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Guidelines for Printed Circuit Board Design

Designing Circuits Using Non-Solid and Solid Aluminum Electrolytic Capacitors

 

Precaution

1. Do not locate any copper traces under the seal/terminal side or body of a capacitor.

2. Avoid locating any heat- producing component near capacitors or under capacitors on the opposite side of the PC board.

3. For surface mount capacitors, design the lands/pads on a PC board according to specifications in the product literature.

Explanatory Notes

1. Do not locate any copper traces under the seal/terminal side of a capacitor or under the body of a capacitor for the following reasons.

1.1 If copper traces on the PC board are located under the rubber seal/terminal side of a non-solid aluminum electrolytic capacitor and electrolyte leakage occurs, the electrolyte may dissolve the traces, bridge the traces or migrate to other traces causing a short-circuit or destroying the PC board.

1.2 Even under normal use, electrolyte vapor diffuses through the rubber seal of a capacitor and accumulates in the area where the capacitor is flush mounted to the PC board. This vapor may adversely affect the circuitry and the capacitor when any copper trace is located under the capacitor.

1.3 Copper traces subjected to the following phenomena and environmental conditions may affect the capacitors and circuit integrity.

(1) Existence of moisture: Moisture accelerates current and causes corrosion. If moisture is a problem, capacitors should be damp-proof coated for protection.

(2) Strength of electrical field: Existence of electric potential accelerates erosion of the traces.

(3) Existence of halogen ions: Halogen ions are accelerator factors and are present in flux activators, cream solder or cleaning agents.

(4) Existence of highly conductive substances: Ag (silver), Cu (copper) and Sn (tin) are most commonly used and may cause circuit problems.

1.4 When designing a double-sided PC board, do not print any traces within the area where the capacitor body will be mounted.

1.5 For an axial lead capacitor, the aluminum can is the cathode and is welded to the negative lead wire. Consequently, do not print any copper trace within the area of the PC board where the capacitor will be mounted. Heat from the traces will break the outer sleeve of the capacitor, expose the aluminum can and cause a short circuit.

2. Locating any heat-producing component near a capacitor or on the reverse side of the PC board under the capacitor will increase the capacitor temperature and may adversely affect the expected lifetime. If estimated lifetime is simply based on ambient temperature and the internal temperature of the capacitor is not considered, the following situations will mislead you in estimating capacitor lifetime.

(1) Radiant heat from nearby components will raise the internal temperature of the capacitor above the ambient temperature.

(2) Heat from a large, heat-producing component located on the reverse side of the PC board will travel into the capacitor through the circuit traces and raise the internal temperature of the capacitor.

3. For surface mount capacitors, printing inaccurate copper land/pad patterns on the PC board will impair solderability and result in poor connections.