Test Coupon: Commonly known as impedance strip






Test Coupon is used to measure the characteristic impedance of the PCB produced by TDR (Time Domain Reflectometer) to meet the design requirements. Generally, the impedance to be controlled is single-ended and differential, so test coupon The line width and line spacing (when there is a differential pair) are the same as the line to be controlled. The most important thing is the position of the ground point when measuring. In order to reduce the inductance of the ground lead, the TDR probe is usually grounded very close to the probe tip, so the distance and way of measuring the signal on the test coupon to the ground point Meet the specifications of the probe used.


▍ gold finger




The gold finger here does not of course refer to the Kato Eagle. The Gold Finger (or Edge Connector) is designed to be used for the oppressive contact with the connector between the connector and the conductive interconnection. Gold is chosen because of its superior conductivity and oxidation resistance. The gold bars in your computer's memory stick or graphics card version are golden fingers.


The question is coming, is the gold on the gold finger gold? Old wu thinks it should be gold, but not pure gold. Why? Should be the hardness of pure gold is not enough, we look at the costume drama, those in order to verify that Jin Yuanbao is not real gold, will use the big incisors to bite to see if there is any tooth print, the old wu do not know if this is God scriptwriter in the ghost Pull, but the golden finger has to deal with the regular insertion and removal, so compared to the "soft gold" of pure gold, the golden finger is usually electroplated "hard gold", here the hard gold is electroplated alloy (that is, Au and other Metal alloy), so the hardness will be harder.




硬 hard gold, soft gold


Hard Gold: Hard Gold; Soft Gold soft Gold


Electroplated soft gold is electroplated to deposit nickel gold on the circuit board, and its thickness control is more flexible. Generally, it is used for the aluminum wire on the COB (Chip On Board), or the contact surface of the mobile phone button. The gold plating gold used for the gold finger or other adapter card and the memory is mostly hard gold because it must be wear-resistant.


Want to understand the origin of hard gold and soft gold, it is best to first understand the process of gold plating. Regardless of the previous pickling process, the purpose of electroplating is basically to plate "gold" on the copper of the circuit board. However, if "gold" and "copper" are in direct contact, there will be a physical reaction of electron migration and diffusion (potential). Poor relationship), so you must first plate a layer of "nickel" as a barrier layer, and then plate gold onto the top of nickel. Therefore, the so-called electroplated gold, the actual name should be called "electroplated nickel gold."


The difference between hard gold and soft gold is the composition of this layer of gold that is finally plated. When gold plating, you can choose to plate pure gold or alloy. Because the hardness of pure gold is soft, it is called "soft gold." "." Because "gold" can form a good alloy with "aluminum", COB will require the thickness of this layer of pure gold when it is used to make aluminum wire.


In addition, if you choose electroplated gold-nickel alloy or gold-cobalt alloy, because the alloy will be harder than pure gold, it is called "hard gold".


▍ Through Hole: Plating Through Hole Abbreviation PTH






Copper foil lines between conductive patterns in different layers of the board are conductive or connected with such holes, but they are not capable of inserting copper-plated holes for component legs or other reinforcing materials. Printed circuit boards (PCBs) are formed by a stack of many copper foil layers. 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 be connected by vias for signal connection, so there is a Chinese via. The title.


The through hole is also the simplest type of hole, because it is only necessary to use the drill bit or the laser to directly drill the circuit board, and the cost is relatively cheap. However, in some cases, some circuit layers do not need to connect these through holes, but the through holes are full-plate through, which will result in waste, especially for the design of high-density HDI boards, the circuit board is inch-inch. Therefore, although the through hole is cheap, sometimes it will use more PCB space.


盲 Blind hole: Blind Via Hole (BVH)


The outermost circuit of the PCB is connected to the adjacent inner layer by a plated hole, and since it is not visible, it is called a "blind hole". In order to increase the space utilization of the PCB circuit layer, a "blind hole" process has emerged.


The blind holes are located on the top and bottom surfaces of the board and have a certain depth for the connection of the surface lines to the underlying inner lines. The depth of the holes generally has a prescribed ratio (aperture). This type of production requires special attention, and the drilling depth must be just right. If you do not pay attention, it will cause plating difficulties in the hole. Therefore, few factories use this method of production. In fact, it is possible to drill holes in the circuit layers that need to be connected in advance, and finally glue them together, but it requires a more precise positioning and alignment device.


埋 Buried hole: Buried Via Hole (BVH)


A buried via is a connection between any circuit layers inside a printed circuit board (PCB), but is not conductive to the outer layer, that is, there is no via extending to the surface of the board.


This production process cannot be achieved by drilling the board after drilling, and drilling must be performed at the time of the individual circuit layers. The inner layer is partially bonded and then plated, and finally all bonded. Since the operation process is more laborious than the original via holes and blind holes, the price is also the most expensive. This fabrication process is typically only used for high-density boards, increasing the space utilization of other circuit layers.


After we have drawn the PCB, we will send it to the PCB board factory for proofing or mass production. When we place an order for the board factory, we will attach a PCB processing technology description document, one of which is to indicate which one to use. PCB surface treatment process, and different PCB surface treatment process, will have a relatively large impact on the final PCB processing price, different PCB surface treatment process will have different charges, below we have some science terminology on PCB surface treatment process .


▍ Why do you need special treatment on the PCB surface?


