PCB Blog - Several issues to be considered in PCB board design

With the vigorous development of the PCB board industry, more and more engineers and technicians have joined the design and manufacture of PCB boards. However, since there are many fields involved in PCB board manufacturing, and a considerable number of PCB board design engineers (Layout staff) I have not engaged in or participated in the production and manufacturing process of PCB boards, resulting in the emphasis on electrical performance and product functions in the design process, but downstream PCB board processing plants receive orders. In the actual production process, there are many problems due to design Without considering the difficulty in product processing, the prolonged processing cycle or the existence of hidden dangers in the product, for such problems that are not conducive to processing and production, for the convenience of expression, from the seven aspects of cutting, drilling, wiring, solder mask, characters, surface treatment and forming analyze:

1. The material opening mainly considers the plate thickness and copper thickness:
For plates with a thickness greater than 0.8MM, the standard series is: 1.0 1.2 1.6 2.0 3.2 MM, and the thickness of the plate less than 0.8MM is not considered a standard series. 0.4 0.6MM, this material is mainly used for the inner layer of multilayer boards. When choosing the thickness of the outer layer, pay attention to the thickness of copper plating, solder mask thickness, surface treatment (tin spraying, gold plating, etc.) and thickness of characters and carbon oil., The tin plate will be thicker than 0.075-0.15MM. For example, when the finished product requires a thickness of 2.0 mm, when the 2.0mm sheet is normally selected for cutting, considering the sheet tolerance and processing tolerance, the finished sheet thickness will reach 2.1-2.3mm. If the design requires that the thickness of the finished product should not be greater than 2.0mm, the board should be made of 1.9mm unconventional board material. The PCB board processing factory needs to temporarily order from the board manufacturer, and the delivery cycle will become very long. When the inner layer is made, the thickness after lamination can be adjusted by the thickness and structural configuration of the prepreg (PP), and the selection range of the core board can be flexible. MM can also be 1.0MM, as long as the thickness of the laminated plate is controlled within a certain range, the thickness requirements of the finished product can be met. In addition, there is the issue of plate thickness tolerance. When considering product assembly tolerance, PCB board designers should also consider the plate thickness tolerance after PCB board processing. There are three main aspects that affect the finished product tolerance, the sheet material tolerance, lamination tolerance and 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±0.18 3.0±0.23 Lamination tolerances are controlled within ±(0.05-0.1) according to different layers and thicknesses between MM. Especially for boards with board edge connectors (such as printed plugs), the thickness and tolerances of the board need to be determined according to the requirements for mating with the connector. The surface copper thickness problem, because the hole copper needs to be completed by chemical immersion copper and electroplating copper, if no special treatment is done, the surface copper thickness will be thickened together when the hole copper is thickened. According to the IPC-A-600G standard, the thickness of copper plating is 20um for grades 1 and 2, and 25um for grade 3. Therefore, when making circuit boards, if the copper thickness requires 1OZ (30.9um) copper thickness, the material will sometimes be cut according to the wire. Select HOZ (15.4um) for width/line spacing, except for the allowable tolerance of 2-3um, it can reach 33.4um. If you choose 1OZ for cutting, the finished copper thickness will reach 47.9um. Other copper thickness calculations can be deduced and so on.

2. Drilling mainly considers the hole size tolerance, the pre-large hole size, the processing problems of the hole to the edge of the board, the non-metallized hole and the design of the positioning hole:
At present, the machining tip of mechanical drilling is 0.2mm, but due to the thickness of copper on the hole wall and the thickness of the protective layer, the design aperture needs to be increased during production, and the tin spray plate needs to be increased by 0.15mm, and the gold plate needs to be increased by 0.1mm. Here The key question is, if the hole diameter is increased, will the distance from the hole to the circuit and copper sheet 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 and the diameter of the pad is 0.35mm. The theoretical calculation shows that 0.075mm on one side of the welding ring can be fully processed, but after the production of the drill nozzle is enlarged according to the tin plate, there is no welding. Ring up. If the pads cannot be made larger by the CAM engineers due to spacing issues, the board cannot be produced. Aperture tolerance problem: At present, most of the domestic drilling rigs have a drilling tolerance of ±0.05mm, plus the tolerance of the coating thickness in the hole, the metallized hole tolerance is controlled at ±0.075mm, and the non-metallized hole tolerance is controlled at ±0.05mm. Another problem that is easy to ignore is the isolation distance from the drilled hole to the inner layer of the copper or wire of the multilayer board. Since the drilling positioning tolerance is ±0.075mm, there is a ±0.1mm tolerance change of the graphic expansion and deformation of the inner laminate during lamination. Therefore, in the design, the distance from the hole edge to the wire or copper skin is guaranteed to be more than 0.15mm for 4-layer boards, and the isolation of 6-layer or 8-layer boards is guaranteed to be more than 0.2mm, which is convenient for production. There are three common ways to make non-metallized holes: dry film sealing or colloidal plugging, so that the copper plated in the hole can be removed during etching because of no corrosion resistance protection. Pay attention to dry film sealing, the pore size should not be larger than 6.0mm, and the plug hole of rubber particles should not be smaller than 11.5mm. In addition, secondary drilling is used to make non-metallized holes. No matter what method is adopted, there must be no copper skin within 0.2mm around the non-metallized hole. The design of positioning holes is often a problem that is easy to ignore. In 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 as much as possible to form a triangle and distribute the holes on the three corners of the circuit board.

