Westcode Semiconductors 07AN01 Application note

Date: 13th January 2010

Issue: 2

WESTCODE

An IXYS Company

Application note for device mounting

instructions

Introduction

This document outlines the minimum mounting conditions for Westcode high power press-pack
diodes, thyristors, GTO’s and IGBT’s.

Recommendations for interface properties

The interfaces to the device pole faces (either heatsink or busbar) must conduct both thermal
and electrical energy from the device. It is important that these interfaces maintain a stable
contact throughout the lifetime of the equipment to ensure reliable operation of the device. Both
the surface geometry and finish are important factors to consider.

Surface roughness

The surface roughness is a measure of the microstructure of the surface and is expressed as a

value as per BS EN ISO 4287:1998+A1 2009. A roughness, Ra ≤1.6µm for all contact

R

a

surfaces is recommended.

Surface flatness

Flatness is a measure of the net variation of a surface defined by two parallel planes. A flatness
of 30µm is required for thyristors, diodes and GTO’s and 10µm for IGBT’s for all interface
surfaces within the clamping structure and clamp force range specified in the device data sheet.

Note 1: The flatness of an IGBT greater than that specified in the clamped condition may be
seen prior to the device being clamped. This is due to differential thermal expansion of the
housing components post manufacture and in no way reflects or impacts the device flatness
once it is clamped to the nominal force.

Note 2: The stack components must only deflect elastically. A flatness greater than the
maximum specified may result if plastic deformation of the contact surfaces (such as cooler
collapse) occurs under loading.

Westcode recommends the use of Fuji Prescale film; see www.fujifilm-prescale.eu
film product to confirm the pressure uniformity of the mechanical design of the assembly. The
film is manufactured in a number of different pressure ranges and should be inserted between
the device contact face and the cooler or heatsink. A good pressure distribution and a non­uniform pressure distribution are shown in figures 1 and 2 respectively.

or a similar

Application Note Type: Mounting instructions Page 1 of 2 January, 2010

WESTCODE An IXYS Company Application Note 07AN01 Issue 2

Figure 1: Example of good pressure

distribution

Figure 2: Example of poor pressure

distribution

Surface finish

In order to maintain a good electrical connection and avoid corrosion over time, Westcode
recommends that all non-aluminium contact surfaces be nickel-plated. Chemical plating is
preferable to electroplating in high reliability applications. Plating depth should be 4-6µm in
accordance with that applied to the device.

Surface preparation prior to assembly

All contact surfaces should be clean and dry prior to assembly. If necessary all non-plated
contact surfaces should be lightly abraded to remove oxide films with a rotary wire brush using
a suitable contact grease to form a slurry or alternatively polished using 3M Scotchbright™ or a
similar product. Note that the slurry produced by the abrasion should be left on the contact
surface until the device is ready to be mounted (to prevent re-oxidation). The contact surfaces
should then be cleaned using ethanol or similar solvent and a lint free cloth. Lint free gloves
should be worn when handling prepared parts. A very thin film of suitable mounting grease,
such as Jetlube SCX13 (Westcode part number XSGSCX13) should be applied to the device.
When the device has been clamped to full load a small bead approximately 0.1mm in diameter
should be squeezed out from between the contact surfaces.

Mounting Force, F

M

The mounting force, F

is the recommended force to be applied for optimal device performance.

M

The data sheet ratings are not guaranteed if the mounting force is lower than that specified in
the data sheet. The thermal impedance and the on-state voltage drop will increase, and the
short circuit current rating will decrease when the force is reduced below the rated value.

Too high a mounting force could reduce the load cycling capability. The mounting force must be
uniformly applied across the whole area of the pole face, this is particularly important for press­pack IGBT’s. Variations in contact pressure of more than 10% across the pole face are not
permitted.

Application support

A full range of clamps, coolers and other assembly parts are available from Westcode.

Application Note Type: Mounting instructions Page 2 of 2 January, 2010