------ELECTRONICS SYSTEMS DDS-27
Printed Circuit Board Design Standards
------Automotive Systems Confidential Page 12 of 139
2.10. FIDUCIALS
Fiducials are used for precise alignment when printing solder paste or adhesive pattern and placing SMC.s. See Section 4.2.9.
2.11. CIRCUIT BOARD CONSTRUCTION
• Circuit board layer stack-up construction is defined in Section 5.1.3.3.
• Copper layers shall be evenly distributed and balanced. This is done to minimize warpage. To balance copper layers, waffling copper is acceptable See Board Design Section 4.0
2.12. CONDUCTOR WIDTH AND THICKNESS
The width and thickness of conductors on the finished printed wiring boards shall be determined on the basis of the current carrying capacity required as determined from charts in IPC-D-275 or its current update.
2.13. TEST POINTS
The test engineer is to assist CAD during circuit board layout with placement and conformance to each paragraph. The test engineer shall be involved and agree with any deviations that may be required. The test engineer is to review and approve the completed design before circuit boards are built.
2.13.1. Test Point Quantity Per Net
2.13.1.1. In-circuit Test
Place one test point on each net. This is not intended to limit test points to one per net. After reviewing the layout, the test engineer may require additional test points. Nets that carry more than 2 amps need to be identified on the Circuit Board Design Requirements form.
2.13.1.2. Functional Test
When the Circuit Board Design Requirements indicate that the board will not be in-circuit tested, test points are not required on every net. Test points are only placed on nets required for functional tests.
2.13.1.3. Grounds
There is to be a minimum of 3 test points on each analog ground, digital ground, and power ground nets. Refer to the test point numbering convention in Section 5.1.5.2.
2.13.1.4. Kelvin Connections
The Manufacturing Test Engineer is to indicate on the Circuit Board Design Requirements form when there is a need to place multiple test pads on each of two nets associated with a component. This is required in order to make a very accurate measurement on a critical component.
2.13.2. Area Density
The Manufacturing Test engineer will verify that there is less than or equal to 15 test points per 625 mm2 (1 in2) when using probes rated for 306 grams (10.8 ounce) pressure. Maximum count per 625 mm2 (1 in2) for other probe pressures: 227 grams (8 ounce) /18 probes, 454 grams (16 ounce) / 8 probes. When too many probes are concentrated in one area, the upward force of test probes exceeds the downward force of the vacuum and will cause the board to flex in the high upward force area.
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------ELECTRONICS SYSTEMS DDS-27
Printed Circuit Board Design Standards
------Automotive Systems Confidential Page 1X of 139
2.13.2.1. Spacing
2.13.2.1.1. Standard
Preferred spacing of test pads should be 2.50 mm from center to center.
2.13.5. Board Access
2.13.5.1. Bottom (WSS/RSS2)
Bottomside access for all nets is required. Topside access is acceptable if the Manufacturing Test Engineer so indicated in the Circuit Board Design Requirements for dual side probing fixture design.
2.15. SPECIAL LAYOUT CONSIDERATIONS
2.15.1.1. Integrated Circuits
Some ICs, for example, Oxygen Sensor Unit (OSU), Dual Oxygen Sensor Unit (DOSU), Hex Oxygen Sensor Unit (HOSU) and their related discrete components (if applicable), require special considerations during the printed circuit board layout phase of the design. Refer to Section 6 (Layout Considerations.
2.15.1.2. High impedance components
When circuits operate at an impedance over 1Meg Ohm (such as certain oscillator circuits), integration into a product requires attention to detail. High impedance circuits can be affected by high humidity and liquid intrusion thus increasing the risk of current leakage failure while the product is operating in the field.
3.0 Component Mounting
• All Component mounting decisions shall be approved by a ------Manufacturing Engineering Pad Design Team. This team shall be appointed by the SMT MTT and Soldering MTT Leaders.
• Details of circuit board layouts are determined by the Pad Design Team, an MTTstaffed team.
4.2.1.2. Solder Mask Set Backs Allowable
A solder mask set back shall equal the registration A solder mask setback of zero is not allowed.
Some component types may require special considerations in solder mask set back. These
requirements are defined in the components section. See below for an example of solder mask set back.
NOTE: These pages contain a brief overview of a very small portion of the circuit board standards contained in the original document and therefore are neither complete nor comprehensive.