recommended practice extensions for gd&t in step-ap210 in … · 2010/8/16 · gd&t in the...

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Recommended practice extensions for

GD&T in STEP-AP210 in the context of

electronic connectors

Jamie Stori

SFM Technology, Inc.

Urbana, Illinois

Kevin Brady

National Institute of Standards and Technology

Information Technology Laboratory

NISTIR 7716

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NISTIR 7716

Recommended practice extensions for

GD&T in STEP-AP210 in the context of

electronic connectors

Jamie Stori

SFM Technology, Inc.

Urbana, Illinois

Kevin Brady


Information Technology Laboratory

U.S. Department of Commerce

John Bryson, Secretary


Patrick D. Gallagher,

Under Secretary of Commerce for Standards and Technology

August 16, 2010

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Recommended practice extensions for GD&T in STEP-AP210 in the context of electronic connectors

Introduction

The purpose of this document is to extend the recommended practices for the

representation of GD&T in STEP AP 210 (ISO-10303:210) in the context of packaged

electronic connectors. Geometric dimensioning and tolerancing (GD&T) is critical to the

design, manufacture, and assembly of electronic products. STEP AP 210 is unique in its

ability to represent detailed usage views of electronic components spanning both the

mechanical (MCAD) and electrical (ECAD) design domains. Within the STEP standards

(ISO-10303), prior attention has been focused on the representation and presentation of

GD&T in the context of three-dimensional (3D) boundary-representation (B-REP)

models. In this context, GD&T annotations are typically associated with faces in the 3D

model.

Vendors of electro-mechanical components such as connectors typically provide detailed

specifications of the mechanical properties of their components, and often will include a

recommended PCB layout for the component. Both the mechanical specification and the

PCB layout recommendation will often include GD&T annotations. This is particularly

relevant in the context of mounting and mating features, such as mounting and alignment

holes and features such as bosses, slots, and studs. Typically, the GD&T annotations are

provided as a technical drawing, most often in .pdf format.

An AP210 model of an electronic component such as a connector will ideally contain

both a detailed three-dimensional geometric representation, suitable for use in mechanical

design, as well as a detailed two-dimensional footprint definition that supports the

application of the component in PCB layout and PCA assembly. Existing

implementations have validated the ability of AP210 to represent the detailed two and

three-dimensional geometric models and product structure of a connector. A prior

recommended practice document1, explored the population of GD&T annotations on a

3D geometric representation of a packaged electronic component represented in AP210.

This document extends that prior work in the context of connectors by proposing a

recommended practice for the representation of critical GD&T annotations within the

footprint definition of a component, and updating the recommended representation of

certain associations and relationships. The concepts discussed will support the GD&T

representation requirements of a typical connector with a combination of both mounting

features and through-hole terminals. Where relevant, elements of the prior recommended

practice have been included in the current document. Many of the supporting details have

not been repeated, however, and the prior document should be referenced as applicable.

1 Stori, J., Brady, K., and Thurman, T, 2009 “Extensions to the recommended practices for GD&T in

STEP-AP210 in the context of packaged electronic components,” NISTIR 7634,

http://www.nist.gov/customcf/get_pdf.cfm?pub_id=903904

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Figure 1. Three mechanisms for associating geometry with a Shape_element

Shape_description_association

represented_characteristic

representation

Shape_element

Geometric_model

Shape_element

Geometric_item_specific_usage

definition

Geometric_modelused_model

Detailed_geometric_model_element

identitied_item

items [i]

Shape_element

Geometric_item_specific_usagedefinition

Detailed_geometric_model_element

identitied_item Chain_based_geometric_item_specific_usage

Geometric_modelnodes L[2:?]

chained_geometric_model_link

undirected_link L[1:?]

Geometric_placement_operation

Geometric_coordinate_space

Geometric_model_relationship

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Notation

ARM AOs will be denoted with a leading uppercase letter in Courier font (i.e. Datum)

while a MIM entity will be displayed in all lowercase notation (i.e. datum).

