ST ACEPACK 1 User manual

November 2017
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www.st.com
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Technical note
Press-fit ACEPACK™ power modules mounting instructions
Introduction
ST introduces the ACEPACK™ Power Module family, designed for easy mounting and reliable
performance in rugged applications. The available module form factors are ACEPACK™ 1 with 33.8 mm
x 48 mm and ACEPACK™ 2 with 56.7 mm x 48 mm body dimensions. Various die selections in silicon
and silicon carbide substrates can be housed in several configurations.
These modules feature a compact, fully isolated, low profile housing able to integrate very high power
density components in a low junction-to-case thermal resistance DBC. Power modules simplify the
design and increase reliability, while PCB size and system costs are optimized.
The following sections provide recommendations for the connection of these modules to a printed circuit
board (PCB) and mounting and dismounting methods to achieve adequate connections, reliability and
performance in typical applications.
Figure 1: ACEPACK™ 1 and ACEPACK™ 2

Contents
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Contents
1PCB requirements...........................................................................3
2Module mounting process in a PCB ..............................................5
2.1 General press- in process.................................................................5
2.2 Multiple modules press-in process....................................................6
3Fixing a PCB to the ACEPACK™ module ......................................8
4Mounting the module to a heat sink...............................................9
4.1 Requirements for a heat sink ............................................................9
4.2 Application of thermal paste..............................................................9
4.3 Heatsink mounting ............................................................................9
5Assembly of the PCB and heat sink.............................................12
6Clearance and creepage distances..............................................14
7Revision history ............................................................................15

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PCB requirements
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1 PCB requirements
The press-fit used in the ACEPACK™ module has been qualified for standard FR4 printed
circuit boards with tin (chemically) (IEC 60352-5 + IEC60747-15). If other handling
technologies are used in the production of printed circuit boards, they would have to be
tested, inspected and qualified.
Figure 2: Plated through holes
An adequate design of the plated through holes (PTH) of a PCB is essential to obtain good
quality press-fit connections. If the finished hole diameter of the PTH is too small, the
press-in force through the plated hole may be too high and cause mechanical damage to
the pins and the PTH. If the finished hole diameter is too large, it may not form a reliable
connection with the pin.
The initial hole diameter prior to plating is important in determining the reliability of press-fit
connections. As per IEC 60352-5 specification, it should be 1.15 mm typical. The thickness
of the copper plating applied to the initial hole shall be minimum 25 μm to maximum 50 μm.
Then, a surface finish of about 1 μm chemical tin is applied to the hole.
The overall hole diameter is typically between 1.00 mm and 1.10 mm.
Table 1: Printed circuit board requirements for press-fit leads option
Press-fit leads option
Min.
Typ.
Max.
Unit
Hole drill diameter
1.12
1.15
1.20
mm
Copper thickness in hole
25
50
µm
Metallization in hole
15
µm
Finished hole diameter
1.00
1.10
mm
Copper thickness of conductors
35
70-105
400
µm

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Press-fit leads option
Min.
Typ.
Max.
Unit
Metallization of circuit board
Tin (chemically) / HAL
Metallization of pin
Tin (galvanic)
Other tin finish technologies should be avoided before verification. The HAL plating method
is not recommended because of uneven plating on the hole.
The electrical and thermal contacts with the circuit board are implemented by means of
cold welding when press-fit pins are used. Permanent deformation takes place as a result
of PCB insertion and this deformation is intended to accommodate the tolerance and
provides the basis for the cold welding. The resulting forces during the press-fit process
ensure that the welded materials on the PCB and pin exhibit a continuously consistent and,
unlike other contact technologies, very small electrical contact resistance, see Figure 3:
"Materials connected together in a gas-tight manner due to the press-in force".
Figure 3: Materials connected together in a gas-tight manner due to the press-in force
A module that has been pressed in and then pressed out again can no longer be pressed
in again. Instead, the module can only be attached to a new printed circuit board by
soldering. The plastic deformation of the press-fit zone does not permit further press-fit
processes.

