Package Mounting Methods
7. Points requiring care for special packages
Points requiring care for power devices
Power loss at the connections during operation of the device is always accompanied by an increase in temperature. Since it
is necessary that the connection temperature be below the rated maximum temperature during use, how much of a margin
to provide for use is an important point in designing system reliability.
Particularly for power devices, since the cases in which they are used at temperatures close to the rated temperatures due
to large power losses, suitable heat sink designs are important. In order to reduce the increase in heat, heat sinks are
attached. At this time, since the dimensional accuracy of the attached parts and the attachment method have a great
influence on thermal characteristics and mechanical stress, which can cause malfunctions or reduced service life,
sufficient care must be taken regarding the following points:
1) Thermal grease
Since the use of thermal greases when attaching the heat sink to the power transistor can reduce the contact thermal
resistance to the levels of R
th (j-c)
shown in Table 5, it is often used. For metal-sealed devices, this was not a problem but for
plastic-sealed power transistors, it is necessary to use care when selecting the thermal grease to use.
Table 5 Examples of contact thermal resistance and insulation plate thermal resistance
R
th(c-f)
(°C/W)
Grease applied
0.5
0.4
3.0
2.3
2.3
0.6
Grease not applied
1.0
1.7
6.0
5.0
4.5
1.5
Shape
Presence/absence of
insulation plate
No insulation plate
TO-126
Mica (100
µm)
No insulation plate
TO-220 (SC-46)
TO-220 Full pack
TOP-3 Full pack
Mica (100
µm)
No insulation plate
No insulation plate
If a highly oil-separated grease is used, the stress of the power cycle, etc. may result in faults such as partially
disconnected wires after a relatively short time. The reason for this is that the silicon oil which forms the base oil of
thermal grease separates out and penetrates into the inside of the transistor through gaps between the plastic resin and the
fins or lead frame, and swells the junction coating resin (JCR), increasing the stress on the connection base of the Al or Au
electrode lead wires and causing them to be disconnected.
Because of this, it is necessary to select a thermal grease which has low oil separation and which is based on silicon oil
which does not swell JCR easily. For example, G-746 (Shin-Etsu Chemical Co., Ltd.) and YG-6260 (Toshiba Silicone
Co., Ltd.) are recommended.
M18
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