Figure 2. Three mechanisms for associating geometry with a Shape_element - MIM

chain_based_item_identified_representation_usage

representationnodes L[2:?]

mapped_item

chained_representation_linkundirected_link L[1:?]

representation_context

representation_relationship

definition

identitied_item

geometric_item_specific_usage

shape_aspect

geometric_representation_item

item_identified_representation_usage

chain_based_geometric_item_specific_usage

definition

identitied_item


shape_aspect

shape_representation used_representation

geometric_representation_item

item_identified_representation_usage

items [i]

shape_aspect

shape_definition_representation

shape_representation

definition

used_representation

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Shape_element

Shape_element is the ARM concept that enables a portion of a shape to be identified or

called out for some particular purpose in model representation. Subtypes of

Shape_element are used heavily in the package model representation, and are also the

critical link between GD&T annotations, and the geometric model. Specific subtypes and

their use will be discussed in further detail as relevant sections. There are several

mechanisms for associated geometric the elements that compose a Shape_element, as

illustrated in Figure 1, above. If there is an explicit Geometric_model that represents

the Shape_element, a Shape_description_association may be employed. If the

Shape_element is defined by a subset of a Geometric_model, either the

Geometric_item_specific_usage or its subtype

Chain_based_geometric_item_specific_usage may be used. Each of these

creates a defining association between the Shape_element and a single

Detailed_geometric_model_element. As will be discussed below, in the case of

three-dimensional geometric models, the model element will often be a face of a

boundary representation. In the case of a two-dimensional footprint definition, the model

element may often be a templated shape. Often, multiple instances of the

Figure 3. The top-level structure of a package model.

Package

Package_bodycontaining_shape

mounting_technology_type(OPT) mounting_technology

Length_data_element(OPT) maximum_seating_plane_installation_offset

Length_data_element(OPT) nominal_mounting_lead_pitch

Length_data_element(OPT) nominal_mounting_lead_span

Length_data_element(OPT) maximum_body_height_above_seating_plane

(INV) body [0:1]

Physical_unit_sh

ape_model

application_environmentshape_environment

shape_extentshape_extent

shape_approximation_levelshape_approximation_level

……

.

Package_terminal

Part_mounting_feature

(INV) package_accesses [0:?]

Physical_unit_

shape_model

containing_shape

Physical_unit_kee

pout_shape_model

……

.

Seating_planecontaining_shape

……

.

Footprint_definition(OPT) reference_package

Part_version

Part

defined_version

of_product containing_shape

Part_usage_view

shape_characterized_definition

Part_mounting_featurecontaining_shape

Part_feature

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Geometric_item_specific_usage will be employed to enumerate the individual

model elements that collectively define the Shape_element.

The “chain-based” subtype of Geometric_item_specific_usage recently incorporated into

the standard to address a long-standing need to call out a specific instanced item within a

representation. For example, in a typical mechanical assembly, there are often multiple

Figure 4. Defining a shape_aspect based on a surface of a terminal in a package model.

package_terminal

package of_shape

package_terminal_t

emplate_definition

product_definition

_shape

definition


definition

used_representation

usage_concept_usage_

relationship

definition

used_representation

mapped_representation

representation_map

Axis2_placement

_3dmapping_source

mapping_target

packaged_part

packaged_part_terminal

relating_product_definition

related_product_definition

property_definition

_relationship

{.name = ‘definition’}

property_definition

definition

property_definition

definition

product_definition_relationship



.name = ‘used package’{.description = ‘join terminal’}

shape_aspect_relationship

relating_shape_aspect

related_shape_aspect

{.name = ‘terminal of package’}

of_shape

shape_definition

_representation

definition


used_representation

description_attribute

described_item

{.attribute_value = 'part feature template shape model’}

axis2_placement_3d

mapping_origin

{.name = ‘3d bound volume shape’}

shape_aspect

definition

nodes [2]

identified_item

nodes [1]

advanced_brep_shape_

representation

manifold_solid_brep

items [1:?]

closed_shell

outer

advanced_face

cfs_faces [1:?]

chain_based_geometric_item

_specific_usage

undirected_link [1]

{.name = ‘3d bound volume shape’}

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instances of the same part. The part may be represented with a single geometric model

(i.e. a B-Rep model). The “chain-based” referencing mechanism, enables the

identification of a specific face on the part model within a specific instance of the part in

the assembly. Specific examples of this identification mechanism will be provided below

in the context of both the package and the footprint. Figure 2 provides the MIM mapping

of the Shape_element concepts of Figure 1.

