Introduction to Thermal Relief BGA

Ball Grid Array (BGA) integrated circuit packages have become increasingly popular in modern electronics due to their high pin density and compact size. However, the dense array of solder balls beneath the package presents challenges when it comes to soldering the BGA to a printed circuit board (PCB). One key challenge is managing the thermal profile during reflow soldering to ensure reliable solder joints form without damaging the package or board.

Thermal relief BGA pad designs help address this challenge by providing a controlled thermal path between the BGA solder balls and the copper planes on the PCB. Thermal relief traces, also known as thermals or spokes, connect the BGA pads to the surrounding copper pour while restricting heat flow. This article will dive into the details of thermal relief BGA pad design, including:

• The purpose and benefits of thermal relief
• Designing thermal relief traces and pads
• Calculating thermal relief trace widths
• Thermal considerations for different BGA sizes
• Frequently asked questions about thermal relief BGA design

Why Use Thermal Relief on BGA Pads?

The main purposes of incorporating thermal relief in BGA pad designs are:

1. Easier soldering: Thermal relief traces slow down heat transfer from the pad to the connected copper plane. This allows the BGA solder balls to melt and wet the pads more readily without the planes acting as large heat sinks.

2. Improved solder joint reliability: By promoting complete reflow and wetting of the solder balls, thermal relief connections help ensure strong, reliable solder joints between the BGA package and PCB. Insufficient thermal relief can lead to poor solder joints.

3. Reduced risk of damage: Gradual heating and cooling of the BGA solder joints mitigates thermal stress on the package and board. Abrupt thermal gradients are more likely to cause defects like pad lifting or laminate cracking.

4. Better rework capability: Thermally relieved pads are easier to rework if needed, since the thermal relief traces can be cut to locally heat a solder joint without affecting the entire plane.

While thermal relief is beneficial in most cases, there are some situations where solid connections may be preferable, such as:

• High current applications where maximum conductivity is required
• Thermally sensitive components that must dissipate heat to the planes
• Controlled impedance traces that require a specific capacitance to the reference plane

In general, though, thermal relief is recommended for BGA pad connections to planes unless there is a compelling reason to use solid connections. The improved solderability and reliability usually outweigh any tradeoffs.

Designing Thermal Relief Traces for BGA Pads

Thermal Relief Pad Patterns

There are several common patterns used for thermal relief BGA pad design:

1. Diagonal cross (X-style): Four traces extend diagonally from the corners of the pad to the plane. This is the most common style.

2. Plus sign (+): Four traces extend orthogonally (top, bottom, left, right) from the pad to the plane.

3. Symmetric spokes: Three or more evenly spaced traces radiate from the pad center to the plane. This can provide more uniform heat transfer than the asymmetric cross or plus styles.

4. Neckdown: The traces are narrower near the pad and widen as they approach the plane. This can further restrict heat flow close to the pad.

Here are examples of these thermal relief patterns:

Pattern	Diagram
Diagonal cross
Plus sign
Symmetric spokes
Neckdown

The choice of thermal pattern depends on factors like BGA pitch, pad size, available space between pads, layer count, and the desired thermal and electrical properties. The diagonal cross is most common since it provides good thermal relief and is easy to route between tightly spaced pads.

Trace Width and Spacing

The width and spacing (air gap) of the thermal relief traces determine the amount of thermal isolation between the pad and plane. Wider traces and smaller air gaps provide a lower thermal resistance path, while narrower traces and larger gaps restrict heat flow more. The size of the BGA package and its solder balls also influence the ideal trace dimensions.

Typical values for thermal relief trace width and spacing are:

BGA Pitch	Solder Ball Diameter	Trace Width	Trace Spacing
1.00 mm	0.60 mm	0.20 – 0.25 mm	0.30 – 0.40 mm
0.80 mm	0.50 mm	0.15 – 0.20 mm	0.25 – 0.30 mm
0.65 mm	0.40 mm	0.12 – 0.15 mm	0.20 – 0.25 mm
0.50 mm	0.30 mm	0.10 – 0.12 mm	0.15 – 0.20 mm

These are general guidelines and the optimal trace dimensions may vary based on the specific BGA package, PCB materials, layer stackup, and soldering process used. It’s always a good idea to start with conservative values and adjust as needed based on testing and experience.

Calculating Thermal Relief Trace Widths

To calculate the maximum thermal relief trace width for a given BGA pad, you can use this formula:

Max Trace Width = (Pad Diameter - (2 * Trace Spacing) - Solder Mask Expansion) / 2

Where:
– Pad Diameter is the size of the BGA pad on the PCB
– Trace Spacing is the desired air gap between traces
– Solder Mask Expansion is the extra space around the copper pad openings in the solder mask layer to account for registration tolerances (typically 0.05 – 0.10 mm)

For example, consider a 0.80 mm pitch BGA with 0.50 mm diameter solder balls and a pad diameter of 0.45 mm. Using a trace spacing of 0.25 mm and mask expansion of 0.075 mm:

Max Trace Width = (0.45 mm - (2 * 0.25 mm) - 0.075 mm) / 2 
                = (0.45 mm - 0.575 mm) / 2
                = -0.125 mm / 2 
                = -0.0625 mm

A negative result means the desired trace spacing is too large for the given pad size. In this case, you would need to either reduce the spacing or increase the pad diameter to accommodate thermal relief traces.

