What Do You Want on Your Tombstone?

ABSTRACT Tombstoning, draw-bridging, bill-boarding, and skewing are all common defects that are caused by movement of components out of their intended position. This is reminiscent of pepperoni lifting up while baking a Tombstone pizza. These defects typically occur with passive components (e.g. 1206, 0805, 0603 Imperial components) and are a common issue for electronics manufacturers. Tombstoning, draw-bridging, bill-boarding and skewing can be caused by the circuit board design, pick and place errors, stencil design, reflow profile, and other potential sources. These potential causes of tombstoning and related defects were studied in order to create solutions for this issue. This paper summarizes the results of this work. Circuit board design can create the potential for tombstoning. One example of this is when the PCB design utilizes one pad set for multiple component sizes such as 0805 and 0603 Imperial components. These pad sets may be too close together for the larger passive components, but also too far apart for the smaller components. Another example is when the pad at one end of the component sits on a ground plane which acts as a heat sink during reflow. The other end of the component sits on a standard copper defined pad. During reflow, there is a temperature differential between these pads which causes the solder paste on the standard copper defined pad to melt first. This can lift or skew the component before the solder paste on the ground plane pad melts. The stencil design is often based on the copper pad layout rather than the component lead layout. When the pad set is farther apart than the component leads, the solder paste is printed off target with respect to the actual component leads. If the component leads sit to the inside of the solder paste bricks, then tombstoning or draw-bridging can occur. If the component leads sit outside of the center of the solder paste bricks, then skewing and mid-chip beading can occur. Proper alignment of the component into the solder paste bricks is necessary to mitigate these issues. A modified stencil design based on both the circuit board pad layout and the component layout can be used to solve tombstoning, draw-bridging, bill-boarding and skewing issues. Use of an anti-tombstoning solder paste can also help reduce the potential for these issues. Anti-tombstoning solder pastes typically employ solder alloys which melt in a wide range thereby allowing the solder paste bricks at both ends of the component to melt at the same time which reduces the potential for tombstoning. Adding a longer soak to the reflow profile can also help with this issue. Soak profiles help to equalize temperature differences from one pad to another thereby helping the solder paste bricks to melt at the same time at both ends of the component. All of these variables are studied in this work and solutions for tombstoning are presented with respect to their effectiveness.