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I have a P1004B746400 B7464 REV.A board which is a 72-pin SIMM RAM with two TI TMS418169DZ chips on it. According to 72-pin SIMM configuration, the pins #1, #39 and #72 are ground Vss and pin #10 is supply Vcc.

However, those ground and supply pins are connected to no where, I mean, there is no trace on the board connecting to those pins. Now, I wonder what I'm missing and how the ground and supply pins work.

Pins #1 and #72 are shown below which have no trace connecting to them.

enter image description here

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    \$\begingroup\$ Notice that the pads have holes. Those holes are copper plated internally that connects the pad to the internal layers as the other answers are describing. \$\endgroup\$ – Trygve Laugstøl May 26 '17 at 9:37
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The board is a multi-layer stackup, probably 4-layer.

This means that there are more layers inside the PCB on which other connections are routed.

You can tell this from the seemingly disappearing routing, but also from the colour of the board. Notice how it is light around the edges (where light can shine through), but then suddenly gets dark. The dark region is where there is more copper inside the board.

The internal layers in this case are most likely just power planes - one of Vcc and one for Vss. All connections for power and ground will connect to one or other of the planes providing nice low impedance power routing.

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The PCB most probalby has 4 layers. The outer 2 which you can see are used to carry the signals and the inner 2 are power layers - 1 for ground and 1 for Vcc. This is the most common approach when designing digital electronics.

Inner layers can be seen as the dark regions inside the board. Vias - a small copper plated holes are used to conduct between layers.

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According to the above pinout, pins 30 and 59 are also VCC. Looks like they designers used one of those for each IC.

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