What is BGA?

A Ball Grid Array (BGA) is a type of surface-mount packaging used for integrated circuits. BGA packages have an array of solder balls on the bottom of the package that connect to the PCB board. The main advantages of using BGA over other packaging styles like QFP are:

• Higher pin count – BGAs can accommodate more I/O pins in a smaller footprint vs QFPs. This allows for integration of more features and functions.
• Better electrical performance – The shorter traces from the solder balls to the PCB allow for better high-speed signal integrity.
• Easier routing – Routing traces out of a BGA is simpler than routing fine pitch leads out of QFPs.
• Better thermal performance – The bottom of the BGA package is directly attached to the PCB ground plane allowing for better heat dissipation.

Some key characteristics of BGA packages:

• Solder ball pitch – Distance between the centers of adjacent solder balls. Common pitches are 1.0mm and 0.8mm.
• Ball count – Number of solder balls in the array. Range from less than 100 up to over 1000 balls for large FPGAs and GPUs.
• Solder ball alloy – Typically SnAgCu (SAC). Other alloys like high-lead can also be used.
• Package substrate – Usually FR-4, but sometimes ceramics or flex circuits are used.
• Package sizes – From small 10x10mm up to 45x45mm and larger. 0.8mm pitch allows high I/O counts.

PCB Design Guidelines for BGA

Here are some key PCB design guidelines when working with BGA packages:

Land Pattern Design

• Use solder mask defined pads (SMD). Solder mask openings slightly larger than metal pads.
• Pad shape should match solder ball shape. Round pads for typical spherical solder balls.
• Pad size is approximately 20% larger than solder ball diameter.
• Include solder paste cutouts in pads for excess solder relief.
• Include fiducials for accurate placement.

Pad Placement

• Follow datasheet recommendations for land pattern dimensions.
• Allow for package warpage and coplanarity variation in pad positions.
• Place pads equidistant from center of package.

Routing

• Use vias-in-pad underneath BGA balls for thermal relief.
• Minimize trace length differences between BGA pads.
• Include ground vias near perimeter pads.
• Route critical signals first, then power, then grounds.

Stencil Design

• Use laser cut, electroformed or etched metal stencils.
• Stencil thickness is typically 0.1mm. Thicker for large BGAs.
• Aperture openings match pad sizes.
• Draft angles help with paste release.

Re-work Considerations

• Include spare parts in BOM for re-work.
• Use nozzle vacuums to remove failed parts.
• Precision placement tools to align new component.
• Use matching solder alloy balls when replacing BGA.

BGA Soldering Process

Here is a typical soldering process used for attaching BGA packages to PCBs:

Solder Paste Printing

• Apply solder paste over pads using stencil.
• Inspect for proper alignment, transfer and aperture fill.
• Type 3 or 4 powder suitable for BGA pitch.

Component Placement

• Use pick and place machine for accurate alignment.
• Vision systems check proper positioning.
• Place other SMT components after BGAs.

Reflow Soldering

• Typical reflow profile: ramp-up to 150C, over 205C for 60 sec, peak 230-260C.
• Nitrogen atmosphere preferred to avoid oxidation.
• Slow cooling to prevent solder ball cracks.
• Inspect for proper solder joint fillet and shape.

Underfill Dispensing

• Apply underfill material after reflow while still warm.
• Underfill helps strengthen solder joints and provides strain relief.
• Capillary action draws underfill material between PCB and package.
• Typical underfill cure temperature around 150°C.

Inspection

• Verify solder balls have proper standoff height.
• Use x-ray imaging to check for voids, shorts and proper placement.
• ICT testing checks for electrical opens and shorts.

Rework

• Hot air Preheat to melt solder joints.
• Use vacuum nozzle to lift failed component.
• Remove residual solder.
• Use solder paste and air flow tool for new component.
• Reflow attachment. Inspect.

BGA Failure Modes and Analysis

Here are common BGA failure modes and analysis methods:

Open Solder Joints

• Cause: Insufficient solder, misalignment, contamination.
• Troubleshoot: X-ray imaging, ICT testing, visual inspection.

Cold/Disturbed Solder Joints

• Cause: Poor reflow profile, thermal stress.
• Troubleshoot: Cross-sectioning, SEM, dye penetration.

Voids in Solder

• Cause: Solder paste quality, reflow issues.
• Troubleshoot: X-ray imaging, thermal profiling, cross-sections.

Solder Ball Cracking

• Cause: Thermal/mechanical stress, intermetallic growth.
• Troubleshoot: Dye penetration testing, cross-sectioning.

Underfill Delamination

• Cause: Contamination, uncured resin, moisture.
• Troubleshoot: Cross-sectioning, die shear testing.

Electromigration

• Cause: High current density, thermal cycling.
• Troubleshoot: Resistance measurement, thermal profiling.

Pad Cratering

• Cause: Excessive solder volume, large balls.
• Troubleshoot: 3D AOI, SEM, cross-sectioning.

Solder Extrusion

• Cause: Excess solder, small apertures.
• Troubleshoot: Visual inspection, 3D AOI.

Assessing failures requires analysis like cross-sectioning to examine solder joint quality and interfacial issues. Troubleshooting often involves adjusting the soldering process and material properties.

Questions and Answers

Q: What are some typical voiding levels for BGA solder joints?

A: Industry standards allow up to 25% voiding in BGA solder joints before it is considered a defect. Anything above 30% voiding could result in potential reliability issues from thermal cycling stress. Voiding above 50% increases risks of joint failure.

Q: How are BGAs attached when reworking the solder joints?

A: Reworking BGAs requires preheating the board, then using a hot gas nozzle to uniformly reflow all solder balls simultaneously. Vacuum suction can help secure the package during rework. Proper profiles, board support and nitrogen all help improve rework yield.

Q: What causes pad cratering defects under BGAs?

A: Pad cratering is usually caused by excess solder volume leading to etching of the pad copper surface during reflow. Larger solder balls and smaller pad openings are more susceptible. It can be mitigated by optimizing paste volume and stencil design.

Q: How thick should solder paste stencils be for 0.8mm pitch BGAs?

A: For 0.8mm pitch BGAs, a stencil thickness around 0.125-0.150mm provides a good balance of paste release and sufficient paste transfer. Laser cut or electropolished stencils help provide precise aperture definition.

Q: What are some key factors in selecting BGA underfill materials?

A: Important properties are coefficient of thermal expansion (CTE) matching to the board and package, low viscosity for flow, high glass transition temperature, and low ionic content. Underfill also needs sufficient adhesion strength, moisture resistance, and reworkability.