Package Model

The key structure of a typical AP210 package model is summarized in Figure 3. Of

particular relevance in the context of GD&T are the common subtypes of

Part_feature critical to a package model representation – body, terminal, and

mounting / mating features. Also of note is the Seating_plane, a

Non_feature_shape_element. All of the preceding are types of Shape_element,

and their geometric representation can vary significantly. For example, a

Package_terminal may be a collection of faces called out from a B-Rep model of the

entire package, or the package terminal may have its own defining geometric model

associated with the template definition of the package terminal. Figure 4 illustrates such a

case, in which an advanced_brep_shape_representation defines the shape of the

package_terminal_template_definition. A specific package_terminal (a

single terminal) is placed with respect to the shape model of the package using a

usage_concept_usage_relationship. A shape_aspect is defined (in red) which

is associated with a single face of this specific package terminal, despite the fact that the

shape_representation of the terminal template is shared among many different

terminals. The chain_based_geometric_item_specific_usage enables this

identification to be made by qualifying the face in the template definition through the

specific terminal placement of the usage_concept_usage_relationship. If it were

desired to call out the entire geometric model of the template definition as a new

shape_aspect, a direct relationship could be established with a

shape_definition_representation. Alternatively, if it was necessary to call out a

specific face of the terminal template definition as a shape_aspect that would apply to

all uses of the template in the package (i.e. any and all package terminals that used the

template), a geometric_item_specific_usage could be used.

Footprint_definition

A “footprint” in the context of PCB design and layout is the collections of features

needed to support assembly and connectivity for a particular component. Footprint

features include the copper pads or lands on the mounting surface of for surface mount

terminals, mounting tabs, ground connections, and heat sinks, specification of compatible

soldermask and solderpaste regions, and drill and via locations and sizes for through-hole

features such as terminals, alignment studs, mounting bolts, etc.

In AP210, the definition of a footprint is represented with the ARM application object

Footprint_definition. A footprint definition is a template that can be instanced

when an individual component is placed into a design. Figure 5 outlines the most

common ARM AOs and relationships composing the typical structure of a footprint

definition. While a footprint definition can exist completely independently of a package

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model, AP210 provides important mechanisms for explicitly linking the features of the

Figure 5. A Footprint_definition is a structured template that is composed of lower-level template elements, including Padstack_definition.

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- 10 -

footprint with those of the package. The

Figure 6. Alternate means of associating a shape_aspect with a csg_solid_2d in a geometric_template.

inter_stratum_feature

_template

single_boundary

_csg_2d_shape_

representation

shape_defintion_

representation

definition

used_representation

{.name = 'ppsm’}

of_shape


definition

used_representation

shape_aspect

feature_of_size


_template

single_boundary

_csg_2d_shape_

representation

shape_defintion_

representation

definition

used_representation

{.name = 'ppsm’}

#25659

items [1:?]

csg_primitive_

solid_2d

identified_item

of_shape


definition

used_representation

#25657

circular_area

tree_root_expression

shape_aspect

feature_of_size

- 11 -

Part_feature_based_template_location enables an explicit relationship to be

established between a placement of a template (for example, a padstack) and the

corresponding Part_feature of the package (for example, a terminal). A

Padstack_definition is typically used to represent the collection of PCB elements

needed to support the application of a particular part feature. For example, a surface

mount terminal may be supported by a padstack containing a surface mount land, a

soldermask cutout, and possible a solder paste mask. A through hole terminal may be

supported by a padstack consisting of a plated passage (a through hole plated for

electrical connectivity), and round pads and soldermask cutouts on both the top and

bottom pcb surfaces. The elements of a padstack definition can be decomposed into

single-stratum features (i.e. lands and soldermask cutouts) and inter-stratum features

