Head-in-pillow is a BGA or CSP defect in which the solder ball and the paste deposit both melt but never coalesce, leaving the ball sitting in a dent in the paste like a head on a pillow. It comes from package or board warpage during reflow, which lifts the ball away long enough for its surface to oxidise, or from weak flux activity. The joint can pass electrical test and then fail in the field, and it is found by X-ray, cross-section or dye-and-pry.
X-ray looks marginal on the corner balls, dye-and-pry two boards and check for head-in-pillow.
Also heard as
- HiP
- head-on-pillow
- ball-and-socket
Related terms
Voiding
Gas pockets trapped inside solder joints, seen as light spots on X-ray, which weaken them and block heat.
Reflow
Soldering surface mount parts by melting printed solder paste in an oven that follows a set temperature profile.
Bow and twist
Warpage of a board, measured as deviation from flat, which upsets assembly and stresses solder joints.
Solder paste
Mix of fine solder powder and flux printed onto pads to hold parts in place and form the joints in reflow.