Component Mounting

Lead length and thermal resistance

Component Lead Length, Lead Temperature vs Power Dissipation
Component Lead Length vs Power Dissipation

Lead length and Heat Transfer;

The graph above shows Maximum Power vs Lead Temperature and Lead Length.
Heat is removed from components by convection, radiation and conduction. The amount of heat released by the main body of the component depends on the shape and size of the body. With axial lead mounted parts heat is removed by conduction as heat travels down both leads of the component to the printed wiring board the part is mounted to. Heat is then conducted into the copper pads used to interface to the component. Assuming equal lead length, heat is conducted down both leads away from the component body in an identical manner because the thermal resistance of the leads is identical. Thermal resistance is proportional to lead length, the shorter the leads the lower the thermal resistance to heat conduction.

Axial Lead Mounting Considerations;
Note that resistors are shown but the text applies to any axial lead component; Resistors, Diodes and so on. Component Sockets should always be avoided because they increase the lead length [component plus socket lead length].

The graph above shows that the maximum power dissipation is achieved when the component has a zero lead length; however that would define a surface mount component using pads for terminals and not leads [as shown by a TO-276 body style or any other lead-less package]. The power dissipation is further reduced as the lead length grows from 0.125, 0.25 and to 0.375 inches, which results in the lowest power dissipation.

3-Terminal Lead-Less Transistor Package
Horizontal-Mount Component Lead Length,
Horizontal Mounting

The leads should be bent near the component body to keep the leads as short as possible, and also insure the body is raised up just above the surface of the PWB. However the leads should not be bent right at the body, which could cause damage to the body to terminal connection point. The short leads between the DO-41 and the Printed Circuit Board [PCB] allow more efficient heat conduction into the board [Horizontal mounting].

Vertical mounting is also possible and results in even more efficient heat conduction than horizontal mounting because of the single short lead. The shorter the leads the lower the thermal resistance between the DO-41 body and the PCB. The lower the thermal resistance the better the heat conduction between the component and the board.

Vertical-Mount Component Lead Length
Vertical Mounting

Axial leaded parts with identical lead lengths are the most common, but the leads are in parallel. A single short lead offers lower thermal resistance than equal length leads.


Thermal Resistance vs Copper Pad Area
Copper Pad Resistance

Related topics:
Types of Diode cases
Diode Derating Guidelines
Diode Derating Curves
Diode Manufacturers

Resistor Manufacturers
Resistor Derating Curves
Resistor Mounting Considerations

Zener Diode Manufacturers
Zener Diode Terms

Through-Hole Diode Lead Temperature
Diode Lead Temperature

Through-Hole Axial Leaded Thermal Impedance
Axial Lead Thermal Impedance
PC motherboard

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Modified 6/13/15
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