Conquering BGA Design Challenges in Modern PCB Layouts:

Ball Grid Arrays (BGAs) are the dominant package type for complex semiconductor devices like FPGAs and microprocessors. Their versatility makes them ideal for a wide range of embedded designs, but their high pin count and small pitch can pose significant challenges in high-density interconnect (HDI) layouts. The following explores strategies for overcoming these hurdles and achieving successful BGA integration.

Planning is Key:

Since BGAs often act as the central processing units, their placement is crucial. While not mandatory, starting your layout with the largest BGA helps determine the layer count and routing strategy for the entire board. Here are some key considerations for a successful BGA layout:

1. Signal Layer Count: The number of signal layers in the stackup directly impacts the number of power/ground planes and the trace width required for routing. A well-planned stackup ensures efficient signal routing and optimal board functionality.

2. Fanout Strategy: Defining how signals enter and exit the BGA, along with any controlled impedance requirements, influences the layer count and ultimately determines how traces are routed within the board.

3. Performance and Qualification: High-reliability designs demand adherence to stringent standards like Class 3/3A or even stricter product-specific criteria. Such specifications may necessitate larger pad sizes exceeding the IPC-6012 Class 3 standard. This can render traditional dog-bone fanout techniques unusable due to tolerance limitations and annular ring/solder mask requirements.

A Three-Step Approach

By keeping these factors in mind early on, we can approach BGA PCB layout in three key steps:

1. Stackup and Fanout Planning: Carefully define the number of signal layers and the routing strategy for BGA connections, considering controlled impedance needs and performance requirements.

2. Component Placement: While not a rigid rule, placing the largest BGA first helps determine the overall layout and routing plan.

3. Detailed Routing: Based on the chosen stackup and fanout strategy, meticulously route signals within the board, ensuring adherence to design specifications and performance targets.

By following these strategies, designers can successfully navigate the challenges of BGA integration and create high-performing, reliable PCB layouts.

Mastering BGA Routing: The Art of Exit Strategy

One of the biggest hurdles in BGA layout involves defining reliable escape routes for signal traces. These routes need to be manufacturable and prevent the need for costly rework after PCB assembly.

High-Layer Complexity:

When dealing with BGAs that have a high layer count, planning these exit routes becomes even more critical. Here, the challenge lies in routing traces efficiently through multiple rows of pins on internal layers.

Signal Speed Matters:

The type of signal being routed also plays a role in exit strategy. High-speed signals necessitate greater spacing between traces to minimize crosstalk, a phenomenon that can disrupt signal integrity. Conversely, slower configuration signals can be routed closer together with a lower risk of interference.

A Look at Multi-Layer Routing:

The example showcases BGA escape routing on two internal layers. Notice how the traces navigate through multiple rows of vias (more than two). This approach is suitable for internal layers where surface pin routing isn't a concern.

Surface Layer Considerations:

Surface layer routing typically involves connecting only to the outermost two rows of pins. This is due to limitations imposed by pad size in the BGA land pattern, the need for maintaining clearances, and the chosen fanout style (often dog-bone fanout).

By understanding these exit strategy considerations, designers can create efficient and reliable BGA layouts that minimize the risk of manufacturing issues and ensure optimal board performance.

Conquering Surface Layer Challenges in BGA Layouts

The top layer, directly below the BGA, requires careful planning to connect numerous pads to internal layers via vias. Here, we explore routing strategies for different BGA types and pin pitches.

The Dog-Bone Advantage:

For larger-pitch BGAs (up to 1 mm), the standard method is the dog-bone fanout. These short, narrow traces connect to vias, enabling access to the outer two rows of surface layer pins beneath the BGA. Additionally, vias provide access to inner pads on deeper layers. Dog-bone fanout remains the go-to method for coarse-pitch BGAs due to its practicality.

Via-in-Pad: Flexibility for Tight Spaces

As pin pitches shrink, the space available for routing traces becomes limited. In such cases, via-in-pad offers greater flexibility on the surface layer. This technique integrates vias directly within the BGA land pattern, minimizing trace length and maximizing space utilization.

