How to choose the parameters of PCB design

There are many parameters to be confirmed when PCB is hit. Size, quantity, plate material, thickness, solder mask color and screen printing color are all relatively easy to understand, but copper thickness, minimum line width/line spacing, minimum aperture, hole opening, hole covering oil, tin spraying and gold sinking are not so easy to understand.

Today, we will talk about these parameters in detail, so that we don't have to worry about choosing parameters in the future, and we don't have to chase engineers to ask questions.


Number of boards/board size
The number of boards is easy to understand, that is, how many boards you have to make. The size of the board is the size of the board, but the PCB board generally does not make the size very large. If necessary, it is often divided into several pieces and then spliced and combined after manufacturing. Of course, PCB boards are generally not made very small. If they are too small, there will be some troubles in the production, testing and use of the boards, which will lead to rising costs.

Layer number of board
Generally, the number of layers will be determined during PCB circuit design, so the corresponding number of layers can be directly selected according to the design requirements. Generally speaking, the higher the number of layers, the more difficult the process and the higher the cost. According to the number of layers, PCB can be divided into single panel, double-faced board and multilayer board.

A single panel is the most basic PCB, where all parts are placed on the same side, while the wires are concentrated on the other side, which is suitable for simple circuits.

Double-panel is widely used, its top and bottom are coated with copper, which can be wired and welded, with an insulating layer in the middle, and the circuits on both sides are connected through vias, which is suitable for general complex circuits.

The biggest difference between multi-layer board and single panel or double panel is that multi-layer board adds internal power layer and grounding layer, but the wiring of multi-layer board is mainly based on top layer and bottom layer, supplemented by intermediate wiring layer. Generally, the number of layers of multilayer boards is even (even layers are more advantageous than odd layers), but the number of layers cannot directly indicate how many wiring layers there are, because sometimes empty layers are added to control the board thickness. The wiring space of the multilayer board is larger, and the EMC (electromagnetic anti-interference) ability is stronger under the condition of reasonable circuit design.

Color of solder resist
Green, blue, black, red, yellow and other colors are commonly used. If there is no special requirement, use green. If it is for your own experiment, it is best to use green, because under normal circumstances, green oil has the best performance.

Screen printing color
White and black are commonly used colors. If the solder resist is green oil, white characters are generally selected. This is mainly because you can see clearly. You can choose the color contrast of the heel plate. If it is white solder resist, you should choose black characters.

Material  type
Material is one of the important factors affecting the quality of PCB. Commonly used is FR-4 (epoxy resin-glass fiber board), which has the advantages of high strength, heat/fire resistance (no combustion), chemical resistance (no corrosion and mildew), moisture resistance, thermal property (low expansion coefficient and high thermal conductivity coefficient), electrical property (no electricity and insulation), so FR-4 has been widely used in the industry.

Material brand
Domestic common plate brands are Shengyi, Jiantao, Jinan Guoji, South Asia, Rogers and so on.

Board thickness
0.8mm, 1.0mm, 1.2mm, 1.6mm, 2.0mm ... There are quite a few commonly used specifications, but if there is no special requirement, it is best to choose a thickness of 1.6 mm.

Copper thickness
0.5oz, 1.0oz, 2.0 oz ... There are actually many common specifications of copper foil, but if the power is not particularly large, you can choose 0.5oz, and if the power is around 100W, you can use 1.0oz. As for the specific selection, there is no unified standard in the industry at present.

Minimum line width/line spacing
Under normal circumstances, the smaller the difficulty, the higher the difficulty. At present, the upper limit of the manufacturing capacity of domestic manufacturers is mainly 3/3mil, and some excellent ones are around 2/2mil.

Minimum aperture
For through holes, it is generally considered in the industry that the minimum is 0.1mm (about 4mil), which is determined by the drilling machine, drill nozzle and plate; If it is a blind hole, the most advanced laser drilling rig can produce a minimum aperture of 25μm (about 1mil). In general, the smaller the hole, the more expensive it is. It is suggested to use 10/18mil and 12/20mil holes for through holes, and 16/24mil holes for power holes.

Solder resistance covering
Solder resist coating, that is, green oil coating, refers to the part of the circuit board that needs green oil, which is mainly used for insulation. There are three main types of solder mask: via window opening, via cover oil and via plug.

Opening a window through a hole means exposing copper. Refers to the printing solder mask, where there is a VIA, windows are opened to expose the copper foil (pad) at both ends of the via;
Through-hole cover oil means that the solder resist printing plate does not open the window, and the copper foil (pad) at both ends of the through-hole is covered with green oil and cannot be soldered;
Plug the via hole is to plug the via hole with something, so that there will be no holes on the surface. Generally, green oil plugs are used, and resin and metal plugs are also useful.
In fact, there is no requirement for aperture for solder mask coverage, but no matter which type is selected, the via hole can't be too large. Generally, the maximum size of the plug hole is 0.5mm, and it can't be plugged again. Moreover, if the hole is large, it will occupy a larger design space and affect the line width.

surface treatment
The copper solder joints on the circuit board are easily oxidized in the air, which leads to poor tin eating or poor contact and reduces the conductivity of the circuit board, so it is necessary to carry out surface treatment on the copper solder joints. Common surface treatment processes in PCB manufacturing include tin spraying, gold precipitation, gold electroplating and osp, and users can choose the most suitable one according to their own needs.

HASL, Tin spraying is a common processing method in the early proofing of printed circuit boards. It can be divided into lead spraying and lead-free spraying. After PCB is completed, the copper surface is completely wetted (tin is completely covered before soldering), which is suitable for lead-free soldering, mature technology and low cost, and suitable for visual inspection and electrical testing.

Gold deposition, that is, gold plating on PCB, can effectively block copper metal and air from oxidation. This gold plating process will produce chemical reaction, so it is also called gold melting. The deposited gold surface is very smooth, the deposited color is very stable, and it has strong environmental erosion resistance. Generally, the thickness of gold is 1-3 Uinch, which is suitable for SMT, electrical testing, switch contact design, aluminum wire bonding and thick plate.

Electroplating gold, that is, electrifying the PCB for gold plating, is similar to sinking gold, but the cost is higher than sinking gold, and its main feature is wear resistance, so electroplating gold is usually chosen for the position that needs to be plugged and unplugged.

Osp, that is, organic solderable protective film, is an organic film grown on the clean bare copper surface by chemical method. This film has oxidation resistance, thermal shock resistance and moisture resistance. It has smooth surface, simple process, convenient operation and maintenance, less pollution and low cost. osp can be said to be the lowest cost in PCB surface treatment process. But the disadvantages are also obvious. First, the protective film is extremely thin and easy to scratch; Second, the protective film will change color or crack after repeated high-temperature welding, which will affect the weldability and reliability; Third, it is not acid-resistant, and the high humidity environment will affect its weldability.

Of course, in addition to the above conventional parameters, the production of PCB will also involve some special processes, including gold finger, impedance, buried blind hole, thick copper plate, multi-layer special laminated structure, special-shaped hole, depth control groove, etc. You can make corresponding designs according to product requirements.

About Maxipcb
Maxipcb inspires and empowers innovators and helps them bring world-changing technologies to life. We provide advanced design, simulation and testing solutions to help engineers complete development and deployment faster in the whole product life cycle, while controlling risks. Our customers are all over the world, such as communication, industrial automation, aerospace and national defense, automobile, semiconductor and general electronics. We join hands with customers to accelerate innovation and create a safe and interconnected world.