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Electronic Design
What problems can't be taken care of in PCB design
Electronic Design
What problems can't be taken care of in PCB design

What problems can't be taken care of in PCB design

2021-11-06
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Author:Downs

1. PCB board drilling mainly considers the hole size tolerance, the pre-large drilling, the processing of the hole to the board edge, 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. The spray 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, 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 issue: At present, most of the domestic drilling rigs have a tolerance of ±0.05mm, plus the tolerance of the plating thickness in the hole, the tolerance of metalized holes is controlled within ±0.075mm, and the tolerance of non-metalized holes is controlled within ±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, 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.

This article mainly explains and analyzes the problems in PCB design that cannot be ignored

pcb board

2. The more troublesome part of solder mask production is the solder mask treatment method on the vias:

In addition to the conductive function of the via, many PCB 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 spray plate is very easy to cause the tin beads to be embedded in the hole, so a considerable part of the product is designed as a through hole plug oil, and the position of the BGA is also treated as the plug oil for the convenience of packaging. 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.

3. Line production mainly considers the influence caused by 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. The conventional compensation of HOZ copper for spraying tin and gold plate is 0.025mm, and the conventional compensation for 1OZ copper thickness is 0.05-0.075mm, and the line width is /Line spacing production and processing capacity is conventionally 0.075/0.075mm. Therefore, in the design, when considering 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.

4. The influence of PCB surface coating (plating) layer on design:

At present, the most widely used conventional surface treatment methods are OSP, gold-plated, immersed gold, and tin-sprayed. We can compare its advantages in terms of cost, solderability, wear resistance, oxidation resistance, production process, drilling and circuit modification, etc. shortcoming.

OSP process: low cost, good conductivity and flatness, but poor oxidation resistance, which is not conducive to storage. The drilling compensation is conventionally made according to 0.1mm, and the HOZ copper thickness line width 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 piece is less than 80MM, it must be considered for delivery in the form of contiguous pieces.