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.
  3. 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

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