paste size vs pad size

In a technology-independent library you shouldn't use a smaller paste pad on your footprint unless the package calls for a specific stencil design (as with QFNs). This is because each board may have different decrement requirements based on the thickness of the stencil and the type of stencil and paste used.

You can either ask the stencil vendor to apply a universal decrement or do what I do and create a manufacturing pastemask layer and use Skill to create a stencil pattern matching the particlar requirements of each board, type of device and stencl technology.

Typically for SMD land paterns like resistors J leaded parts etc, the paste size is the same as the physical land. On items like power IC'S that have a ground bonding pad the manufacturer of the part will have specific requirements for the paste mask.

Now something that is not very obvious. On a square or rectangular pad there is a trend to make the pad convex or have a slight arc at the end of each pad. The reason is this. When a stencil is created for the paste mask, the laser cant create square holes or rectangular holes but instead the ends of the holes will be rounded slightly.

Manufacturers of stencils can compensate for this, but the ideal is to have the pad shape the same as what the stencil manufacturer can actually make so as to yeild the best solder paste coverage and easy release of the stencil from the board.

Thanks Scott

Thanks.

I'm curious about the shape you are talking about. Do you just mean rounded corners? Perhaps this is more of an issue on very small parts?

The corners are not really rounded, but the top of the rectangle is convex, basically a slight arc at the top of the pad... Similar to how the stencil is made using a laser cut.

Thanks Scott