There are seven issues that need to be considered in PCB circuit board design! For the convenience of expression, analyze from seven aspects: cutting, drilling, wiring, solder mask, characters, surface treatment and forming:
一、Mainly consider plate thickness and copper thickness when cutting materials:
Sheet thickness greater than 0.8MM. Standard series: 1.01.21.62.03.2MM sheet thickness less than 0.8MM is not considered a standard series. Thickness can be determined according to needs, but the commonly used thickness is: 0.10.150.20.30.40. 6MM, this material is mainly used for the inner layer of the multilayer board.
Pay attention to the selection of board thickness when designing the outer layer. Production and processing need to increase the thickness of copper plating, solder mask thickness, surface treatment (tin spraying, gold plating, etc.) , the tin plate will be thicker by 0.075-0.15MM. For example, when the finished product requires a plate thickness of 2.0 mm during design, when the 2.0mm plate is normally used for cutting; considering the plate tolerance and processing tolerance, the finished plate thickness will reach 2.1-2.3mm If the design must require that the thickness of the finished board should not be greater than 2.0mm, the board should be selected as 1.9mm unconventional board to make PCB processing factories need to temporarily order from the board manufacturer, and the delivery cycle will become very long.
When making the inner layer, the thickness after lamination can be adjusted through the thickness and structural configuration of the prepreg (PP). The selection range of the core board can be more flexible. For example, the thickness of the finished board is required to be 1.6mm, and the selection of the board (core board) can be 1.2MM It can also be 1.0MM, as long as the thickness of the laminated board is controlled within a certain range, it can meet the thickness requirements of the finished product.
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. The impact on the finished product tolerance is mainly due to three aspects: board material tolerance, lamination tolerance and outer layer thickening tolerance. Several conventional plate tolerances are now provided for reference: (0.8-1.0)±0.1(1.2-1.6)10.132.0+0.183.0+0.23 Lamination tolerances are controlled at ±(0.05-0.1) according to different layers and plate thicknesses ) Between MM. Especially boards with board edge connectors (such as printed plugs) need to determine the thickness and tolerance of the board according to the requirements for matching with the connector.
The surface copper thickness problem, because the hole copper needs to be completed by electroless copper plating and electroplating copper, if no special treatment is done, the surface copper thickness will increase when the hole copper is thickened. According to the IPC-A-600G standard, the minimum copper plating thickness is 20um for grades 1 and 2, and 25um for grade 3. Therefore, in the production of circuit boards, if the copper thickness requires 10Z (minimum 30.9um) copper thickness, sometimes the material will be cut according to Select HOZ (minimum 15.4um) for line width/line spacing, and the minimum allowable tolerance of 2-3um can reach 33.4um. If you choose 10Z material, the minimum copper thickness of the finished product will reach 47.9um. Other copper thickness calculations can be analogized in turn .
二、Drilling mainly considers the size tolerance of the hole, the pre-size of the drilling, the processing of the hole to the edge of the board, the processing of the non-metallized hole, and the design of the positioning hole:
At present, the smallest processing drill bit for mechanical drilling is 0.2mm, but due to the thickness of the hole wall copper and the thickness of the protective layer, the design hole diameter needs to be increased during production, and the tin plate needs to be increased by 0.15mm, and the gold plate needs to be increased by 0.1mm. The key question here is that if the hole diameter is enlarged, does the distance from the hole to the circuit and the copper skin meet the processing requirements? Is the welding ring of the circuit pad originally designed enough? The diameter of the plate is 0.35mm, and theoretical calculation shows that the welding ring can be processed with a single side of 0.075mm, but if the drill is enlarged according to the tin plate, there will be no welding ring in production. If the pads cannot be enlarged due to spacing problems, the board cannot be processed and produced.
Aperture tolerance problem: At present, the tolerance of most drilling rigs in China is controlled at ±0.05mm, plus the tolerance of the coating thickness in the hole, the tolerance of metallized holes is controlled at +0.075mm, and the tolerance of non-metallized holes is controlled at ±0.05mm.
Another problem that is easily overlooked is the isolation distance from the drilled hole to the inner layer of the copper skin or wire of the multilayer board. Since the positioning tolerance of the drilled hole is ±0.075mm, there is a tolerance change of ±0.1mm for the expansion and contraction deformation of the inner laminated board during lamination. . Therefore, when designing, the distance from the edge of the hole to the wire or copper skin is guaranteed to be more than 0.15mm for 4-layer boards, and the isolation for 6-layer or 8-layer boards is guaranteed to be more than 0.2mm to facilitate production.
There are three common ways to make non-metallized holes, dry film sealing or rubber plugging, so that the copper plated in the hole can be removed during etching because there is no corrosion resistance protection. Note that the diameter of the dry film sealing hole should not be greater than 6.0mm, and the plug hole of the rubber particle should not be smaller than 11.5mm. In addition, the secondary drilling is used to make the non-metallized hole. No matter what method is used to make non-metallized holes, there must be no copper skin within 0.2mm.
The design of positioning holes is often a problem that is easily overlooked. During the process of circuit board processing, testing, shape punching or electric milling, holes larger than 1.5mm need to be used as positioning holes for board fixing. When designing, it is necessary to consider distributing the holes on the three corners of the circuit board as triangular as possible.
三、The circuit production mainly considers the influence caused by circuit etching
Due to the influence of side erosion, copper thickness and different processing techniques are considered during production and processing. It is necessary to pre-roughen the line. The conventional compensation for HASL and immersion gold boards is 0.025mm for HOZ copper, and the conventional compensation for 10Z copper thickness is 0.05-0.075mm. The line spacing production and processing capacity is generally 0.075/0.075mm. Therefore, the compensation problem during production needs to be considered when considering the most line width/line spacing wiring during design.
The gold-plated board does not need to be compensated because the line width of the gold-plated layer on the circuit does not decrease after etching. However, it should be noted that the side etching still exists. Therefore, the line width of the copper skin under the gold layer will be smaller than the line width of the gold layer. If the copper thickness is too thick or the etching is excessive, it will easily cause the gold surface to collapse, resulting in poor welding.
For lines with characteristic impedance requirements, the line width/line spacing requirements will be more stringent.
四、the more troublesome solder mask production is the solder mask treatment method on the via hole