(holes, vias, etc.). Consider a common critical feature in the application of a connector –

a mounting hole or plated through hole to accommodate a connector terminal. A drilled

hole would be represented in a footprint as an inter_stratum_feature_template

or a specialized subtype such as a component_termination_passage_template. It

will commonly be desirable to annotate such a feature with either a dimensional tolerance

Figure 7. Associating a shape_aspect with an individual placement of a csg_solid_2d within a padstack_definition

padstack_definition

shape_represent

ation

shape_defintion_

representation

definition

used_representation

representation_map


axis2_placement_2d

mapping_origin

[.name = 'origin’]

{.name = 'ppsm’}

assembly_compon

ent_usage



_template


mapped_item

product_definition_

shape

definition

definition

shape_represent

ation

used_representation

items [1:?]

shape_definition_

representation

{.name = 'tlistt’}

axis2_placement_2d

mapping_target

single_boundary

_csg_2d_shape_

representation

shape_defintion_

representation

definition

used_representation

representation_map


axis2_placement_2d

mapping_origin


mapping_source

{.name = 'ppsm’}

items [i]

{.name = ‘tlist’}

#25659

items [1:?]

csg_primitive_

solid_2d

identified_item

of_shape


_specific_usage

definition nodes [1] nodes [2]undirected_link [1]

#25657

#25669

#170535

#25653

circular_area


shape_aspect

feature_of_size

- 12 -

(i.e. diameter) or a positional tolerance. A shape_aspect must be defined to support the

GD&T annotation. Figure 6, Figure 7and Figure 8 illustrates various scenarios for the

identification of a shape_aspect associated with a drilled hole within a

footprint_definition.

The two scenarios illustrated in Figure 6 are functionally equivalent in this case, as the

shape_representation of the template contains only the geometric specification of

the hole’s cross section (i.e. a circle). Either representation implies that a dimension or

tolerance is associated with all occurrences of the template. This is a common and

desirable scenario – often a series of equivalent holes are to be created to support a series

of equivalent terminals. A common dimensional and/or locational tolerance would apply

to each instance of the hole individually.

Figure 7 illustrates a significantly less common scenario. In this instance, the

shape_aspect has been qualified by a specific hole placement within a

padstack_definition. The associated GD&T annotation would apply to all

occurrences of the padstack.

Figure 8. Associating a shape_aspect with an individual placement of a csg_solid_2d within a footprint_definition

footprint_definition


padstack_definition


assembly_comp

onent_usage

package_terminal


relating_product_definition related_product_definition property_definiti

on_relationship

{.name = ‘reference feature’}

property_definition

definition

property_definition

definition

mapped_item

product_definition_

shape

definition

definition

shape_represent

ation

used_representation

items [1:?]

shape_definition_r

epresentation


axis2_placement_2d

mapping_target

shape_represent

ation

Shape_defintion

_representation

definition

used_representation

representation_map


axis2_placement_2d

mapping_origin


mapping_source

{.name = 'ppsm’}

shape_represent

ation

description_attribute

{.attribute_value =

'footprint_definition_shape_model’}

described_item

Shape_defintion

_representation

definition

used_representation

items [i]

assembly_compon

ent_usage



_template


mapped_item

product_definition_

shape

definition

definition

shape_represent

ation

used_representation

items [1:?]

shape_definition_

representation


axis2_placement_2d

mapping_target

single_boundary

_csg_2d_shape_

representation

shape_defintion_

representation

definition

used_representation

representation_map


axis2_placement_2d

mapping_origin


mapping_source

{.name = 'ppsm’}

items [i]


#25659

items [1:?]

csg_primitive_

solid_2d

identified_item

nodes [1]

of_shape


_specific_usage

undirected_link [1]definition

nodes [2] nodes [3]undirected_link [2]

#25657

#25669

#170535

#25653

#26161

#170770

#25644

circular_area


shape_aspect

feature_of_size

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If it is necessary to call out a specific instance of the hole within a

footprint_definition as a feature for GD&T association, a

chain_based_geometric_item_specific_usage could be employed to qualify the

individual inter-stratum feature within a specific placement of the padstack definition.

