PCB Tech - What does double-layer circuit board mean? What are the main points of double-layer circuit board design?

With the continuous development of the electronics industry, the early single-layer circuit boards can no longer meet the needs of most electronic products. At present, there are more and more double-layer circuit boards in use, but some friends still don’t know what a double-layer circuit is. Board and what points should be paid attention to in design, let's talk about it in detail below.

Double layer circuit board introduction
Double-layer circuit boards refer to copper on both sides and metalized holes, that is to say, there are copper on both sides, and there is copper in the holes. For both sides of the circuit board, the copper in the hole is particularly important, because the earliest The most difficult thing is that there is copper in the hole (how to have copper on the copper-free hole wall).This is the most important basis for distinguishing between double-sided and single-sided.

A circuit board has two sides, the top layer and the bottom layer. This is the double-layer circuit board. The double-layer circuit board is a double-sided copper-clad PCB board. The double-layer circuit board has copper-clad wires and traces on both sides, and the lines between the two layers can be connected through via holes to form the required network connection.

Double-sided circuit board material classification
General PCB board materials can be divided into two categories: rigid substrate materials and flexible substrate materials. Generally, the rigid substrate material is copper clad laminate, which is made of reinforced material (Reinforeing Material), impregnated with resin adhesive, dried, cut, and laminated to form a blank, then covered with copper foil, and steel plate is used as a mold. It is made by high temperature and high pressure forming process.

There are many ways to classify copper clad laminates. According to the different reinforcing materials of the board, it can be divided into five categories: paper base, glass fiber cloth base, composite base (CEM series), multi-layer laminate base and special material base (ceramic, metal core base, etc.).

If classified according to the different resin adhesives used in the board, common paper-based CCIs include phenolic resin (XPc, XxxPC, FR-1, FR-2, etc.), epoxy resin (FE-3), polyester resin, etc. Various types.

Common glass fiber cloth base CCL has epoxy resin (FR-4, FR-5), which is currently the most widely used type of glass fiber cloth base.

In addition, there are other special resins (with glass fiber cloth, polyamide fiber, non-woven fabric, etc. as additional materials): bismaleimide modified triazine resin (BT), polyimide resin (PI), Diphenylene ether resin (PPO), maleic anhydride imine-styrene resin (MS), polycyanate resin, polyolefin resin, etc.
Two-layer circuit board design points
1. There must be a reasonable direction

Such as input/output, AC/DC, strong/weak signal, high frequency/low frequency, high voltage/low voltage, etc. Their direction should be linear (or separated) and must not blend with each other. Its purpose is to prevent mutual interference. The best trend is in a straight line, but it is generally not easy to achieve. The most unfavorable trend is a circle. Fortunately, isolation can be set to improve. For DC, small signal, low voltage PCB design requirements can be lower. So "reasonable" is relative.

2, choose a good grounding point

A small grounding point, I don’t know how many engineers and technicians have talked about it, which shows its importance. Under normal circumstances, a common ground is required, such as: multiple ground wires of the forward amplifier should be merged and then connected to the main ground, and so on.

In reality, it is difficult to achieve this completely due to various restrictions, but we should try our best to follow it. This problem is quite flexible in practice. Everyone has their own set of solutions. It is easy to understand if they can explain it for a specific circuit board. You can also learn about the related article "How to Design the Ground Wire of the PCB Board".

3. Reasonably arrange power supply filter/decoupling capacitors

Generally, only a number of power filter/decoupling capacitors are drawn in the schematic diagram, but it is not pointed out where they should be connected. In fact, these capacitors are provided for switching devices (gate circuits) or other components that require filtering/decoupling. These capacitors should be placed as close to these components as possible, and too far away will have no effect. Interestingly, when the power supply filter/decoupling capacitors are arranged properly, the problem of the grounding point becomes less obvious.

4. There is a requirement for the line diameter to be the appropriate size of the buried through hole

If conditions permit, wide lines should never be made thin; high-voltage and high-frequency lines should be round and slippery, without sharp chamfers, and corners should not be at right angles. The ground wire should be as wide as possible, and it is best to use a large area of copper, which can greatly improve the problem of grounding points. The size of the pad or via is too small, or the size of the pad and the hole size are not properly matched.

The former is unfavorable for manual drilling, and the latter is unfavorable for CNC drilling. It is easy to drill the pad into a "c" shape, but to drill off the pad. The wire is too thin, and the large area of the unwiring area is not provided with copper, which is easy to cause uneven corrosion. That is, when the unwiring area is corroded, the thin wire is likely to be over corroded, or it may appear to be broken, or completely broken. Therefore, the role of setting copper is not only to increase the area of the ground wire and anti-interference.

5, the number of vias, solder joints and line density

Some problems are not easy to be found in the early stage of circuit production. They tend to emerge in the later stage. For example, if there are too many vias, a slight mistake in the copper sinking process will bury hidden dangers. Therefore, the design should minimize the wire hole. The density of parallel lines in the same direction is too large, and it is easy to join together when welding.

Therefore, the linear density should be determined according to the level of the welding process. The distance of the solder joints is too small, which is not conducive to manual welding, and the welding quality can only be solved by reducing the work efficiency. Otherwise, hidden dangers will remain. Therefore, the minimum distance of solder joints should be determined by comprehensive consideration of the quality and work efficiency of the welding personnel.