A cross-section through the failed joint shows a distinct boundary between the solder ball on the part and the solder paste on the circuit board, rather like a section through a head resting on a pillow.
This is particularly a concern when using lead-free solder, which requires a higher processing temperature.
During heating, the board and components undergo thermal expansion, can flex, and some of the balls can be lifted off the paste.
flux with higher activation temperature, together with the wetting characteristics of the solder ball, are the most significant mitigation factors.
Diagnosis of head-in-pillow defects may require use of X-rays or EOTPR (Electro Optical Terahertz Pulse Reflectometry), since the solder joints are hidden between the integrated circuit package and the printed circuit board.