A ball grid array (BGA) is a type of surface-mount integrated circuit package with solder ball terminals arranged in a grid pattern on the underside of the package. BGAs allow for higher density pin counts, improved electrical performance, and efficient heat dissipation compared to other package styles.
Some key characteristics of BGA packages:
- Solder balls act as terminals and electrically connect the package to a PCB
- Ball pitches range from 0.5mm up to 2mm typically
- Package sizes range from 10mm x 10mm up to 45mm x 45mm or larger
- High pin counts - often several hundreds up to over a thousand
- Materials used include ceramics, plastics, laminate materials
- Popular package types include PBGA (plastic BGA), CBGA (ceramic BGA), TBGA (tape BGA)
BGA Land Pattern Definition
The land pattern on a PCB refers to the metal pads and traces that the BGA solder balls connect to. This includes:
Solder pads - the exposed metallic areas that the solder balls contact and attach to. Usually made of copper.
Solder mask openings - Areas around the pads that are left exposed without solder mask coating so that solder can adhere to the pad.
Vias - Plated through holes that connect BGA pads to inner signal layers on multilayer PCBs.
Properly designing the land pattern is critical for good solder joint reliability. The dimensions and geometry of pads and openings must match the BGA solder ball specifications.
| Component | Description |
|---|---|
| Pad diameter | Width of a single solder pad |
| Pad pitch | Distance between the centers of adjacent pads in the grid |
| Solder mask opening | Exposed ring area around the pad without coating |
| Anti-pad diameter | Area without copper around the pads |
BGA Footprint Components
The BGA footprint refers to all of the land patterns and layers that interact with the solder balls, including:
Top Layer Pads: Exposed pads on the outer layer where balls attach.
Internal Layers: Additional signal layers in multilayer boards with via pads.
Bottom Layer Pads: Large thermal pads that dissipate heat (optional).
Solder Mask Expansions: Clearance allowance for solder mask openings.
Assembly Pastes: Solder paste areas that contact each solder ball.
Pin/Ball Map: Pinout diagram showing the function of each terminal point.
Body Outline: Clearance area under the package body footprint.
The component layers combine to form the complete footprint that matches the specifications of a BGA package.
BGA Land Pattern Design Rules and Guidelines
Properly designing the land pattern or "footprint" for a BGA package ensures good solder joint integrity and reliability. Here are some key guidelines:
Solder Pad Geometry
The solder pads on the PCB must match the diameter, pitch, layout and count of the BGA solder balls:
- Pad diameter slightly larger than solder ball diameter
- Pad spacing/pitch matches solder ball layout
- Pad count equals total number of balls
If pads are mismatched, balls cannot connect properly.
Solder Mask Openings
The openings in the solder mask around pads should provide adequate solder coating:
- Opening diameter larger than pad
- Additional clearance allowance specified
- 0.1-0.2mm annular ring typical
Insufficient openings can impede ball/pad solder fillet formation.
Anti-pad Dimensions
Anti-pads or "solder thieves" help avoid solder bridging between neighboring pads.
- Anti-pads form moat between pads
- Typical anti-pad 0.15-0.3mm clearance
Via Land Patterns
Vias in pads anchor joints to internal layers.
- Optional thermal relief cutouts
- Tenting vias prevents solder wicking into holes
- Multiple vias help heat dissipation
Solder Mask Expansion
Additional margin added to openings to account for image registration tolerances during PCB fabrication.
- Expansions typically 0.075-0.150mm
- Helps ensure openings align properly over pads
Table visualizing the key land pattern design parameters:
| Land Pattern Feature | Typical Values | Function |
|---|---|---|
| Pad Diameter | Ball Diameter + 0.15mm | Make contact surface for solder balls |
| Pad Pitch | Equal to Ball Pitch | Match grid layout of package |
| Solder Mask Openings | Pad Dia. + 0.175mm | Expose metal pads for soldering |
| Anti-Pads | 0.15mm-0.3mm annular clearance | Prevent solder bridges between pads |
| Solder Mask Expansion | 0.075mm-0.150mm outward | Compensate for image alignment variations |
Additional Design Rules
Further PCB design guidelines for BGA footprints:
- Reference identifier markings for assembly
- Polarity markers to denote pin 1 side
- Body outline to check clearance fit
- Break away tabs help board assembly
- Fiducial markers to aid component placement
- Test/probe points to access signals
Careful attention to all aspects of footprint design ensures a BGA package mounts and solders properly on a PCB.
Creating BGA Footprints in CAD Software
Printed circuit board (PCB) design software is used to create custom footprints matched to BGA packages. Steps include:
1. Import Package Model
If available, import 3D STEP or IPC-7351C package models from the manufacturer into PCB layout tools. Models contain detailed pinout, body and footprint information to leverage.
2. Define Layer Stacks
Set up internal and external layers needed in the footprint cross section, such as solder pads, paste layers, mask expansions, etc.
3. Draw Land Pattern Geometry
Add the required pad, openings and clearance shapes onto corresponding PCB layers.
4. Assign Pinout
Designate pad connectivity by assigning package ball numbers to land pattern pads.
5. Add Keepout Areas
Define assembly or routing exclusion areas under package body outline.
6. Include Markings
Add layer-specific text and symbols like polarity marks, identifiers and fiducials.
7. Check Rules
Validate footprint versus IPCD-7351C standards or device specifications.
Once created, footprints can be saved to reusable libraries for placement across designs.match ref
Below is a diagram of a sample BGA footprint cross-section in CAD software:
As illustrated, a BGA footprint contains the solder pads, solder mask expansions, paste layers and other features required to mount the package.
BGA Assembly Process and Reflow Soldering
To mount a BGA component on a PCB, solder balls on the package must be reflowed and fused with the footprint pads using solder paste. Key steps include:
PCB Pad Preparation
Expose fresh solderable metal on pads and apply solder paste using stencils. Nano coating helps anchor paste.
BGA Placement
Use optical systems to align balls perfectly with paste deposits on pads. Apply tack flux.
Reflow Attachment
Heat assembly above liquidus temperature (~220°C) to melt the solder paste and alloy balls and pads.
Inspection
Verify all ball connections formed proper fillets without shorts between pads using X-ray imaging.
Proper footprint design along with precision assembly and soldering is imperative for reliable BGA connectivity.
Conclusion
In summary, the BGA land pattern constitutes the layer of pad and aperture geometries on the PCB, while the footprint includes all of the land layers in addition to keepouts, pastes and markings used for assembly. Precisely matching PCB footprints to BGA packages based on specifications is critical prior to fabrication. This helps ensure sound solder joints after reflow attachment during the soldering process. Careful design verification and assembly practices enable reliable interconnections.
Frequently Asked Questions
What is the main purpose of a BGA footprint?
The primary purpose of a BGA footprint is to define compatible pad geometries and clearances on a PCB to match the layout of solder balls underneath BGA packages. This provides suitable areas for ball/pad interconnection after reflow.
