Multilayer PCB design method (M)

The above is the general principles of the PCB layout and wiring, but in practice, the layout of the components and wiring is still a very flexible work, the layout and the connections of the components is not the only way, the result of the layout to a large extent depends on the designer's experience and thoughts. In a manner of speaking, there is no a standard to judge the layout and wiring scheme of right and wrong, and can only compare relatively better. So the above layout and wiring principle as a design reference only, the practice is the only standard of judgment.
11.2.3 The special requirements of multilayer PCB layout and wiring
Compared with the simple single plate and double board, multilayer PCB layout and wiring has its unique requirements. For multilayer PCB layout, sum up is to arrange the different power supply and the layout of the types of components. Its purpose is to bring electricity in the back of the segmentation is convenient, but also can effectively improve the anti-interference ability of between components.
The so-called reasonable arrangement of the power supply and to the layout of the type of components, is to use the same power level and the same type of components together as far as possible. For example, when a + on circuit principle diagram, + 5 V, 3.3 V? 5 v, + 15 v,? 15 v, voltage grade, the designer should use the same voltage grade of components placed centrally in the circuit board of a certain area.

Ms Grace Zhang
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Oversea sales
Wonderful PCB
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The characteristic and kinds of PCB surface finishing

As for human living environment asks is ceaseless rise, currently involved in the process of PCB board production environment problem is particularly prominent.At present about lead and bromine is one of the hottest;Utilizing and halide will influence the development of PCB in many ways.Although for now, the change of the PCB surface treatment technology and is not very big, still more distant things like, but should notice: long-term slow change will result in great changes.In the case of environmental protection calls for more and more high, the surface of the PCB process changes will happen in the future.
Surface treatment for the purpose of surface treatment of the most basic purpose is to guarantee good weldability or performance.Due to the nature of copper in air tend to exist in the form of oxide, is unlikely to keep to the original copper for a long time, so you need to other copper processing.While in the later assembly, strong flux can be used to remove most of the copper oxide, but strong flux itself is not easy to remove, so the industry generally don't use strong flux.
Common five kinds of surface treatment technology now has a lot of PCB surface treatment technology, common is hot air leveling（HASL), organic coating, electroless nickel/immersion gold(ENIG), immersion silver and tin the five kinds of craft.Because the PCB production  and material cost and process, so the different pcb design will use the different surface finishing. The cheap electronics  and simple products, we will use HASL or OSP, but for the expensive and high-density electronics products, we will use the immersion gold or silver, the pcb cost will higher, but it will better for components assembly.

Aluminum alloy and aluminum common surface treatment

Aluminum alloy and aluminum usually need to meet the demands of different surface treatment, common treatment over the surface of aluminum alloy electroplating, painting, drawing, anode, sandblast, passivation, polishing, oxidation film processing, etc.
1. Sandblasting, main effect is the surface cleaning, sand blast before coating (paint or coating) can increase the surface roughness, has certain contribution to improve adhesion, but the contribution is limited, as chemical processing before painting.

2. Color: there are two main types of coloring of aluminum process: one is aluminum oxidation coloring process, the other one is aluminum paint electrophoresis process. In the oxide film is formed on a variety of color, to meet certain requirements, such as optical instrument parts are commonly used in black, commemorative medallion is made with gold, etc.

3. Conductive oxide (chromate conversion coating) - for both protection and conductive occasions.

4. Chemical oxidation, with thin oxide film, thickness of about 0.5 ~ 4 microns, and porous, soft, has good adsorption, can be used as the bottom layer of organic coating, but its wear resistance and corrosion resistance than the anode oxidation membrane;

Aluminum pcb and aluminum alloy chemical oxidation process according to its nature of the solution can be divided into two types: alkaline oxidation and acid oxidation.
According to the layer properties can be divided into: oxide film, phosphate film, chromate film, chromic acid, phosphate film.

5. Electrochemical oxidation, chemical oxidation of aluminum and aluminum alloy processing equipment is simple, convenient operation, high efficiency, no electricity, the applicable scope is wide, without being limited by the size and shape of the parts. Oxide film thickness of about 5 ~ 20 microns (hard anodic oxidation film thickness up to 60 ~ 200 microns), has high hardness, good heat resistance and insulation, corrosion resistant ability higher than that of chemical oxidation film, porous, has good adsorption capacity.

6. Spraying: used for equipment of external protection, decoration, usually on the basis of the oxidation. Aluminum should carry on the pretreatment before coating to coating and artifacts in combination with strong, generally there are three measures.
1. Phosphate method (phosphate) 2.Chromium (without chrome) 3.Chemical oxidation.

Immersion tin

For nowadays all the solder material is tin-based, the tin layer can be matched with any type of solder, In this point, immersion tin process is very promising. However years ago, PCB boards will exits tin whiskers after immersion tin process and tin migration and tin whiskers shall bring reliability problems in the welding which limited the use of immersion tin process. Later we found the solution that adding tin organic additives to the immersion tin liquid which makes tin layer to become granular structure, well solved the problems, and also make better thermal stability and weldability.
Immersion tin process can form the flat copper-tin intermetallic compounds, this feature makes the immersion tin has the same good weldability as hot air leveling but without the headaches flatness problem; Also do not have proliferation issues between gold metal compared to Electroless Nickel / Immersion;
But the immersion tin plate can not be stored for a long time.

Through Hole and SMT

Through Hole Mounting: This mounting technique involves different components which have lead wires that are led to the board through holes, hence the name. In this method, leads rely on holes in a Multilayer pcb. The leads are then finally soldered to offer permanent mounting. The technology finds usage in DIP aka Dual Inline Packaging and Pin Grid Array Package.

Surface Mounting: This is a relatively new mounting method worked out seeing a tremendous increase in the number of pins in Integrated Circuits. As demand for smaller weight and size of packaged weight increased, there was felt a need for a new, improved technology that came to be known as surface mount technology (SMT). In this leads are soldered on PCB surface directly rather than using hole mounting. The technology is used in Flatpack Packages and Chip Carrier Packages.

There are some points that make SMT different from Through Hole, which have been briefly summarized below:
1.SMT has helped significantly in solving the space problems that were commonly noticed with the Through Hole mounting.
2.The pin count has increased greatly in SMT when compared to its older counterparts.
3.In SMT, the components are leadless and are directly mounted to the board surface. In Through Hole the components have lead wires that are taken to the wiring boards via
holes.
4.The Pads on the surface in SMT are not used for connection of layers on the printed wiring boards.
5.The components in the Though Hole Technology are larger which leads to lower component density per unit area. The packing density that can be achieved with SMT is very high as this allows mounting component on both sides, when needed.
6.SMT has made possible applications that seemed impossible with through hole.
7.SMT is suitable for high volume production that gives access to lowered cost of unit assemblies which is not possible with through hole technology.
8.With SMT acquiring higher circuit speed is easier due to the reduced size. SMT fulfils one of the prime marketing requirements while assisting in making high performance circuits in a very small size.
9.SMT has a disadvantage as the capital investment involved in its machinery and production is higher.
10.Designing, production, skill and technology required in implementing SMT is very advanced when compared to through hole technology.

When comparing both, SMT surpasses Through Hole in advantages, which clearly define and justify the majority (90%) use of this technology in board assemblies. Through Hole however is anticipated to remain in use in testing and prototype applications that may need manual adjustments and replacements. Its complete disappearance from the scene seems to be ruled out.