Tips & Tricks – Why Do Components Tombstone

Table of Contents

Understanding Component Tombstoning

What is Component Tombstoning?

Component tombstoning is a phenomenon in electronics manufacturing where components, particularly small passive components like resistors and capacitors, suddenly stand up on end during the reflow soldering process. Instead of remaining flat on the printed circuit board (PCB) as intended, the components lift up and stand vertically like a tombstone, hence the term “tombstoning.”

Tombstoning can cause open circuits, short circuits, and other defects that impact the functionality and reliability of the assembled PCBs. It is a common issue in surface mount technology (SMT) assembly and can be frustrating for manufacturers to troubleshoot and prevent.

Why Does Tombstoning Occur?

There are several factors that can contribute to component tombstoning:

Uneven heating: If one side of a component heats up and reflows before the other side, the surface tension of the molten solder can pull the component up into a vertical position. Uneven heating can be caused by:
Poor oven profiling with uneven heat distribution
Shadowing effects from taller nearby components
Different thermal masses of copper pads and traces on each side of the component
Misalignment: If a component is placed slightly off-center or rotated on its pads, it may tombstone as the solder melts and pulls it into alignment. Placement inaccuracy can result from:
Incorrect component feeding in the pick-and-place machine
Nozzle wear or damage causing misplacement
Stretched or deformed carrier tapes allowing components to shift position
Pad design: The size, shape, and spacing of the solder pads can impact tombstoning risk. Some problematic pad configurations include:
Pads that are too small to hold the component in place
Pads with significantly different sizes that cause uneven solder volume and wetting forces
Pads spaced too far apart allowing the component to tip over easily
Component geometry: Some components are more prone to tombstoning than others based on their physical characteristics, such as:
Small, lightweight, symmetrical chips like 0201 resistors and capacitors
Components with high aspect ratios (length much greater than width)
Devices with end terminations that have a small contact area with the pads
Solder paste: The solder paste’s composition and printing process can affect tombstoning, including:
Paste formulation with inadequate tackiness to hold components in place
Insufficient or excessive paste volume causing weak or unbalanced solder joints
Poor paste transfer through the stencil due to clogged apertures or incorrect pressure

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Strategies to Prevent Tombstoning

PCB Design Guidelines

Proper PCB design is critical for minimizing tombstone risk. Follow these guidelines when designing pads and footprints for susceptible components:

Pad Size and Spacing

Use pads that match the component’s end termination size as closely as possible. Avoid undersized pads.
Increase pad width to provide more stability and solder wetting area. Wider pads help counteract tipping forces.
Minimize pad spacing to limit the component’s ability to pivot upwards. Bring pads as close together as manufacturing capabilities allow.

Here are some recommended pad dimensions for common chip sizes:

Chip Size	Pad Length	Pad Width	Pad Spacing
1206	1.0-1.2 mm	1.0-1.2 mm	0.5-0.8 mm
0805	0.8-1.0 mm	0.8-1.0 mm	0.4-0.6 mm
0603	0.6-0.8 mm	0.6-0.8 mm	0.3-0.5 mm
0402	0.4-0.6 mm	0.4-0.5 mm	0.2-0.4 mm
0201	0.2-0.4 mm	0.2-0.3 mm	0.1-0.3 mm

Pad Geometry

Use rectangular pads instead of rounded pads. Rectangles provide more wetting area and stability.
Add non-solderable anchor extensions to pads to help hold components in place. The extensions should be at least 50% of the pad width.
Avoid significantly unequal pad sizes that create unbalanced wetting forces. Use the same size pads on both ends when possible.
Orient rectangular pads perpendicular to the tombstone axis to maximize wetting force. Pads parallel to the tombstone direction are less effective at holding components down.

Placement Orientation

Rotate tombstone-prone components 90° so the shorter edge is parallel to the convection flow in the reflow oven. This helps both ends heat and reflow simultaneously.
Avoid placing susceptible components broadside to the airflow which promotes uneven heating.
Do not orient rectangular chips with the long axis hanging off the edge of the board. Overhanging components are more likely to tombstone.

Stencil Design Optimization

Effective stencil design ensures proper solder paste volume and release onto the pads. Optimize your stencils with these techniques:

Aperture Size and Shape

Calculate theoretical paste volumes and area ratios for your pad sizes. Aim for 0.66 to 0.90 area ratio for most components.
Adjust aperture width and length to achieve target paste volume. Increasing width has more impact than increasing length.
Use home plate shaped apertures instead of simple rectangles. The tapered home plate design promotes paste transfer and release.

