Introduction to BGA Technology

Ball Grid Array (BGA) is an advanced surface-mount packaging technology used for integrated circuits. BGAs provide more interconnection pins than can be put on a dual in-line or flat package. BGA packages have the I/O pads arranged in a grid pattern on the bottom of the package.

Key advantages of BGA technology include:

• Reduced package size compared to fine-pitch QFPs
• Good electrical and thermal performance
• Large ball pitch allows easy PCB Routing
• Very thin profile, lightweight packages
• Lower thermal resistance compared to QFPs
• Shorter I/O lengths reduce inductance
• Available in various ball counts (up to >1000)

Common BGA Package Sizes

Package Type	Body Size (mm)	Ball Pitch (mm)	Ball Count
BGA	23 x 23	1.27	256
BGA	27 x 27	1.27	352
BGA	31 x 31	1.27	480
BGA	35 x 35	1.27	676
BGA	40 x 40	1.27	900
BGA	45 x 45	1.27	1156

BGA Assembly Process Flow

The basic process flow for BGA assembly includes:

1. Solder Paste Printing: Solder paste is applied to the PCB through a laser-cut Stencil using automated printing equipment. The stencil has apertures that match the BGA land pattern.

2. BGA Placement: An automated pick-and-place machine uses vision alignment to accurately place the BGA component onto the solder paste deposits on the PCB.

3. Reflow Soldering: The populated PCB passes through a multi-zone convection reflow oven. The solder paste melts and forms solder joints between the BGA balls and PCB pads as it cools.

4. Inspection: Post-reflow, the assembled PCB undergoes automated optical inspection (AOI) to verify component presence, position, and solder joint quality. X-ray inspection may also be used to check for opens, shorts, and solder voids.

5. Rework (if needed): BGAs that fail inspection are removed and replaced using specialized BGA rework equipment. This involves site redressing, paste printing, component placement, and localized reflow.

6. Test: Functional circuit testing verifies that the assembled PCB operates to specifications. In-circuit test (ICT) and flying probe test are commonly used test methods.

Typical BGA Assembly Equipment

Process Step	Equipment Type
Paste Printing	Automated stencil printer
BGA Placement	High-speed pick-and-place
Reflow	Convection reflow oven
Inspection	AOI system, X-ray
BGA Rework	Rework station with vision alignment
Test	ICT system, flying probe tester

BGA PCB Design Considerations

Proper PCB design is critical for successful BGA assembly. Some key BGA layout guidelines include:

Pad & Solder Mask Design

• Pad size should be 80-90% of ball diameter
• Solder mask opening should equal pad size for SMD pads
• Use Non Solder Mask Defined (NSMD) pads for best results
• Round or oval pads are preferred over square pads

PCB Footprint

• Place thermal and mechanical vias outside BGA footprint if possible
• Avoid vias under BGA pads as they can cause assembly issues
• Provide adequate clearance around BGA for assembly & rework
• A non-solder mask defined (NSMD) pad is recommended for BGA lands

Routing & Planes

• Route high-speed signals on inner layers
• Maintain Controlled impedance where needed
• Avoid routing between BGA pads at fine pitch
• Provide ground planes on adjacent layers for signal integrity
• Ensure proper plane cuts/splits for BGA power & ground planes

Solder Paste Stencil Design

• Stencil aperture should be 80-90% of pad size
• Minimize stencil aperture area ratio (width/thickness)
• Provide adequate aperture spacing to avoid solder bridging
• Consider home plate or window pane aperture designs
• Laser-cut stencils provide the best printing results

BGA Inspection Methods

Inspecting BGA solder joints is challenging due to the lack of visible solder fillets. Specialized inspection techniques are needed to verify joint integrity.

Automated Optical Inspection (AOI)

AOI systems use cameras and image analysis software to inspect BGA joints. They can detect missing balls, misaligned components, and excessive solder. However, AOI cannot see under the component to fully verify joint quality.

X-Ray Inspection

X-ray is the only non-destructive way to inspect hidden BGA solder joints. 2D X-ray provides a top-down view to check for opens, shorts, and voids. 3D X-ray (CT) generates a 3D reconstructed image to measure joint volume and percent voiding.

