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QSMP
QS FamilyQFN Style Solder-Down
Computer-on-Modules• Solder-down version• 27mm square• 2.3mm total height• QFN type lead style
◦ 1mm pitch◦ 100 pads◦ Thermal pad
• Visual solder joint inspection possible after soldering• Single-sided assembly• High speed design compliant• 3.3V power supply
Key Features• Processor STM32MP1 Series
Dual-Core Arm® Cortex®-A7 650MHzCortex-M4 209MHz
• RAM 128MB up to 512MB DDR3L• ROM 4GB eMMC
SLC NAND on request
• Grade Industrial• Temperature -25°C to 85°C (eMMC)
STM32 MP15xC onlySTM32 MP157 only n/a on STM32 MP151
2020
-02-
19
QSMP
QFN Style Computer On ModuleAdvantages
Defined Return Path
The reason PCB layout becomes more and more important isbecause of the trend to faster, higher integrated, smallerformfactors, and lower power electronic circuits. The higherthe switching frequencies are, the more radiation may occuron a PCB. With good layout, many EMI problems can beminimized to meet the required specifications.
When a module or component is used in a design, thesupplier specifies the basis for such a layout. It‘s not onlythe pinout which should lead to an easy wiring without theneed for crossings. He has also provide a proper solution forthe signal path back to the module. If this return path,mostly the ground plane, cannot be connected near thesignal pin, the return current has to take another way andthis may result in a loop area. The larger the area, the moreradiation and EMI problems may occur.
Ka-Ro QSCOM modules uses a large ground pad on thebottom side. With this a defined ground plane connection isavailable for all signals. In addition to have a good returnpath for all signals this large ground pad can be used forcooling.
Easy Wiring - Even 2-layer printed circuitboards can be used.
With a solid ground plane on the bottom layer, high speedsignals can be routed on the top layer at a definedimpedance. However, this is only possible if a peripheral orplug can be connected directly without crossing the routing.
Advanced Soldering
Using a large solder pad underneath the component has notonly electrical and thermal advantages. This is also used tohold the component at a defined height during soldering,without the solder being compressed by the weight, whichcould result in short circuits.
See figure above for the suggested module layout. Thefive 1mm pads in the square GND pad cutout can beomitted if no JTAG Boundary Scan test is used. Thesolder mask openings are shown below.
The ground pad solder mask on the bottom side of theQSCOM module is divided into sections for a betterreliability of the solder joint and self-alignment of thecomponent.
If the via holes used on the application board have adiameter larger than 0.3 mm, it is recommended tomask the via holes to prevent solder wicking throughthe via holes. Solders have a habit of filling holes andleaving voids in the thermal pad solder junction, as wellas forming solder balls on the other side of theapplication board which can in some cases beproblematic. The 0.7mm wide solder mask stripes canbe used to arrange the vias as shown here:
Ka-Ro QSCOM modules are compatible with industrialstandard reflow profile for Pb-free solders. Ka-Ro willgive following recommendations for soldering themodule to ensure reliable solder joint and operation ofthe module after soldering. Since the profile used isprocess and layout dependent, the optimum profileshould be studied case by case. Thus followingrecommendations should be taken as a starting pointguide.
• Refer to technical documentations of particular solder paste for reflow profile configurations
• Avoid using more than one flow.• A 150μm 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
Aperture size of the stencil is 1:1 with the pad size.Four 1.7mm diameter bumps are used for each of the4.3mm square GND pads sections giving a 50% solderpaste padding. The lower component settling with thisensures that the pads at the edge are always solderedeven at vertical misalignment by distortion or warping.
Thermal Considerations
The QSCOM module consume more than 1 W of DCpower. In any application where high ambienttemperatures for more than a few seconds can occur, itis important that a sufficient cooling surface is providedto dissipate the heat. The thermal pad at the bottom ofthe module must be connected to the application boardground planes by soldering. The application boardshould provide a number of vias under and around thepad to conduct the produced heat to the board groundplanes, and preferably to a copper surface on the otherside of the board in order to conduct and spread theheat. The module internal thermal resistance should inmost cases be negligible compared to the thermalresistance from the module into air, and commonequations for surface area required for cooling can beused to estimate the temperature rise of the module.Only copper planes on the circuit board surfaces with asolid thermal connection to the module ground pad willdissipate heat. For an application with high load themaximum allowed ambient temperature should bereduced due to inherent heating of the module,especially with small fully plastic enclosed applicationswhere heat transfer to ambient air is low due to lowthermal conductivity of plastic. The module measuredon the evaluation board exhibits a temperature rise ofabout 20°C above ambient temperature. Aninsufficiently cooled module will rapidly heat beyondoperating range in ambient room temperature.