Aperture Placement

Center apertures on the pads for maximum paste coverage and stability.
Avoid aperture overhangs off the edge of the pad. Paste should be printed entirely on the solderable surface.
Shift apertures slightly away from the component center to create more paste volume on the outer edges of the pads. The extra paste helps hold the component down.

Stencil Thickness

Use a stencil thickness that balances sufficient paste volume and clean release onto the pads. Thicker stencils deposit more volume.
Start with a 4-5 mil (0.10-0.12 mm) thick stencil for most standard SMT Stencils. Go thicker for large components and thinner for fine-pitch.
Increase thickness incrementally if paste volume is insufficient. Excessive thickness can cause bridges and shorts.

Here is a quick reference for starter stencil thicknesses and aperture sizes:

Pitch	Stencil Thickness	Aperture Width
>0.65 mm	5-6 mil	90% pad width
0.5-0.65 mm	4-5 mil	85% pad width
0.4-0.5 mm	3-4 mil	80% pad width
<0.4 mm	2-3 mil	70% pad width

Solder Paste Selection

The right solder paste can provide better tack strength and reflow consistency to resist tombstoning. Consider these factors when choosing a paste:

Powder Type and Size

Use paste with a powder size suitable for the component pitch and volume requirements. Type 3 and 4 are most common.
Finer powder sizes (e.g. Type 5, 6) have higher metal content and better printing definition but less tack strength.
Coarser powders (e.g. Type 3, 4) provide stronger tack to hold components but may cause more solder defects. Match powder size to aperture size.

Flux Activity

Select a paste flux with an activity level appropriate for your surface finishes and reflow profile. Higher activity promotes wetting.
Avoid excessively active fluxes that can cause graping and solder balling. These defects weaken solder joints.
Consider no-clean pastes to minimize post-reflow residue while still providing good activity and tack strength.

Tack Strength

Evaluate pastes for their ability to hold components in place before and during reflow. High tack strength resists tombstoning forces.
Conduct tack tests by measuring the force needed to dislodge components from printed paste deposits. Compare paste options quantitatively.
Be aware that tack strength decreases over time as the paste dries out and solvents evaporate. Use paste promptly after printing.

Reflow Profile Optimization

Careful reflow profiling is essential to ensure all areas of the PCB heat and cool uniformly to prevent tombstoning. Follow these guidelines:

Profile Type

Use a ramp-to-peak (RTP) profile instead of a ramp-soak-spike (RSS) profile when possible. RTP profiles have a slower, more linear heating rate.
Avoid long soak zones that can dry out paste and reduce tack strength before reflow. Minimize soak time if used.
Target a peak temperature 20-30°C above the solder’s melting point to achieve good wetting and coalescence. Higher peaks help components self-align.

Heating Rate

Maintain a consistent heating rate of 1-2°C/second throughout the profile. Avoid sudden jumps or dips in temperature.
Limit the temperature difference across the PCB to less than 10°C at any point in the profile. Use thermocouples to measure multiple locations.
Adjust top and bottom heater settings to balance the heat input to each side of the board. Apply more heat to the side with greater thermal mass.

Cooling Rate

Control the cooling rate after reflow to minimize thermal shock and stress on components. Aim for a rate of 2-4°C/second.
Avoid forced cooling until the solder has solidified. Let PCBs cool naturally in the oven or in still air first.
Use a convection reflow oven with top and bottom cooling zones for the most uniform cooling across the PCB.

Here is an example reflow profile suitable for lead-free SAC305 solder paste:

Zone	Time	Temperature
Preheat	60-90 sec	150-180°C
Soak	60-90 sec	180-200°C
Reflow	60-90 sec	230-250°C
Peak	10-30 sec	245-260°C
Cooling	60-120 sec	<180°C

Inspection and Rework Techniques

Post-Reflow Inspection Methods

Catching tombstoned components quickly is key to preventing them from reaching later production stages or customers. Use these inspection techniques post-reflow:

Visual Inspection

Train operators to visually recognize tombstoned components on PCBs exiting the reflow oven. Tombstones are usually evident to the naked eye.
Use magnification aids like eye loupes, microscopes, or high-resolution cameras to inspect smaller components.
Pay close attention to components at high risk for tombstoning like chips near the PCB edge or under large devices.

