http://onsemi.com
41
Surface Finish
Surface finish is the average of the deviations both above and below the mean value of surface height.
For minimum interface resistance, a finish in the range of 50 µin. to 60 µin. is satisfactory. A finer finish
is costly to achieve and does not significantly lower contact resistance. Tests conducted by Thermalloy
using a copper TO–204 (TO–3) package with a typical 32 µin. finish, showed that heatsink finishes between
16 µin. and 64 µin. caused less than ±2.5% difference in interface thermal resistance when the voids and
scratches were filled with a thermal joint compound.(3) Most commercially available cast or extruded
heatsinks will require spotfacing when used in high power applications. In general, milled or machined
surfaces are satisfactory if prepared with tools in good working condition.
Mounting Holes
Mounting holes generally should only be large enough to allow clearance of the fastener. The larger
thick flange type packages having mounting holes removed from the semiconductor die location, such as
the TO–204AA, may successfully be used with larger holes to accommodate an insulating bushing, but
many plastic encapsulated packages are intolerant of this condition. For these packages, a smaller screw
size must be used such that the hole for the bushing does not exceed the hole in the package.
Punched mounting holes have been a source of trouble because if not properly done, the area around
a punched hole is depressed in the process. This “crater” in the heatsink around the mounting hole can
cause two problems. The device can be damaged by distortion of the package as the mounting pressure
attempts to conform it to the shape of the heatsink indentation, or the device may only bridge the crater
and leave a significant percentage of its heat–dissipating surface out of contact with the heatsink. The first
effect may often be detected immediately by visual cracks in the package (if plastic), but usually an
unnatural stress is imposed, which results in an early–life failure. The second effect results in hotter
operation and is not manifested until much later.
Although punched holes are seldom acceptable in the relatively thick material used for extruded
aluminum heatsinks, several manufacturers are capable of properly utilizing the capabilities inherent in
both fine–edge blanking or sheared–through holes when applied to sheet metal as commonly used for
stamped heatsinks. The holes are pierced using Class A progressive dies mounted on four–post die sets
equipped with proper pressure pads and holding fixtures.
When mounting holes are drilled, a general practice with extruded aluminum, surface cleanup is
important. Chamfers must be avoided because they reduce heat transfer surface and increase mounting
stress. Ho w ever, the edges must be broken to remove burrs which cause poor contact between device and
heatsink and may puncture isolation material.
Surface Treatment
Many aluminum heatsinks are black–anodized to improve radiation ability and prevent corrosion.
Anodizing results in significant electrical but negligible thermal insulation. It need only be removed from
the mounting area when electrical contact is required. Heatsinks are also available which have a nickel
plated copper insert under the semiconductor mounting area. No treatment of this surface is necessary.
Another treated aluminum finish is iridite, or chromate–acid dip, which offers low resistance because
of its thin surface, yet has good electrical properties because it resists oxidation. It need only be cleaned
of the oils and films that collect in the manufacture and storage of the sinks, a practice which should be
applied to all heatsinks.
For economy, paint is sometimes used for sinks; removal of the paint where the semiconductor is
attached is usually required because of the paint’s high thermal resistance. However, when it is necessary
to insulate the semiconductor package from the heatsink, hard anodized or painted surfaces allow an easy
installation for low voltage applications. Some manufacturers will provide anodized or painted surfaces
meeting specific insulation voltage requirements, usually up to 400 V.
It is also necessary that the surface be free from all foreign material, film, and oxide (freshly bared
aluminum forms an oxide layer in a few seconds). Immediately prior to assembly, it is a good practice to
polish the mounting area with No. 000 steel wool, followed by an acetone or alcohol rinse.
________________________________
(3) Catalog #87–HS–9 (1987), page 8, Thermalloy, Inc., P.O. Box 810839, Dallas, Texas 75381–0839.