Additional information
8 of 31
Revision 4.0
2021-10-11
Recommendations for board assembly of Infineon transistor outline
type packages
Package description
𝑅
𝑡ℎ 𝑗𝑎
= 𝑅
𝑡ℎ 𝑗𝑐
+ 𝑅
𝑡ℎ 𝑐𝑠
+ 𝑅
𝑡ℎ 𝑠
+ 𝑅
𝑡ℎ 𝑠𝑎
(1)
R
th ja
thermal resistance junction to ambient (K/W)
R
th jc
thermal resistance junction to case (K/W) – specified in relevant datasheet
R
th cs
thermal resistance case to sink (K/W)
R
th s
thermal resistance sink (K/W)
R
th sa
thermal resistance sink to ambient (K/W)
When mounting a package on an external heat spreader, it is important to consider the interface resistance
R
thcs
. In real application cases, R
thcs
will exceed the ideal value of 0. That is caused by small air gaps between the
package and the heat spreader that can have the following reasons:
Surface roughness and waviness of package heat sink and external heat spreader
Slight tilt between package heat sink and external heat spreader
For insulated packages such as the PG-TO220 FullPAK, the use of thermal grease is recommended to fill the air
gap between the package and the heat sink. Measurements have shown that the usage of thermal grease
reduces the interface resistance by 1.2 to 1.5 K/W.
In many applications, the package must be electrically isolated from its mounting surface. The isolation
material has a relatively high thermal resistance, which raises junction operating temperatures.
For further information about thermal performance, please refer to the
Thermal performance of surface mount
semiconductor package
s
document that is available on the Infineon web page [2]. Please also feel free to
contact your local sales, application, or quality engineer.
Termination design
The gullwing lead is considered to be one of the most reliable terminations for SMD. To form gullwing
terminations, the leads are bent outwards at the tip. These bent input/output (I/O) lead foot and heel areas
form the seating plane that is then soldered to the PCB. The leads as well as the heat sink pad feature a
solderable surface for board mounting. The distance between the I/O lead seating plane and the exposed pad
landing area at the package bottom is defined as the package stand-off. The single-ended gullwing lead row of
TO SMD will result in a non-symmetric stand-off between the lead landing area and the heat sink pad.
The solder joints are mainly formed underneath the package. The tips of such terminations often feature bare
copper (e.g. cut edges) and are therefore not intended to wet with solder by design according to IPC-A-610 [7].
Figure 7
Cross-section of a HSOG package lead with proper solder connection to board.