3. Circuit fabrication mainly considers the influence of circuit etching
Due to the influence of lateral corrosion, the copper thickness and different processing techniques are considered during production and processing, and the circuit needs to be pre-thickened. The conventional compensation for HOZ copper of spray tin and immersion gold plate is 0.025mm, the conventional compensation for 1OZ copper thickness is 0.05-0.075mm, and the line width is 0.05-0.075mm. /Line spacing production and processing capacity is conventional 0.075/0.075mm. Therefore, when considering line width/line spacing wiring, it is necessary to consider the compensation problem during production. Since the gold-plated plate does not need to remove the gold-plated layer on the line after etching, the line width is not reduced, so no compensation is required. However, it should be noted that since the side etching still exists, 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, the gold surface will easily collapse, resulting in poor soldering. For lines with characteristic impedance requirements, the line width/line spacing requirements will be more stringent.

4. The more troublesome 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 a finished online test point after assembling components, and even a few are designed as component plug-in holes. Conventional vias are designed to cover oil in order to prevent tinning during soldering. If they are used as test points or plug-in holes, windows must be opened. However, it is very easy to cause tin beads to be hidden in the hole by spraying the tin plate via cover oil. Therefore, a considerable part of the products are designed to be plugged with oil. In order to facilitate the packaging of the BGA position, it is also treated according to the plugged oil. However, when the aperture is larger than 0.6mm, it will increase the difficulty of plugging the oil (the plug is not full). Therefore, the tin-spraying plate is also designed as a half-open window with a single side 0.065mm larger than the aperture, and the hole wall and hole edge are within the range of 0.065mm. Spray with tin.

5. The addition of pads and related marks on the characters is mainly considered in character processing.
As the component layout is getting denser and denser, and it is necessary to consider that the pads cannot be placed on the character printing, at least ensure that the distance between the characters and the pads is more than 0.15mm, and sometimes the component frame and component symbols cannot be completely distributed on the circuit board. Most of the film is completed by the machine, so if it is really impossible to adjust during the design, you can consider printing only the character frame, not the component symbols. Commonly added contents of the mark include supplier identification, UL demonstration identification, flame retardant grade, anti-static mark, production cycle, customer-specified identification and so on. It is necessary to clarify the meaning of each mark, and set aside and designate the additional location.

6. The influence of the surface coating (plating) layer of the PCB board on the design:
At present, the widely used conventional surface treatment methods include OSP gold plating, immersion gold and tin spraying. We can compare their advantages and disadvantages from the aspects of cost, weldability, wear resistance, oxidation resistance and production process, drilling and circuit modification. OSP process: low cost, good conductivity and flatness, but poor oxidation resistance, which is not conducive to preservation. The drilling compensation is usually made by 0.1mm, and the HOZ copper thickness line width is compensated by 0.025mm. Considering that it is easy to be oxidized and contaminated with dust, the OSP process should be completed after forming and cleaning. When the size of a single piece is less than 80MM, the form of continuous piece must be considered delivery. Nickel-gold electroplating process: good oxidation resistance and wear resistance, when used for plugs or contact points, the thickness of the gold layer is greater than or equal to 1.3um, and 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 to make a gold plate, the copper layer under the surface gold layer is very likely to cause excessive etching and collapse, resulting in solderability problems. Because gold plating requires current assistance, the gold plating process is designed before etching, and the complete surface treatment also plays the role of corrosion resistance. After etching, the process of removing corrosion resistance is reduced, which is also the reason why the line width is not compensated. Electroless nickel gold plating (immersion gold) process: good oxidation resistance, good toughness, flat coating is widely used in SMT boards, drilling compensation is made according to 0.15mm, HOZ copper thickness line width compensation is 0.025mm, because the immersion gold process is designed in After solder mask, corrosion resistance protection needs to be used before etching, and corrosion resistance needs to be removed after etching. Therefore, the line width compensation is more than that of gold-plated boards, so gold is immersed after solder mask. For boards with a large area of copper, the gold salt consumed by the immersion gold board is significantly lower than that of the gold-plated board. Spray tin plate (63 tin/37 lead) process: oxidation resistance, relatively toughness, poor flatness, drilling compensation is made by 0.15mm, HOZ copper thickness line width compensation is 0.025mm, the process is basically the same as immersion gold, At present, it is a common surface treatment method. Since the EU proposed the ROHS directive, refusing to use six harmful substances containing lead, mercury, cadmium, hexavalent chromium, polybrominated diphenyl ethers (PBDE) and polybrominated biphenyls (PBB), the surface treatment launched a spray of pure tin (tin copper ), spraying pure tin (tin-silver-copper), immersion silver and immersion tin and other new processes to replace the lead-tin spraying process.

7. Jigsaw puzzle, shape production is also a problem that is difficult to consider comprehensively when designing:
First of all, it is necessary to consider the convenience of processing. The time interval of the electric milling shape should be based on a milling cutter diameter (normally 1.6 1.2 1.0 0.8). The size of the punching groove should be larger than 0.8mm. If V-CUT is used for connection, the distance between the edge line and the copper sheet must be 0.3mm from the center of the V-CUT. Secondly, the utilization rate of the large material should be considered. Fixed, the commonly used sheet specifications are 930X1245, 1040X1245, 1090X1245 and other specifications, if the delivery unit is unreasonable, it is easy to cause PCB board material waste.