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Several issues that should be considered in PCB design
2021-09-29
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Author:Kavie

Several issues that should be considered in PCB design

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    1. Cutting materials mainly consider the issue of plate thickness and copper thickness:

    The standard series is 1.0 1.2 1.6 2.0 3.2 MM for the thickness of the sheet material greater than 0.8MM. The thickness of the sheet material is less than 0.8MM and does not count as the standard series. The thickness can be determined according to the needs, but the commonly used thicknesses are: 0.1 0.15 0.2 0.3 0.4 0.6MM, this material is mainly used for the inner layer of multilayer boards. When designing the outer layer, pay attention to the thickness of the plate. The production and processing need to increase the thickness of copper plating, solder mask thickness, surface treatment (tin spraying, gold plating, etc.) thickness and the thickness of characters, carbon oil, etc. The actual production of sheet metal will be thicker than 0.05-0.1MM , The tin plate will be thicker than 0.075-0.15MM. For example, when the finished product requires a thickness of 2.0 mm in the design, and when the 2.0mm sheet is normally selected for cutting, the thickness of the finished product will reach between 2.1-2.3mm in consideration of the tolerance of the sheet and the processing tolerance. If the design must require the thickness of the finished product to be no greater than At 2.0mm, the plate should be made of 1.9mm unconventional plate material. PCB processing plants need to temporarily order from the plate manufacturer, and the delivery cycle will become very long. When the inner layer is made, the thickness after lamination can be adjusted through the thickness and structure configuration of the prepreg (PP). The selection range of the core board can be flexible. For example, the thickness of the finished board is 1.6mm, and the choice of the board (core board) can be 1.2 MM can also be 1.0MM, as long as the thickness of the laminated plate is controlled within a certain range, the thickness of the finished plate can be met. The other is the board thickness tolerance issue. PCB designers should consider the board thickness tolerance after PCB processing while considering the product assembly tolerance. There are three main aspects that affect the tolerance of the finished product, including the incoming sheet tolerance, the lamination tolerance and the outer layer thickening tolerance. Several conventional sheet tolerances are now provided for reference: (0.8-1.0)±0.1 (1.2-1.6)±0.13 2.0±0.183.0±0.23 The lamination tolerance depends on the number of layers and thickness, and the tolerance is controlled within ±(0.05-0.1 ) Between MM. Especially for boards with board edge connectors (such as printed plugs), the thickness and tolerance of the board need to be determined according to the requirements of matching with the connector. The surface copper thickness problem, because the hole copper needs to be completed by chemical copper plating and copper electroplating, if no special treatment is done, the surface copper thickness will be thicker when the hole copper is thickened. According to the IPC-A-600G standard, the thickness of the small copper plating layer is 20um for level 1, 2 and 25um for level 3. Therefore, in the production of circuit boards, if the copper thickness requires 1OZ (smaller 30.9um) copper thickness, the cutting may sometimes choose HOZ (smaller 15.4um) cutting according to the line width/line spacing, removing the allowable tolerance of 2-3um, It can reach 33.4um. If you choose 1OZ cutting, the thickness of the finished copper will reach 47.9um. Other copper thickness calculations can be deduced by analogy.

    2. Drilling mainly considers the hole size tolerance, the pre-enlargement of the hole, the processing problems of the hole to the edge of the board, the non-metalized hole and the design of the positioning hole:

    At present, the small machining drill bit for mechanical drilling is 0.2mm, but due to the copper thickness of the hole wall and the thickness of the protective layer, the design aperture needs to be enlarged during production. The key question here is, if the diameter of the hole is enlarged, will the distance between the hole and the circuit and the copper skin meet the processing requirements? Is the originally designed solder ring of the circuit pad enough? For example, the diameter of the via hole is 0.2mm during the design. The diameter of the pad is 0.35mm. The theoretical calculation shows that 0.075mm on one side of the solder ring can be processed completely, but after the drill is enlarged according to the tin plate, there is no solder ring. If the pads cannot be enlarged by the CAM engineers due to the spacing issue, the board cannot be processed and produced. Aperture tolerance problem: At present, most of the drilling tolerances of internal drilling rigs are controlled at ±0.05mm, plus the tolerance of the plating thickness in the hole, the tolerance of metalized holes is controlled at ±0.075mm, and the tolerance of non-metalized holes is controlled at ±0.05mm. Another problem that is easy to overlook is the isolation distance between the drilled hole and the inner layer of the copper or wire of the multilayer board. Since the drilling positioning tolerance is ±0.075mm, there is a tolerance change of ±0.1mm for the expansion and contraction of the pattern after the inner laminate during lamination. . Therefore, in the design, the distance from the edge of the hole to the line or the copper skin is guaranteed to be above 0.15mm for the 4-layer board, and the isolation of the 6-layer or 8-layer board is guaranteed to be above 0.2mm to facilitate production. There are three common ways to make non-metallized holes, dry film sealing or rubber particle plugging, so that the copper plated in the hole is not protected by corrosion resistance, and the copper layer on the hole wall can be removed during etching. Pay attention to the dry film sealing, the hole diameter should not be greater than 6.0mm, and the rubber plug hole should not be less than 11.5mm. The other is to use secondary drilling to make non-metallized holes. No matter what method is adopted, there must be no copper skin around the non-metallized hole within 0.2mm. The design of positioning holes is often a problem that is easy to overlook. In the process of circuit board processing, testing, shape punching or electric milling all need to use holes larger than 1.5mm as the positioning holes for the board. When designing, it is necessary to consider as much as possible to distribute the holes on the three corners of the circuit board in a triangle shape.

    3. Circuit production mainly considers the influence caused by circuit etching. Due to the influence of side corrosion, copper thickness and different processing techniques are considered during production and processing, and a certain pre-roughness of the circuit is required.

    The conventional compensation of HOZ copper for spray tin and gold plate is 0.025mm, the conventional compensation for 1OZ copper thickness is 0.05-0.075mm, and the line width/line spacing production capacity is conventionally 0.075/0.075mm. Therefore, in the design, when considering the line width/line distance wiring, it is necessary to consider the compensation problem during production. The gold-plated board does not need to remove the gold-plated layer on the circuit after etching, and the line width is not reduced, so there is no need for compensation. However, it should be noted that because side etching still exists, the width of the copper skin under the gold layer will be smaller than the width of the gold layer. If the copper thickness is too thick or the etching is too much, the gold surface will easily collapse, resulting in poor soldering. For circuits with characteristic impedance requirements, the line width/line spacing requirements will be more stringent.

The above are a few issues that should be considered when PCB design, ipcb also provides PCB manufacturers and PCB manufacturing technology