The provides the ability to attach annotation that are specific to an individual hole, and

would often be employed in the definition of a datum, for example (the datum feature is

one particular hole).

Shape_elements for GD&T

Features of the model relevant for GD&T are represented by the Shape_element AO.

Figure 9 shows several of the important subtypes of Shape_element. When it is desired

to treat multiple disjoint regions as a single feature, a Composite_shape_element

may be employed. A Composite_shape_element has two important subtypes – the

Composite_group_shape_element and the Composite_unit_shape_element. A

Composite_unit_shape_element is used to aggregate multiple Shape_elements

that are to be treated as a unit. For example, multiple surfaces of a single feature-of-size.

A Composite_group_shape_element is used when it is desired to apply a property

to each of the constituent elements individually.

It is often common to require a reference to derived geometry, such as a center plane of a

feature. Such a feature would be modeled through the appropriate subtype of

Derived_non_feature_shape_element (a Shape_element that is not on the

physical boundary of the part). Several examples for the representation of

shape_elements in the context of a package model were previously documented in

NISTIR 7634.

Shape_element

Centre_of_symmetry

Derived_non_feature_

shape_element

Centre_plane

Non_feature_shape

_element

Derived_shape_element

Datum_shape_element

Datum

Composite_shape

_element

Composite_group_

shape_element

Composite_unit_

shape_element

Shape_element_composing

_relationship

composing_relationships INV [2:?]

Shape_element

derived_from [1:?]

Shape_element_deriving

_relationship

deriving_relationships INV [1:?]

Shape_element

related

Shape_element

related

relating relating

Figure 9. Several relevant subtypes of the Shape_element AO for GD&T

../../EclipseWorkspace/stepmod/data/modules/shape_property_assignment/sys/4_info_reqs.xml#shape_property_assignment_arm.shape_element

- 14 -

Feature-of-size

The representation of a “feature of size” is critical to the successful application and

interpretation of many common dimensions and tolerances. Certain common GD&T

annotations, such as a positional tolerance must be applied to a feature of size. A feature

of size must have opposed points and contain a reproducible median point, axis, or plane.

When a positional or perpendicularity tolerance is applied to a feature of size, it is

controlling the feature’s center point, axis, or plane. The most common feature of size is

referred to as a “Regular Feature of Size,” defined in the 2009 edition of the ASME

Y14.5 standard as follows:

“one cylindrical or spherical surface, a circular element, and a set of two

opposed parallel elements or opposed parallel surfaces, each of which is

associated with a directly toleranced dimension.”

In the context of a three-dimensional B-Rep model of a solid, a feature of size would

typically reference either a cylindrical face, or two parallel opposed planar faces. In a 3D

B-Rep, there is typically no functional or feature-based decomposition of the geometric

model available, and individual faces of a complex model would be called out through a

geometric_item_specific_usage or its chain-based subtype.

In the context of two-dimensional ECAD geometry, developing a clear representation of

a feature-of-size is critical. Within a footprint, there is a parallel structure between the

footprint design elements and their geometric representation. The shape representations

in a footprint definition are most commonly either a csg_2d_shape_representation

or a subtype. The majority of individual geometric elements will be represented as a

subtype of primitive_2d. Figure 11 details the common subtypes of primitive_2d

and their key attributes.

The representation of the individual geometric elements are particularly relevant in the

context of the feature of size. A circular feature of size can be unambiguously defined by

Figure 10. The 2D geometric elements of an ECAD footprint are typically represented with a csg_2d_shape_representation.

csg_2d_shape_representation

csg_2d_shape_select

items S[1:?]

mapped_item

axis2_placement_2d

csg_solid_2d

csg_primitive_solid_2d

primitive_2dtree_root_expression


boolean_result_2d

single_area_csg_2d_shape_representation

planar_path_shape_representation_with_parameters


- 15 -

associating a shape_aspect with a shape_representation containing the

circular_area entity. However, the primitive_2d representations do not support

the unambiguous definition of a feature of size corresponding to two opposed parallel

surfaces. Nevertheless, positional tolerances and length / width dimensional tolerances on

the variety of common pad geometries shown in Figure 11 are very common.

