Dimension Examples for QFN

Terminology Reference Symbol Description
Nominal dimensions D x E Combination of package length D and package width E
Package length D Largest dimension of the package length excluding terminals
Package width E Largest dimension of the package width excluding terminals
Overall length HD
or
HD
Length from the tip of the terminal to another tip of the terminal on the opposite side
Overall width HE
or
HE
Width from the tip of the terminal to another tip of the terminal on the opposite side
Seating plane S The surface determined when the package is placed on a horizontal surface
Datum surface A A Calculated by crossing 2 straight lines by the Least Squares Method from both lengthwise and then widthwise terminal center positions, then making the X axis (lengthwise) surface datum A.
Datum surface B B Calculate by crossing 2 straight lines by the Least Squares Method from both the widthwise and lengthwise terminal center positions, then making the Y axis (widthwise) surface datum B.
Seated height A Height from the seating plane to the highest point of the package ( including package warp )
1st standoff height A1 Distance from the seating plane to the base plane
Terminal pitch e Terminal pitch. This represents theoretical reference dimension.
Terminal width b Width of terminal with plating
Terminal width b1 Width of terminal before plating
Length of soldered part Lp Effective projected length of terminal attachment defined
Tolerance value of terminal center position Positional tolerance for center axis of the terminal defined by the datum S (seating plane) , A and B.
Using the maximum material principle, show the tolerance for maximum terminal width. When terminal width is narrow, allowable tolerance is increased by the difference with the maximum width.
Coplanarity y Uniformity of the bottom-most surface of the terminal relative to the datum S (seating plane).
Parallelism of package top surface y1 Parallelism of package top surface relative to the datum S (seating plane).
Positional tolerance of terminal tips t Positional tolerance for tips of terminal defined by the datum S (seating plane), A and B.
Terminal thickness c Thickness of each terminal(including plating)
Terminal thickness c1 Thickness of terminal before plating
Package overhang ZD
or
ZD
Distance from the true position of the outer-most terminal to the package edge
Package overhang ZE
or
ZE
Distance from the true position of the outer-most terminal to the package edge