Back drilling Technology Overview
Back drilling, or controlled depth counter boring, is a process where plating is removed from the unused portion of the via. Multi-layer Printed Circuit Boards (PCBs) are processed in a standard manner, adding a secondary drilling operation after plating using PCB CNC drilling equipment with controlled depth capabilities. CNC drill files created from customer data allow this process to be automated and repeatable.
The energy that is transitioning through the plated through hole uses the hole as a transmission line element whose parameters are defined by the physical dimensions of the structure (pad size, anti-pad size, dielectric), It then reaches an impedance matched stripline layer. Some of the energy is transmitted into this layer and some continues to travel down the remaining via (reflects back to the driver).
Any portion of the plated through hole below the exiting stripline layer can be looked at as an open transmission element referred to as a stub. Back drilling, or counter boring, the via is an alternative technique to minimize the stub. Back drilling is simply drilling out the unused portion of a via to a controlled depth on the same type of mechanical drill equipment used to initially produce the PCB.
An important parameter is the secondary drill diameter. This drill diameter must be greater in diameter than the primary drill to allow removal of all the electrodeposited plated metal, typically copper with an additional surface finish.
Minimization of secondary drill diameter is important to avoid reduction of routing channels which compromise hole to trace spacing in the pin fields.
The current recommendation is 0.008”- 0.010 over original drill diameter depending upon depth. Drill-to-copper” spacing ranges from 0.008” – 0.006”.
Back drilling is a trade-off between manufacturing cost and electrical performance. Contributors to back drill depth variation is mechanical depth and layer position.
An optimized process achieves a 3-sigma overall variation of +/- 0.005” to nominal target depth. At least a 0.010” target nominal depth before the last layer connected is recommended.
Back drilling of Printed Circuit Board
PCB Back drills design rules
Primary-drill to copper: 0.012” Standard, 0.009” Advanced
Back-drill diameter: Primary drill + 0.008”, Advanced + 0.006”
Back-drill to copper: 0.008” Standard, Advanced 0.006”
PCB back drill design rules single lamination constructions
Back Drilling Parameters
Drill 0.010” (250 µm) over original drill diameter
Z tolerance = + /- 0.005” (150 µm)
Drill location tolerance = +/- 0.002” (50 µm)
PCB thickness = +/- 10%
PCB Back Drilling
Back Drilling Goal is to minimize Stub Length
Variation in Stub Length depends upon:
- Variation in Mechanical Drill depth
- Variation in PCB overall thickness
- Variation in Layer Depth
Via Stub
Cerra Systems Printed Circuit Board Fabrication capabilities support high-frequency PCB, High-temperature Boards, Thick PCB, ultra-thin PCB, heavy copper PCB, Metal Core PCB, HDI boards with Blind Vias, Buried Vias, Micro Vias, Embedded passives, bonded heat sink, Impedance Control, Depth control drilling, Back drilling, Edge Plated PCB, Bump Pads, cavity with ledge, Via on pad and stacked micro-Via technology.
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