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Difficulty: HardMotherboards Form Factors and Connectors

A technician is installing a Micro-ATX motherboard into a computer case that previously housed a standard ATX motherboard. After securing the motherboard with mounting screws and connecting all power and front-panel cables, the technician attempts to power on the system. The power supply fans momentarily spin for less than a second before immediately shutting down, and the motherboard status light indicates a power fault protection state. Which of the following is the most likely cause of this issue?

  1. An unused brass standoff left in the chassis from the previous ATX motherboard configuration is shorting against the underside of the Micro-ATX board.Answer
  2. B
    The technician connected a key-keyed 8-pin PCIe power cable into the motherboard's 8-pin EPS12V CPU power connector.
  3. C
    The motherboard requires an M.2 SATA SSD to be populated in slot M.2_1 to complete the main system power circuit prior to POST initialization.
  4. D
    The standard 24-pin ATX main power cable delivers inadequate wattage because it lacks the secondary 4-pin auxiliary power extension required for Micro-ATX form factors.

Answer

An unused brass standoff left in the chassis from the previous ATX motherboard configuration is shorting against the underside of the Micro-ATX board.
Standard ATX motherboards (12 x 9.6 inches) utilize additional standoff locations on the computer case chassis that extend beyond the lower perimeter of a Micro-ATX motherboard (9.6 x 9.6 inches). If a technician fails to remove the leftover standoff from the ATX layout, the metal standoff presses against electrical traces or component solder pins on the underside of the Micro-ATX motherboard. When power is applied, this creates an immediate short circuit to the grounded case chassis, triggering the power supply's short-circuit protection (SCP) mechanism and causing an immediate power-off state.

Step-by-Step Solution

1
Analyze the physical dimension and mounting footprint differences between form factors.
Standard ATX motherboards measure 12 x 9.6 inches and use up to 9 or 10 mounting standoffs, whereas Micro-ATX boards measure 9.6 x 9.6 inches and use up to 8 standoffs in slightly different layout grids.
When replacing a larger motherboard form factor with a smaller one, standoff locations that do not align with the new motherboard's reinforced screw holes must be removed from the case tray.
2
Evaluate the reported symptom of immediate shutdown after power-on.
Fans spinning for under a second followed by system shutdown indicates short-circuit protection (SCP) or over-current protection (OCP) within the power supply unit.
Direct contact between a metal standoff and electrical traces on the back of the motherboard creates a zero-resistance path to ground, forcing the power supply to trip safety shutoffs.

Key Concept

Motherboard Form Factor Mounting & Standoff Alignment
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Topics

Motherboards Form Factors and Connectors

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