PCB Tech - What problems cannot be ignored in pcb design

With the development of the PCB industry, the better, the better, more and more engineering and technical personnel are joining PCB design and PCB manufacturing, but because PCB manufacturing involves a wider range of fields, and a considerable number of PCB design engineers (Layout personnel) Did not engage in or participated in the PCB manufacturing process, which led to the emphasis on electrical performance and product functions in the design process, but downstream PCB processing plants received orders, and there were many problems in the actual production process because the design did not consider It causes difficulty in product processing, prolonged processing cycle or hidden product hazards.

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 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.

When designing the PCB outer layer, pay attention to the thickness of the board. 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 0.05-0.1 MM, the tin plate will be 0.075-0.15MM thicker. 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.

The other is the PCB thickness tolerance. PCB designers should consider the product assembly tolerance while considering the PCB thickness tolerance after PCB processing. There are three main aspects that affect the tolerance of the finished product: the tolerance of the sheet material, the tolerance of the laminate and the tolerance of the outer layer thickening. . 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 problem of copper thickness on the PCB surface, 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, in the production of circuit boards, if the copper thickness requires 1OZ (minimum 30.9um) copper thickness, the cutting may sometimes select HOZ (minimum 15.4um) cutting according to the line width/line spacing, removing the allowable tolerance of 2-3um, the minimum It can reach 33.4um. If 1OZ cutting is selected, the minimum thickness of the finished copper will reach 47.9um. Other copper thickness calculations can be deduced by analogy.

2. PCB board drilling mainly considers the hole size tolerance, the pre-enlargement of the drilling, the processing problems 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.