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One, the planning of the circuit board layer
In the EMC design of the PCB, the first thing involved is the layer setting. The number of layers of the circuit board is composed of the number of power layers, the number of ground layers, and the number of signal layers. The relative positions of the power layer, ground layer, and signal layer and the division of power and ground planes are very important to the EMC indicators of the circuit board.
Number of layers
The power of the circuit board, the type of grounding, the density of the signal, the operating frequency of the circuit, the number of signals with special wiring requirements, as well as the performance index requirements and cost tolerance of the integrated circuit board, ensure the number of layers of the circuit board. In the case of demanding EMC indicators and relatively affordable costs, an appropriate increase in grounding is one of the effective methods for PCB EMC design.
1: The number of layers of power and ground
The number of power supply layers is determined by the number of types. For a PCB powered by a single power supply, one power plane is sufficient. For multiple power supplies, if they are not interlaced, power layer division can be adopted (to ensure that the key signal wiring of adjacent layers does not cross the division area). For circuit boards with interleaved power sources (multiple power sources and interleaved with each other), two or more power planes must be considered. The setting of each power plane must meet the following conditions:
A single power source or multiple power sources that are not interleaved with each other.
The key signal of the adjacent layer does not cross the partition. In addition to meeting the requirements of the power plane, the number of ground layers must also be considered.
There is a relatively complete ground plane below the component surface (layer 2 or the second to last).
Key signals such as high frequency, high speed, clock, etc. have adjacent ground planes.
The critical power supply has a corresponding ground plane adjacent to each other.
2: The number of signal layers in the Altium Designer software. After the netlist is loaded, the EDA software can provide layout and routing parameter reports. From this parameter, the number of layers required for the signal can be roughly judged; experienced EDA engineers According to the above parameters, the working frequency of the circuit, the number of signals with special wiring requirements, and the performance index requirements and cost tolerance of the circuit board can be combined to finally determine the number of signal layers.
The number of signal layers depends on the function implementation. From the EMC perspective, shielding or isolation measures for key signal networks (strong radiation networks and small and weak signals that are susceptible to interference) need to be considered.
2. What is solder mask opening?
The PCB solder layer is a solder mask, which refers to the part of the printed circuit board that is going to turn green. In fact, the solder layer uses negative output, so after the shape of the solder layer is mapped to the board, it is not on the unsintered oil-resistant solder but on the copper skin.
Process requirements of the welding layer
The barrier layer plays an important role in controlling soldering defects during the reflow soldering process, and the PCB designer should minimize the spacing or air gap around the solder pad features.
Although many process engineers prefer to separate the solder layer from all pad features on the board, special consideration needs to be given to the lead pitch and the pad size of closely spaced elements. Although unpartitioned barrier openings or windows on four-sided qfp are acceptable, it can be more difficult to control solder bridges between component pins. For the resistance layer of bga, the barrier layer provided by many companies does not touch the pads, but covers any features between the pads to prevent tin bridges. Most surface mount PCBs are covered with a solder layer, but if the thickness is greater than 0.04mm (), it may affect the application of solder paste. The placement of surface PCBs, especially those that use closely spaced components, requires a low-profile photosensitive solder layer.
The process of welding layer to produce welding materials must be used through liquid wet process or dry film lamination. The thickness of the dry film resistor material is 0.07-0.1mm (0.03-0.04"), which can be used for some surface mount products, but it is not recommended to use this material for tightly spaced applications. Few companies provide sufficiently thin dry Membrane is required to meet strict spacing standards, but there are several companies that can provide liquid photosensitive soldering materials. Generally, the opening of the barrier pad should be 0.15mm (0.006"). This creates a gap of 0.07 mm (0.003 inch) on all sides of the pad. Thin liquid photosensitive soldering materials are economical and are usually specified for surface mount applications, providing precise feature sizes and gaps.
Understand that the opening shielding window of the PCB resistance welding layer refers to the size of the part that needs to be welded to expose the copper, that is, the size of the part that does not cover the ink, and the cover line refers to the size and amount of the welding oil cover line. In the production process, the distance of the cover line is too small, which will cause condensation. The reason is that the window of the PCB resistance welding layer has dropped the slot. The aperture is opened: because many customers do not need the ink plug hole, if the window hole is not opened, the ink will enter the hole. . (This is for small holes) If the large hole is filled with ink, the customer will not be able to wear the key. In addition, if it is a gold board, the window must be opened. When soldering the PCB, the window will be opened with a PAD (ie copper) solder paste layer: the customer needs soldering and surface treatment.
