onsemi AND90206 Full Plastic Case Modules Installation Guide

onsemi AND90206 Full Plastic Case Modules

Product Information

The F1/F2 Full Plastic Case Modules with Press-fit Pins are a type of module that features a full plastic case and press-fit pins. These modules are designed for various applications and offer several features, including mounting with press-fit pins, tolling design, PCB specifications, and thermal interface material.

Features

• Mounting with Pressfit Pins
• Tolling Design
• PCB Specifications
• Thermal Interface Material

Product Usage Instructions

Specification of PCB
The minimum PCB thickness for the F1/F2 modules is 1.6 mm. It is recommended to have solder mask on both sides of the PCB. The recommended PCB hole plating options include immersion tin, immersion silver, electroless nickel immersion gold (ENIG), and organic solderable preservative (OSP). HAL plating is not recommended. For detailed PCB specifications, please refer to Table 1.

Design Restrictions within Mounting Area
During the press-in process, PCB bending can cause mechanical stress to other PCB components. To ensure safe mounting, a recommended minimum space of 4 mm between the center of the plated through hole and the edge of the component should be maintained. Please refer to Figure 4 for layout restrictions.

PCB Fixing and Dimensions
Spacers should be used to fix the PCB to the heatsink. The number of space posts is not specified, but they should be designed symmetrically, considering the weight of the PCB components. PCB bending or flexing should be avoided, and the distance from the space post to the module (dimension x) should be at least 10 mm from the module’s outer edges.

Press-in Process
The press-fit connection provides a strong mechanical and electrical connection between the module and the PCB. To achieve suitable press-fit connections, various types of presses are available, ranging from simple toggle presses to automated pneumatic presses. When performing the press-in process, it is recommended to monitor the press-in/press-out distance, speed, and force to ensure mechanical stability and high reliability of the connection. The travel distance should be controlled to ensure proper placement of the press-fit zone of the pins in the plated through hole. The speed should follow the recommendations set by the IEC standard.

Mounting Guideline for

• F1/F2 Full Plastic Case
• Modules with Press-fit Pins

AND90206/D

Introduction

This application note deals with mounting instructions of modules F1/F2 with full plastic case and press−fit pins. It includes specification of press−fit tooling thermal interface material and mounting with thermal interface material.

Features

• Mounting with Pressfit Pins
• Tolling Design
• PCB Specifications
• Thermal Interface Material

AND90206/D

Specification of PCB
Minimum PCB thickness is 1.6 mm. Solder mask is recommended on both sides of the PCB. Recommended PCB hole plating options include immersion tin, immersion silver, electroless nickel immersion gold ENIG), and organic solder able preservative (OSP). HAL plating is not recommended. For PCB specifications please see Table 1.

Table 1. PCB SPECIFICATIONS FOR F1 AND F2 MODULES WITH 1.2 MM PRESS−FIT PINS

Design Restrictions within Mounting Area

PCB bending during the press−in process causes mechanical stress to other PCB components, such as capacitors and resistors. Experiments to verify a safe minimum distance between passive components and the plated through hole were conducted with FR4 PCB. Various sizes (0603, 0805, 1206, 1210, 1812, and 2220) of mechanically sensitive components were evaluated. Based on experimental results, the recommended minimum space between center of the plated through hole and the edge of the component is 4 mm, as shown in Figure 4.

PCB Fixing and Dimensions
Spacers should be used to fix the PCB to the heatsink. Number of space posts is not given. Space posts positions should be designed symmetrically, weight of the PCB components should be considered. PCB bending or flexing should be avoided. Distance from the space post to the module (dimension x) is recommended to be at least 10 mm from module outer edges.

Length of the space posts should match the length of press−in tool distance keeper – see section Press−in tool for F2 package.
Recommended length of the space post is 12.4 mm + 0 mm 0.05 mm. An air gap may be present between the module and PCB (as on Figure 5), this airgap allows tolerance of the module case.

Press−in Process
The press−fit connection generates a good electrical, and strong mechanical connection between the module and the PCB. This section deals with the mounting process to achieve suitable press−fit connections here are several types of presses available: from simple toggle presses to the automated pneumatic presses shown in Figure 6.

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 also influences the quality of the press−fit connection; therefore, speed recommended by IEC standard should be applied.

General Press−in Process

Use of tooling with distance keeper is recommended. Figure 8. Shows the general sequence of press in procedure. Press−in procedure. The press−in tool is comprised of two parts: the upper press−in tool is flat (or with special design for modules with TIM. Please see Press−in tool for module with PCM section) to contact with the module backside evenly and the lower press−in tool has engraved spaces to accommodate the press−fit pins and PCB components. The two parts of the tool need to be aligned to each other. In the first step of the assembly, the printed circuit board is placed on the alignment pins of the lower part of the press−in tool (a). Then, the module is placed on top of the printed circuit board using the alignment pins (b).

