During soldering of surface mount components on printed circuit boards, a defect known as “tombstoning” sometimes occurs. This involves components standing upright on one end rather than lying flat flush to the board after soldering.
Tombstoned parts often completely fail from lack of soldering or induce shorts, rendering the PCB inoperable. This article provides an in-depth look at the causes of tombstoning along with prevention and troubleshooting tips. Read on to learn how to avoid and address this serious assembly issue.
What is PCB Tombstoning?
Tombstoning refers to surface mount device (SMD) components becoming vertically raised during reflow soldering, balanced precariously on one end pad rather than properly seated flat on the board.
The name comes from the visual resemblance to a tombstone standing upright above a grave. Affected parts may only have one termination soldered while the other end lifts away from the pad, causing open circuits.
Adjacent tombstoned components can also short against each other, preventing proper functioning of the affected circuitry. This phenomenon typically afflicts smaller, lighter package styles like 0402 discretes or QFN ICs.
What Causes Tombstoning Defects?
While the exact triggers vary, tombstoning ultimately results from imbalanced soldering forces, especially during reflow:
Uneven Wetting Forces
- One pad wetting faster creates unequal lift force on one end.
- Surface condition differences prevent simultaneous wetting.
- Temperature variations across component cause uneven expansion.
- Nearby parts or board features create localized hot/cool zones.
Poorly Balanced Design
- Asymmetric pad shapes and sizes induce uneven wetting.
- Different thermal mass of large pads, planes shifts center of mass.
Solder Volume Imbalance
- Excess solder volume on one pad lifts that end during reflow.
- Too little solder leaves weakly anchored end.
- Misaligned parts increase susceptibility by positioning components off-center on pads.
Understanding the root cause is key to mitigating the risk of tombstoning defects during assembly.
Tombstone Defect Failure Modes
Depending on which end of the component lifts from the pad, different failure modes manifest:
Lifted Lead Failure
- The smaller, weaker pin end tombstones leaving that termination unsoldered
- Causes an open circuit condition on that pin
Lifted Body Failure
- The larger pad and thermal mass keep one end down while body tombstones
- Can create electrical shorts between lifted end and adjacent components
- Risk of part simply falling out if weak retention on one pad
Either mode results in defective PCBs. Catching and resolving tombstoning is vital to avoid scrap and rework.
PCB Design Strategies to Avoid Tombstoning
Careful board layout practices can significantly reduce tombstoning risk during assembly:
- Use equal pad shapes and sizes on both terminations.
- Places even wetting forces on each end during reflow.
Balanced Thermal Masses
- Ensure thermal planes and traces do not bias heating to one side.
- Isolate pads from large copper regions if needed.
Limit Nearby Tall Components
- Adjacent tall parts can restrict even airflow causing localized cooling.
- Give clearance between small SMDs and tall components.
Place Pairs Together
- Group pairs of 0402 or similar caps in single location.
- Mutual proximity provides more even heating.
- Keep SMDs clear of taller parts which can cause shadowed cool zones.
- Use spacers under taller components if needed.
Proactive design is the first line of defense against tombstoning issues.
Solder Paste Printing Strategies
In addition to layout, optimizing the solder paste printing process helps reduce tombstone defects:
- Print equal paste amounts on both sets of pads
- Could involve reducing volume on larger pads
- Print all pads on one side first, then other side
- Avoids excessive moistening of pad printed first
- Ensure stencil apertures align accurately over pads
- Allows paste release without shifting part
Clean Mask and Board
- Remove all residues to enable free paste release
- Prevent pads from becoming tacky causing part sticking
Break Surface Tension
- Use tacky flux or pierce the paste surface after print
- Allows even release of part onto pads
Refined stencil design paired with tightly controlled paste printing reduces the sources of uneven solder forces.
Solder Reflow Process Optimization
Lastly, the solder reflow process itself strongly influences tombstoning potential:
- Use lower max temp ramp rates around 1-2°C/second.
- Minimizes thermal shock risk from sudden heating.
- Allow sufficient time above liquidus for even wetting.
- 30-60 seconds recommended for even solder flow.
- Avoid excessive max reflow temperatures beyond needs.
- High peaks increase thermal gradients across assemblies.
- Optimize forced air cooling to bring down temperature gradually.
- Rapid cooling can freeze one joint first inducing tombstones.
- Strategically shield parts near edges or heat sinks to balance heating.
- Prevents small parts from dropping excessively in temp.
Smooth, gradual profiling backed by thermal process controls limits partisan heating that enables tombstoning.
Rework Strategies for Tombstoned Components
Despite best efforts, tombstones still occasionally occur:
- Attempt to tack down lifted end by re-introducing heat.
- Chance of success if pad and end still wetted.
Remove and Replace
- For severe tombstones, remove and replace part.
- May require cleaning pads first.
- If tombstone is marginal, tweak part position to flatten.
- Risks weakening existing termination.
- Use thermally resistant adhesive to secure down lifted end.
- Not preferred for electrical reliability.
Reworking tombstoned parts is very case-specific depending on severity, adjacent parts risk, etc. Having plans in place for addressing tombstones during assembly reduces downtime and rework costs.
Process Validation Methods
To confidently verify no tombstoning risk, several validation techniques exist:
- Thermal profiling using thermocouples to quantify gradients
- Solder paste deposit inspection for volume and positioning
- 2D X-ray inspection of solder fillets across terminals
- High speed video of reflow to identify lifting precisely
- Thermal shock testing under exaggerated conditions
- Corner ball shear testing to measure joint strength difference
- Monitoring first pass yield and visual inspection for occurrences
Validating improvements through controlled testing ensures mitigations successfully eliminate tombstoning.
Can tombstoning occur during hand soldering or is it only reflow?
While more prevalent in reflow, uneven heating during manual soldering can also cause tombstoning if proper technique is not followed.
Are there component package styles more prone to tombstones?
Due to small size and low mass, 0402 discretes, fine pitch QFNs, 0201 chip components, and microBGAs are most susceptible.
Does higher or lower solder paste deposition help avoid tombstoning?
Too much solder volume exacerbates lifting forces. But too little risks voiding. Well-balanced volumes matched to pad sizes is ideal.
Can conformal coating help hold down tombstoned parts?
Once present, conformal coating can mechanically anchor lifted component ends. But it cannot inherently prevent tombstoning itself.
How does part density affect tombstone risk on a populated PCB?
Higher component densities induce more significant neighboring part thermal effects during reflow which can create gradients leading to tombstones.
Tombstoning remains a serious assembly defect afflicting PCBs. Through careful design, controlled process engineering, validation testing, and diligent inspection, the risks can be minimized. But occasionally rework will still be required if observed. Understanding the pathways to both prevent and address tombstones makes completing complex PCB assemblies more robust.