A locking push pin and methods of using the locking push pin to assemble a heatsink assembly, are described. The locking push pin includes a body lumen extending through a pin body, and several prongs radially outward from the body lumen. Peripheral surfaces of the prongs are separated by a body slot radially outward from the body lumen. The locking push pin includes a pin lock having a branch that extends through the body lumen and into the body slot between the peripheral surfaces. The pin lock advances from an unlocked configuration in which the branch is in the body slot proximal to a distal end of the prongs, allowing the prongs to deflect radially inward, to a locked configuration in which the branch is in the body slot at the distal end of the prongs, blocking deflection of the prongs. Other embodiments are also described and claimed.