Introduction
Ball grid array (BGA) packages have become a mainstay of modern electronics, offering high density interconnection in a small footprint. But properly laying out a printed circuit board for a BGA device does require special considerations versus other package styles. This article will provide guidance on key factors when designing BGAs including pad dimensions, placement, routing, thermal design, and board-level reliability. Following these PCB design recommendations will help ensure successful implementation of BGA packages.
Overview of BGA Packages
First, a quick overview of BGA technology:
- Package surface mounts to PCB via an array of solder balls
- Ball pitch ranges from 0.5mm to over 1.5mm
- High density interconnection – Over 1,000 pads/balls
- Package sizes from 5x5mm up to 55x55mm or larger
- Lower inductance versus wire bonding
- Relatively low profile package
- Common for FPGAs, ASICs, GPUs, large microprocessors
The main PCB design challenges include pad layout, escape routing, and thermal management.
PCB Pad Design
The pad design on the PCB mating surface is critical for reliability. Considerations include:
Pad Size
Pad diameters range from 0.20mm to 0.30mm larger than ball diameter. Larger balls need more solder volume and fillet height.
Pad Stencil Design
Stencil openings are typically 0.05mm smaller than pads to optimize solder application. Avoid excess solder volume.
Solder Mask Defined vs Non-Solder Mask Defined
Solder mask defined (SMD) pads reduce solder bridging risks. Non-solder mask defined (NSMD) pads improve self-alignment during reflow.
Anti-Pad Size
Anti-pads surround pads where solder mask is pulled away. Make anti-pads 0.05-0.10mm larger than pads for adequate solder mask web to avoid bridging.
Copper finishes
Immersion silver or ENIG are common final finishes for BGA pads. Avoid HASL for fine pitch BGAs.
Thermal Pad Design
Larger BGAs often have a solid central thermal pad on the package underside:
- Provides primary thermal path into PCB
- Often requires thermal vias under pad into ground plane
- Use continuous copper fill for maximum heat transfer
- May require solder mask openings for paste application
Follow all package recommendations for thermal pad design and soldering process.
BGA Placement
BGA placement significantly impacts PCB routing complexity:
- Central locations simplify routing to all quadrants
- Near board edges complicates routing
- Critical signals may warrant inner locations
- High speed I/O aligned with specific stackup layers
- Assign IO to ease high-density routing
Place BGAs early before surrounding component placement restricts options.
Thermal Analysis
Confirm sufficient cooling of each BGA during temperature analysis:
- Ensure rated temperature range is not exceeded
- Check for temperature gradients across package
- Leverage thermal vias under pads if needed
- Incorporate recommended thermal pad soldering process
- Force air cooling may be required in some cases
BGAs often require careful thermal design due to concentrated heat generation.
Fan-Out Routing
Escape routing from dense BGA pads can be challenging:
- Stagger rows when possible to ease fan-out
- Plan pad/ball assignments to optimize flow
- Via-in-pad on the package or PCB may be needed
- Match I/O signals to appropriate stackup layers
- Utilize vias and relief cutouts under BGA to ease congestion
- Use wider routing traces exiting pads then neck down
- Allow sufficient routing channels surrounding BGA
Test routing critical nets before finalizing BGA footprint placement.
High Speed Design
Treat BGAs as fixed points when designing impedance controlled routes:
- Minimize length of high-speed signals near BGA
- Place BGA to ease same-layer routing of critical signals
- Identify return paths under device footprint in stackup
- Provide shielding traces or ground fills around signals
- Contain high speed routes within component quadrant if possible
BGAs warrant careful signal integrity analysis given their fixed interconnect.
Board-Level Reliability
Several factors impact the long-term solder joint reliability of BGAs:
- Coefficient of thermal expansion (CTE) mismatch
- Warpage and coplanarity
- Solder volumes and fillet height
- Solder joint fatigue over thermal cycles
- Moisture ingression leading to fractures
- Pad cratering during thermal excursions
Consult the device datasheet and application notes for recommended design rules and process guidance to enhance BGA reliability.
Design for Manufacturing
Coordinate with your assembly vendor when optimizing BGA design:
- Specify qualified solder paste & reflow profile
- Require automated optical inspection after placement
- Perform X-ray inspection of solder joints
- Implement thermal relieving processes if needed
- Apply conformal coating for moisture prevention
- Utilize underfill material if available for package
- Qualify process together before full production
Design, assembly process, and materials selection all contribute to robust BGA solder connections and field reliability.
Design Review Guidelines
When reviewing a BGA design, consider the following checkpoints:
- Pad dimensions match datasheet recommendations
- Solder mask expansion follows IPC guidelines
- Stencil design enables optimum solder volume per IPC
- Thermal pad design facilitates effective heat transfer
- Thermal analysis confirms temperature ranges
- Placement allows required routing channels
- Impedance control implemented for high speed routes
- Fan-out routing uses vias, traces, layers effectively
- DFM guidelines adhered to aid manufacturability
- Reliability rules and recommendations followed
Thoroughly vetting the design avoids integration or production issues down the line.
Conclusion
Designing a PCB for a ball grid array device involves special considerations for routing, thermal management, manufacturability, and reliability. Following IPC guidelines and package-specific recommendations helps ensure your BGA implementation meets performance and quality standards. While requiring more planning, close collaboration between designer and manufacturer enables successfully deploying BGAs and gaining the benefits of the high-density interconnect technology in your products.
Frequently Asked Questions
Q: How fine of a pitch is achievable with newer BGA packages?
A: Packaging advances are enabling finer BGA pitches below 1mm, including 0.8mm and 0.65mm. This provides interconnect densities over 2500 pads/balls. However, PCB fabrication and assembly requires tighter tolerances at finer pitches which can increase cost.
Q: What are common solder ball materials used with BGA packages?
A: Solder ball alloys are typically eutectic SnAgCu (SAC). High lead solder is still used for some applications requiring high reliability. Lead-free solders are becoming standard due to regulatory pressures to eliminate lead.
Q: What are indications of potential BGA solder joint defects?
A: Excessive voids in solder joints, pad cratering, non-uniform or missing solder fillets, solder bridging, thermal pad dry joints, and cracked joints are defects that can lead to failures. X-ray inspection after assembly is recommended to identify issues.
Q: How many PCB layers are typically required for complex BGA designs?
A: High density BGA designs often require at least 6 to 8 layers. Critical signals need routing on inner layers with reference planes above and below. More layers provides additional routing channels to relieve congestion under devices.
Q: What are common causes of solder joint failures in BGA packages?
A: Thermal expansion mismatch, mechanical stresses, vibration, solder voids, dry joints, poor pad design, and moisture absorption can all contribute to eventual BGA solder joint failure over temperature cycling in the field. Following reliability design rules helps mitigate risks.
| BGA Pitch | Layout | Formula |
| 1.00 mm | In Line | a +c + d <=0.53mm |
| 1.00 mm | Diagonal | a +c + d <=0.94mm |
| 0.8 mm | In Line | a +c + d <=0.46mm |
| 0.8 mm | Diagonal | a +c + d <=0.68mm |