To support this need, the addition of a subtype of Shape_element is proposed for

consideration, that would enable the optional specification of an orientation for the

feature of size.

Figure 11. Common 2D geometric elements used in an ECAD footprint representation.

rectangular_area

axis2_placement_2d

positive_length_measure


position

x

y

circular_area

cartesian_point


centre

radius

area_with_outer_

boundary

primitive_2d

composite_curvebase_curve

path_area_with_parameters

polygonal_area

primitive_2d_with_inner_boundary

cartesian_pointbounds L[3:?]

curvemapping_target

- 16 -

ENTITY Feature_of_size

SUBTYPE OF (Shape_element);

direction_ratios : Optional LIST[2:3] OF length_measure;

END_ENTITY;

In the above, the optional direction_ratios enable specification of the orientation. If an

orientation is provided, it is assumed that two opposed parallel surfaces are contained

within the associated geometry normal to the specified direction.

Representation of geometric dimensions, tolerances, and datums.

The top-level ARM application objects and attributes used in the representation of a

geometric dimension and tolerance are outlined in Figure 12 and Figure 14. For

additional detail in the context of packaged components regarding the mapping of

Figure 12. The top-level subtypes and key entities of the Geometric_dimension AO.

Geometric_dimension

element_with_dimension_selectis_applied_to

Shape_element

dimension_value_select

dimension_value

Numerical_item_with_unit

Dimension_value_with_limitation

Tolerance_range

Value_limit

Size_dimension

Angular_size_dimension Curved_size_dimension Diameter_size_dimension

Radial_size_dimension

Thickness_size_dimension

Externally_defined_size_di

mensionHeight_size_dimension

Length_size_dimension

Dimensional_size_based_

on_opposing_boundaries

Width_size_dimension

Location_dimension

Angular_location_dimension Curved_distance_dimension Linear_distance_dimension

placed_element_select

Shape_element

placed_element_select

Shape_element

origin

target

BOOLEANdirected (OPT)

../../AP210ed2/wg3n2602_10303_210_ed2_IS/iso10303_210/data/modules/value_with_unit/sys/4_info_reqs.htm#value_with_unit_arm.length_measure

- 17 -

common representations, refer to NISTIR 7634.

Per ASME Y14.5 (2009), a datum is a theoretically exact point, line or plane derived

from geometric counterpart(s) on the physical model. The geometric counterpart is

known as a datum feature. The AP210 ARM supports a family of application objects

(AOs) that are subtypes of Datum. Several of the commonly used subtypes that are most

relevant for the domain of packaged electronic components are documented in Figure 13.

In many cases, a datum feature will correspond to either a surface feature of a part or a

“feature of size” of a part. When it is required that a specific region of a surface be used

to determine the datum, a datum target may be used. Many times, a feature of size is used

as a datum feature in defining a datum.

Tolerance_condition

Geometric_tolerance

Angularity_tolerance Circular_runout_tolerance Coaxiality_tolerance Concentricity_tolerance

Parallelism_tolerance

Perpendicularity_tolerance

Symmetry_tolerance Total_runout_tolerance

Value_with_unittolerance_value

Shape_element

applied_to

(OPT) modification

Cylindricity_tolerance Flatness_tolerance

Position_tolerance Roundness_tolerance

Line_profile_tolerance

Straightness_tolerance

Surface_profile_tolerance

Figure 14. The top-level subtypes of the Geometric_tolerance AO.

Figure 13. Selected subtypes of the Datum application object

Datum

Single_datum

Datum_defined_by_

derived_shape

Derived_geometry

Datum_plane

Datum_defined_by

_feature

Datum_defined_by

_targets

Shape_element

Datum_feature

defined_by defined_by [1:?]

Datum_target

Shape_elementderived_from [1:?]

constructive_element_selectgeometry

Common_datum

- 18 -

Application to a common connector configuration

Figure illustrates a recommended GD&T based dimensioning and tolerancing scheme

for a common connector and the corresponding PCB mating features. A connector vendor

will often provide both tolerance specifications for the connector as well as the

corresponding PCB layout. In the absence of a recommended PCB layout, the dimensions

and tolerances for compatible mating features on the PCB can often be derived from the

provided connector specifications. The assumptions, variables, and relationships needed

for this example, based on simple tolerance stack-up analysis are provided in the

Appendix.