HDI for High-Density Routing

With even smaller pin pitches, achieving the necessary trace width for routing to the BGA on each layer becomes increasingly challenging, especially for controlled impedance signals. This may necessitate the use of thinner laminates and, ultimately, High-Density Interconnect (HDI) techniques to ensure proper routing capabilities. As a result, the preferred fanout style transitions from dog-bone to via-in-pad for finer-pitch BGAs.

Exploring Breakout Methods:

For further exploration of BGA fanout styles and alternative breakout methods, I recommend consulting the following excellent industry resource:

Pfiel, C. BGA Breakouts and Routing: Effective Design Methods for Very Large BGAs, 2nd Edition. Mentor Graphics (2010).

Powering Up Your BGA Design: Ground & Power Strategies:

Large BGAs often have numerous dedicated ground and power pins. This is especially true for high-performance processors that require multiple high-speed interfaces. Furthermore, the component might necessitate power from multiple voltage supplies, demanding dedicated routing for each voltage level.

Power Delivery with Power Rails

The most efficient method for managing BGA power connections is by utilizing power rails. These are typically implemented on one or two dedicated plane layers within the PCB stackup. Positioning the power and ground planes on adjacent layers with minimal dielectric separation fosters optimal power integrity by creating high interplane capacitance.

Beyond Exit Routes: Routing Complexity Beneath the BGA

While escape routing is a crucial consideration for BGA layouts, it's just one aspect of the routing performed near BGA pins. Several other routing tasks may be required within this area, including:

• Power Rail Connections: Establishing connections from BGA power pins to dedicated power rails on the designated plane layers.

• Grounding: Connecting ground pins to the ground plane layer(s).

• Inter-Pin Routing: Routing signals between BGA pins on the same layer, which may coexist with power/ground polygons.

The illustration below exemplifies a scenario where inter-pin routing and power/ground polygons coexist on the same layer beneath a BGA.

This revised text avoids directly mentioning "exit routes" and focuses on overall routing complexity near the BGA. It also emphasizes the importance of power delivery and grounding strategies specific to BGA design.

Optimizing Your BGA Design: Stackup Strategies

The BGA's pinout and I/O count play a critical role in determining the ideal layer count for your PCB stackup. Here's a breakdown of the key factors involved:

1. Controlled Impedance Routing:

Once the trace width required for controlled impedance lines is established, the necessary layer thickness to maintain that impedance can be determined. This directly impacts the overall layer count.

2. Signal Layer Calculation:

By analyzing the BGA's row structure and adding the number of signal layers required based on trace width and impedance needs, you can calculate the total number of signal layers needed in the stackup.

3. Efficient Routing Techniques:

The first two outermost BGA rows typically don't necessitate vias, allowing for routing on the surface layer using techniques like dog-bone fanout, via-in-pad, or other alternatives. This pattern can be applied throughout the BGA to estimate the layer count required for complete signal breakout.

4. Ground Plane Integration:

Ground pins are often strategically placed between signal pins for isolation purposes. This philosophy extends to the PCB stackup, where ground planes are interleaved between signal layers for optimal signal integrity.

The illustration showcases a BGA with some interior rows where pins have been removed. This allows for routing signals to reach these inner pins, potentially increasing the number of accessible rows from internal layers.

Stackup Example:

In the provided example, the central square of the flip-chip BGA likely houses power and ground connections, suggesting a minimum of two dedicated layers for this purpose. Combining these layers with the BGA routing requirements and the base layer results in a minimum stackup of at least 6 layers for complete BGA breakout and routing.

By carefully considering these factors, designers can optimize their BGA PCB stackup for efficient routing, signal integrity, and overall board performance.

Mastering BGA Design: Resources for Success

Integrating BGAs into your PCB design presents a unique set of challenges. However, with the right knowledge and resources, you can achieve successful layouts.

This guide has explored key BGA design strategies, including:

• Exit Route Planning: Determining efficient and manufacturable routes for signal traces exiting the BGA.

• Surface Layer Routing: Strategies for connecting BGA pads to internal layers via vias, considering factors like pin pitch and fanout styles.

• Power and Ground Management: Utilizing power rails and strategically placed ground planes for optimal power integrity.

• Stackup Optimization: Determining the ideal PCB layer count based on BGA pinout, signal integrity requirements, and routing techniques.