IEC 60191-1-2018 pdf free.Mechanical standardization of semiconductor devices- Part 1: General rules for the preparation of outline drawings of discrete devices
4.3Methods for locating the datum
These methods are listed below in order of preference. When more than one of thesemethods is possible for a given device，the method appearing earliest in the list should beused. When none of the following methods is possible,the method best suited to the deviceshould be used.
The datum is:
a) the radial line through the centre of the mechanical or visual index;
b) the radial line midway between the two terminals which obviously comprise a gap in anotherwise equally spaced circular terminal array;
c) the radial line 180° from the locating radius of the most isolated terminal;d) the radial line 180° from the mid-point of, in order of preference:
– the two most widely spaced terminals;
– the two most closely spaced terminals;
e) the radial line through the centre of the index terminal. ln order of preference, the indexterminal is defined as that having:
– the smallest cross-sectional area at the point of emergence from the case;
– the greatest axial length,when one terminal is noticeably longer than the others;- the smallest axial length, when one terminal is noticeably shorter than the others;
f) datum C is the plane formed by at least three apexes at the bottom of the SMD package
body that exhibit the greatest perpendicular distance from the package bottom;
g) datum A is the plane perpendicular to Datum C and wraps one edge of the package body;h) datum A is the plane perpendicular to Datum C and crosses the centre line of the package body;
4.4 Numbering of terminals4.4.1General
Where possible,device terminals should be identified by numbers according to the systemoutlined in 4.4.2 to 4.4.7.In all instances, terminals are considered as being viewed from theirfree ends.
4.4.2single-ended devices with terminals in a linear array4.4.2.1Symmetrical linear array
The terminal nearest the reference mark should be numbered as No.1, the other terminalsshould be numbered progressively from terminal No.1.
4.4.2.2Asymmetrical linear array
The terminals should be numbered progressively from the end having the most terminals.
4.4.3 Single-ended devices with terminals in a circular array
The terminal, the centre of which is past the datum, should be numbered as No.1，the other terminals should be numbered progressively and in a clockwise sequence from No.1.
Where a terminal is situated in the centre of the base, this should be known as the centreterminal and shall not be given a number.
Where omission of one terminal in an otherwise equally spaced array identifies the datum,the position of the omitted terminal should not be numbered; but，in a fixed modularcircular array, any location of an omitted terminal that does not define a datum should benumbered.
4.4.4Double-ended devices
Terminals on both end views should be numbered without duplication of numbers.IEC 60191-1 pdf download.