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PCB backplane design and testing points

Shenzhen Inno Circuit Co.,Ltd | Updated: Jun 05, 2018

PCB backplane design and testing points

 The ever-increasing user demand for increasingly complex large-size backplanes that can operate at unprecedented high bandwidth has led to the need for equipment processing capabilities beyond conventional PCB manufacturing lines. In particular, the backplane is larger, heavier, and thicker, requiring more layers and perforations than standard PCBs. In addition, the required line widths and tolerances are more sophisticated, requiring hybrid bus architectures and assembly techniques.

The backplane has always been a specialized product in the PCB manufacturing industry. Its design parameters are very different from most other circuit boards. The production needs to meet some of the most demanding requirements. The noise margin and signal integrity also require the backplane design to follow unique design rules. These characteristics of the backplane have led to great differences in manufacturing requirements such as equipment specifications and equipment processing.

Backplane size and weight requirements for conveyor systems

The biggest difference between a conventional PCB and a backplane is the size and weight of the board, as well as the processing problems of a large and heavy raw material panel. The standard size of PCB manufacturing equipment is typically 24x24 inches. Users, especially telecommunication users, require larger backplanes. This has led to the need for confirmation and purchase of large-size board conveyors. Designers had to add extra layers of copper to solve the routing problem of large pin-count connectors, increasing the number of backplane layers. Harsh EMC and impedance conditions also require increased layers in the design to ensure adequate shielding, reduce crosstalk, and improve signal integrity.

When a high-power application card is inserted into the backplane, the thickness of the copper layer must be moderate in order to provide the required current to ensure that the card works properly. All of these factors lead to an increase in the average weight of the backplane, which requires that the conveyor belt and other conveyor systems must not only be able to safely transport large-sized raw material boards, but must also take into account the fact that they are weight-increasing.

The need for backplanes with thinner core layers and more layers brings with it two opposite requirements for the transport system. Conveyor belts and conveyors must on the one hand be able to pick up and deliver large gauge sheets of less than 0.10 mm (0.004 in.) in thickness without damage, and on the other hand they must be able to carry 10 mm (0.394 in) and 25 kg (56 lb). The board does not fall off the board.

The difference between the plate thickness (0.1mm, 0.004 inches) of the inner layer and the thickness (up to 10mm, 0.39 inches) of the finished backsheet means that the delivery system must be strong enough to safely hold them. Move through the processing area. Because the backplane is thicker than conventional PCBs and the number of drilled holes is much larger, it can easily cause the fluid to flow out. With a 30,000-drilled 10mm-thick large-size backplane, it is easy to carry out a small amount of fluid absorbed in the guide hole by the surface tension. In order to minimize the amount of liquid carried and to eliminate the possibility of any dried impurities remaining at the pilot hole, it is extremely important to use a high-pressure flushing and air blower method to clean the borehole.

Layer alignment

As user applications require more and more layers, alignment between layers becomes very important. Inter-layer alignment requires tolerance convergence. The board size change ambassador is more demanding in this convergence. All patterning processes are produced in a controlled temperature and humidity environment. The exposure equipment is in the same environment, and the alignment tolerance of the front and rear of the whole area should be kept as 0.0125mm (0.0005 inches). In order to achieve this precision requirement, CCD cameras must be used to complete the alignment of the front and rear layouts.

After etching, the inner plate was perforated using a four-hole drilling system. Perforation through the core plate maintains a positional accuracy of 0.025mm (0.001 inch) and a repeatability of 0.0125mm (0.0005 inch). The perforations are then inserted with needles to align the etched inner layer and bond the inner layers together.

Initially, the use of such etched post-piercing methods can fully ensure the alignment of the drilled and etched copper plates to form a sturdy, ring-shaped design. However, as users require more and more lines to be laid in a smaller area for PCB layout, in order to keep the fixed cost of the board constant, the size of the etched copper plate is required to be smaller, thereby requiring the inter-layer copper plate to be better Counterpoint. To achieve this goal, you can use the X-ray drilling machine. The device can achieve a position accuracy of 0.025mm (0.001 inch) for drilling a hole in a 1092 × 813mm (43 × 32 inch) plate of the largest specification. There are two usages:

1. Observe the etched copper on each layer with an X-ray machine and use drill holes to determine an optimal position.

2. The drill machine stores statistical data and records deviations and divergence of the alignment data from theoretical values. This SPC data is fed back to the previous processing steps such as raw material selection, processing parameters, and layout drawing to help reduce its rate of change and continuously improve the process.

Although the plating process is similar to any standard plating process, due to the unique characteristics of the large-size backplane, there are two major differences that must be considered.

Fixtures and conveyor equipment must be able to transport large size plates and heavy plates at the same time. The 1092x813mm (43x32 inch) large format raw material substrate can weigh up to 25 kg (56 lbs). The substrate must be securely gripped during transport and processing. The design of the tank must be deep enough to accommodate the board and uniform electroplating characteristics must be maintained throughout the tank.

In the past, users had specified a press-fit connector for the backplane, and therefore the dependence on copper plating uniformity was too heavy. The thickness of the back plate produces a variation of 0.8 mm to 10.0 mm (0.03 inches to 0.394 inches). The presence of various aspect ratios and the increased size of the substrate make the uniformity of electroplating an important indicator. In order to achieve the desired uniform performance, periodic reverse ("pulsed") plating control equipment must be used. In addition, necessary agitation must be performed to keep plating conditions as uniform as possible.

In addition to requiring uniform plating thickness for drilling, backplane designers generally have different requirements for copper uniformity on the outer surface. Some designs etch very few signal lines on the outer layer. On the other hand, in the face of high-speed data rate and the need for impedance control circuits, it will become necessary to set up near-solid copper sheets for the EMC shielding layer.


Since the user requires more layers, it is important to ensure that the inner layer of the etched layer is identified and isolated before adhesion. In order to achieve effective and repeatable control of the backplane impedance, the width, thickness, and tolerances of the etched lines are key indicators. At this time, the AOI method may be used to ensure that the etched copper pattern matches the design data. Using the impedance model, the line width tolerance is set on the AOI to determine and control the sensitivity of the impedance to line width variation.

The large multi-drilled backplanes and the tendency to place active circuits on the backplanes jointly advance the need for rigorous testing of bare boards prior to component loading for efficient production.

The increase in the number of drilled holes in the backplane means that the bare board test fixture will become very complicated, although the use of special fixtures can greatly reduce the unit test time. To shorten the production process and prototyping time, the use of double-sided flying probe probes and programming with raw design data ensures consistency with user design requirements, reduces costs, and shortens time to market.