Whenever feasible, it is recommended that the seating plane be treated as a primary

datum plane. In this example, the connector locating pins serve as the secondary and

tertiary datum features in the feature control frame for the connector, and the

corresponding mating holes in the PCB are the secondary and tertiary datum features in

the feature control frame for the PCB.

The detailed population of the GD&T annotations will be dependent on the geometric

representation of the package and footprint. Assuming a single B-Rep model of the

connector is provided by the vendor, datum A (coincident with seating plane) could be

established with a Datum_defined_by_feature based on datum feature A – a

Shape_element associated with the appropriate face of the geometric model through a

Geometric_item_specific_usage. ( a datum related to a datum_feature through

a shape_aspect_relationship) Datum B and C could each be established through a

Datum_defined_by_derived_shape (in this case, a centre_of_symmetry)with

reference to the cylindrical face of the mounting pin. In the event that the two mounting

pins shared a common geometric model, it would be necessary to employ a

chain_based_geometric_item_specific_usage to (see Figure 4) discriminate

between the two occurrences of the face.

In terms of the footprint definition, the two mounting holes would likely share a common

padstack definition. A shape_aspect similar to that of Figure 8 would be employed to

establish the secondary and tertiary datum features. There is no explicit geometric

representation of the seating plane within the footprint definition. The primary datum

feature should be established through reference to the seating plane.

It is desirable that the tolerance on the diameter of the two mounting holes in the PCB be

symmetric. In this case, this tolerance could be associated directly with the

inter_stratum_feature_template for the mounting holes (see Figure 6). A single

padstack definition that incorporates this inter-stratum feature template could be

instanced twice in the footprint definition for the two mounting holes. The

perpedicularity and positional tolerance would rely on the same shape_aspect used to

define the secondary and tertiary datum features. Finally, the positional tolerance of the

36 terminal mating holes could be associated directly with the

inter_stratum_feature_template for the terminal mating holes.

- 19 -

Figure Error!. Representative connector with dimension and tolerance variables, and corresponding PCB layout.

[Source: Bryan Fischer, Advanced Dimensional Management LLC]

U n its : m m

C o n n e c to r - w ith D im e n s io n a n d T o le ra n c e V a r ia b le s

N O T E S (U N L E S S O T H E R W IS E S P E C IF IE D ):

1 . IN T E R P R E T D R A W IN G P E R IS O 1 1 0 1 :2 0 0 4 .

A

0 ,1

Ø P n o m ,c ± S T c

C

1 1 ,2

3 6 x Ø P n o m ,p ± S T p

1,5

0,5

2

4

1,5

0,5

1 ,6

Ø Pn o m ,b

± S T b

B

1 ,1

1 4

1 ,4

13 2 x

4,5

0 ,2 5 A B C

A BØ T 2 c

A B CØ T 2p

AØ T 2 b

P C B - C o n n e c to r M o u n tin g D e ta il

N O T E S (U N L E S S O T H E R W IS E S P E C IF IE D ):

1 . IN T E R P R E T D R A W IN G P E R IS O 1 1 0 1 :2 0 0 4 .

U n its : m m

D

P C B S U R F A C E

W IT H IN R E S T R IC T E D A R E A Ø Hn o m ,f

± S T f

F

1 1 ,2

3 6 x Ø Hn o m ,h

± S T h

1,5

0,5

2

4

1,5

0,5

1 ,6

Ø H n o m ,e ± S T e

E

1 ,1

1 4

1 ,4

13 2 x

DØ T1 e D EØ T1 f

D E FØ T1 h

- 20 -

Appendix – Assumptions, variables, and formulae related to the connector example of Figure . [Source: Bryan Fischer, Advanced Dimensional Management LLC]

Assumptions:

The connector is located by locator pins in mating holes in the PCB.

The connector locating pins are referenced as the secondary and tertiary datum features in the positional tolerance feature control frame that controls the other (numbered) connector pins.

