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PCBComputer consists of two boards - keyboard board and main board:
Main board consist of several layers of conductors. On the outer layers of the board almost no conductors. In the following photo showing main board solder side. Under the top layer of the PCB slightly visible conductors:
Computer board consists of eight layers. Two layers - is the top and bottom boards, and 6 layers inside. You can look at the plateau near the RF encoder: the layers are numbered from 1 to 8 (the photographs RF encoder board with sealed-off for the convenience of shooting):
The figures are best seen when viewed through the circuit board to the light source:
Depending on the year of manufacture of computer circuit boards are slightly different from each other. For example, in some computers production in 1992 keyboard was connected to the mainboard with connectors. In this case, for repair the keyboard can be easily detached from the main board.
In most computers, the keyboard has been soldered to the mainboard:
Perhaps it was done to reduce the cost of production. Additionally, with a shock and bumps could disconnect keyboard connector, so just wires soldered to the mainboard.