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Module mounting process in a PCB
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2 Module mounting process in a PCB
2.1 General press- in process
The press-fit process is a cost-effective way to assemble power modules without
introducing additional thermal loads. The press-fit connection generates a strong
mechanical and a good electrical connection between the module and the PCB.
Figure 4: Types of equipment for press-in
Generally, a module can be pressed in until the stand-offs on the four corners of the
module touch the PCB.
A press-in tool that records the necessary force and the travel distance is recommended to
ensure appropriate quality. If possible, monitor the press-in/press-out distance, speed, and
force to achieve mechanical stability and high reliability of the press-fit connection:
the travel distance during the press-in process should be controlled to ensure that the
press-fit zone of the pins sits properly in the plated through hole.
the speed influences the quality of the press-fit connection. The speed should not be
lower than 25 mm/min according to IEC 60352-5. A lower speed can lead to increased
press-in forces and deformation of the pins or a non gas-tight connection.
Typical press-in forces vary with the finished hole diameter of your PCB and more in
general on the contact area between press-fit pin and plated through hole. Based on the
PCB requirements listed above, the following forces and speeds are recommended:
Table 2: Press-fit requirements in a printed circuit board
Press-fit requirements in a printed circuit board
Min.
Typ.
Max.
Unit
Press-fit speed
25
mm/min
Press-in force(each pin)
60
90
120
N
The maximum applied force per module during pressing should not exceed 4 kN.

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Attention should also be paid to other components like resistors, diodes or capacitors that
need to be assembled on the PCB area next to the ACEPACK™ module. PCB bending
during press-in processes can cause mechanical stress to other PCB components.
It is recommended to leave at least 4 mm between the edge of these components and the
middle of the PTH.
Figure 5: Minimum suggested distance between PTH and components
2.2 Multiple modules press-in process
When multiple modules are assembled on the same PCB and the same heat sink, the
height tolerances among the modules must be minimized to guarantee adequate contact
on the heatsink and avoid mechanical stress on the PCB. In this case, a press-in tool with
distance keepers is required to ensure the same distance between PCB and the top of the
modules, as shown in Figure 6: "Press-in process for multiple modules PCB".
Figure 6: Press-in process for multiple modules PCB
During the process, once the distance keeper contacts the surface of the PCB, the press-in
force rises and the press-in process can be stopped, thus preventing direct contact

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Module mounting process in a PCB
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between the case and the PCB. The distance keepers and the other board components
should be designed so that no contact among them can occur during the press-in process.
The module and the PCB are first placed on the lower press-in tool and the module is then
aligned to the PCB with the guide pins. Then, the upper press-in tool can start moving
down the module until the distance keepers touch the surface of the PCB.

Fixing a PCB to the ACEPACK™ module
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3 Fixing a PCB to the ACEPACK™ module
Figure 7: ACEPACK fixing PCB and cross-section
Figure 8: Guide hole for the screws in ACEPACK 1 and ACEPACK 2
After the PCB mounting process, it is recommended to fix the PCB to the module with
screws to ensure reliable contact. If an air gap remains between the module stand offs and
the PCB, do not screw to avoid PCB bending. The figures above show the details for
screwing the module. The effective length of a screw excluding the PCB thickness should
be 4.0-8.0 mm. Screwdriver speed lower than 300 rpm and screwdriver torque 0.4 Nm ±
10% is recommended.
Screwing into a plastic cavity is a delicate operation, and care has to be taken to avoid
stand-off damage. We suggest using M2.5 x L self-tapping screws, with length L depending
on the PCB thickness (stand off thread has to be between 4 and 8 mm). The screws will
self-tap into the stand-off cavity. The vertical position of the screw must also be maintained
to prevent lateral insertion. A crosswise sequence for mounting the screws is suggested
and the screwdriver should have a slow rotating speed. Typical mounting torque is 0.45 Nm
± 10 %.
Do not exceed 1 Nm to avoid screw/plastic damage.

