- Backplane PCBs: Introduction
Generally a Backplane PCB is a collection of multipleconnectors placed and connected on one PCB. By nature its responsibility is to carry signals like a cable from one connector to other but enough good health such as single0ended, differential paired signals, and power supply rails and return paths. The PCBs with semiconductor ICs are directly press-fit over a backplane PCB. These insertable cards or PCBs are called daughter cards. It makes the pins of sourceconnectors to be connected exactly on the required pinsof destination connectors.
A backplane PCB,in complexity is somehow similar to a motherboard PCB in respect of PCB design parameters but it has manydifferences in manufacturing process and functionality. A motherboard, well known to be densely routed signalsboard with high pin cunt semiconductor ICs. On the other enda backplane is considered an enough connectivity board on the “back” of boards. But a backplane PCB is much more than a connectivity board especially for high datarate communication systems. A backplane PCB transports allelectrical connectivity of signals and powersin between daughter boards.It also holds and supports daughter cards over it which can be press-fit or extract. It increases throughput of system on the cost of increased design and manufacturing effort. Although, the early times backplanes do have wire-wrapped connectorsand sockets but data speed in newer systems has realized to utilize advance PCBs materials and manufacturing skills for backplane PCBs. It originated further PCB manufacturing constraints alongwith the existing ones. PC (Printed Circuit) based Backplanes are preferred over wire-wrapped backplanes due to their increased reliability and data carriage capability.In common practice Backplanes don’tcomprises of semiconductor chips over them. However, they have small components like resistors, capacitors, filters and chassis ground circuitry over them.
- Backplane Manufacturing Difficulties:
The Backplane PCBs have enhanced data throughput, signal travelling quality, ease of assembly and production but still it increased challenges for PCB manufacturers.
- PCB Thickness
The Backplane PCBs thickness increases because it consists of several high speed controlled impedance transmission lines, supply rail planes ad return paths.Transmission of high frequency data over longer traces requires wide signals traces to reduce signal attenuations. It needs thicker dielectric capable to maintain impedance and keep the signal absorption lowest. Increasing trace width decreases signal density in a layer in per unit area of PCB. On the other hand increasing dielectric thickness increases overall PCB thickness.
Insertion of more signallayers added to power planes and returnplanesincreases layer count that finallyapproaches to 20 or even more.It becomes challenging to maintain standard thickness of PCB for a manufacturer with given materials and impedance matching parameters.
High speed fine PCB drilling for vias and connectors in backplanes works well till a thickness of 5 to 6mm. However, larger drill diameters are required for thicker PCBs. Higher drill aspect ratios such as 8:1 constraintsmanufacturer to volumetric production so thicker PCBs require multiple drill passes for a single via.
It is a common practice to through daughter cared input power supplies by backplanes for examples ±5V, ±12V and ±24V DC. Many a times each power rail needs a separate return path plane to reduce PCB commotion. In such a way it may need upto 12-layers for power supplies.Framework computers having high current power supplies which produce IR losses over the planes copper. Thatneedsheat sinks or heat radiator fans for temperature compensation. Their leading layers need thicker copper compared to other internal layers.With higher layer count PCB layer registration, via alignment, pad-hole alignment and pressing a thicker PCB also becomes difficult.
Overall a high-speed backplane PCB increases entire manufacturing process through routing complexity, etching, layer registration, thickness, weight, material selection, and cost and production process.
- PCB Pressing
In thicker backplanes manufacturing drilling becomes more complicated. A lengthy drill hole requires multiple times drilling the hole. It decreases drilling accuracy ad PCB yield percentage. It degrades high frequency performance.
- High density etching
In a backplane PCB due to high count of connectors so their mounting holes and pads use significant part of PCB real-estate. The PCB designers strive to increase signal routing density maintaining impedance, trace length, instead of increasing number of layers. The increased signals density increases etching complexity. It pushes the trace, via-hole, pad-hole, annular ring geometries to the fabrication tolerance limits.
- Impedance matching
With high speed data transmissions in the backplane PCBs impedance matched PCB fabrication becomes a challenge.Data transmission in GB per second range is generally not recommended over general FR-4 materials. The dielectric constant and dissipation factor dictate the material selection for backplane PCBs. This creates a challenge for PCB manufacturer to provide PCBs on demanded materials. The transmission line impedance, via impedance and connector pad to pin impedance also becomes a challenge. A new material type needs PCB manufacturing process to be slightly amendedat manufacturer facility.
- Via back-drilling
As unused copper piece in PCB can act as stub in high frequency signals.The similar stub-effect may occur due to unused via barrel copper and annular ring. For this purpose PCB manufacturers are pushed to back-drill PCB. It requiresremoving the redundant via barrel from a thru-hole via in a PCB. The manufacturer removes thatvia copper after PCB fabrication is complete byre-drilling the target holes with a different drill size, leaving a certain length after last layer connection. It slows the production and causes inaccuracies in PCBs.
- Connectors Alignment
A high speed backplane has good impedance matched connectors over it. In general a backplane has around 50 impedance controlled differential pairs per square inch. So the total differential pair length approaches to 500 pairs in a plane.The high density SMD and through-hole connectors allow smaller PCB size but they need many blind press-fit insertion and extraction of daughter cards from it. It harmsdurability of PCB. The manufacturerconstraint arises to maintain a hole-locations and geometry of connectors as well as maintaining the signal integrity throughout the PCB.
- Layer registration
Layer registration in high speed high density PCBs causes low accuracy during copper etching, lamination, drilling and dimensional stability. The signal integrity and PCB yield at smaller sizes becomes more challenging for manufacturer.
- Aspect Ratio
Aspect ratio is the ratio of drill size to PCB thickness. Dense signals routing,higher copper layers count, and high vias and connector through-holes count in a backplane makes itsaspect ratio high that leads manufacturingdifficulties.PCB design engineers strive to maintain an acceptable ratio figure effective for PCB assembly and durability. Manufacturers are pushedto manufacture PCBs with 10:1 aspect ratio. However, 6:1 aspect ratio is a common number.
Other common problems which are related to backplane PCBs manufacturing of backplane PCBs are CAF (copper anodic filament), impractical stackups (such as odd-numbered layers PCB design), thickness control of PCB and high-pressure compression, incomplete manufacturing information, incorrect blind and buried vias placement, improper layer-pairs, environmental impacts on PCB core and pre-preg materials, stringentrequirementsforced by international body regulations.