A common issue among Macbook logic boards that do not chime, is no CPU vcore! Vcore is a primary CPU power rail; vcore the CPU will not do anything, and the laptop will not do anything. When troubleshooting the CPU vcore circuit, a specific point of interest are the TON pads of the CPU vcore buck regulator controller. Of course, to plug ourselves, we fix this regularly with our Macbook logic board component level repair service.
A buck regulator is a type of basic switching power supply that takes a higher DC voltage and converts it into a lower DC voltage. Two mosfets, one between a power supply and output, the other between output and ground, are controlled by a controller IC, and the speed of switching is what determines output voltage. Slower switching means lower voltage, faster switching means higher voltage.
The controller IC can work with a wide variety of input power supplies. Obviously, given the same speed of switching, if you start with a higher voltage, the output voltage will be higher. The way this chip can know to put out a specific voltage regardless of a large variety of input voltages across different systems is by referencing the input voltage. This is done by the TON pin of the CPU buck controller IC.
The TON pin references the higher voltage power supply that the buck regulator uses to create the lower output voltage. It does this through a resistor, and this resistor is usually the first to corrode because it is attached to a high voltage power line. Voltage at this pin sets switching frequency within the chip. Without voltage being present on this pin, the buck regulator never turns on, and you get no CPU vcore.