Aluminum electrolytic capacitors remain the practical way to store bulk energy on many power-input and bulk-rail nets. They offer high capacitance per cost and a wide voltage range. They also punish casual layout and careless SMT heat: polarity mistakes, excess reflow temperature, and ignored ripple current still dominate early field returns that get blamed on "a bad batch."
If your overseas PCBA builds keep seeing bulged cans or dry-out after months, start with lifetime math and thermal distance, not only with supplier accusations.
What sits inside the can
An aluminum electrolytic is polarized. Etched aluminum foil raises surface area, an oxide film acts as the dielectric, and electrolyte completes the structure with separator paper between foils. After impregnation, the element is sealed in a metal can or, in some styles, a resin body.
That liquid electrolyte is why soldering heat is a reliability input. Peak reflow temperature and preheat ramp rate can stress seals and chemistry in ways chip resistors never experience. Datasheet reflow limits are process requirements, not suggestions.
SMD versions often sit vertically on a plastic or rubber base that makes automated placement practical. Through-hole versions still appear where height, ripple, or mechanical preference wins. Both need clear polarity marks on the part and on the PCB silkscreen or assembly drawing.
Where they earn their keep
Typical uses include bulk input filtering, rail smoothing, energy hold-up, and low- to mid-frequency decoupling where MLCCs cannot economically provide the capacitance. Consumer power sections, industrial supplies, LED drivers, and general PCBA designs still rely on them heavily.
Selection should start from capacitance and voltage, then immediately include ripple current rating, ESR, temperature rating, lifetime hours at stated temperature, can size, and mounting style. A substitute with the same uF and voltage but weaker ripple or temperature rating is not equivalent.
SMT and rework discipline
Confirm polarity before placement: the dark stripe on many electrolytics marks the negative terminal and must match the board mark. Reverse mounting can mean leakage, overheating, or violent failure.
Use a reflow profile suited to the capacitor family, not the hottest lead-free curve the board's other parts can survive. For hand work, keep iron contact short; prolonged localized heat cooks seals. Tall cans in vibration environments may need adhesive support so solder joints are not the only mechanical anchor.
Keep electrolytics away from hot MOSFETs, inductors, and heatsink shadows when the layout allows. Lifetime roughly tracks electrolyte temperature; a cooler can is often worth more than a marginally larger capacitance.
BOM notes that prevent silent lifetime cuts
Write lifetime target and ambient assumption next to critical bulk parts. List approved alternates only when ripple, temperature, and size are equal or better. Mark polarity and keep-outs on the assembly drawing. During shortages, reject "same uF" swaps that demote thermal ratings without a documented lifetime review.
For assembly context, align capacitor constraints with SMT PCB assembly profiling and, when turnkey sourcing is used, with components purchasing rules so alternates cannot silently enter the kit.