Guidance on Cracking a 0.5mm Ball Grid Array (BGA)
Precision Design Key to Successful BGA Breakout in HDI PCB
Designing a Ball Grid Array (BGA) breakout for High Density Interconnect (HDI) Printed Circuit Boards (PCBs) requires meticulous planning and adherence to strict design rules. Here's a breakdown of the crucial steps and considerations for a 0.5mm pitch BGA breakout:
1. Breakout Pad and Annular Ring Calculations
The BGA pads are typically around 0.25 mm to 0.3 mm in diameter to allow for sufficient space for solder ball placement and breakout routing. The annular ring, a copper ring around the drilled via or pad hole, is essential for reliable electrical and mechanical connections. For microvias or HDI vias, a minimum annular ring of 0.075 mm (75 µm) or more is recommended.
2. Copper Spoke Width (Trace Width from Pads)
The routing traces, or "spokes," from each BGA pad must fit within the breakout area without violating spacing rules. In HDI technology, copper trace width is usually kept to 0.075 mm to 0.1 mm, with a minimum spacing of 0.075 mm to 0.1 mm between adjacent traces.
3. Design Rules for HDI Technology
Microvias between layers are commonly used, with a minimum drill size around 0.1 mm to 0.15 mm. Blind and buried vias are recommended to break out from the inner BGA balls, minimizing crosstalk and congestion. The layer stackup should allow routing immediately below the BGA using via-in-pad techniques. HDI rules generally specify a minimum trace/space of 75 µm or better.
Table: Key Parameters and Values
| Parameter | Typical Value / Range | Purpose/Notes | |------------------------|----------------------------|----------------------------------| | BGA Pad Diameter | 0.25 mm – 0.3 mm | Allows solder ball and breakout | | Annular Ring Width | ≥ 0.075 mm (75 µm) | Reliable via/pad connection | | Copper Spoke Width | 0.075 mm – 0.1 mm | Routing traces from pads | | Trace/Space Min | 0.075 mm – 0.1 mm | HDI fabrication limits | | Microvia Diameter | 0.1 mm – 0.15 mm | Via-in-pad breakout | | PCB Layer Stackup | Include HDI layers for breakout | Blind/buried vias recommended |
This guideline applies to typical 0.5mm pitch chip scale BGAs like those in the LFXP10C series, which have standard die-up configurations requiring precise breakout. Using materials like Shengyi S1140F can help with thermal and mechanical stability in HDI designs.
Consult closely with your PCB fabricator to tailor annular ring and trace widths to their capabilities, as HDI rules can vary. The drill-to-copper spacing on a .5mm BGA with the given sizes is 5.3425 mils. The board thickness for this BGA breakout should be 62 mils or less. The pad size for the via-in-pad should be 10.68 mils. The design software can be set up to suppress unconnected pads and remove the pads on non-functional pins. If the via ties to the plane, it will be a solid tie (no thermal relief). The BGA breakout should not have more than eight layers. If the via does not tie to the plane or to a signal trace on this layer, the pad on this layer can be removed. The trace width in the BGA area should be 3 mils, and the minimum trace width is 3 mils. The example demonstrates a .5mm BGA breakout method.
Technology plays a crucial role in the successful BGA breakout in HDI PCBs, as precision design techniques such as the via-in-pad method are employed to ensure optimal electrical and mechanical connections. Adherence to HDI design rules, including the use of appropriate copper trace widths and annular rings, is essential for reliable performance.
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