Application Notes
AN1004
Allow for adequate ventilation. If possible, route heat sinks to out-
side of assembly for maximum airflow.
Mounting Surface Selection
* Screw head must not touch
the epoxy body of the device
* Mounting
screw
Proper mounting surface selection is essential to efficient trans-
fer of heat from the semiconductor device to the heat sink and
from the heat sink to the ambient. The most popular heat sinks
are flat aluminum plates or finned extruded aluminum heat sinks.
The mounting surface should be clean and free from burrs or
scratches. It should be flat within 0.002 inch per inch, and a sur-
face finish of 30 to 60 microinches is acceptable. Surfaces with a
higher degree of polish do not produce better thermal conductiv-
ity.
6-32
Heatsink
Lockwasher
6-32 Nut
High potential appication
using Isolated TO-220
On heavy aluminum heatsinks
Figure AN1004.7
TO-220 Mounting
Many aluminum heat sinks are black anodized to improve ther-
mal emissivity and prevent corrosion. Anodizing results in high
electrical but negligible thermal insulation. This is an excellent
choice for isolated TO-220 devices. For applications of TO-202
devices where electrical connection to the common anode tab is
required, the anodization should be removed. Iridite or chromate
acid dip finish offers low electrical and thermal resistance. Either
TO-202 or isolated TO-220 devices may be mounted directly to
this surface, regardless of application. Both finishes should be
cleaned prior to use to remove manufacturing oils and films.
Some of the more economical heat sinks are painted black. Due
to the high thermal resistance of paint, the paint should be
removed in the area where the semiconductor is attached.
Bare aluminum should be buffed with #000 steel wool and fol-
lowed with an acetone or alcohol rinse. Immediately, thermal
grease should be applied to the surface and the device mounted
down to prevent dust or metal particles from lodging in the critical
interface area.
For good thermal contact, the use of thermal grease is essential
to fill the air pockets between the semiconductor and the mount-
ing surface. This decreases the thermal resistance by 20%. For
example, a typical TO-220 with RθJC of 1.2 °C/W may be lowered
to 1 °C/W by using thermal grease.
Punched holes are not acceptable due to cratering around the
hole which can cause the device to be pulled into the crater by
the fastener or can leave a significant portion of the device out of
contact with the heat sink. The first effect may cause immediate
damage to the package and early failure, while the second can
create higher operating temperatures which will shorten operat-
ing life. Punched holes are quite acceptable in thin metal plates
where fine-edge blanking or sheared-through holes are
employed.
Drilled holes must have a properly prepared surface. Excessive
chamfering is not acceptable as it may create a crater effect.
Edges must be deburred to promote good contact and avoid
puncturing isolation materials.
For high-voltage applications, it is recommended that only the
metal portion of the TO-220 package (as viewed from the bottom
of the package) be in contact with the heat sink. This will provide
maximum oversurface distance and prevent a high voltage path
over the plastic case to a grounded heat sink.
TO-202
The mounting hole for the Teccor TO-202 devices should not
exceed 0.112” (4/40) clearance. (Figure AN1004.8) Since tab is
electrically common with anode, heat sink may or may not need
to be electrically isolated from tab. If not, use 4/40 screw with
lock washer and nut. Mounting torque is 6 inch-lbs.
Teccor recommends Dow-Corning 340 as a proven effective ther-
mal grease. Fibrous applicators are not recommended as they
may tend to leave lint or dust in the interface area. Ensure that
the grease is spread adequately across the device mounting sur-
face, and torque down the device to specification.
A
B
Appropriate
Screw
Contact Teccor Applications Engineering for assistance in choos-
ing and using the proper heat sink for specific application.
Tab
Form
4/40 Nylon
Bushing
Hardware And Methods
Mica
Insulator
TO-220
The mounting hole for the Teccor TO-220 devices should not
exceed 0.140” (6/32) clearance. (Figure AN1004.7) No insulating
bushings are needed for the L Package (isolated) devices as the
tab is electrically isolated from the semiconductor chip. 6/32
mounting hardware, especially round head or Fillister machine
screws, is recommended and should be torqued to a value of
6 inch-lbs.
Nut
Heat Sink
Heat Sink
at Case
Potential
Compression
Washer
Figure AN1004.8
TO-202 Mounting
A nylon bushing and mica insulation are required to insulate the
tab in an isolated application. A compression washer is recom-
mended to avoid damage to the bushing. Do not attempt to
mount non-formed tabs to a plane surface, as the resulting strain
on the case may cause it or the semiconductor chip assembly to
fail. Teccor has the facilities and expertise to properly tab form
TO-202 devices for the convenience of the consumer.
©2002 Teccor Electronics
Thyristor Product Catalog
AN1004 - 3
http://www.teccor.com
+1 972-580-7777
TECCOR [ TECCOR ELECTRONICS ]
