Board Details | ISO Form #48
Printed Wiring Board | Industry Design Standards and ACC Design Specifics
ACC Electronix follows the guidelines and standards put forth by IPC (Association Connecting
Electronics Industries). Because of their general acceptance by the electronics industry as a viable
set of working standards, we recommend that developers of printed wiring board assemblies also
follow these guidelines where possible. The standard titled IPC-2221 (current revision) establishes
the generic requirements for the design of printed boards and other forms of component mounting or
interconnecting structures, whether single-sided, double-sided or multilayer. Following this standard
assures the design team that the product is producible, testable, reliable and cost-efficient.
To acquire a copy of the standard, ACC Electronix recommends that developers contact IPC by
either phone (1-847-615-7100) or internet (www.ipc.org) at their on-line store under the heading of
“Design Standards”. IPC also provides a wealth of associated standards that are more specific to
the unique design needs one might encounter during product development.
ACC Electronix has assembly equipment requiring some specific items for accurate, repeatable
manufacturing. These include guidelines for overall board size and panel or array parameters. In
the case of assemblies incorporating surface-mount devices, properly sized and positioned tooling
holes and fiducial marks are required. Accurate inclusion of these in your design will facilitate good
compatibility between our processes and your assemblies. The following sections will provide you
with these guidelines. Of course, there are exceptions to every rule. If you would like to discuss any
of these points for elaboration or clarification, we encourage you to contact ACC Electronix without
hesitation at (309)888-9990.
BOARD SIZE AND PANEL CONSTRUCTION
Generally, a PC Board or Panel/Array should not exceed 510mm (20.78 in.) in the X-dimension or
454mm (17.87 in.) in the Y-dimension for maximum efficiency and accuracy. These figures
represent the gating restrictions associated with the automated equipment at ACC Electronix.
Circuit boards where the maximum dimension in either direction is less than 76.2mm (3.00 in.)
cannot be processed through our automated equipment without special arrangements. Boards
falling within this category can either be incorporated into an array of multiple boards, or a custom
pallet can be designed to securely hold the individual boards during processing.
Consideration must be given to component and trace clearance along the top and bottom edges
of the board or panel. Clearance along the top edge should be no less than 5mm (0.197 in.).
Clearance along the bottom edge should be no less than 7mm (0.275 in.). The inclusion of
breakaway tooling rails is encouraged to meet these specifications. These clearances allow for
manufacturability on ACC’s equipment, which may require edge-to-component clearances and
tooling holes. Any multiple of smaller boards that will fit within these parameters is acceptable.
Routed panels will tend to sag more than scored panels during reflow. Therefore, consideration
should be given to keeping routed panel dimensions smaller than the maximum size. ACC will be
glad to review and discuss your design with you to help assure good design for manufacturability.
TOOLING HOLES
Tooling holes are required for assembly of boards incorporating surface mount devices. Tooling
holes (2) must be located with their centers at exactly +5mm Y (0.197 in.) and
-5mm X (0.197 in.) from the lower, right outer corner as well as +5mm Y and +5mm X from the
lower, left outer corner of the board or panel (including breakaway rails, if present.) The tooling
holes must be 3.175mm (.125 in.) diameter.
FIDUCIAL PATTERN AND PLACEMENT
Fiducial marks are required for automated assembly of boards incorporating surface mount
devices. Fiducial marks provide common measurable points for all steps in the assembly process.
This allows each piece of equipment used for assembly to accurately locate the circuit pattern.
There are three types of fiducial marks. These are:
1. Panel or Global Fiducial Marks
Fiducial marks used to locate the overall position of a panel or an array of boards. When a
multi-image circuit is processed in panel form, the fiducial marks are referred to as Panel or
Global Fiducial Marks.
2. Local Fiducial Marks
Fiducial marks used to locate the position of individual circuit boards within an array.
B. Component Fiducial Marks
Fiducial marks used to locate the position of an individual component.
A Fiducial mark is a printed artwork feature which is created in the same process as the circuit
artwork. The fiducial and a circuit pattern artwork must be etched in the same step. The optimum
fiducial mark should be a filled circle with a diameter of 1.27mm (.050 in.). A restricted area with no
traces, solder mask or silkscreen must be present around the fiducial mark with a minimum
diameter of twice the fiducial dimension or 2.54mm (.100 in.). A preferred clearance would be three
times the diameter of the fiducial.
Panels (arrays) should have a minimum of three (3) Panel Fiducial marks. Each PC board within
an array should also have a minimum of two (2) Local Fiducial Marks. This is especially true for
high-density arrays. These fiducial marks are required for correction of non-linear distortions
(scaling, stretch and twist). These should be located in a triangular position as far apart as possible
on the circuit or panel. The ideal locations are in the lower-right, lower-left and upper-right or
upper-left corners of any board or array. Fiducial patterns must be located on any board surfaces
where SMT components are to be placed (top, bottom or both).
Fiducial marks, including their preferred clearance area, should not be located within 3.18mm (0.125 in.) from any perimeter edge of the board or panel (including breakaway rail) and no more than 200mm (7.874 in.) distance from a second fiducial mark. Larger boards may require more than three (3) fiducial marks for accurate placement.
Centroid Data, also called Pick and Place Data, is required to properly program the pick and place
equipment. This data is not to be confused with pad data. Centroid data (exact, geometric center
of a component) provides the reference designator, X and Y coordinates relative to the zero datum
point along with component rotation for accurate placement of each component on a specific
assembly.
This data is obtainable from the board designer. To program without this information requires
“bomb sighting”. This process utilizes the “cross hairs” of a vision system or software package. It
requires manually locating the approximate center of a component and returning the coordinates.
This is not as accurate as using placement data and will substantially increase the cost of technical
labor which translates into elevated programming costs to the customer.
A complete CAD file of the project is the best collection of data. Some of the equipment utilized by
ACC Electronix, Inc. for component placement can be automatically programmed directly from the
appropriate CAD file. If that is unavailable, a complete set of Gerber files, related bill of materials
and placement data is typically adequate.
STENCIL
Stencils with outside frame dimensions of up to 29” X 29” are acceptable. It is typically best to have
stencils for your assembly ordered by ACC Electronix. This allows us to make the appropriate
selections of thickness, material, aperture variations, finish, special manufacturing considerations
and frame for best manufacturing practices.
FINAL
We encourage all of our customers (potential and current) to contact us with any questions you
might have related to design and manufacturability. We can be reached at:
PH: (309)-888-9990
FX: (309)-452-0893
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http://www.accelectronix.com
