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I'm comparing two datasheets from Micron and Infineon to the Sparkfun and Adafruit footprints for TSSOP-28

Both the footprints use an SMD pad size of 1.0x0.3 mm, while the manufacturers recommend something closer to 1.3x0.4.

Why would they do this? Should I undersize my own footprints?

Infineon Microchip

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  • \$\begingroup\$ Because they have man-decades of engineering experience and know where they can make changes without negatively affecting the finished product. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 30 '16 at 18:49
  • \$\begingroup\$ These things are complex and they depend on the soldering profile and paste granule size also. Don't trust any source blindly and also consult your PCB manufacturer. Although, for TSSOP it's not that critical, since the paste can also flow upwards in the reflow soldering... \$\endgroup\$ – PkP Oct 30 '16 at 18:56
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TSSOP-28 package is manufactured by many companies, and so many datasheets exist involving this package. I am sure if you will take a dozen of datasheets detailing footprint for this type of package, you will have at least two versions of pad sizes.

In your particular case I think someone, creating devices for these chips, took TSSOP-28 package from ref-packages library and applied to the device without comparing to particular datasheet.

I had a case when I designed footprint for the chip as manufacturer advised, and soldering chip by hand was simply nightmare - because pads were just short for proper access by soldering iron to put solder onto.

Thus if manufacturer recommends longer pads I would follow its advice. Regarding width of pads - up to you, depends on hand soldering skill you have and flux you use.

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There are probably different ways to answer this, but my opinion: most Sparkfun and Adafruit customers are probably hobbyists and small manufacturers, who will not necessarily have access to high-volume soldering process. They will probably solder by hand, or use inexpensive automatic soldering systems. So they need more spacing between pads to prevent solder bridges, and make cleaning flux easier.

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