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Mounting the module to a heat sink
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4 Mounting the module to a heat sink
The heat produced by a module must be dissipated to avoid overheating and consequent
damage. The thermal performance of a module in combination with a heat sink can be
characterized by the sum of all thermal resistances along the thermal path: junction-to-
case, case-to-heatsink,and heatsink-to-ambient, as shown in the figure below.
Figure 9: Dissipation mechanism
Proper contact between the module substrate and the surface of the heat sink is crucial for
managing the overall thermal efficiency of the system. Thermal Interface Materials (TIMs)
are thermally conductive materials used to allow proper matching of the two surfaces and
improve heat transfer.
4.1 Requirements for a heat sink
In order to maximize heat transfer efficiency, the heat sink contact surface must be flat and
clean. The required heat sink surface qualities to achieve good thermal conductivity must
be according to DIN 4768-1. Roughness (Rz) should be 10 μm or less and flatness, based
on a length of 100 mm, should be 50 μm or less. In particular, the flatness must be less
than above value in the module mounting area, including the two clamp screws.
Furthermore, the interface surface of the heat sink must be free of particles and
contamination.
4.2 Application of thermal paste
Thermal paste thickness strongly affects the thermal resistivity between the module and the
heat sink. An even layer of 80 μm is recommended. Thermal grease quantity is sufficient if
a small amount of it can be seen out of the module after the heat sink mounting process.
4.3 Heatsink mounting
Heatsink mounting on the modules requires aligning the clamp screw holes of the module
with the two thread holes on the heatsink.

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Figure 10: Screw clamping zone dimensions
In the figure below shows one-step and three-step fastening methods.
In the one-step method, the two screws are simultaneously fastened to avoid tilting on one
side of module. It is recommended to synchronize the two electric screwdrivers with the
same rpm and maximum torque.
In the three-step method:
1. fasten the first screw only until it touches the screw clamp and does not provoke tilting
of the module.
2. fasten the second screw to the final torque (see Table 3: "Technical data of the
mounting screw").
3. fasten the first screw to full torque, securing the module to the heatsink.

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Mounting the module to a heat sink
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Figure 11: Module to heatsink fastening method
Table 3: Technical data of the mounting screw
Description
Value
Mounting screw
M4
Recommended mounting tourque
2.0 –2.3 Nm

Assembly of the PCB and heat sink
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5 Assembly of the PCB and heat sink
After modules are assembled to the PCB and the heat sink, the overall structural integrity
needs to be considered to avoid mechanical stress to any of the system components. If the
PCB is large and heavy with other components assembled to it, there is some risk that the
PCB can bend, creating mechanical stress on the module and the PCB. When multiple
modules are applied to the same PCB, height tolerance between modules can result in the
mechanical stresses on the board and modules. To reduce stress, space posts should be
added on the heat sink to prevent movement of the PCB.
The recommended height of the space posts is 12.4 (+0/-0.1) mm. The effective distance
between center of stand-off and the space post (= X) is 50 mm minimum, as shown in the
following figure.
Figure 12: Assembly procedure when space posts are used
Figure 13: "Heat sink mounting" shows the overall assembly procedure when space posts
are used. First the PCB with the modules is placed onto the heatsink (a). Then the modules
are fastened to the heatsink through the screw clamps (b). Finally the PCB is fixed on the
space posts.

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Assembly of the PCB and heat sink
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Figure 13: Heat sink mounting

Clearance and creepage distances
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6 Clearance and creepage distances
When defining the layout of the PCB, application-specific standards, especially regarding
clearance and creepage distances, must be considered. This is particularly important for
the area of the screw clamp located under the printed circuit board. In order to meet the
respective requirements regarding clearance and creepage distances, current carrying
devices or through-holes in this area should be avoided, or additional isolation measures
like lacquering must be taken.
Figure 14: Air path between clip and PCB
The minimum clearance distance between the screw and the PCB depends on the screw
itself. The distance is 6.8 mm with a hexagon socket head screw according to DIN 912, a
washer according to DIN 125 and the clamp, see Figure 14: "Air path between clip and
PCB".
The clearance and creepage distances specified in the datasheet are minimum values,
irrespective of other devices that may be mounted close to the module.
In any case, the application-specific clearance and creepage distances must be checked
and compared with relevant standards and guaranteed through appropriate construction
methods, if required.

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Revision history
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7 Revision history
Table 4: Document revision history
Date
Revision
Changes
07-Nov-2017
1
Initial release.

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