What is Head-in-Pillow effect?

In BGA solder joints, head-in-pillow (HIP) refers to the bad structure and phenomenon. According to the section picture of the slice, it resembles the shape of a person's head leaning on a pillow. The head-in-pillow (HIP) effect is mainly used to describe the BGA parts of the circuit board during the high temperature process of reflow (Reflow). Due to the high temperature, the BGA carrier board or the circuit board will warp and warp (War page) or deform for other reasons, separating the solder ball (Ball) from the solder paste on the circuit board.

After the board passes through the reflow zone, the temperature slowly drops, and the deformation of the IC substrate/body and the circuit board gradually returns to its original state (sometimes it won't). Because the temperature is lower than the melting point of the solder ball and solder paste, they have cooled and solidified again from their molten state. Due to the inability to fuse the two, a weld shape similar to a head resting on a pillow is formed.

The majority of HIP devices are packaged in BGA/CSP or POP formats. Based on the results of the slicing and dyeing experiments, the BGA solder balls are in contact with the solder on the PCB pad, but there are obvious boundaries that prevent valid electrical and mechanical connections. The problem may be detected through online functional testing (open circuit), but it may not be detected (weak connection), resulting in reliability problems and affecting the product's service life and reliability.