The ever-increasing demand for large, complex backplanes that can operate at unprecedented high bandwidths has led to the need for processing capabilities beyond the capabilities of 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 refined, requiring hybrid bus architecture 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 boards. Some of the most demanding requirements are met in production. The noise tolerance and signal integrity also require the backplane design to follow the unique design rules. These characteristics of the backplane lead to huge differences in manufacturing requirements such as equipment specifications and equipment processing.
Backplane size and weight requirements for the conveyor system
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 large and heavy raw material substrates. The standard size of PCB manufacturing equipment is typically 24x24 inches. Users, especially telecommunications users, require larger backplane sizes. This has driven the need for confirmation and acquisition of large-size board transport tools. In order to solve the problem of the wiring of the large pin count connector, the designer has to add an additional copper layer to increase 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 to provide the required current to ensure proper operation of the card. All of these factors result in an increase in the average weight of the backing plate, which requires that the conveyor belt and other conveyor systems must not only be able to safely transfer large-sized raw material sheets, but must also take into account the fact that they are weight-increasing.
The user's need for a thinner, more layered backsheet results in two opposite requirements for the delivery system. Conveyor belts and conveyors must be able to pick and transport large gauge sheets of less than 0.10 mm (0.004 in.) thick without damage on the one hand, and must be capable of transporting 10 mm (0.394 in) thick and 25 kg (56 lb) weights on the other hand. The board does not drop the board.
The difference between the thickness of the inner plates (0.1mm, 0.004 inches) and the thickness of the finished back plate (up to 10mm, 0.39 inches) means that the conveyor system must be strong enough to safely carry them. Transfer through the processing area. Since the backing plate is thicker than the conventional PCB and the number of drilled holes is much larger, it is easy to cause the outflow of the working fluid. With a 30,000-drilled 10mm thick gauge backsheet, it is easy to carry out a small amount of machining fluid that is attracted to the pilot hole by surface tension. In order to minimize the amount of liquid carried and to eliminate any possibility of drying impurities at the pilot holes, it is extremely important to clean the boreholes by means of high pressure flushing and air blower.
Layer alignment
Since user applications require more and more layers, the alignment between layers becomes very important. Inter-layer alignment requires tolerance convergence. The larger the board size makes this convergence requirement even more demanding. All layout 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 back images of the entire area should be kept at 0.0125 mm (0.0005 inches). In order to achieve this accuracy requirement, a CCD camera is required to complete the alignment of the front and rear layout.
After etching, the inner layer plate is perforated using a four-hole system. The perforation is passed through the core plate, the positional accuracy is maintained at 0.025 mm (0.001 inch), and the repeatability is 0.0125 mm (0.0005 inch). The perforations are then inserted through the pins to align the etched inner layers while bonding the inner layers together.
Initially, the use of this post-etch perforation method fully ensures alignment of the drilled and etched copper plates to form a robust loop design. However, as the user requires more and more lines to be laid in a smaller area in the PCB routing, in order to keep the fixed cost of the board unchanged, the size of the etched copper board is required to be smaller, thereby requiring the interlayer copper board to be better. Counterpoint. In order to achieve this goal, an X-ray drilling machine can be used. The device is capable of drilling a hole with a maximum accuracy of 0.025 mm (0.001 inch) on a 1092 x 813 mm (43 x 32 inch) maximum size board. There are two ways to use it:
1. Observe the etched copper on each layer with an X-ray machine and determine the best position by drilling.
2. The drilling machine stores statistical data and records the deviation and divergence of the alignment data from the theoretical value. This SPC data is fed back to the previous processing steps such as raw material selection, processing parameters and layout drawing to help reduce the rate of change and continuously improve the process.
Although the electroplating process is similar to any standard plating process, there are two major differences that must be considered due to the unique characteristics of the large format backsheet.
Clamps and conveyors must be capable of transporting both large and heavy plates simultaneously. The large format raw material substrate of 1092x813mm (43x32 inches) 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 maintain uniform plating characteristics throughout the box.
In the past, users have specified press-fit connectors for the backplane, which has placed too much emphasis on the uniformity of copper plating. The backsheet thickness produces a variation of 0.8 mm to 10.0 mm (0.03 inch to 0.394 inch). The existence of various aspect ratios and the size of the substrate become large, making the uniformity index of the plating become critical. In order to achieve the required uniform performance, a periodic reverse ("pulse" plating control device must be used. In addition, the necessary agitation must be performed to keep the plating conditions as uniform as possible.
In addition to the uniform thickness of the plating required for drilling, backplane designers generally have different requirements for the uniformity of copper 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 impedance control circuit requirements, the outer layer of nearly solid copper foil will become necessary for EMC shielding.
Detection
Since the user requires more layers, it is important to ensure defect identification and isolation of the inner layer of the etch layer prior to bonding. Etching line width, thickness, and tolerance are key indicators for effective and repeatable control of backplane impedance. At this time, the AOI method can be used to ensure the matching of the etched copper pattern with the design data. Using the impedance model, the linewidth tolerance is set on the AOI to determine and control the sensitivity of the impedance to linewidth variations.
The trend toward large-size, multi-drilled backplanes and the placement of active circuits on the backplanes has spurred the need for rigorous inspection of bare boards prior to component loading for efficient production.
An increase in the number of drilled holes on the backplane means that the bare board test fixture will become very complicated, although the use of dedicated fixtures can greatly reduce the unit test time. To reduce production time and prototyping time, double-sided flying probe probing fixtures, programmed with raw design data, ensure consistency with user design requirements, reduce costs and reduce time to market.
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Label: Backplane PCB Pressing Design Rule Design Data
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