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Seven issues that need to be considered in PCB circuit board design
2021-09-01
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Author:Aure

Seven issues that need to be considered in PCB circuit board design

Seven issues need to be considered in PCB circuit board design! For ease of expression, analysis from seven aspects: cutting, drilling, wiring, solder mask, characters, surface treatment and forming:
1. Cutting materials mainly consider the issue of plate thickness and copper thickness:

The standard series of plates with a thickness of more than 0.8MM are: 1.0 1.2 1.6 2.0 3.2 MM, and the thickness of a plate less than 0.8MM is not considered as a standard series. The thickness can be determined according to 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, characters, carbon oil and other thickness. 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 during the design, when the 2.0mm sheet is normally selected for cutting, the thickness of the finished sheet will reach 2.1-2.3mm, taking into account the tolerance of the sheet and the processing tolerance. In the meantime, if the design requires that the thickness of the finished plate should not be greater than 2.0mm, the plate should be made of 1.9mm unconventional plate material. The PCB processing plant needs to temporarily order from the plate manufacturer, and the delivery cycle will become very long.


Seven issues that need to be considered in PCB circuit board design

When the inner layer is made, the thickness after lamination can be adjusted by 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 required to be 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. PCB designers must 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 material tolerance, lamination tolerance and outer 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±0.18 3.0±0.23 Laminating tolerances are controlled within ±(0.05-0.1) according to different layers and thicknesses 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 minimum copper plating thickness is 20um for level 1, 2 and 25um for level 3. Therefore, if the copper thickness is required to be 1OZ (minimum 30.9um) when the copper thickness is required during circuit board production, the cutting may sometimes Choose HOZ (minimum 15.4um) cutting material according to the line width/line distance, remove the allowable tolerance of 2-3um, the smallest can reach 33.4um, if you choose 1OZ cutting, the minimum thickness of the finished copper will reach 47.9um. Other copper thickness calculations are available And so on.

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

At present, the smallest 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, and the spray tin plate needs to be increased by 0.15mm. The gold plate needs to be increased by 0.1mm. 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 solder ring of the originally designed circuit pad enough? For example, the diameter of the via hole is 0.2mm, The diameter of the pad is 0.35mm. The theoretical calculation shows that 0.075mm on one side of the welding ring can be processed completely, but after the drill is enlarged according to the tin plate, there is no welding 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 domestic 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 ignore is the isolation distance between the drill hole and the inner copper layer of the multilayer board. Since the hole positioning tolerance is ±0.075mm, there is a tolerance change of ±0.1mm for the expansion and contraction of 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. In addition, secondary drilling is used to make non-metallized holes. No matter what method is adopted, the non-metallized hole must be free of copper in the range of 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.

Third, the line production mainly considers the impact of line 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 line is required. Conventional compensation for HOZ copper for spraying tin and gold plate is 0.025mm, and conventional compensation for 1OZ copper thickness is 0.05-0.075mm, line width /Line spacing production and processing capacity is conventionally 0.075/0.075mm. Therefore, when designing the most line width/line spacing 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.

Fourth, the more troublesome part of the solder mask production is the solder mask treatment method on the vias:

In addition to the conductive function of the via, many PCB board design engineers will design it as an online test point for the finished product after assembling the components, and even a very small number of them are also designed as component plug-in holes. In the conventional via design, in order to prevent the soldering from being tinted, it will be designed as a cover oil. If it is a test point or a plug-in hole, the window must be opened.

However, the through hole cover oil of the tin-sprayed circuit board is very easy to cause the tin beads to be embedded in the hole, so a considerable part of the products are designed as the through hole plug oil, and the position of the BGA is also treated as the plug oil for the convenience of packaging the BGA. But when the hole diameter is larger than 0.6mm, it will increase the difficulty of plugging oil (the plug is not full). Therefore, the spray tin plate is also designed as a half-open window with a larger than the hole diameter of 0.065mm on one side, and the hole wall and hole edge are within the range of 0.065mm. Spray tin.

Five, the character processing mainly considers the addition of pads and related marks on the characters.

