The following is a Study Guide Snapshot on the topic PRINTED
BOARD & PRINTED BOARD ASSEMBLY VIEWING PRINCIPLES. The  topic
relates to QODs #119; #120; #121.

When only through-hole assembly existed, the viewing principles
were fairly straight forward. One viewed the board from the
component side; the opposite side was the solder side. With the
advent of surface mounting, the definition was no longer as
clear. New terms were needed as were new rules for determining
which side was which. The term  Primary Side  replaced component
side; the term  Secondary Side replaced solder side. The primary
side is that side of the packaging and interconnecting structure
(printed board) that is so defined on the master drawing. It is
usually the side that contains the most complex or the most
number of components. The secondary side is opposite the primary
side.

The design dictates which side is called what; design develops
the assembly drawing so in many cases the reasons for selecting
one side over the other is related to the intensity of the
assembly operation. It is conceivable that the primary side could
be the side opposite the through-hole components, even though
that side has only one component; a microprocessor. That is the
side the designer wanted to view, as the layout proceeded.
Primary and Secondary sides should not be confused with primary
and secondary datums. The datums are used for dimensioning
purposes; the primary datum plane is that side which is opposite
the primary side.

Datum features are used to position the printed board in relation
to a set of three mutually perpendicular planes. Typically,
printed board drawings are oriented with layer one (1) facing up.
This orientation establishes the backside of the printed board as
the first (primary) of the three required datum planes according
to ANSI Y14.5. The other two datum planes (secondary and
tertiary) are typically established using holes or etched
features of the board. These are then the datum features that set
up the planes from which all dimensions are determined. All of
these conditions are mainly used to convey intent to the
manufacturer of the board or the assembler. They take that
information and establish the panels for board fabrication and
board assembly. Never the less the master drawing descriptions
form the final accept/reject criterion for the final product.

Understanding how layers are viewed and numbered is important to
maintain consistency in communication between design and
manufacturing. Usually conductors are numbered sequentially
starting with the primary side as layer one (1). If there are no
conductors or lands on the primary side then the next conductive
layer becomes layer one. Nonconductive layers are numbered after
all the conductive layers have been identified, starting over at
the primary side. Nonconductive layers are those for which there
is an image or dimensional configuration in the
secondary/tertiary datum planes. Legend, solder mask, cover coat,
in fact any layer that remains with the board is numbered
according to these principles. Thus, if a 12 layer multilayer
board has solder mask on both sides, the solder mask near the
primary side becomes layer thirteen (13).

Sequential layer conventions only apply to things that stay with
the board. Data layers for temporary masking, solder-paste
stencils, hole drilling templates can be numbered in any fashion
as, to this date, there is no industry accepted consensus
standard
Resources: IPC-T-50 Specific Terms and ANSI Y14.5 para 4.5.2;
4.5.3; 4.5.3.1; and IPC-D-275 para 3.3.1; 5.2.3; figure 3-1;
5-6A; 6.1.1; 6.2.1; 6.2 and IPC-D-350 para 4.5 table 4-1;   figure 4-2.
     

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Please send comments, constructive critique, or suggestions to
Lisa Williams at [log in to unmask] 

Dieter Bergman