What are Mid-Chip Solder Balls?

Mid-Chip Solder Balls occur when some solder paste detaches from the bulk deposits on the pads and gets stuck under the component. When this cools, it looks like a ball of solder sticking out from under the component. The mid-chip balling defect usually occurs in surface mount / chip components like resistors, inductors and capacitors. This defect reduces the aesthetics of the board more than reliability but sometimes can result in an electrical short between the component leads. Hence, removal of the mid-chip ball is recommended.

Mid-chip balling usually occurs when using excessive solder paste is used and/or when the solder paste has a poor hot slump value and/or insufficient wetting.

The selection of stencil aperture size, aperture thickness, aperture shape decides the amount of solder paste deposits. Research shows that a stencil that is triangular in shape, has a small pad-aperture size and small aperture thickness will ensure the minimum deposit of solder paste and reduce the possibility of mid-chip solder balling defects.

Triangular Aperture

Square Aperture

The selection of appropriate solder pastes for an application dictates the formation of mid-chip balling to an extent. The deciding factors are the hot slump value, the flux nature, and the presence of halogen in the solder paste. The solder paste having an excessive hot slump value will become the cause for solder paste detach from the deposits. The hot slump value of the solder paste can be minimized by adding a gelling agent or increasing the metal content. The reflow atmosphere can also play a role in mid-chip balling. A Nitrogen reflow atmosphere instead of air, will reduce the mid-chip balling defect. Also, an appropriate reflow profile will reduce mid-chip balling.

Effect of pad-aperture ratio on 1206 pads

Result: Lowest pad to aperture ratio will reduce mid-chip balling.

Effect of stencil thickness on 1206 pads

Result: Lower stencil thickness will reduce the mid-chip balling defects.

Effect of pad coverage and aperture shape on 1206

Result: Triangle shape will reduce the amount of paste deposit. Hence, reduce the mid-chip balling.