A printed circuit board (PCB) tombstone is a defect that occurs during the soldering process where one side of the component lifts off the board, causing the component to stand vertically like a tombstone. It is one of the most common soldering defects seen in PCB assembly. Tombstoning can result in open circuits and connection failures, making tombstone rework an important process in PCB manufacturing.

This article will provide an in-depth explanation of what a PCB tombstone is, what causes it, how to identify it, the effects of tombstoning, and how to prevent and fix tombstone defects. We will also look at tools for tombstone rework and analysis.

What Causes Tombstoning on PCBs?

Tombstoning occurs when there is uneven solder paste deposition or uneven heating of a component during reflow soldering. This leads to one pad heating faster than the other. As the solder paste melts, it pulls the component towards the pad that heats up first, lifting it off the board.

Here are some of the main causes of tombstoning:

Uneven Solder Paste Deposition

• Too much solder paste on one pad and not enough on the other pad. This uneven deposit causes one side to heat faster.
• Misalignment of solder stencil during printing. If the stencil is not properly aligned, more paste will be deposited on one pad.
• Damaged/clogged stencil apertures. Uneven paste volume on pads.
• Poor stencil design. Differences in pad shapes and sizes can lead to uneven deposit.
• Warped PCBs. Can cause variation in paste deposit.

Uneven Heating During Reflow

• PCB design issues like differences in pad shapes, sizes, and thermal masses. Causes one pad to heat faster.
• Non-uniform heating in the reflow oven. Hotspots lead to uneven heating of pads.
• PCB orientation issues in reflow oven. Leads to uneven heating.
• Solder mask issues. Can cause thermal mismatch between pads.

Other Contributing Factors

• Excessive solder paste volume. Can exacerbate tombstoning.
• Parts with large size differences between pads/terminals. Makes tombstoning more likely.
• Poor pad design. Insufficient paste containment area leads to uneven deposit.
• Component placement issues. Misalignments exacerbate paste volume differences.
• Highly tilted components. Aggravates tombstoning.

So in summary, anything that contributes to uneven solder paste deposit or uneven heating of the component pads during reflow can cause tombstoning. Addressing these issues is key to preventing tombstones.

How to Identify Tombstoning

Identifying tombstoning is relatively straightforward visually, but can be easy to miss on crowded PCBs. Here are some tips for effective tombstone detection:

• Visual inspection of solder joints. Look for components that are tilted/lifted from the board.
• Look along component rows. Tombstones are often easier to spot when looking down rows.
• Inspect under magnification. Use a microscope or high-res camera for detailed inspection.
• Pay attention to problematic components. Large connectors and capacitors are prone to tombstoning.
• Do side-angle inspections. Easier to spot lifted leads from the side.
• Automated optical inspection (AOI). AOIs are great for reliable tombstone detection.
• X-ray inspection. Provides internal views of solder joints for identifying tombstones.
• Electrical testing. Can help find tombstones that cause electrical failures.

Signs of Tombstoning:

• Component lifted on one side, tilted vertically
• One side of leads/pads without solder fillet
• Shadows under lifted component side
• Disrupted solder joint on one side
• Broken joints due to leads lifting from board

Catching tombstones early using the right inspection methods is important to minimize rework costs and avoid reliability issues.

The Effects of Tombstoning on PCBs

Tombstoning can have several negative effects on the reliability and performance of printed circuit boards:

Open Solder Connections

Since tombstoning lifts one side of the component off the pad, it often leads to open solder joints on that side. This breaks electrical connectivity and can lead to circuit failures.

Weak Mechanical Attachment

With only one side soldered down, the component has a weak mechanical attachment to the board which can break under shock/vibration.

Thermal Stress

The uneven attachment can induce mechanical stress on the component and joints during thermal cycling due to CTE mismatches. This can cause solder fatigue and joint fracture over time.

Short Circuits

In severe cases, the lifted component leads can contact adjacent components or traces, creating short circuits.

Difficulty Reworking

Once tombstoned, a component can be very difficult to properly re-solder back down to the pads without causing further issues.

EMI Problems

Poor solder connections and open pads caused by tombstoning can allow EMI leakage or make circuits more susceptible to interference.

Failed Assemblies

Severe cases of multiple tombstoning may require entire PCB assemblies to be discarded if rework is impractical. This leads to added material costs.

Problems from tombstoning may not appear immediately but can show up during testing, QA inspections, or later in the field as latent defects. So tombstone prevention is key for avoiding delayed failures.

How to Prevent Tombstoning

With so many potential factors contributing to tombstoning, a combination of design and process improvements is required for effective prevention. Here are some tombstone prevention tips:

PCB Design Considerations

• Match pad shapes and sizes on each side of component. Equal thermal mass prevents uneven heating.
• Add extra solder or thermal reliefs to larger pads to slow heating.
• Minimize pad size differences within reason. Avoid large size mismatches.
• Use less thermally conductive pad finishes like ENIG or Imm Ag.
• Ensure good thermal symmetry between pads.
• Add footprints only where needed. Extra pads increase tombstoning risks.

Solder Paste Printing

• Utilize stencils designed to minimize volume differences between pads. Step apertures, balanced areas, etc.
• Properly align stencil and PCB. Use guide rails or fiducials for accuracy.
• Clean stencil regularly to prevent clogging and uneven paste deposit.
• Replace damaged stencil apertures causing volume inconsistencies.
• Optimize print process settings. Speed, pressure, separation, etc.
• Only print paste once. Multiple prints increase volume unpredictability.

