PCB Technical - PCB backplane manufacturing technology

The backplane has always been a specialized product in the PCB manufacturing industry. Its design parameters are very different from most other circuit boards, and some stringent requirements need to be met during production. The noise tolerance and signal integrity also require the backplane design to comply with unique design rules. These characteristics of the backplane lead to huge differences in equipment specifications and equipment processing and other manufacturing requirements. Future backplanes will be larger and more complex, and will require an unprecedented high clock frequency and bandwidth range. The number of signal lines (tracks) and the number of nodes will continue to increase: it will no longer be uncommon for a backplane to contain more than 50,000 nodes.

1. The needs of users

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

2. Requirements of the size and weight of the backplane to the conveying 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 the large and heavy raw material substrate (panel). The standard size of PCB manufacturing equipment is typically 24x24 inches. However, users, especially telecommunications users, require larger backplanes. This has promoted the confirmation and purchase of large-size board conveying tools. Designers have to add additional copper layers to solve the routing problem of the large-pin-count connector, which increases the number of backplane layers. The harsh EMC and impedance conditions also require an increase in the number of layers in the design to ensure adequate shielding, reduce crosstalk, and improve signal integrity.

When a card with high power consumption is inserted into the backplane, the thickness of the copper layer must be moderate to provide the required current to ensure that the card can work normally. All these factors lead to an increase in the average weight of the backplane, which requires that conveyor belts and other conveying systems must not only be able to safely transfer large-size raw boards, but also must take into account the fact that their weight has increased.

The user's need for a thinner layer core and a more layered backplane brings about two opposite requirements for the conveying system. Conveyor belts and conveying devices must be able to pick up and transport large-format thin plates with a thickness of less than 0.10mm (0.004 inches) without damage on the one hand, and must also be able to transport 10mm (0.394 inches) thick and 25 kg (56 pounds) weight on the other hand. The board does not fall off the board.

The difference between the thickness of the inner plates (0.1mm, 0.004 inches) and the thickness of the final backplane (up to 10mm, 0.39 inches) is two orders of magnitude, which means that the conveying system must be strong enough to safely transport them Transfer through the processing area. Since the backplane is thicker than the conventional PCB, and the number of holes is much larger, it is easy to cause the process fluid to flow out. The 10mm thick back plate with 30,000 holes can easily take out a small amount of working fluid that is adsorbed in the guide hole by surface tension. In order to minimize the amount of liquid carried and eliminate the possibility of any drying impurities remaining at the guide hole, it is extremely important to clean the borehole by means of high pressure flushing and air blower.

Third, the counterpoint of the layers

As user applications require more and more board layers, the alignment between layers becomes very important. Interlayer alignment requires tolerance convergence. The board size has become more demanding for this convergence requirement. All layout processes are produced in a certain temperature and humidity controlled environment. The exposure equipment is in the same environment, and the alignment tolerance of the front image and the back image of the entire area must be maintained at 0.0125mm (0.0005 inches). In order to achieve this accuracy requirement, a CCD camera needs to be used to complete the alignment of the front and rear layout.

After etching, a four-drilling system is used to perforate the inner plate. The perforation passes through the core plate, and the position accuracy is maintained at 0.025mm (0.001 inch), and the repeatability is 0.0125mm (0.0005 inch). Then insert a pin into the perforation to align the etched inner layer while bonding the inner layer together.

Initially, the use of this post-etching perforation method can fully ensure the alignment of the drilled hole and the etched copper plate, forming a solid ring-shaped design structure. However, as users require more and more circuits to be laid in a smaller area in terms of PCB routing, in order to keep the fixed cost of the board unchanged, the size of the etched copper plate is required to be smaller, which requires a better interlayer copper plate. Counterpoint. To achieve this goal, an X-ray drilling machine can be purchased. The device can drill a hole on the board of the largest size of 1092*813mm (43*32 inches) with a position accuracy of 0.025mm (0.001 inch). There are two usages:

1. Observe the etched copper on each layer with an X-ray machine, and determine the best position with the help of drilling holes.

2. The drilling machine stores statistical data and records the deviation and divergence of the alignment data relative to the theoretical value. This SPC data is fed back to the previous processing procedures, such as the selection of raw materials, processing parameters and layout drawing, etc., 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 main differences that must be considered due to the unique characteristics of large-format backplanes.

Jigs and conveying equipment must be able to convey large-size boards and heavy boards at the same time. The weight of a large-format raw material substrate of 1092x813mm (43x32 inches) can reach 25 kg (56 lb). The substrate must be able to be securely gripped during transportation and processing. The design of the tank must be deep enough to accommodate the board, and uniform plating characteristics must be maintained throughout the tank.

Since the backplane is thicker and heavier than conventional PCB boards, its heat capacity is correspondingly larger. In view of the slow cooling rate of the backplane, the length of the reflow oven should be lengthened. It also needs to be forced air-cooled at the outlet to reduce the temperature of the back plate to a level that can be safely operated.