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PCB Design For Manufacturing & The Importance of Fiducial Points

03 April, 2020

A correct electronic board design not only helps you avoid unnecessary costs in long-term production, but also helps you achieve the target quality. We have explained the importance of design for manufacturing in another article. In this article, we are sharing the points of consideration during the PCB design phase and the importance of fiducial points.

PRELIMINARY ASSESSMENT
Before the beginning of design, the determination of the following points regarding PCB by the designer will provide insight into the manufacturing processes and the implementation of manufacturability arrangements. With the manufacturing processes to be followed for PCB determined, suitable selection of the layout, pad size/shape, panelization, breakaway tab and similar arrangements will be ensured.

1- Which sheating materials will be used for the PCB ?
a. Only Through Hole Mounted
b. Only Surface Mounted
c. Both Through Hole and Surface Mounted
2- On which side / sides of the PCB will the materials be located ?
a. Only Top Side
b. Both Top side and Bottom side
3- With what will Axial Inserter and Radial Inserter be used ?
a. Only with Through Hole material.
b. With top side SMD material.
c. With bottom side SMD material.
4- Will there be and adhesive application on the bottom side ?
5- Will Selective Wave Soldering be used ?

With example questions such as above, the path to be followed for the electronic manufacturing processes will be determined with the table below, and the type of manufacturability arrangements to be used for the design to be created may be decided.

PCB FIDUCIAL POINTS

When the PCB Design begins, if any automated machinery is to be used in the manufacturing processes, fiducial points or holes must be added on top of the PCB. Today, there are AOI machines that scan an area of 60cm⊃2; with 10um resolution, and chip driving machines with a capacity of 40.000 components per hour with a tolerance of +/- 0.025mm. In order for the machines to process the PCB, the machine operator or engineer prepares a CAD / CAM program in a language that the machine is capable of understanding. According to this program, the machine can perform processes along 2, 3, 4 or even 5 Axes. For the machines to operate with the incredible accuracy as stated above, in addition to their capacity, the design of the PCB must also be compatible with this technology. In order for the machine to recognize the layout of the PCB and perform the relevant process with 10um or 0.025mm tolerance, the program must be matched with the PCB. This matching is possible with fiducial points or holes (for Axial and Radial machinery). If there are no fiducial points on the PCB, in accordance with the appropriate locations, numbers or shapes, the machine will be left blind and will be unable to perform accurate matching or any matching at all.
Some of the manufacturing errors that can be encountered in such a case are overhangs, wrong positioning, missing components, short circuits and tombstones. If it is mandatory to use a PCB, the fiducial points of which were designed incorrectly during the design phase, an attempt will be made to utilize fixtures or the existing holes on the PCB, resulting with extra time and cost spent. In order to avoid such manufacturing issues, the fiducial points on the PCB must be dimensioned, formed and positioned correctly and must be included in the CAD data (Pick and Place / Gerber) created at the end of the design phase.

PCB FIDUCIAL POINT APPLICATIONS

Fiducial Point Application for SMD Manufacturing
The point must be solid, with a diameter of 1-3 mm (preferably 1.5 mm), and round-shaped. There should be a solder mask that is at least twice the diameter. There should be no Tracks, Vias or Silkscreens on the Fiducial. X, Y coordinates must always be provided with the CAD data.

Fiducial Point Application for Axial and Radial Manufacturing
If THT Axial and Radial inserters are to be used in production, 2 holes with a diameter of 4mm must be drilled (may vary according to the machine's pin diameter). These holes must be positioned along the PCB's longitudinal strip and at a distance of 5mm from the edge of the transverse axis (may vary according to the machine's pin layout).

Fiducial Point Layout

The following points must be considered when adding fiducials for SMD manufacturing; For the machines to perform accurate centering, there must be at least two fiducials located on the diagonal corners rather than on the same X, Y coordinates.If the PCB is not to be panelized and manufactured as a single component, the minimum distance from the board edges must be 5mm.If the PCB is to be manufactured as a panelized components, Fiducials to be located on breakaway tabs must be positioned along the PCB's transverse strip and at a distance of 5mm from the edge of the longitudinal axis.

The fiducial points must not be located on the bottom of the components (SMD-THT). Fiducial points at the bottom of the component will be rendered ineffective during the machinery processes that follow the inclusion of the component (SMD, AOI, Flying Probe Tester etc.). There should be no material leg holes, vias or similar shapes of the same size near the fiducial points. Similar shapes located near the fiducials will reduce the decision-making speed of the machine during the matching process, or cause the wrong point to be selected as the Fiducial point.

Cem Kanımdan

Production Manager