Because the component layout is getting denser, and it is necessary to consider that the pad cannot be placed when the characters are printed, at least ensure that the distance between the characters and the pad is more than 0.15mm, sometimes the component frame and component symbols cannot be completely distributed on the circuit board. Fortunately, it is now pasted. Most of the film is complete by the machine, so if it is really impossible to adjust the design, you can consider printing only the character frame instead of the component symbol.

Commonly added contents of the mark include supplier identification, UL demonstration mark, flame-retardant rating, anti-static mark, production cycle, customer-specified identification, and so on. The meaning of each sign must be clarified, and it is best to leave it aside and specify where to place it.

Sixth, the influence of the surface coating (plating) layer of the PCB board on the design:

At present, the most widely used conventional surface treatment methods include OSP gold plating, immersion gold, and tin spraying.
We can compare the advantages and disadvantages of each in terms of cost, weldability, wear resistance, oxidation resistance, different production processes, drilling and circuit modification.

OSP process: low cost, good conductivity and flatness, but poor oxidation resistance, which is not conducive to storage. The drilling compensation is conventionally made by 0.1mm, and the HOZ copper thickness compensation is 0.025mm. Considering that it is extremely easy to be oxidized and contaminated with dust, the OSP process is completed after the forming and cleaning. When the size of the single chip is less than 80MM, the splicing form must be considered delivery.

Nickel-gold electroplating process: good oxidation resistance and wear resistance. When used in plugs or contact points, the thickness of the gold layer is greater than or equal to 1.3um. The thickness of the gold layer used for welding is usually 0.05-0.1um, but the relative solderability Poor. The drilling compensation is made according to 0.1mm, and the line width is not compensated. Note that when the copper thickness is more than 1OZ, the copper layer under the surface gold layer is likely to cause excessive etching and collapse, causing solderability problems. Gold plating requires current assistance. The gold plating process is designed before etching. The complete surface treatment also plays a role of corrosion resistance. After etching, the process of removing corrosion resistance is reduced, which is why the line width is not compensated.

Electroless nickel gold plating (immersion gold) process: good oxidation resistance, good toughness, smooth plating is widely used in SMT boards, drilling compensation is made at 0.15mm, and HOZ copper thickness compensation is 0.025mm, because the immersion gold process is designed After solder mask, you need to use corrosion resistance protection before etching. After etching, you need to remove the corrosion resistance. Therefore, the line width compensation is more than that of the gold-plated board. For large-area copper-clad boards, the amount of gold salt consumed by immersion gold boards is significantly lower than that of gold-plated boards.

Spray tin plate (63 tin/37 lead) process: relatively best oxidation resistance, toughness, poor flatness, drilling compensation is made at 0.15mm, HOZ copper thickness line width compensation is 0.025mm, the process is basically the same as that of sinking gold Consistent, it is currently the most common surface treatment method.

As the EU put forward the ROHS directive, it refused to use six hazardous substances containing lead, mercury, cadmium, hexavalent chromium, polybrominated diphenyl ethers (PBDE) and polybrominated biphenyls (PBB). The surface treatment introduced pure tin (tin copper ) ), spraying pure tin (tin-silver-copper), immersion silver and immersion tin, etc. to replace the lead-tin spraying process.

7. Jigsaw puzzle and shape making are also difficult to consider comprehensively when designing:

First of all, the ease of processing should be considered when the board is assembled. The time distance of the electric milling shape should be assembled according to a milling cutter diameter (conventional 1.6 1.2 1.0 0.8). When punching the board shape, pay attention to whether the distance from the hole and line to the edge of the board is greater than a board thickness. The minimum punching size must be greater than 0.8mm. If the V-CUT connection is used, the edge of the board and the copper must be 0.3mm away from the center of the V-CUT.

Secondly, we must consider the problem of the utilization rate of large materials. Because the specifications of large materials are relatively fixed, the commonly used sheet materials are 930X1245, 1040X1245, 1090X1245 and other specifications. If the delivery unit is unreasonably assembled, it is easy to cause waste of sheet materials.