Automated Optical Inspection (AOI)

Implement 2D or 3D AOI machines after reflow to detect component presence, position, and orientation defects including tombstones.
Program AOI with high-quality reference images of properly soldered components. Use angled lighting to highlight component body and solder fillet shape.
Set AOI parameters to flag components lifted above a certain height threshold or tilted beyond a certain angle.

X-Ray Inspection

Utilize X-ray inspection for critical high-reliability boards to detect partially tombstoned components with a solder joint on one end.
Focus X-ray on areas obscured from optical inspection like components under RF shields or large chips.
Evaluate head-in-pillow defects where the component end is soldered but not fully collapsed, indicating incomplete reflow on that side.

Rework and Repair Procedures

When tombstones are found, a decision must be made to scrap the board or rework the defective component. If rework is chosen, follow these procedures:

Component Removal

Apply a small amount of no-clean flux to the tombstoned component’s solder joints. Flux helps with heat transfer and wetting.
Use a hot air pencil or soldering iron to carefully reflow and remove the component. Avoid disturbing adjacent components.
Clean any excess solder from the pads with solder wick. Do not use a solder sucker which can damage the pads.

Solder Replenishment

Print solder paste on the clean, bare pads using a mini stencil or solder paste dispensing syringe. Follow original paste deposit size.
Alternatively, apply solder spheres or flux-cored wire to the pads for lower-volume rework. Achieve a uniform solder amount on both pads.
Inspect the printed solder to ensure even volume and no bridging before placing a new component.

Component Replacement

Place a new component squarely on the solder-printed pads. Check alignment and orientation before soldering.
Reflow the component using a hot air pencil or localized conduction heating. Follow the recommended profile for the component and board.
Allow the solder to cool and solidify completely before cleaning any residual flux. Inspect the new solder joints to confirm full wetting and no defects.

Frequently Asked Questions

What are the most common tombstoned components?

The components most susceptible to tombstoning are small, lightweight, two-terminal chips like resistors and capacitors in 0402, 0201, and 01005 footprints. These chips have little mass to resist the lifting forces of the surface tension of the molten solder, especially if they are misaligned or heated unevenly.

Can tombstoning be prevented entirely?

While it is impossible to completely eliminate all risk of tombstoning, the likelihood can be significantly reduced by implementing the PCB design, stencil design, paste selection, and reflow profiling best practices described in this guide. Tombstoning rates of less than 10 defects per million opportunities (DPMO) are achievable with the right process controls.

Does pad finish affect tombstone risk?

Pad surface finish can influence tombstone risk in several ways. Plated finishes like ENIG (Electroless Nickel Immersion Gold) tend to have slower wetting and heat transfer compared to bare copper, which can contribute to uneven reflow. Thicker finishes like ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold) can create an uneven surface that causes components to tilt during placement. In general, HASL (Hot Air Solder Leveling) and OSP (Organic Solderability Preservative) finishes have the lowest tombstone risk due to their good solderability and planarity.

What is the cost impact of tombstoned components?

The cost of component tombstoning can add up quickly in a high-volume production environment. Yield loss from scrapped PCBs, rework labor time, and potential quality escapes all eat into profitability. In some cases, tombstoned components that go undetected can cause premature failures in the field, leading to expensive warranty returns and damage to a company’s reputation. Investing in tombstone prevention, detection, and rework can have a high ROI.

How does tombstoning differ from drawbridging?

Tombstoning and drawbridging are related but distinct component soldering defects. In tombstoning, one end of the component lifts off the pad entirely while the other end remains soldered. In drawbridging, the component lifts up slightly on both ends, like a drawbridge, while still maintaining a solder joint on each side. Drawbridging is usually caused by excessive solder paste volume rather than uneven heating. Both defects can be addressed with similar PCB design and process optimizations.

Conclusion

Tombstoning may be a frightening prospect for electronics assemblers, but it doesn’t have to be a death knell for your production yields. By understanding the causes of tombstoning and implementing strategic design and process improvements, the risk of components rising from the dead can be effectively managed.

Focus your tombstone prevention efforts on four key areas: PCB pad design, stencil aperture optimization, solder paste material selection, and reflow oven profiling. Work closely with your PCB and stencil suppliers to ensure robust footprints and paste volumes. Partner with your solder paste and reflow oven vendors to select high-tack materials and develop consistent reflow profiles.

Catching and correcting the tombstones that do occur is critical to quality control. Establish clear visual and automated inspection criteria and rework procedures to keep defects from haunting your products downstream. Train your team to identify and remedy tombstoned components safely and efficiently.

With a holistic tombstone taming approach that includes both prevention and detection,