A method includes loading a first package component on a concave boat, and placing a second package component over the first package component. A load clamp is placed over the second package component, wherein the load clamp is supported by a temperature-variable spacer of the concave boat. A reflow step is performed to bond the second package component to the first package component. During a temperature-elevation step of the reflow step, the temperature-variable spacer is softened in response to an increase in temperature, and a height of the softened temperature-variable spacer is reduced, until the load clamp is stopped by a rigid spacer of the concave boat.