Component placement

11.7.1. Board layout

To maximize performance of the ATE operator or repair technician during diagnostic probing, the preferred overall layout of the board is for the main components (for example, integrated circuits) to be placed on a rectangular grid (see Figure 11.6). As discussed in Section 11.7.3, all components should be oriented in the same direction with pin-1 of each device being placed on a grid intersection. Components that are too large to fit within a single position on the grid can spread into neighbouring positions.

Note that Figure 11.6 shows a number of locations on the standard grid reserved for test support components. Since an amount of additional circuitry may be required to render the design testable according to the standards defined in this book, it is good practice to reserve space for the extra components from the outset — and to release it later if not required.

Reserving a consistent set of locations for boards in a product range has benefits in terms of reduced cost of providing ATE interface fixtures.

* l u i i i c u ^IK^UH LAYOUT

11.7.2. Component mounting

The preferred style of assembly is single-sided component mounting on plated-through-hole printed circuit boards, using either through-hole or surface-mount components. Other styles of assembly, notably double-sided component mounting, significantly increase the cost of building ATE interface fixtures and complicate the ATE operator's task during diagnostic probing.

Where components must be mounted on both sides of an assembled board it is essential that the following factors are considered during component placement:

O Where practical, all integrated circuits mounted on the bottom side should meet IEEE Std 1149.1, since this removes the need for physical probing of their pins.

O Components on the bottom side may be in a vacuum during testing, which will inhibit cooling. These components must therefore be able to tolerate being in the vacuum for in excess of 15'minutes (to allow for fault diagnosis time).

O Components on the bottom side must not obstruct the operation of the bed-of-nails fixture. In particular, they must be clear of test lands (see Section 11.9) and their height must be less than 4.00 mm (0.160 inches) preferred, 9.00 mm (0.360 inches) absolute maximum.

11.7.3. Component orientation

The ease and accuracy with which an ATE or repair operator can locate a specified component pin for probing or examination can be significantly decreased if all components do not have the same orientation on the board.

(Generally, a good operator can achieve around five probes per minute on a well laid out board.)

To allow maximum diagnostic throughput to be achieved, all components must have the same orientation, with their axes parallel to either the X or the Y axis of the board. Note also that for reliable flow soldering, all dual-in-line packages should have the same orientation.

Where it is necessary to adopt different component orientations, the following rules must be obeyed:

O all packages of the same style (for example, all dual-in-line packaged devices) should have the same orientation;

O package orientations should be at 90° intervals — that is, component axes must be parallel to the X or Y axis of the board; and

d the position of pin 1 and the flow of pin numbers (that is, clockwise or anti-clockwise) should be prominently marked beside each component.

COMPONENT PLACEMENT 161

11.7.4. Space between components

The rules contained in this sub-section must be followed unless neighbouring components conform to ANSI/IEEE Std 1149 J.

To allow for accurate and reliable contact between a guided probe, chip clip, and so on, and a component pin or printed circuit track, there must be at least 3.810 mm (0.150 inches) separation between component pins and adjacent components, measured perpendicular to the side of the component on which the pin is located. The space between adjacent sides of components which do not have connections must exceed 1.27 mm (0.050 inches).

These requirements are illustrated in Figure 11.7. Note that where sockets are used, the spacings are relative to the outer socket walls.

Figure 11.7 Space between components.

1 0 / fRINTED CIRCUIT LAYOUT %

11.7.5. Rules for specific component types Adjustable components

Adjustable components must be placed on the top side of the board. All adjustable components must be mounted so that they can be adjusted with a screwdriver or other instrument held perpendicular to the printed circuit board. The direction of adjustment should be the same for all components (for example, clockwise/anticlockwise, left-to-right, top-to-bottom, and so on).

Select-on-test components

Select-on-test components must be placed on the top side of the board. The mounting arrangement must be compatible with the number of insertions and removals anticipated during the product's life.

Indicators

Light-emitting diodes and other indicators must be placed so that they are visible when the board is mounted on the ATE.

Fuses and circuit breakers

All fuses and circuit breakers must be visible during testing, and capable of being reset or replaced without any dismantling.

Links and switches

Links, switches, and so on, must be placed on the top side of the board and be capable of being changed or operated without any dismantling.