PCB back board design and inspection points

PCB back board design and inspection points

User's requirements for large-size backplane that can work at high bandwidth leads to the need to exceed the processing capacity of conventional PCB manufacturing lines. In particular, the backboard size is larger, heavier, thicker, more layers and drillings than standard PCB requirements. In addition, the line width and tolerance required are more fine, need to use the hybrid bus structure and assembly technology are needed.

Backboard is always a professional nature of the product in the PCB manufacturing industry, its design parameters are very different from most other boards. Need to meet some demanding requirements in production. In terms of noise tolerance and signal integrity, backboard design is also required to conform to specific design rules. These characteristics of the backboard lead to great differences in the manufacturing requirements such as equipment specification and equipment processing.

Dimensions and weights of backboard-requirements for conveying systems

The standard size of the PCB manufacturing equipment is typical 24x24 inch. Users, especially telecommunications users, require a larger size of the backboard. This promotes the identification and acquisition requirements of large size board conveying tools. The designer has to increase the copper layer in order to solve the problem of the wiring of the large PIN number connector and increase the number of the back board. Stringent EMC and impedance conditions also require the addition of layers in the design to ensure adequate shielding, reduce crosstalk, and improve signal integrity.
When a large power application card is inserted into the backboard, the thickness of the copper layer must be moderate to provide the required current. All these factors lead to an increase in the average weight of the back board. This requires that conveyor belts and other conveyor systems not only be able to safely transfer large-scale raw material boards. And the fact that it has to be weighed.
The need for more thin layers and more layers of backboard has brought about two diametrically opposed requirements for the conveyor system, conveyor belts and conveyors must be capable of conveying large sheet sheets of a thickness less than 0.10mm (0.004 inches). On the other hand, must be able to transport 10mm (0.394 inches) thick, 25 kilograms (56 lb) Heavy board
Because the backboard is thicker than conventional PCB, and the number of drilling holes is much more, so it is easy to cause the outflow of processing liquid. 10mm thick back board with 30,000 drilled holes that can easily be brought out by a little liquid adsorbed on the guide hole by surface tension. To minimize and eliminate the possibility of any dry impurities remaining at the guide hole. It is very important to use the method of high pressure flushing and air blower to clean the drilling hole.

Alignment of layers

As user applications require more and more board layers, the alignment between layers becomes very important. Tolerance convergence is required between interlayer alignment. All the layout processes are produced in a controlled environment with a certain temperature and humidity. Exposure devices in the same environment, the entire area before and after the map of the alignment tolerance should be kept to 0.0125mm (0.0005 inches). In order to achieve this precision requirement, CCD camera is needed to complete the alignment of the front and rear graphs.

Along with the user in the PCB line requirements in a smaller area to lay more and more lines, in order to maintain the fixed cost of the board unchanged, It requires a smaller size of the etched copper board. Which requires that the laminate be better aligned. To achieve this goal, the acquisition of X-ray drilling machine can be used. The device is able to achieve 0.025mm (0.001 inch) accuracy of drilling a hole in the 1092x813mm (43x32 inch) maximum size board.


Because the user requires more layers. Therefore, it is very important to ensure the defect identification and isolation of the etching layer on the inner layer before bonding. In order to realize the effective and repeatable control of back-board impedance, etching line width, thickness and tolerances become key indicators. At this time, the AOI method can be used to ensure the matching of etched copper pattern and design data. using impedance Models, by setting the line width tolerance on the AOI, so as to determine and control the sensitivity of impedance to line width change.

It is necessary to carry out a rigorous inspection of the bare board before carrying out the component filling in order to produce efficiently.

The increase in the number of drilled holes in the back board means that the bare board test fixture will become very complex, to shorten the production process and prototype manufacturing time, using double-sided flying needle detection fixture. Programming with raw design data. Ensures consistency with user design requirements, reduces cost, and shortens time to market.