Thermal interface material

AN3232

12/26

Doc ID 17594 Rev 3

6

Thermal interface material

Careful consideration must be paid to the actual interface layer between the backside of the

package and the mounting base. Cleanliness is key: particulates, grease, and oxidation

should be removed using standard practices.

A thermal interface material (TIM) of good thermal conductivity should be used in order to fill

any discontinuities between the package and mounting base surface. The decision of which

TIM material to use is based on many factors. Many types of TIMs are available with an

incredible range of thermal, electrical, and mechanical properties. A few are listed in

Table 3.

In all cases, the chosen TIM should be no thicker than necessary and applied as evenly as

possible. For liquidus compounds, it is easiest to apply it to the backside of the package,

keeping in mind that excessive TIM thickness results in a higher thermal resistance than

using no TIM at all. Application of pressure displaces liquidus TIMs in regions of metal-to-

metal contact, and fills in any remaining air gaps, thus resulting in excellent electrical and

thermal contact.

For foils and soldering, it is best to specify an appropriately sized “preform”, with a thickness

of approximately 0.003 in(0.08 mm), and apply it to the core layer or heatsink.

In soldering of Au plated components which have not been pre-treated by HSD or tinning,

enough solder should be used so that the final Au content of the multi-eutectic alloy is less

than 4 % by weight, to avoid “gold embrittlement” issues. Excessive Au content in Pb and

Pb-free alloys can result in a brittle solder joint which degrades over time with continuous

power/temperature cycling.

Table 3.

Common TIMs

Type

Description

Supplier

Paste or grease

WPSII (silicone-free)

Austerlitz Electronics

Type 340

Dow Corning

Type 120(silicone based)

Wakefield thermal solutions

Foil or film based

Indium preform

Indium

Solder based

Pb or Pb-free preform

Various

Pb or Pb-free paste

Various