A local, functional datum reference frame is established at each location a connector mates with the PCB.

The holes in the PCB that mate with the connector’s locating pins are referenced as the secondary and tertiary datum features in the positional tolerance feature control frame that controls the holes that correspond to the numbered pins on the connector.

The secondary and tertiary datum features are the same size and have the same form, size, orientation, and location tolerances applied as applicable.

The secondary and tertiary datum features on the connector engage the corresponding locating holes in the PCB at the same time and play an equal role in locating the connector.

Maximum assembly shift is possible between the locating pins on the connector and the corresponding holes in the PCB.

The assembly shift manifests itself at assembly as translational variation – rotational assembly shift is not addressed in the calculations. In this example, because the locating pins and holes are farther apart than the working pins and holes, the effect of rotational assembly shift will be less than the effect of translational assembly shift.

The dimensioning and tolerancing schemes and calculation methods proposed here apply to other applications.

Equal-bilateral size tolerances applied to all holes and pins.

The pins on the connector and the holes in the PCB are cylindrical features.

- 21 -

Variables:

PCB

He,f Hole (MMC, smallest) – Datum Features E and F on PCB

Hh Hole (MMC, smallest) – PCB Holes that interface with numbered connector pins

Hnom,e,f Hole (nominal, stated size) – Datum Features E and F on PCB

Hnom,h Hole (nominal, stated size) – PCB Holes that interface with numbered connector pins

Hlmc,e,f Hole (LMC, largest size) – Datum Features E and F on PCB

STe,f Size Tolerance for Datum Features E and F on PCB

STh Size Tolerance for PCB Holes that interface with numbered connector pins

T1,e,f Perpendicularity or Positional Tolerance applied to Datum Features E and F on PCB

T1,h Positional Tolerance applied to the PCB Holes that interface with numbered connector pins

Connector

Pb,c Pin (MMC, largest) – Datum Features B and C on PCB

Pp Pin (MMC, largest) – Numbered connector pins

Pnom,b,c Pin (nominal, stated size) – Datum Features B and C on PCB

Pnom,p Pin (nominal, stated size) – Numbered connector pins

Plmc,b,c Pin (LMC, smallest size) – Datum Features B and C on PCB

STb,c Size Tolerance for Datum Features B and C on PCB

STp Size Tolerance for numbered connector pins

T2b,c Perpendicularity or Positional Tolerance applied to Datum Features B and C on connector

T2,p Positional Tolerance applied to numbered connector pins

Assembly

AS Assembly Shift (Maximum Clearance between Hlmc,e,f and Plmc,b,c)

- 22 -

Formulas:

Calculate and Verify Fit, Size, Size Tolerance, or Geometric Tolerances for Alignment Features: Corresponding Datum Features on Connector and PCB: Use Fixed Fastener Formula

He,f = Pb,c + T1,e,f + T2,b,c

Calculate and Verify Positional Tolerance for Holes and Numbered Pins: Working Features: Use Modified Fixed Fastener Formula

Hh = Pp + T1,h + T2,p + AS

Calculate Hnom,e,f: Nominal Hole Size for the Datum Feature Holes on the PCB

Hnom,e,f = He,f + STe,f

Calculate Hlmc,e,f: Largest Size for Datum Feature Holes on the PCB

Hlmc,e,f = Hnom,e,f + STe,f

Calculate Pnom,b,c: Nominal Pin Size for the Datum Feature Pins on the Connector

Pnom,b,c = Pb,c - STb,c

Calculate Plmc,b,c: Smallest Datum Feature Pin Size for Connector

Plmc,b,c = Pnom,b,c - STb,c

Calculate AS: Assembly Shift: Worst-Case Variation between LMC Datum Features on PCB and Connector

AS = Hlmc,e,f - Plmc,b,c

Calculate Hnom,h: Nominal Size for Working Holes in PCB (Correspond to Numbered Pins on Connector)

Hnom,h = Hh + STh

Calculate Pnom,p: Nominal Size for Numbered Pins on Connector

Pnom,p = Pp - STp

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