Apertures for the thermal pad of the QFN component
Long ago, IPC published the IPC-7093 Implementation of the Design and Assembly Process for Bottom Leaded Components, from which some basic recommendations can be drawn regarding the shape of apertures under QFN.
QFN, like all components, is designed with electrical, mechanical and thermal requirements in mind. The larger thermal pad on the underside of the QFN is designed not only to provide an electrical connection, but also to efficiently dissipate thermal energy to the underlying copper plane. It is important to follow the IC supplier's recommendations regarding the amount of solder paste required on the underside of the QFN and on all other solder joints of this type of case.
When designing apertures under the QFN, make sure that the paste pattern beneath the component allows the paste to degas, leaving paths that can lead to volatiles evacuated during the brazing process. In addition, you should also make sure that too much paste under the QFN does not push the chip too high, which could result in missing connections or poor solder connections on the pads around the perimeter of the component.
Gary Freedman of Colab Instruments talks about this as follows: 'It's hard to talk about very specific aperture shapes without knowing the exact component size, lead pattern, or PCB layout. So you should always start by reading the technical specifications provided by the manufacturer of the IC, which can be further optimized if necessary. The finished formula should exactly match what is recommended in the datasheet to meet the reliability requirements for that particular QFN. '
Brien Bush, application engineer at Cirtronics Corp., has a very similar opinion: 'There is no one size fits all for thermal pads in QFN. Their size depends on the housing, thermal pad size, PCB thickness, number of vias, etc. Some component vendors include template design guidelines in the component specification. Conversely, if you cannot find template hints for your specific layout, you can try looking for a component with similar housing design and thermal properties. Also, most template vendors are familiar with these guidelines, and they can help too. '
However, if you have to decide on the shape of the aperture for QFN yourself, we have gathered a handful of advice from PCBA specialists. Stephanie Nash of Integrated Ideas & Technologies, Inc. it is recommended to slightly reduce the circumference of the print, but not less than 0.0025 inch on each side. Then create an aperture pattern that will cover the remaining area by 65-70%. For optimal release of the paste, we recommend the use of square holes with a radius in the corner. Further, it is good practice to design a pattern around the open vias to allow outgassing.
Tony Lentz of FCT's recommendations are slightly different: he recommends reducing the total area of the printed paste to 50-60% of the size of the thermal pad, measured based on the copper layer on the PCB, and using 5 circular holes with a total surface area of 50 -60% of the calculated rectangle. The aperture sizes change with the QFN thermal pad size, i.e. larger pads require larger paste deposits. 'We call this pattern a 5-dice pattern because the printed solder paste looks like dots on a cube. This design also creates effective escape routes for volatile substances, minimizing the possibility of void formation, 'says Tony Lentz.
You can also find a lot on this in the recent article 'Void Reduction in QFN' by Tim O'Neill.