QFN Style Computer On Module Advantages¶
Since industrial applications lead to faster, higher integrated and lower power electronic circuits with a smaller formfactor, the PCB layout becomes more and more important. With a good layout many EMI problems can be minimized to meet the required specifications. It’s not only the pinout which should lead to an easy wiring without the need for crossings. It has also to provide a proper solution for the signal path back to the module. If this return path, mostly the ground plane, cannot be connected near the signal pin, the return current has to take another way and this may result in a loop area.
With the special design of KaRo’s QSMP Series you profit from different advantages.
- Large ground pad on the bottom side makes a defined ground plane connection available for all signals.
- The pinout leads to an easy wiring without crossing.
- Ground will be connected near the signal pin to avoid loop areas.
- High speed signals can be routed on the top layer at a defined impedance.
- Ground pad holds the component at a defined height during soldering.
EMC Quality by Design¶
The reason PCB layout becomes more and more important is because of the trend to faster, higher integrated, smaller formfactors, and lower power electronic circuits. The higher the switching frequencies are, the more radiation may occur on a PCB. With good layout, many EMI problems can be minimized to meet the required specifications.
When a module or component is used in a design, the supplier specifies the basis for such a layout. It‘s not only the pinout which should lead to an easy wiring without the need for crossings. He also has to provide a proper solution for the signal path back to the module. If this return path, mostly the ground plane, cannot be connected near the signal pin, the return current has to take another way and this may result in a loop area. The larger the area, the more radiation and EMI problems may occur.
Ka-Ro QSCOM modules uses a large ground pad on the bottom side. With this a defined ground plane connection is available for all signals. In addition to have a good return path for all signals this large ground pad can be used for cooling.
Using a large solder pad underneath the component has not only electrical and thermal advantages. It is also used to hold the component at a defined height during soldering, without the solder being compressed by the weight of the components, which could result in short circuits.
The five 1mm pads in the square GND pad cutout can be omitted if no JTAG Boundary Scan test is used.
The ground pad solder mask on the bottom side of the QSCOM module is divided into sections for a better reliability of the solder joint and self-alignment of the component.
If the via holes used on the application board have a diameter larger than 0.3 mm, it is recommended to mask the via holes to prevent solder wicking through the via holes. Solders have a habit of filling holes and leaving voids in the thermal pad solder junction, as well as forming solder balls on the other side of the application board which can in some cases be problematic. The 0.7mm wide solder mask stripes can be used to arrange the vias.
Ka-Ro QSCOM modules are compatible with industrial standard reflow profile for Pb-free solders. Ka-Ro will give following recommendations for soldering the module to ensure reliable solder joint and operation of the module after soldering. Since the profile used is process and layout dependent, the optimum profile should be studied case by case. Thus following recommendations should be taken as a starting point guide.
- Refer to technical documentations of particular solder paste for reflow profile configurations
- Avoid using more than one flow.
- A 120μm stencil thickness is recommended.
- Aperture size of the stencil should be 1:1 with the pad size.
- A low residue, “no clean” solder paste should be used due to low mounted height of the component.
Recommended stencil design¶
The QSCOM module consume more than 1 W of DC power. In any application where high ambient temperatures for more than a few seconds can occur, it is important that a sufficient cooling surface is provided to dissipate the heat. The thermal pad at the bottom of the module must be connected to the application board ground planes by soldering. The application board should provide a number of vias under and around the pad to conduct the produced heat to the board ground planes, and preferably to a copper surface on the other side of the board in order to conduct and spread the heat. The module internal thermal resistance should in most cases be negligible compared to the thermal resistance from the module into air, and common equations for surface area required for cooling can be used to estimate the temperature rise of the module. Only copper planes on the circuit board surfaces with a solid thermal connection to the module ground pad will dissipate heat. For an application with high load the maximum allowed ambient temperature should be reduced due to inherent heating of the module, especially with small fully plastic enclosed applications where heat transfer to ambient air is low due to low thermal conductivity of plastic. The module measured on the evaluation board exhibits a temperature rise of about 20°C above ambient temperature. An insufficiently cooled module will rapidly heat beyond operating range in ambient room temperature.