Component Placement

• Avoid misalignments that create paste volume imbalances between pads.
• Place sensitive parts symmetrically. Don’t introduce tilts.
• Apply consistent pressure when placing components. No shifting.
• Ensure good wetting of pads. Clean off excess solder, grease, etc.

Reflow Process Optimization

• Tune reflow profile to ensure even component heating.
• Add zones/ramps to balance heating and cooling rates.
• Maintain uniform oven temperatures. Monitor and fix hotspots.
• Use carrier trays if needed to improve cap heating.
• Only pass boards through oven once. Multiple passes increase variability.
• Orient boards to promote even heating based on oven design.

With carefully designed PCBs and optimized assembly processes, tombstoning risks can be minimized. But some amount of inspection and rework is still required.

Tools for Tombstone Rework and Analysis

To efficiently rework and analyze tombstone defects, having the right tools is critical:

Soldering/Rework Stations

Highly controllable soldering irons allow selective heating of pads. Hot gas tools reflow individual joints. Good fume extraction is essential.

Soldering Hand Tools

Soldering tweezers and strippers allow fixing lifted pads. Vacuum pickup tools help reposition components.

Microscopes

Stereo microscopes with good magnification are needed for analyzing tombstones and performing precise rework.

X-Ray Inspection

X-ray provides unique internal views of tombstoned joints helpful for failure analysis and process improvements.

Thermal Profiling

Analyzing thermal profiles during reflow helps identify sources of uneven heating contributing to tombstones.

AOI and SPI

Automated optical and solder paste inspection systems are vital for quickly finding tombstone defects.

Adhesives/Glues

Adhesives help secure repaired components to prevent recurrence of tombstoning. Specific glues designed for SMT rework are available.

Rework Stations

Dedicated rework stations with camera systems aid in tombstone analysis and repair, especially for ultra-fine pitch components.

The right rework tools also make repair of tombstones much faster and eliminate the need to scrap affected boards in many cases.

Tombstone Rework and Repair Methods

Once tombstoned components are identified, properly repairing them can be tricky:

Remove and Replace Component

For simple components, it may be easiest to completely remove and replace the affected part with a new one using soldering iron or hot gas.

Reflow Entire Component

Use hot gas or directed IR heating to uniformly reflow solder under the entire component, allowing realignment.

Targeted Pad Heating

Selectively heat pad that is lifted while pressing down on component to resolder that side.

Add Solder to Lifted Pads

Flow extra solder onto open pad/lead to form new joint fillet. May require flux.

Scrape and Reshape Joint

For plated through-hole parts, scrape off any extruded solder before resoldering pad.

Apply Adhesive Under Component

Secure repaired component with small amount of rework glue to prevent retombstoning.

Redress Solder Paste

For recurring tombstones, stencil in small amount of fresh paste before re-reflow.

Board Heating During Repair

Keeping board warm helps avoid shock/stress to component during tombstone rework.

With practice and microscopic inspection, even severely tombstoned components can often be salvaged, making replacement unnecessary.

Examples of Components Prone to Tombstoning

While any component with mismatched pads can tombstone, here are some common component types where tombstoning is frequently observed:

• Capacitors – Due to size differences between pads and vulnerability to thermal shock. Leaded caps are especially prone.
• Connectors – Large connectors often have wide variances in pad and lead sizes between sides making them tombstone sensitive.
• Quad Flat Packs (QFPs) – Center pads being offset from corners allows one side to pull component up during reflow.
• Plastic Leaded Chip Carriers (PLCCs) – Vulnerable to tombstoning cracks and lifting during reflow due to thermal stress on the plastic body.
• Gull Wing Packages – Can lever up on one side if paste volume is off between pads.
• Leadless Components – Greater solder surface area makes these sensitive to paste deposit discrepancies.
• Crystals – Their high aspect ratio makes evenly soldering both ends difficult.
• Odd Shaped Parts – Any asymmetry tends to increase susceptibility to lifting on one side.

Knowing the most tombstone prone components allow focusing prevention and inspection efforts there.

FQA

What is the main cause of tombstoning during reflow soldering?

The main cause of tombstoning is uneven solder paste deposition or uneven heating of the pads during reflow, leading to one side pulling the component up as the solder paste melts. Contributing factors include stencil/PCB misalignments, inconsistent paste volumes between pads, board hotspots, and differences in pad sizes or shapes.

How can I identify tombstoned components on a PCB assembly?

Look for components tilting vertically, with one side lifted off the board. Visually inspect solder joints for missing fillets or open pads on one side. Side angle views and magnification make identification easier. Electrical testing can also find failures from opens. Automated optical inspection (AOI) is very effective at reliably detecting tombstones.

Does tombstoning always need to be repaired?

It depends on the severity. Minor cases may still make acceptable electrical and mechanical connections. However, tombstones often lead to open joints or weak attachment so repair is recommended in most cases. Reworking tombstones early helps prevent failures down the line.

What’s the best way to prevent tombstoning defects?

Prevention involves addressing both design and process factors. On the design side, match component pad sizes, use thermal reliefs, and ensure symmetry. For processes, optimize stencil printing, paste volumes, component placement, and reflow profile. Keeping boards and components as thermally balanced as possible during reflow prevents tombstones.

How do you rework a tombstoned component?

Methods include completely removing and replacing the component, reflowing the entire joint, heating just the lifted pad while pressing the component flat, adding solder to open pads, and securing with adhesive. The best method depends on the part and extent of tombstoning. Rework stations with hot gas or infrared tools make rework easier.