It is necessary to check if the module and the printed circuit board are in alignment. In the next step, the press−in force is applied via the upper part of the press−in tool to the backside of the module evenly. The module should be pressed−in with a speed of 25~50 mm/min until the distance keepers of the upper tool touch the PCB while press−in distance and force are monitored at the same time (c). It is required to adjust the traveling distance of the press to avoid damages to the module case due to pressure being applied. A simple manual press does not use a distance sensing system, so a distance keeper should be designed on the press−in tool to terminate press−in process appropriately. When the distance keeper contacts the surface, press−in force rises sharply, and the press−in process can be terminated by reaching the limit of the press−in force. The distance keeper should be designed to avoid the collision with other PCB components.

Figure 9. Distance Keeper of the Upper Tool Design
The total press−in force is the result of the number of pins in a module, multiplied with the force required for a single pin. Press−in forces lower than 40 N/pin mean that press−fit pin may have a less secure connection in the plated through hole. The primary reason for the low press−fit force is that the diameter of plated through hole is too large for the press−fit pins. Press−in forces higher than 80 N/pin can cause mechanical damage to the press−fit terminal, the PTH, or to the tracks on the PCB. The recommended press−in speed ranges from 25 mm/min to 50 mm/min in accordance with the recommendations in IEC 60352−5.

The press−fit pins must be pressed into the holes of the PCB to the correct depth. The center of the press−fit zone has to be at least 0.5 mm below the top surface and at least 0.5 mm above the bottom surface of the PCB (Figure 10).

Figure 11. Force vs. Distance during Pressfit

Process

• Force measured in a module is a sum of all pins being pressed at once (Figure 11).
• Force measurement per one pin was conducted, the press−fit pin was mounted in 1.6 mm thick PCB with chemical Sn surface finish. Test conditions and PCB specification are compliant with IEC 60352−5.

Table 2. SINGLE PRESS−FIT PIN TEST

1.02 mm

| f1.11 –

1.12 mm

| f0.98 –

1.02 mm

| f1.11 –

1.12 mm

 | (min)| (max)| (min)| (max)
Minimum| 49.8 N| 40.9 N| 29.4 N| 38.3 N
Mean| 55.9 N| 47.4 N| 40.5 N| 48.2 N
Maximum| 62.1 N| 57.1 N| 48.0 N| 65.9 N
Sample size| 20| 20| 19| 20

Press−in Tool for Module with PCM
To prevent damages on PCM peripheral spacers and point spacers are required. This spacers should not touch PCM on the DBC. 0.4 mm is recommended for the height of spacers. Distance keeper of the press−in tool needs to be 0.4 mm taller than no PCM cases.

Figure 12. Press−fit Tool for F2 Module with PCM

Peripheral spacer and point spacer prevent pre−applied PCM from being deformed or damaged during press−in process. Peripheral spacer distributes mechanical stress by press−in process over the edge of

DBC

Press−out Process
In some situations, it is necessary to remove power modules from the PCB. It is possible to disconnect the contact between module pins and PTH. The press−out process can be performed with the same equipment used in the press−in process. Careful handling in the press−out process is essential to avoid mechanical damage to both the module and the PCB. PCB can be re−used once with a new module.

Please note : in case a module which was pressed out of a PCB should be used again, it is necessary to solder the module to the PCB; this is because the press−fit zone will remain deformed after the press−out process. An additional press−in cycle will result in low holding forces between the press−fit pin and PCB hole.

Figure 15. Press−out Process of Power Module

• Step 1
  Put the printed circuit board assembled with modules on the lower press-out tool.

• Step 2
  Upper press-out tool moves downward and start to press the projecting part of the press-fit pins.

• Step 3
  Process should be terminated as soon as the upper press-out tool contacts the surface of PCB. Module falls down to the lower tools disassembled with PCB.

Heatsink Specification
The following surface qualities are required for the heatsink to achieve a good thermal conductivity, according to DIN 4768−1. Roughness (Rz) should be 10 m or less and flatness, based on a length of 100 mm, should be 50 or less. The heatsink should have no contamination, unevenness, and burrs on the surface contacting the module.
The interface surface of the heatsink must be free of particles and contamination. Avoid handling the heatsink surface with bare hands or contacting any foreign materials. If it is necessary to remove contamination from heatsink, cleaning can be accomplished using dry cloth soaked with solvent, such as isopropyl or ethylene alcohol.

Figure 16. Heatsink Surface Specification

Thermal Grease
Thermal grease can be applied to the heatsink or the module substrate using a rubber roller or spatula or by screen printing. Alternatively, apply thermal paste by screen printing, for example using a honeycomb pattern. The recommended thermal paste thickness is 80−180 m. Thickness of the TIM layer more than this recommendation will unnecessarily increase thermal resistance. When applying thermal grease, the material must be applied uniformly on the whole surface which is in contact to the module substrate surface. If the module is re−mounted, surfaces should be cleaned, and TIM needs be applied again.

Pre−applied Phase–Change Material
Modules can be pre−applied with phase−change material. Typical thickness of the TIM layer is 160 m and its thermal conductivity is 3.4 W/mK.

