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AN1004

Application Notes

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AN1004 - 4

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Thyristor Product Catalog

TO-218

The mounting hole for the TO-218 device should not exceed

0.164” (8/32) clearance. Isolated versions of TO-218 do not

require any insulating material since mounting tab is electrically

isolated from the semiconductor chip. Round lead or Fillister

machine screws are recommended. Maximum torque to be

applied to mounting tab should not exceed 8 inch-lbs.

The same precautions given for the TO-220 package concerning

punched holes, drilled holes, and proper prepared heat sink

mounting surface apply to the TO-218 package. Also for high-

voltage applications, it is recommended that only the metal por-

tion of the mounting surface of the TO-218 package be in contact

with heat sink. This achieves maximum oversurface distance to

prevent a high-voltage path over the device body to grounded

heat sink.

General Mounting Notes

Care must be taken on both packages at all times to avoid strain

to the tab or leads. For easy insertion of the part onto the board

or heat sink, avoid axial strain on the leads. Carefully measure

mounting holes for the tab and the leads, and do any forming of

the tab or leads before mounting. Refer to the “Lead Form

Dimensions” section of this catalog before attempting lead form

operations.

Rivets may be used for less demanding and more economical

applications. 1/8" all-aluminum pop rivets can be used on both

TO-220 and TO-202 packages. Use a 0.129”-0.133” (#30) drill for

the hole and insert the rivet from the top side, as shown in Figure

AN1004.9. An insertion tool, similar to a “USM” PRG 430 hand

riveter, is recommended. A wide selection of grip ranges is avail-

able, depending upon the thickness of the heat sink material. Use

an appropriate grip range to securely anchor the device, yet not

deform the mounting tab. The recommended rivet tool has a pro-

truding nipple that will allow easy insertion of the rivet and keep

the tool clear of the plastic case of the device.

Figure AN1004.9

Pop Riveting Technique

A Milford #511 (Milford Group, Milford, CT) semi-tubular steel

rivet set into a 0.129" receiving hole with a riveting machine simi-

lar to a Milford S256 is also acceptable. Contact the rivet

machine manufacturer for exact details on application and set-up

for optimum results.

Pneumatic or other impact riveting devices are not recommended

due to the shock they may apply to the device.

Under no circumstance should any tool or hardware come into

contact with the case. The case should not be used as a brace

for any rotation or shearing force during mounting or in use. Non-

standard size screws, nuts, and rivets are easily obtainable to

avoid clearance problems.

Always use an accurate torque wrench to mount devices. No gain

is achieved by overtorquing devices. In fact, overtorquing may

cause the tab and case to deform or rupture, seriously damaging

the device. The curve shown in Figure AN1004.10 illustrates the

effect of proper torque.

Figure AN1004.10

Effect of Torque to Sink Thermal Resistance

With proper care, the mounting tab of a device can be soldered to

a surface. However, the heat required to accomplish this opera-

tion can damage or destroy the semiconductor chip or internal

assembly. See “Surface Mount Soldering Recommendations”

(AN1005) in this catalog.

Spring-steel clips can be used to replace torqued hardware in

assembling thyristors to heat sinks. Clips snap into heat sink

slots to hold the device in place for PC board insertion. Clips are

available in several sizes for various heat sink thicknesses and

thyristor case styles from Aavid Thermalloy in Concord, New

Hampshire. A typical heatsink is shown in Figure AN1004.11

Figure AN1004.11

Typical Heat Sink Using Clips

1/2 Rated

Torque

Rated

Torque

Torque – inch-lbs

˚C/Watt

C-S

Effect of Torque on Case to Sink

Thermal Resistance

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