Reflow Profile Deltas across a Fully Loaded Panel
Thermal deltas across fully loaded panels require profile soak expansion and localized copper balance to keep peak temperatures within a five degree window.
Tombstoning Prevention
Defect Mitigation
The implementation of design and process controls avoids the vertical lifting of one end of a surface mount component during the soldering process. Most tombstoning prevention strategies focus on balancing the wetting forces acting on the two terminations of a small capacitor or resistor. This imbalance occurs when the solder at one end of the part melts and wets the pad before the other, pulling the component upright.
Proper pad design is the most effective defense.
Reducing Pads
Reducing the size of the solder pads or increasing the distance between them can help synchronize the wetting of the two ends. If the copper traces connected to the pads are of unequal width, they will act as different sized heat sinks, leading to uneven heating. Tombstoning prevention involves using thermal relief patterns to ensure that both pads reach the reflow temperature at the same time.
Modern solder pastes with a wider melting range are also used to provide a more gradual wetting process.
Optimizing Soak
Optimizing the reflow oven’s temperature profile to include a longer soak zone allows the entire board to reach thermal equilibrium. This step reduces the temperature gradients that are the root cause of many assembly defects.