What Can You Learn from Unwrapping an Electrolytic Capacitor?

[PeteS in CA]

In any electronic product, good design, proper components, and good workmanship are keys to a high quality product. The same is true of the individual components used in the end product. While there is much that is not visible to the eye, many aspects of quality - good or bad - are visible. In an earlier article, http://www.badcaps.net/forum/showthread.php?t=979, I described the process of unwrapping an electrolytic capacitor and what will be found.

Several aspects of quality can be seen on the outside of an electrolytic capacitor. The rubber plug at the base of the can should have space around the leads to permit proper solder flow, and channels to permit the flow of flux cleaner under the installed capacitor. Some cheapies (e.g. the Lelons I’ve unwrapped) don’t. The plastic sleeve on the outside of the capacitor should look seamless, the markings - ratings, logo and series, and date code - should be clear and undistorted. While the sleeve doesn’t affect the function of the part, it may be an indicator of the degree of care with which the part was made. After removing the sleeve, examine the can. The rubber plug must be securely retained, both by a compression around the circumference of the can and by bending over the edge of the can. If the rubber plug isn’t securely retained (e.g., cheapies skimp the bent-over lip), electrolyte can leak or evaporate, leading to premature failure of the part. Also, look at the can itself for scratches or scarring caused by the machinery used to make the part. Scratches and scars indicate poorly adjusted machines.

Once the can has been opened to permit removal of the core, more can be seen. If the can isn’t more or less filled by the core, this could lead to several possible problems with the part. If the core can move within the can, shock or vibration can damage the core more easily. Also, a reasonably filled can permits better dissipation of heat from the core through a better thermal path to the case and ambient air. Better heat dissipation can mean a longer useful life. A core that doesn’t fill the can may be a clue that the manufacturer skimped on foil and over-etched the foil used in the part to compensate for the lost surface area. Over-etched foil is more brittle and may be susceptible to cracking, which can cause opens, reduced capacitance, or hot-spots where cracks have narrowed the ripple current path. Major, name-brand, electrolytic capacitor manufacturers use tape to keep the core tightly wound. The width of the tape should be approximately the same as the height of the core. Using narrow tape or using the separator paper, glued, can lead to the core unraveling and an increase in the part’s ESR and/or a decrease in its capacitance (because the electrolyte is the real cathode of the part and conducts the ripple current, it must be in full contact with the anode foil for full capacitance and ripple current conduction).

Once the foils and separator paper strips are unrolled, there is still more to see. While the “peaks” and “valleys” etched into the foils are microscopic, the “finish” of each foil - color and sheen - should be consistent over the length of each foil. Irregularities may indicate production process damage due to poor handling or foreign debris. Closely examine the lead flags that are swaged onto the foils. Having multiple swages on each flag decreases the contact resistance between the flag and the foil; fewer swages may be less expensive, but at the potential cost of higher contact resistance. Did the manufacturer cover the flag with an extra layer of separator paper? This may mean that the manufacturer’s swaging process creates jagged points that can cause shorts; the extra slips of paper are intended to prevent those shorts, but may not always be sufficient. Look closely at the areas of foil around the lead flags. Cracks or tears in the foils may mean either that the foil is brittle due to over-etching or that the swaging machine exerted excessive force.

[yanz]

interesting.. i was chopping the caps and couldn't really understand what's with the jelly, paper, the color, etc..

[arneson]

Just to add something interesting about the rubber plug.
I just opened one up that had an added piece embedded in the rubber.
Rubber was black but small phenalic reinforcement facing out on the bottom was brown.
It seemed to be an improved design of rubber plug.

[kc8adu]

i had some real oddball brands from a gould motor control with what appears to be epoxy on the bottom.likely to keep it from drying out or to prevent freon tf from getting in and causing corrosion.