The mechanics of tombstoning are based on molten solder wetting forces, component mass and geometry. If solder paste under one termination melts and starts wetting to the termination before the other side does, the wetting action force can pull that component up on its end. Typically, the smaller and lighter the component is, the more susceptible it is to tombstoning.
Factors in tombstoning can be classified into two main categories: design-related (pad design; pad definition, mask or metal; thermal balance between pads; unfilled microvias in the pads) and process-related factors (numerous, and often difficult to quantify). Many DoEs have been performed and published on the topic, often focusing on best practices for design and assembly.
Most 0201 assembly processes were optimized prior to the transition to Pb-free processing. Increasing production of Pb-free products and the associated tighter process windows now dictate a reassessment of the key parameters that can affect tombstone formation.
A review of published studies and high-volume production experiences uncovered 49 potential parameters that can influence tombstone defects. The investigators then used a cause-and-effect (C&E) matrix approach (rating influence 0, 3 or 9) to identify the following 12 parameters as the most influential:
The experiment was partitioned into two phases. The experimental matrix for Phase 1 is shown in Table 1. Factors listed in the top half were designed into the test vehicle (Figure 1). Factors in the table’s bottom half were varied during the assembly process.
Test vehicle. The TV contained 48 individual test cells. Four different pad designs were used, each appearing twelve times on the test vehicle, six times per row of cells on two rows. Each cell had 50 resistors and 50 capacitors oriented at 0 and 90°, a total of 200 per block (Figure 2). Eight cells per padstack were assembled per board, for a total of 1600 placements per padstack per board. In Phase 1, all four padstacks were used, resulting in 6400 component placements per board.