Conversely, to calculate the maximum air gap between thermal relief traces for a given trace width:

Max Trace Spacing = (Pad Diameter - Solder Mask Expansion - (2 * Trace Width)) / 2  

Using the same 0.80 mm BGA example with 0.15 mm wide traces:

Max Trace Spacing = (0.45 mm - 0.075 mm - (2 * 0.15 mm)) / 2
                  = (0.45 mm - 0.375 mm) / 2  
                  = 0.075 mm / 2
                  = 0.0375 mm

This indicates that the maximum spacing between 0.15 mm thermal relief traces on a 0.45 mm pad is only 0.0375 mm. Any larger gap would result in insufficient copper support for the solder mask openings.

These calculations provide a starting point for thermal relief trace sizing, but it’s important to also consider manufacturing capabilities, especially solder mask registration and etching tolerances. Overly narrow traces or spaces may be difficult to fabricate reliably. It’s always best to consult with your PCB manufacturer and assembly provider to determine the optimum thermal relief dimensions for your design.

Thermal Considerations for Different BGA Sizes

The physical size of a BGA package has a significant impact on its thermal behavior during soldering. Larger BGAs with higher pin counts and more thermal mass require more heat energy to reach reflow temperatures. However, they also have more solder balls connecting to the PCB, which can help dissipate heat.

Smaller BGAs may reflow more quickly due to their lower thermal mass, but they have fewer solder balls and less contact area with the board. This can make them more susceptible to uneven heating or heat sinking effects from copper planes.

Here are some general thermal relief guidelines for different BGA sizes:

Large BGAs (>400 pins)

• Use wider thermal relief traces (0.20 – 0.30 mm) to promote heat transfer to the solder balls
• Provide larger air gaps between traces (0.30 – 0.50 mm) to maintain thermal isolation from planes
• Consider additional thermal vias near the BGA pads to conduct heat through the board
• Use thermally enhanced PCB materials like thick copper planes or metal core substrates
• Ensure good thermal contact between the BGA package and heatsink or spreader on the top side

Medium BGAs (100-400 pins)

• Use moderate thermal relief trace widths (0.15 – 0.25 mm)
• Provide air gaps between traces proportional to trace width (0.25 – 0.40 mm)
• Add thermal vias selectively near larger pads or hot spots
• Consider thermally enhanced PCB materials for high-power devices

Small BGAs (<100 pins)

• Use narrower thermal relief traces (0.10 – 0.20 mm) to avoid heat sinking
• Provide smaller air gaps between traces (0.20 – 0.30 mm) for thermal isolation
• Thermal vias may not be needed unless device power is high
• Standard PCB materials are usually sufficient

As with trace sizing, these are general recommendations and the specific thermal relief design will depend on the actual BGA package, PCB stackup, power dissipation, and operating environment. Thermal simulation and testing can help optimize the thermal relief design for a given application.

It’s also worth noting that the reflow soldering profile plays a key role in achieving good results with thermally relieved BGA pads. The ramp rates, soak times, peak temperatures, and cooling rates should be tailored to the specific BGA package and board design. Proper profiling and process control are essential for reliable soldering of thermally relieved BGAs.

Frequently Asked Questions

1. What is the purpose of thermal relief on BGA pads?

Thermal relief traces provide a controlled thermal path between BGA pads and connected copper planes. They restrict heat flow to the planes during soldering, allowing the solder balls to melt and wet the pads more easily. This promotes reliable solder joints and reduces the risk of thermal damage to the package or board.

2. How do I choose the right thermal relief pattern for my BGA?

The most common thermal relief pattern is the diagonal cross or “X” style, which provides good thermal isolation and is easy to route between tightly spaced pads. Other patterns like the plus sign, symmetric spokes, or neckdown traces may be used depending on factors like BGA pitch, pad size, layer count, and desired thermal and electrical properties.

3. What thermal relief trace width and spacing should I use for my BGA?

Thermal relief trace width and spacing depend on the BGA package size, solder ball diameter, pad size, and other design factors. Typical values range from 0.10 mm to 0.30 mm for trace width and 0.15 mm to 0.50 mm for trace spacing. Larger BGAs generally use wider traces and spaces than smaller ones. It’s best to start with conservative values and adjust based on testing and experience.

4. How can I calculate the maximum thermal relief trace width for a given BGA pad?

The maximum thermal relief trace width can be calculated using this formula:

Max Trace Width = (Pad Diameter - (2 * Trace Spacing) - Solder Mask Expansion) / 2

Where pad diameter is the size of the BGA pad on the PCB, trace spacing is the desired air gap between traces, and solder mask expansion is the extra space around the copper openings in the solder mask layer.

5. What special thermal considerations are there for different BGA sizes?

Larger BGAs have more thermal mass and may require wider thermal relief traces, larger air gaps, additional thermal vias, and thermally enhanced PCB materials to achieve reliable soldering. Smaller BGAs have lower thermal mass but are more susceptible to heat sinking effects, so narrower traces and smaller gaps are often used. The specific thermal relief design should be optimized based on thermal simulations and testing of the actual BGA package and PCB assembly.

Conclusion

Thermal relief BGA pad design is a crucial aspect of PCB layout for BGA packages. By providing a controlled thermal path between the solder balls and copper planes, thermal relief traces promote reliable solder joints and reduce the risk of thermal damage during reflow.

The key elements of thermal relief BGA design include selecting an appropriate pad pattern, sizing the traces and air gaps based on the BGA package and PCB parameters, and considering the thermal requirements of different BGA sizes. Formulas for calculating maximum trace widths and spacings can provide a starting point, but the specific thermal relief dimensions should be optimized through simulations and testing.

As BGA packages continue to shrink in size and increase in complexity, effective thermal relief design will remain essential for achieving high-quality, reliable PCB assemblies. By understanding the principles and tradeoffs involved in thermal relief BGA pad layout, PCB designers can create robust, manufacturable designs that meet the demands of today’s advanced electronic products.