Physical Deformation
The out-of-plane distortion of an electronic component package occurs due to internal stresses generated during temperature changes. This condition, known as package warpage, causes the corners or edges of the component to bend away from the board. It alters the distance between the package pads and the solder paste.
Thermal Behavior
Different rates of thermal expansion among the materials that make up the package cause these internal forces. Silicon dies, copper lead frames, and plastic molding compounds all expand and contract at different rates as they heat up and cool down. High reflow temperatures worsen package warpage because they exceed the glass transition temperature of the polymer materials.
This deformation is often dynamic, changing in magnitude as the board moves through the reflow oven.
Defect Formation
Solder joints fail to form properly when the deformed component prevents uniform contact between the pads and the solder paste. Head-in-pillow defects and solder bridging occur frequently on assemblies affected by package warpage. To mitigate these failures, board assemblers use ball grid arrays with larger solder balls or optimize their reflow profiles to reduce the peak temperature.
Inspecting the assembled boards with automated x-ray equipment is required to detect the open circuits and shorts that result from this issue.
