The capacitors in the middle of the photograph are not a pretty sight, especially if found on your mother-board! Notice the bulging tops and the discoloration due to the leaking electrolyte. (Photo courtesy Carey Holzman - www.careyholzman.com
Early in 2002, stories began appearing in on-line news services and news groups about the high failure rates of electrolytic capacitors used on PC motherboards. Technicians were reporting that the capacitors were rupturing, leaking and even exploding like never before.
Initially, there were only two clues to the mystery. First, the
failing capacitors were more often that not to be found in the power supply section of motherboards. The capacitors used in this area are characterised by their need to have very low ESR (Equivalent Series Resistance- see panel).
Second, most of the failing capacitors were identified as
Taiwanese in origin. That's not too surprising at first glance, as Taiwan manufactures about 30% of the world's aluminium electrolytics (22.5 billion a year).
In September, "Passive Industry Components Magazine" published
a story that exposed the reasons behind the unusually high failure rates. They reported that the failures were directly related to the use of faulty electrolytes in the manufacturing process.
The story describing how the electrolytes came to be faulty
reads like a lot of fiction. It begins in Japan, at a major capacitor manufacturer. A materials scientist for the Japanese company resigned and went to work for a Chinese capacitor manufacturer. While there, he reproduced one of the electrolytes used in his former employer's premium (low-ESR) aluminium
Staff working with the scientist then defected, taking the
secret electrolyte formula with them. They used the formula to manufacture their own electrolyte, which they subsequently flogged to major Taiwanese capacitor manufacturers at bargain prices. Unfortunately, their reproduction of the formula was flawed and the rest is history.
A bad case of the squirts
So how does the faulty electrolyte cause early failure? Here we
can only speculate. Some stories have stated that the flawed formula causes electrolysis, which in turn generates lots of hydrogen. Eventually, gas pressure ruptures the can or breaches the rubber end seal. We are inclined to think that since the electrolyte is used in low-ESR capacitors, the typically high ripple currents and resulting heat causes decomposition of the electrolyte. Either way, the result is the same - the capacitor eventually leaks or ruptures.
Apart from eventual self-destruction, leakage from these
capacitors can also damage nearby components and circuit board copper, as the electrolyte compounds are quite corrosive.
When these capacitors fail, the signs are generally quite
obvious. The top of the case may be split open or "bulged" upward and/or the can may be dislodged from its base (the rubber seal). There may also be an unpleasant smell and signs of electrolyte leakage nearby.
Even more obvious is the muffled explosion followed by the
blank screen. Thankfully, we've heard that this failure mode is quite rare!
However, before catastrophic failure eventuates, all kinds of
annoying symptoms can occur. These can range from intermittent boot failures to lock-ups in Windows. Eventually, the affected PC will refuse to boot at all, perhaps even before the culprits have showed themselves!
Unfortunately, there is no certain way of determining whether a
particular motherboard is affected by this problem or not - until it fails. However, some of the big name manufacturers have identified their high-risk machines and have notified their customers accordingly.
What to do
If you've purchased a PC during the last 18 months or so,
especially if it's not one of the big name brands, then it's worthwhile doing a quick check for any visible signs of the capacitor problem.
This motherboard stopped working and was returned to the workshop for repair. It wouldn't have taken the techos long to identify the problem. Check the bulging tops on these suckers! On some boards, up to a dozen capacitors have to be replaced.
We should point out here that any number of other hardware or
software problems can cause the symptoms mentioned above. Just because your PC has problem "X" does not mean that it is caused by faulty motherboard capacitors!
The capacitors to look for will be the largest (highest capacitance) types on the motherboard. Generally, they'll be situated somewhere near the power supply connectors. Also, you'll probably notice one or two toroidal inductors in close proximity. As designs vary so much between manufacturers and models, it's impossible for us to be more specific.
Unless you're experienced with such things, we don't recommend
that you disassemble your PC. You'll probably be able to get a pretty good idea whether any of the larger capacitors are swollen or leaking with the board in-situ. A bright flashlight will help you here.
It's important to note that these capacitors are failing within
a short period of use. This means that even if you're unlucky enough to be affected, it should be covered under your system's warranty!
OK, so your motherboard is out of warranty and you've
determined that it has faulty capacitors. What do you do?
First up, you'll need access to the appropriate desoldering
equipment in order to extract the offending parts without damaging your motherboard. Just as importantly, you'll need to identify and source suitable replacements.
As mentioned earlier, these capacitors are of a specific type;
they have very low ESR. To understand the need for this requirement, let's take a brief look at the circuitry involved.
Probably due to the fact that CPU core voltages change so
often, designers have been forced to implement sections of the power supply circuitry on the motherboard. The standard power supply box still provides the usual 12V, 5V and 3.3V rails, but the lower voltages for the CPU core are provided by further step-down circuitry on the motherboard.
This on-board switchmode step-down circuitry runs at high
frequencies (over 100kHz) to minimise the required inductance and filter capacitance. A key ingredient in this recipe is physically small electrolytic capacitors that can handle high ripple currents at high frequencies. In short, they must have very low impedance at the switching frequency.
As you can probably imagine, capacitors designed for use in
mains filtering applications are not up to the job at all. They are physically much larger for the same capacitance and have high characteristic impedance at high frequencies. This would result in unacceptable amounts of ripple voltage and self-heating, leading to early failure.
The "Rubycon" (Japanese) brand ZL and ZA series ultra-low
impedance capacitors will be suitable in most cases. They're available locally from Farnell Electronic Components - see Farnell's web site at www.farnell.com for more information.
Caution: the standard ZL and ZA series may be marginally larger in diameter than the original parts (10mm versus 8mm).
Any electronics equipment that incorporates high-frequency
switchmode circuitry could be affected by this problem, including power supplies, monitors and games consoles, to name a few.
Only time will tell!
"Normal" Electrolytic Capacitor Failure
This ESR meter is ideal for checking the health of electrolytic capacitors. You can buy the kit from Dick Smith Electronics.
All electrolytic capacitors have a finite life, measured in
thousands of hours. Unlike the exceptional cases discussed in this article,
there are usually no external signs that a capacitor is nearing its end of life.
However, it is possible to determine whether a capacitor is serviceable or not
by measuring it's ESR (Equivalent Series Resistance).
ESR is the sum of in-phase AC resistance, including the
resistance of the dielectric, plates, electrolytic material and leads at a
particular frequency. As the name implies, ESR acts just like a resistor in
series with the capacitance.
Towards end of life, a capacitor's ESR begins to increase as
its dielectric losses increase. To test a capacitor's ESR, you need an ESR
meter; an ordinary capacitance meter usually won't indicate a problem. These
instruments are an indispensable part of any technicians toolbox.
Commercial ESR meters are expensive. However, an excellent unit
was described by Bob Parker in the January 1996 edition of "Electronics
Australia". Kits for this project are still available from Dick Smith
Electronics and of course reprints of the original article can be obtained from SILICON CHIP.