|Ask the Experts|
September 8, 2020
Stencil Pattern for Thermal Pads on QFNs
We need more information regarding the stencil pattern for thermal pads on QFNs. We know why it is important to "mesh" but need more info regarding what is best aperture size, number of apertures, etc. Can you point us to information?
|Expert Panel Responses|
Although, a little dated, the IPC has published a document from which some basic recommendations can be gleaned.
IPC-7093 - Design and Assembly Process Implementation for Bottom Termination Components
Technical Support Engineer
QFNs, like all devices, are designed with electrical, mechanical and thermal requirements in mind. The enlarged "ground" pad on the bottom of the QFN is engineered not only to provide the electrical connection but also to aid in dissipation of thermal energy to the underlying copper plane. It is important to follow the IC vendor's recommendation for the amount of solder or solder paste required on the underside of the device and for all other solder connections on the package.
Ensure that if you "mesh" (I assume you mean provide "window pane" layout under the device to aid paste outgassing) the pattern compensates for the lack of paste in the pane frame areas ("streets"). Also make sure that the volume of paste under the QFN doesn't buoy the device too high so as to cause opens or poor solder joints on peripheral connections.
Without knowing exact device size, contact patterns and PCB layout, it is impossible to recommend aperture size details for your application. Always start with the IC package manufacturer's technical data sheet for the solder paste requirements and optimize from there. Your finished pattern should closely approximate what is recommended in the data sheet in order to meet reliability requirements for that particular QFN.
There are some basic rules for QFN pad arrays. A widely used one is to use square pads of 1 mm to 2 mm in length and to provide a distance between these pads of 0.4 mm. Your stencil house should be able to help you more on your specific components.
Senior Manufacturing Engineer
Recommended aperture design for QFN thermal pads is 40-50% reduction of paste printed using window pane pattern. Also to try to keep paste out of open via's.
Vice President Technology
There is no one size fits all for thermal pads on QFN's. This is going to depend on the package, size of the thermal pad, board thickness, number of via's etc.
Some component suppliers will have stencil design guidelines in the data sheet for the specific part. You will need to verify the volume of solder is correct.
If you cannot find stencil guidelines for your particular part, you may have to try looking at a part with a similar package design and thermal properties. Also most stencil suppliers are familiar with these guidelines, they may help as well.
Hope this helps.
Manufacturing Applications Specialist
It is recommended to reduce the perimeter of the ground pad slightly before making additional modifications.
Once the perimeter is reduced (I suggest a minimum of 0.0025" per side), create a pattern that results in 65-70% coverage of the remaining area. Square apertures with a radius at the corner is recommended for optimal paste release. It is also good practice to design the pattern around open via's to allow for out gassing.
Integrated Ideas & Technologies, Inc.
We recommend reducing the total printed paste area to 50-60% of the ground pad based on the board copper layer and using 5 round apertures who's combined volume adds up to 50-60%. These aperture sizes will change with the size of the ground pad on the QFN, e.g. larger ground pads require larger printed paste dots.
We call this pattern a 5-dice pattern because the printed solder paste looks like the dots on dice. This pattern allows for maximum gas escape routes in order to minimize the potential for voiding.
IPC 7525 should give you everything that you need to know about apertures design.
Now, for your particular case, this spec has more details: IPC-7093 standard, "Design and Assembly Process Implementation forBottom Termination Components".
On the same time take in consideration the new stencil manufacturing technologies that provide increased paste release.
Engineering and Operations Management
IPC 7525, details aperture designs for QFN's. However PCB DFM will allow you to make to calculate this correct volume requirement.
General rules are as follows,
50% to 60% reduction to allow the component to solder and not float during reflow.
Meshing can be done in any shape provided scopping will not occur.
Avoid using large thermal via's in your design. If large via's are required then calculate the volume required to fill the via hole plus the 50% required for the soldering of the terminal plane.
Some via's can be plugged by your PCB manufacture, make use of this. If you are unsure, contact; Asahitec Stencils Pvt.Ltd., www.asahitec.in for any stencil DFM and stencil manufacturing. they are the best in providing DFM input for your product.
Process Engineering Manager - Electronics
Altech UEC, South Africa
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