It comes pre−applied on the DBC surface ready to be mounted on the heatsink. The honeycomb pattern allows the press tool to touch copper DBC and allows PCM to spread on a whole surface. For proper TIM spreading in application, TIM spreading temperature must be achieved on the TIM material, this temperature is specified as minimum 45C. For best spreading results it is recommended to apply 80C for 20 minutes.

Screw Specification
When using screws with flat washers

• Metric screw: M4 (recommended screw type DIN7984)
• Flat washer: D = 8 mm ISO 7092 (DIN 433)
• Spring washer: D = 8 mm DIN 127 or DIN 128
• Mounting torque : 1.6−2.0 Nm
• Screw holes on heatsink need to be countersunk. A torque wrench shall be used to tighten the mounting screws at the specified torque. Excessive torque may result in damage or degradation of the device. The inaccuracy of torque wrench tightening method can range up to 12%. This must be considered to prevent over−tightening the fastener. Due to excessive temperature fluctuations washers should be used to prevent the loosening of the screws. After accurate tightening of the screws the spring washer exerts a constant force on the joint. The flat washer distributes this force on the plastic surface. When using screws with pre−assembled washers : Screws with pre−assembled washers (SEMS or kombi screws) combine the screw and the washers into a single component. These screws eliminate the need to slip the washers into place by hand, boosting the speed and efficiency of the assembly process. The specifications of these screws are provided below:
  • Screw size : M4 according to DIN 6900 (ISO 10644; JIS B1188)
  • Flat washer: According to DIN 6902 Type C (ISO 10673 Type S; JIS B1256)
  • Washer outer diameter : 8 mm diameter can be fitted onto the module
  • Split lock spring washer : According to DIN 6905 (JIS B1251)
  • Mounting torque range : 1.6−2.0 Nm
  • Recommended insertion of screw thread in the heatsink is 6 mm.

Methods of Screw Clamping
There are two recommended screw clamping methods which apply to all modules. The F1 module is used as an example. Figure 18 describes one method for fastening the module to the heatsink. Fasten two screws simultaneously to prevent tilting or rising of one side of module during fastening. Electric screwdrivers can tighten the screws with the specified torque. Screw holes on heatsink need to be countersunk. If method 1 cannot be applied, the method as described in Figure 19 is also acceptable. Fasten the first screw loosely to prevent tilting or rising of the module (step 1). Then insert the second screw with final torque to be fully tightened with the heatsink (step 2). Finally, apply full torque to the first screw for solid tightening with the heatsink. For F1/F2 packages using metal clips, the torque is between 1.6−2.0 Nm using M4 screws.

Assembly of Multiple Modules on the PCB and Heatsink
The overall structure of the mounted module should be considered. If the PCB is large and heavy with other components assembled to it, there is some risk the PCB can bend, creating mechanical stress to 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 heatsink, as illustrated on figure 19 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. If distance keepers are used during the press−in process, resulting in tighter height tolerances; distances between the stand−off of the case and the space post (= X) smaller than 50 mm can be used. Figure 19 shows the assembly procedure when space posts are used and the overall assembly structure: Modules are first pressed into the PCB following the recommendations introduced in Section “Heatsink Surface” before heatsink mounting. Maintaining tight height tolerances between module and PCB is important. Next, the thermal interface material is applied. Then the modules and the PCB are placed on the heatsink (a). Then the module is mounted onto the heatsink via the module plastic clamp. Finally, the PCB needs to be fixed on the space posts (c).

• Step 1 Place modules assembled with printed circuit board and TIM onto the heat sink.
• Step 2 Fasten the screw clamps of modules to the heat sink.
• Step 3 Screwing PCB with space post.

Figure 20. Assembling Multiple Modules on a PCB

Handling of PCM Pre−applied Module
During a transport and storage of the modules extreme thermal and mechanical shock should be avoided. The tray is designed to prevent direct contact to the PCM layer of the module substrate as shown in the Figure 21. PCM_ pre applied modules should be stored in this tray box before its use. Table 3 shows recommended storage conditions.

Table 3. RECOMMENDED STORAGE CONDITIONS FOR MODULES WITH PCM

PCM printing layer should be treated as a functional area of the module and be protected from damage or removing when handling or mounting. PCM pre−applied module is delivered in a tray box with a tight cover.

Figure 21. Modules in a Transport Tray Box

It is recommended to open the cover carefully side−by side to prevent mechanical damage to the PCM layer. Also during the assembly process attention is needed not to touch the PCM layer directly. If the PCM layer is contaminated or more thant 10% of the entire printed area is damaged, then it is recommended to remove the PCM layer according to instructions stated below. In case of rework prior to operation of modules, standard PCB / heat sink disassembly process can be applied. If the module was operated and the PCM melted and distributed already, then bond strength between heat sink and the module may be strong, and the module cannot be easily removed from the heatsink. In such cases it is recommended to use a knife to detach the module or apply some heat (45−60C) to re−melt PCM to detach the module easily. Use a soft plastic scraper to remove the PCM layer from the module back side and heat sink. For the removal of remaining PCM residues it is recommended to use microfiber cloth and isoprophyle alcohol.

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