STMicroelectronics TN1250 Press Fit ACEPACK Power Modules Mounting Installation Guide
- June 13, 2024
- STMicroelectronics
Table of Contents
STMicroelectronics TN1250 Press Fit ACEPACK Power Modules Mounting
Product Information
The ACEPACKTM power modules are designed for press-fit mounting on standard FR4-printed circuit boards. These modules are suitable for a range of applications and provide reliable connections.
The PCB requirements for the ACEPACKTM module include:
- Standard FR4 printed circuit boards
- Tin-plated through holes (PTH) with specified dimensions
- Surface finish of chemical tin on the holes
- Metallization of circuit board and pin
Product Usage Instructions
PCB Requirements
The ACEPACKTM module is compatible with standard FR4-printed circuit
boards. If alternative handling technologies are used, they need to be tested
and qualified.
Ensure that the plated through holes (PTH) on the PCB are designed properly:
- The finished hole diameter should be between 1.00 mm and 1.10 mm.
- The initial hole diameter before plating should be 1.15 mm.
- The copper plating thickness in the hole should be between 25 m and 50 m.
- A surface finish of about 1 m chemical tin should be applied to the hole.
Module Mounting Process in a PCB
General Press-in Process
To mount the ACEPACKTM module, follow these steps:
- Ensure that the PCB meets the requirements mentioned earlier.
- Align the module with the plated through holes (PTH) on the PCB.
- Apply a press-in force to insert the module into the PTH.
- Ensure that the module is securely attached to the PCB.
Multiple Modules Press-in Process
If multiple ACEPACKTM modules need to be mounted on the PCB, follow these
steps:
- Ensure that each module meets the PCB requirements.
- Align all the modules with their respective plated through holes (PTH) on the PCB.
- Apply a press-in force to insert all the modules into their PTH simultaneously.
- Ensure that all the modules are securely attached to the PCB.
Fixing a PCB to the ACEPACKTM Module
Instructions for fixing the PCB to the ACEPACKTM module are not provided in
the given text extract from the user manual. Please refer to the complete user
manual for detailed instructions on this process.
Mounting the Module to a Heat Sink
The ACEPACKTM module can be mounted to a heat sink using the following
steps:
- Ensure that the heat sink meets the specified requirements.
- Apply thermal paste to the contact surface of the module.
- Mount the module onto the heat sink, ensuring proper alignment.
Note : Detailed instructions for heat sink mounting are not provided in the given text extract. Please refer to the complete user manual for more information.
For additional information, including clearance and creepage distances, and revision history, please refer to the complete user manual.
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.
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.
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 before 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 a minimum of 25 μm to a maximum of 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 the 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
Metallization of the 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 oof coldwelding 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 providess the basis for 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 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.
Module mounting process in a PCB
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.
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
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.
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 the PCB and the top of the modules, as shown in 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 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
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. A screwdriver speed lower than 300 rpm and a screwdriver torqueof 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 willself-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. The typicall mounting torque is 0.45 Nm ± 10 %.
Do not exceed 1 Nm to avoid screw/plastic damage.
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.
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.
Requirements for a heat sink
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 the 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.
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.
Heatsink mounting
Heatsink mounting on the modules requires aligning the clamp screw holes of
the module with the two thread holes on the heatsink.
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 the module. It is recommended to synchronize the two electric screwdrivers with the same rpm and maximum torque.
In the three-step method:
- fasten the first screw only until it touches the screw clamp and does not provoke tilting of the module.
- fasten the second screw to the final torque (see Table 3: “Technical data of the mounting screw”).
- fasten the first screw to full torque, securing the module to the heatsink.
Table 3: Technical data of the mounting screw
| Description | Value |
|---|---|
| Mounting screw | M4 |
| Recommended mounting torque | 2.0 – 2.3 Nm |
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 mechanical stresses
on the board and modules. To reduce stress, space posts should be added to the
heat sink to prevent movement of the PCB.N The recommended height of the space
posts is 12.4 (+0/-0.1) mm. The effective distance between the centre of the
stand-off and the space post (= X) is 50 mm minimum, as shown in the following
figure.
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.
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. 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.
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.
Revision history
Table 4 : Document revision history
| Date | Revision | Changes |
|---|---|---|
| 07-Nov-2017 | 1 | Initial release. |
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