Key BGA Defects

Common BGA assembly defects include:

• Opens: Electrical discontinuity, often due to poor wetting or insufficient solder volume
• Shorts: Electrical connection between adjacent balls, resulting from excessive solder or poor stencil design
• Voids: Trapped air pockets or outgassing in solder joint, caused by improper reflow profile or contamination
• Misalignment: Lateral or rotational misplacement of BGA component, due to poor vision alignment or stencil registration
• Head-in-pillow: Open caused by BGA ball and PCB pad not collapsing together during reflow, often due to oxidized solder balls or poor wetting

BGA Rework & Repair

BGA components that fail inspection may require rework. The BGA rework process includes the following steps:

1. Component Removal: Localized hot air heating is used to reflow the solder joints and remove the defective BGA. Proper temperature control is critical to avoid PCB damage.

2. Site Redressing: Residual solder is removed from the BGA pads using a combination of heat and vacuum. A soldering iron may be used for site touch-up.

3. Solder Paste Application: Solder paste is applied to the BGA pads by miniature stencil printing or dispensing. Precise alignment and volume control are critical.

4. Component Placement: A replacement BGA is placed onto the solder paste deposits using a rework system with vision alignment. Placement pressure and time must be carefully controlled.

5. Reflow: Localized hot air or infrared heating is used to reflow the solder paste and form new solder joints. The heating profile should match the paste specifications.

6. Inspection: X-ray inspection verifies that the reworked BGA joints are free of defects. If not, the entire rework process is repeated.

BGA Rework Challenges

Successful BGA rework requires specialized equipment and skilled operators. Some challenges include:

• Controlling heating to avoid damaging PCB or adjacent components
• Preventing pad lifting or laminate cracking from excessive heat
• Difficulty of applying uniform solder paste deposits
• Maintaining placement accuracy for large, high ball count BGAs
• Longer reflow times needed to ensure full collapse of large BGAs
• Repeated reflow can degrade PCB pads, making defects more likely

Conclusion

In conclusion, Ball Grid Array technology provides many benefits for today’s advanced electronic assemblies. However, the small size and hidden solder joints of BGAs create unique challenges for PCB design, assembly, inspection, and rework.

By understanding the technical requirements and process guidelines for BGA assembly, manufacturers can take advantage of this packaging technology while ensuring high quality and reliability. This includes designing robust and manufacturable PCBs, utilizing precision assembly equipment, and having specialized inspection and rework capabilities in place.

As BGAs continue to shrink in size and increase in complexity, staying up-to-date on the latest techniques and best practices will be critical for electronics assembly success.

Frequently Asked Questions

What is a Ball Grid Array (BGA)?

A Ball Grid Array is a surface-mount package where the interconnects between the IC and the PCB are an array of metal spheres (balls) on the bottom side of the component. The balls are made of solder and collapse during reflow to form the electrical and mechanical connection.

What are the advantages of BGA packaging?

BGAs offer several advantages compared to other SMT packages, including:
– Reduced size due to interconnects being under the component
– Higher I/O density with ability to have hundreds of interconnects
– Shorter lead lengths for improved electrical performance
– Lower profile and lighter weight than leaded packages
– More PCB routing space between BGA pads compared to leaded packages

What equipment is needed for BGA assembly?

The key equipment needed for BGA assembly includes:
– Stencil printer for solder paste application
– Pick-and-place machine with vision alignment for component placement
– Reflow oven with multi-zone heating for solder joint formation
– Automated Optical Inspection (AOI) for post-reflow defect detection
– X-ray inspection system for checking hidden BGA solder joints
– Rework station with precision heating and alignment for BGA repairs

What are some common BGA assembly defects?

Some of the most common BGA assembly defects are:
– Opens due to poor solder wetting or insufficient paste volume
– Shorts from solder bridges between adjacent balls
– Voids in solder joints caused by paste outgassing or contamination
– Component misalignment from poor vision or stencil registration
– Head-in-pillow defects where ball and pad don’t fully collapse together

How can you inspect BGA solder joints?

BGA solder joints are hidden underneath the component, making inspection challenging. The two main methods for BGA inspection are:

1. Automated Optical Inspection (AOI): Uses cameras and image analysis to check for missing balls, gross misalignment, and excess solder. Cannot see under the component.

2. X-Ray Inspection: Uses X-rays to image the solder joints under the component. 2D X-ray checks for opens, shorts, and voids. 3D X-ray can measure joint volume and percent voiding.