
AN1004
Application Notes
http://www.teccor.com
+1 972-580-7777 
AN1004 - 4
2002 Teccor Electronics
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
θ