In several fora it was recommended that the affected board should be washed under the tap with water. I am quite skeptical about that though, because the water could easily dissolve the dried-up residue of the electrolyte, which would then merrily wick itself deeper into the board wherever there isn't any conformal coating (immediately around the ICs pins, for example). Getting it out in that case might prove next to impossible.
When capacitor electrolyte contamination is the issue, removing it only at the affected spots by thoroughly scraping the residue and already damaged coating off the board (before applying any liquids), then thoroughly cleaning just that spot with a cotton swab soaked in isopropyl alcohol (IPA)  logically seems much safer. After drying with hot air (at about 140 degrees centigrade applied locally for about 5 - 10 minutes), one can seal the spot with acrylic varnish, removing the possibility of future moisture ingress.
A hasty cleaning action with the wrong means or method might make things a lot worse, so beware..!

30 January 2011

The Leaking Electrolytic Capacitor Plague and my Tek TDS-460

There are many good things in life, but bad ones tend to take more of humanity's time. There is compassion, love, science, understanding, but people spend most of their life - if not all of it - in greedy bellicose hunger, hating, believing in ghosts (holy or otherwise), and missing the - equally valid - other guy's perspective. The news every day provide ample proof.
Having put that off my chest, I had better now focus  my "elegant tapestry of quotations, musings, aphorisms, and autobiographical reflections" to the more mundane matter of  The Capacitor Plague, which, although completely unknown to the Medical Science, has caused many a frustrated consumer's tear to flow. In short, electrolyte leakage from bad (bad, bad) electrolytic capacitors destroys thousands of electronic devices every day, by corroding copper traces and creating parasitic conductivity on printed circuit boards. The plague doesn't discriminate and victims appear in every class of electronic devices. I have seen my car's ECU, personal computers, power supplies, test instruments, transceivers (and my friend's Stavros Sony ICF-80) die a gruesome death by the infamous Capacitor Plague.
In all godlike modesty, I have been able to resurrect most of the victims, save a few that were truly beyond redemption, as the time and cost to fix them was more than getting a new one. The latest unfortunate victim in my troubled experience was my beloved Tek TDS-460 digitising oscilloscope, a true work of four-channel art, which suddenly (and scaringly) started failing the self-test and showing erratic triggering (quite blasphemous for a prestigious Tek stallion). As the original SMD electrolytic capacitors, with a tarnished history of leaking, had been very wisely replaced with new ones before I acquired the instrument, I was almost certain that the malfunction couldn't be attributed to the electrolytics - although the symptoms - in an eerie way - pointed straight to that direction. Turns out that the Plague is like a time-bomb, and a very delayed one sometimes.

After thoroughly cleaning the (amazingly beautiful) acquisition board with isopropylic alcohol around the electrolytics, the symptoms went away (to my great joy), but only to come back to haunt me a couple of days later. I immediately repeated the cleaning and drying - same story. I replaced some of the caps in despair - nada. So I was clearly missing something. My trusty 4X Russian magnifying glass in hand, I started examining the PCB in detail around the electrolytics - and I finally spotted what you see in the pictures (click to enlarge), near the pins of U82 and U140. The discoloration was well concealed UNDER the conformal coating of the PCB and very hard to see, due to the light reflecting off the epoxy coating and the glare obscuring the surface under.
As there were no signs of electrolyte leakage from the new capacitors, the damage must have started with the old, leaky caps, and it took years to finally manifest itself as a malfunction of the instrument. The electrolyte had crawled under the conformal coating and gradually compromised the insulation, eventually disturbing trigger control potentials and making the self-test fail (it logged the error message: "trigComparatorTest, TRIGA status after trigger: exp(ected) = 1, act(ual) = 0).  The dried-up murderous electrolyte residue shows best in the second photo, of U82 (the first is of U140).
After scraping off the coating and affected spots under it with the point of a scalpel, the symptoms vanished, and this time I think it will be for good. I couldn't find any more such discolorations elsewhere on the board, but there's always the inaccessible area under the SMD  integrated circuits, so I shouldn't be unduly surprised if...!

Well, there's a nice thought to keep a test instrument lover twisting and turning in his bed at night...

27 January 2011

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!!