SMB (DO-214AA) and SOD-123 packages carry TVS and small rectifier functions, and their small mass makes them more sensitive to the reflow process than the power packages beside them. This guide covers the process sensitivity, the pads, the tombstone defect, and the inspection that keep a small package reliable on a production board.
Small Packages, Big Process Sensitivity
A small body heats faster, reaches the solder melting temperature sooner, and carries less mass to resist the wetting forces of the molten paste. The same profile tuned for a large power device can skew a SOD-123, and the same pad geometry that works for an SMB termination can lift a smaller one. The process sensitivity is the reason small-signal and TVS packages need their own pad and reflow review, separate from the board’s large-package settings.
The two packages sit at different points on that sensitivity scale. SMB (DO-214AA) has wider terminations and more body mass, so it tolerates a moderate imbalance in pad or paste volume. SOD-123 is smaller, lighter, and terminates on two narrow ends, which makes it the classic tombstone candidate when the two ends do not wet at the same moment. The difference is not a quality ranking; it is a process input, and the board’s design rules should reflect it.
The TVS role makes the process stake higher than a signal path: a damaged or weakly soldered protection part fails quietly, and the protected circuit loses its clamp without an obvious symptom. A part like the ASMBJ28CA—the 28 V, 600 W TVS in SMB (DO-214AA) on the Good-Ark site—is a good reference for the assembly discussion, because its job is precisely the one that must not be left to chance at the solder joint.
Reflow Profiles and Peak Temperatures
For small packages, the profile’s important parameters are the ramp into the peak, the peak temperature itself, and the cooling rate. Lead-free assembly typically needs a peak in the 245–260 °C range, and the package’s moisture-sensitivity classification sets the upper bound under J-STD-020; the general profile method is covered by the soldering guide, while the small-package point here is that the board’s largest and smallest parts must both stay inside their limits.
The ramp rate matters differently for a small body: there is little thermal mass to create a large gradient across the package, so the risk shifts to the paste. A fast ramp past the melting point can cause paste spatter and solder balling around the terminations, while a controlled ramp and a proper soak give the flux time to act. A common starting profile for mixed boards uses a ramp near 1–3 °C/s, a soak in the 150–200 °C band, and a peak dwell long enough for the solder to wet—then the actual numbers are confirmed with thermocouples on the smallest package and signed off by the assembly house.
The cooling rate sets the joint’s microstructure and residual stress. A slow cool gives a coarser grain and lower residual stress, which matters more for the thermal-cycling life of the joint; a rapid cool can leave the termination stressed. The profile verification places a thermocouple on the SMB or SOD-123 body, not only on the large parts, because the small body reaches the peak faster and the measured peak is the value that must stay inside the limit.
Pad Design for SMB and SOD-123
The pad pair is the design’s first defense against tombstoning. The two terminations need equal pad geometry, equal copper area, and equal paste aperture, so that the wetting forces at both ends arrive at the same time and with the same magnitude. A difference in pad width, a ground plane that sinks heat from one pad only, or a paste aperture that favors one end is enough to tip the balance.
| Pad factor | SMB (DO-214AA) | SOD-123 |
|---|---|---|
| Termination span | Wide, two ends | Narrow, two ends |
| Pad symmetry | Required, both ends | Required, both ends |
| Paste aperture | Matched to each pad | Reduced and matched |
| Heat sinking | Balanced copper under both pads | No plane under one pad only |
| Stencil | Confirm against package drawing | Confirm against package drawing |
The table’s message is symmetry: the pad pair is designed as a pair, with the copper and the stencil treated as one system. A thermal via under one termination or a long trace to one pad changes the heat flow at that end, and the two ends then wet at different moments. For SOD-123 the paste volume is often reduced through the aperture rather than by shrinking the pad, because a smaller aperture keeps the joint geometry while controlling the paste-to-mass ratio.
The final pad values come from the package drawing and the assembly house’s process qualification; the article’s table is the decision logic, not a replacement for the drawing.
Process Qualification for a New Board.
The first article of a new board is the process’s proof: the profile is verified with thermocouples on the SMB and SOD-123 bodies, the paste deposit is checked against the aperture, and the placement is confirmed within the pad. The reflow result is inspected, and the electrical check—the TVS clamp on the assembled board—closes the loop. The qualification record ties the process settings to the electrical result, and it is the reference for every later board revision.
Tombstoning and Solder Joint Quality
Tombstoning happens when one termination wets and pulls while the other has not yet wet, rotating the part onto its end. The trigger list reads like a layout review: unequal pad sizes, unequal paste apertures, placement offset toward one pad, a thermal imbalance from copper or planes, and a reflow gradient across the board. Each trigger moves one end ahead of the other, and the fix addresses the imbalance, not just the placement machine.
The quality checks continue after the part stands upright: the fillet should show wetting on both terminations, the part should sit flat and centered, and the paste should not bridge to adjacent parts. Solder balling around a small termination is a process signal, usually from a fast ramp or an expired paste; voids inside the joint are harder to see and matter most when the joint also carries heat, as it does in a TVS that conducts surge current through the SMB terminations.
Inspection Criteria
Inspection for small packages follows the assembly acceptance standard, with the same criteria applied at the small scale: solder coverage on the termination, fillet shape, absence of cracks and bridging, and correct placement within the pad. Automated optical inspection is the production tool, and the first-article inspection verifies the profile and the pads before the line runs.
The electrical check closes the process: for a TVS, the clamp behavior is verified on the assembled board, because a joint with marginal wetting can pass visual inspection and still add resistance that shifts the clamp under surge current. The verification is done at the board level with the actual surge test, and the record ties the assembly settings to the electrical result.
Design note. The reflow values and the pad factors are starting points for a small-package process; the final peak, pad dimensions, and apertures are confirmed against the package drawing, the MSL classification, and the assembly house’s qualification. The general profile method and moisture handling are covered in the soldering and moisture guides.
Frequently Asked Questions
Why are small packages more process-sensitive?
A small body heats faster and has less mass to resist the wetting forces, so pad, paste, and thermal imbalances that a large package tolerates can lift or tombstone a SOD-123.
What is the profile rule for SMB and SOD-123?
Stay inside the package’s J-STD-020 peak limit, verify the ramp and soak with a thermocouple on the small body, and control the cooling rate for joint stress.
How do I prevent tombstoning?
Design the two pads as a pair—equal geometry, equal copper, equal paste—and remove thermal imbalances that let one termination wet before the other.
What does inspection check?
Solder coverage, fillet shape, placement, cracks, and bridging under the assembly standard, plus an electrical clamp check for TVS parts.
Why verify the TVS electrically after assembly?
A marginal joint can pass visual inspection and still add resistance that shifts the clamp under surge, so the assembled-board surge test is the real proof.
Conclusion
Small packages reward a deliberate process: matched pads, controlled paste, a profile verified on the smallest body, and inspection that ends with an electrical check. The acceptance criteria are the same as for any assembly, applied at a scale where symmetry is the deciding factor.
Review the Schottky rectifier diodes category on the Good-Ark site for the small-package context, and send Good-Ark your board stack and assembly profile for a process review of your small-package placement.