AN3232
Application note
Mounting recommendations
for STAC
®
boltdown packages
Introduction
RF power transistors are amongst the highest power density devices in the semiconductor
industry. It is crucial to the reliability and performance of such devices to consider
mechanical stress and thermal and electrical resistance within the application environment.
The general purpose of this application note is to provide guidelines for mounting various
types of STAC
®
packages in amplifiers or application boards (PCB) by means of bolting or
by soldering. Specific attention is paid to the STAC244B and STAC265B boltdown styles and
the STAC244F and STAC265F flangeless styles, which are used to encapsulate numerous
VDMOS and LDMOS technology products.
This application note is intended to provide mounting tips and design guidelines. For actual
data please refer to the relevant product datasheet.
STAC is a registered trademarks of STMicroelectronics.
Figure 1.
STAC boltdown packages
August 2011
Doc ID 17594 Rev 3
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www.st.com
Contents
AN3232
Contents
1
2
Epoxy sealed, non-hermetic RF power packages . . . . . . . . . . . . . . . . . 5
Exceptional thermal performance potential of the STAC package
concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Heatsink selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Core preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Mounting base surface conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Thermal interface material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Seating plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Printed circuit board (PCB) considerations . . . . . . . . . . . . . . . . . . . . . 15
Package attachment to core by means of boltdown method . . . . . . . 16
9.1
9.2
9.3
Required hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Procedure summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3
4
5
6
7
8
9
10
Package attachment to thermal base by means of soldering . . . . . . . 20
10.1
10.2
Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Solder reflow equipment and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
11
12
13
Electrical connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
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Doc ID 17594 Rev 3
AN3232
List of tables
List of tables
Table 1.
Table 2.
Table 3.
Table 4.
Table 5.
Table 6.
Preferred copper core thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Surface conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Common TIMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Recommended screw torque . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Pb-free process - package classification reflow temperatures . . . . . . . . . . . . . . . . . . . . . . 21
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
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List of figures
AN3232
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
STAC boltdown packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Drying times . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Infrared imaging of a STAC package . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Pocket depth consideration (mm) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Lead bending . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Package cut and pads layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Ideal screw-center spacing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
First boltdown mounting steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
SEHO FDS “MAXIPOWER” reflow oven . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Component level temperature profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
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Doc ID 17594 Rev 3
AN3232
Epoxy sealed, non-hermetic RF power packages
1
Epoxy sealed, non-hermetic RF power packages
Epoxy sealed products, such as the STAC package family, should be received in an N2-
backfilled, vacuum-sealed ESD bag containing a desiccant. This decreases the possibility of
moisture uptake by the package materials during transit and long-term storage. STAC
packages carry a moisture sensitivity rating of 3 (MSL3), but have demonstrated capabilities
of up to MSL1.
Even with such shipping methods, it is advised to store epoxy sealed packages in the
original containers, or in a dry box, until required for soldering. When the environmental
history of devices is not well-known, such as after a prolonged storage period, it is a well-
practiced safety measure to bake non-hermetic packages for 24 hrs at 125 °C prior to
soldering operations. These conditions can be accelerated or lengthened as a function of
temperature, up to the limits imposed by shipping containers, by trays, or by device
maximum ratings. Refer to
Figure 2
for recommended baking conditions.
Figure 2.
Drying times
Under no circumstances should any epoxy-sealed package be treated as a hermetically
sealed package, due to the fact that such sealing materials are not impervious to moisture
ingress. However, it is important to point out that RF Power packages are typically
constructed using engineering ceramics and polymers which, on their own, demonstrate
high resistance to moisture penetration, such as the proprietary liquid crystal polymer
materials upon which STAC packages are realized.
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