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PCB Tech

PCB Tech

PCB Tech

PCB Tech

These words of PCB design and manufacturing industry!
2021-10-07
View:22
Author:Downs

It is used to use TDR to measure whether the characteristic impedance of the produced PCB meets the design requirements. Generally, the impedance to be controlled includes single-ended and differential pairs. Therefore, the trace width and line spacing on the test coupon (with differential Time-correction) should be the same as the wire to be controlled, and the most important thing is the location of the grounding point during measurement.

Golden finger

The gold finger is used to press and contact the connection between the connector shrapnel and conduct conductive interconnection. The reason why gold is selected is because of its superior conductivity and oxidation resistance. The row of memory sticks or graphics card versions in your computer is brilliant The thing is gold finger.

Hard gold, soft gold

Electroplating of soft gold is to deposit nickel and gold on the circuit board by electroplating, and its thickness control is more flexible. Generally, it is used for aluminum wire on COB (Chip On Board), or the contact surface of mobile phone keys, while gold fingers or other adapter cards, and most of the electroplated gold used for memory is hard gold, because it must be wear-resistant.

The difference between hard gold and soft gold is the composition of the last layer of gold that is plated on. You can choose to electroplate pure gold or alloy when plating. Because pure gold is softer, it is also called "soft gold." Because "gold" can form a good alloy with "aluminum", COB will particularly require the thickness of this layer of pure gold when making aluminum wires.

The copper foil layers cannot communicate with each other because each layer of copper foil is covered with an insulating layer, so they need to rely on vias for signal linking, so there is a Chinese via Title.

The through hole is also the simplest kind of hole, because when making it, you only need to use a drill or a laser to directly drill the circuit board, and the cost is relatively cheap.

Blind Hole: Blind Via Hole (BVH)

The outermost circuit of the PCB is connected to the adjacent inner layer with a plated hole. Because the opposite side cannot be seen, it is called a "blind hole". In order to increase the space utilization of the PCB circuit layer, the "blind via" process was developed.

pcb board

Blind holes are located on the top and bottom surfaces of the circuit board and have a certain depth. They are used to connect the surface line with the underlying inner line. The depth of the hole generally has a specified ratio (aperture).

Buried via: Buried Via Hole (BVH)

Buried vias are the connections between any circuit layers inside the printed circuit board (PCB), but they are not connected to the outer layer, that is, they do not have the meaning of via holes extending to the surface of the circuit board.

Let's compare the advantages and disadvantages and applicable scenarios of different PCB surface treatment processes.

Bare copper

Advantages: low cost, smooth surface, good weldability (without being oxidized).

Disadvantages: It is easy to be affected by acid and humidity, and cannot be stored for a long time. It should be used up within 2 hours after unpacking, because copper is easily oxidized when exposed to the air; it cannot be used for double-sided boards because the second side after the first reflow soldering It's already oxidized. If there is a test point, solder paste must be printed to prevent oxidation, otherwise it will not be in good contact with the probe.

Spray tin plate (HASL, Hot Air Solder Levelling, hot air levelling)

Advantages: lower price and good welding performance.

Disadvantages: Not suitable for welding pins with fine gaps and components that are too small, because the surface flatness of the spray tin plate is poor. Solder bead is easy to produce in PCB processing, and it is easy to cause short circuit to fine pitch components. When used in the double-sided SMT process, because the second side has undergone a high-temperature reflow soldering, it is very easy to spray tin and re-melt, resulting in tin beads or similar droplets that are affected by gravity into spherical tin dots, which make the surface even worse. Flattening affects welding problems.

The tin spraying process used to dominate the PCB surface treatment process. However, the tin spraying process is an excellent process for larger components and wires with larger spacing. In high-density PCBs, the flatness of the tin-spraying process will affect subsequent assembly; therefore, the tin-spraying process is generally not used for HDI boards. With the advancement of technology, the industry now has a tin spraying process suitable for assembling QFPs and BGAs with smaller pitches, but there are fewer practical applications.

OSP (Organic Soldering Preservative, anti-oxidation)

Advantages: It has all the advantages of bare copper welding. The expired (three months) board can also be resurfaced, but usually only once.

Disadvantages: easily affected by acid and humidity. When used in secondary reflow soldering, it needs to be completed within a certain period of time. Usually, the effect of the second reflow soldering will be relatively poor. If the storage time exceeds three months, it must be resurfaced. It must be used up within 24 hours after opening the package. OSP is an insulating layer, so the test point must be printed with solder paste to remove the original OSP layer to contact the pin point for electrical testing.

The OSP process can be used on low-tech PCBs as well as high-tech PCBs, such as PCBs for single-sided TVs and boards for high-density chip packaging. For BGA, OSP has more applications. If PCB does not have functional requirements for surface connection or limitation of storage period, OSP process will be the most ideal surface treatment process. However, OSP is not suitable for a small number of diverse products, nor for products with inaccurate demand estimates. If the company's circuit board inventory often exceeds six months, it is really not recommended to use OSP surface treatment boards.

Immersion Gold (ENIG, Electroless Nickel Immersion Gold)

Advantages: It is not easy to oxidize, can be stored for a long time, and the surface is flat, suitable for welding small gap pins and components with small solder joints. The first choice for the button PCB board. Reflow soldering can be repeated many times without reducing its solderability. It can be used as a substrate for COB (Chip On Board) wire bonding.

Disadvantages: high cost, poor welding strength, because the electroless nickel plating process is used, it is easy to have the problem of black disk. The nickel layer will oxidize over time, and long-term reliability is a problem.

Immersion gold process is different from OSP process. It is mainly used on boards that have functional requirements for connection and a longer storage period, such as button contact areas, edge connection areas of router housings, and the electrical properties of the elastic connection of the chip processor. Contact area. Due to the flatness problem of the spray tin process and the removal of the flux of the OSP process.

Immersion Gold (ENIG, Electroless Nickel Immersion Gold)

Immersion Silver is cheaper than Immersion Gold. If the PCB has connection functional requirements and needs to reduce costs, Immersion Silver is a good choice; coupled with Immersion Silver's good flatness and contact, it is better to choose Immersion Silver technology.

Shen Xi (ENIG, Electroless Nickel Immersion Gold)

Immersion gold is different from the crystal structure formed by gold plating. Immersion gold boards are easier to weld than gold-plated boards and will not cause poor welding;

The immersion gold board only has nickel and gold on the pad, and the signal transmission in the skin effect is on the copper layer without affecting the signal;

Immersion gold is denser than gold-plated crystal structure, and it is not easy to oxidize;

The immersion gold board only has nickel and gold on the pads, and will not produce gold wires that cause shortness;

The immersion gold board only has nickel and gold on the pad, and the solder mask and the copper layer on the circuit are more firmly combined;

Immersion gold is golden yellow, more yellow and better-looking than gold plating;

Immersion gold is softer than gold plating, so it is not as good as gold plating in terms of wear resistance. For gold finger plates, the effect of gold plating is better.