Because copper is easily oxidized in the air, the copper oxide layer has a great influence on the soldering, and it is easy to form false soldering and soldering. In severe cases, the pads and components cannot be soldered. Therefore, the PCB is manufactured. When there is a process, a layer of material is applied (plated) on the surface of the pad to protect the pad from oxidation.
At present, the PCB processing techniques of domestic board factories include: spray (HASL, hot air solder leveling), sinking tin, immersion silver, OSP (anti-oxidation), chemical immersion gold (ENIG), electroplating gold, etc. Of course, there are some special PCB surface treatments for special applications.


Compared with different PCB surface treatment processes, their cost is different, of course, the occasions used are different. Only the right ones are not expensive. At present, there is no perfect PCB surface treatment process suitable for all application scenarios (here is the price/performance ratio). That is, we can satisfy all PCB application scenarios at the lowest price, so there are so many processes to choose from. Of course, each process has its own merits. The existence is reasonable. The key is to know them. Good for them.


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


Bare copper plate






Advantages: low cost, smooth surface and good weldability (without oxidation).
Disadvantages: It is easy to be affected by acid and humidity. It can't be put out for a long time. It needs to be used within 2 hours after unpacking, because copper is easily oxidized when exposed to air; it can't be used in double panel because it is the second side after the first reflow. It has already oxidized. If there is a test point, solder paste must be applied to prevent oxidation, otherwise the subsequent contact with the probe will not be good.


Tin plate (HASL, Hot Air Solder Levelling)






Advantages: low price and good welding performance.
Disadvantages: It is not suitable for soldering pins with small gaps and too small components because the surface of the tin plate is poor. Solder bead is easy to be produced 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 been subjected to a high-temperature reflow soldering, it is very prone to re-melting the tin to produce tin balls or ball-like tin spots similar to water droplets that are dripped by gravity, resulting in a surface that is less Leveling and thus affecting the welding problem.


The tin-spraying process once dominated the PCB surface treatment process. In the 1980s, more than three-quarters of PCBs used tin-spraying, but the industry has been reducing the use of tin-spraying processes for the past decade. The tin-spraying process is dirty, unpleasant, and dangerous, and has never been a favorite process, but the tin-spraying process is an excellent process for larger components and larger pitch wires. In a PCB with a higher density, the flatness of the tin-spraying process will affect subsequent assembly; therefore, the HDI board generally does not use a tin-spraying process. With the advancement of technology, the industry has now developed a tin-spraying process suitable for assembling QFPs and BGAs with smaller pitches, but the practical application is less. At present, some factories adopt the OSP process and the immersion gold process to replace the tin-spraying process; the technical development also makes some factories adopt the process of sinking tin and sinking silver. Coupled with the trend of lead-free in recent years, the use of tin-spraying technology is further limited. Although so-called lead-free soldering has appeared, this may involve compatibility issues with the device.


OSP (Organic Soldering Preservative)








Advantages: With all the advantages of bare copper plate soldering, expired (three months) boards can also be resurfaced, but usually limited to one time.
Disadvantages: susceptible to 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 poor. If the storage time exceeds three months, it must be resurfaced. After opening the package, it needs to be used within 24 hours. The OSP is an insulating layer, so the solder paste must be applied to the test point to remove the original OSP layer in order to contact the pin point for electrical testing.


The OSP process can be used on low-tech PCBs or on high-tech PCBs, such as single-sided TV PCBs and high-density chip packaging boards. For the BGA, there are more OSP applications. The OSP process is the ideal surface treatment process if the PCB has no surface connection functional requirements or storage period limitations. However, OSP is not suitable for use on a small number of products, nor is it suitable for products with inaccurate demand estimates. If the inventory of the company's internal circuit board often exceeds six months, it is really not recommended to use OSP surface treated boards.


Shen Jin (ENIG, Electroless Nickel Immersion Gold)






Advantages: It is not easy to oxidize, can be stored for a long time, and the surface is flat. It is suitable for soldering fine gap pins and components with small solder joints. The first choice for button PCB board. Reflow soldering can be repeated multiple times and it is less likely to reduce solderability. Can be used as a substrate for COB (Chip On Board) wire bonding.
Disadvantages: high cost, poor soldering strength, because the use of electroless nickel process, easy to have black disk problems. The nickel layer oxidizes over time and long-term reliability is a problem.


The immersion gold process is different from the OSP process in that it is mainly used on boards with connection functional requirements and long storage periods, such as the button contact area, the edge connection area of the router housing, and the electrical connection of the chip processor. Contact area. Due to the flatness of the tin-spraying process and the removal of the flux in the OSP process, the immersion gold was widely used in the 1990s; later, due to the appearance of black disks and brittle nickel-phosphorus alloys, the application of the immersion process was reduced. However, almost every high-tech PCB factory currently has an inverted gold line. In view of the fact that the solder joints become brittle when the copper-tin intermetallic compound is removed, many problems occur in the relatively brittle nickel-tin intermetallic compound. Therefore, almost all portable electronic products (such as mobile phones) use copper-tin intermetallic compound solder joints formed by OSP, immersion silver or immersion tin, and use immersion gold to form a button area, a contact area and a shielding area of EMI, so-called selectivity. Shen Jin craft.


ENIG, Electroless Nickel Immersion Gold




Shenyin is cheaper than Shenjin. If the PCB has connectivity requirements and needs to reduce costs, Shenyin is a good choice; plus the good flatness and contact of the sinking silver, it should choose the silver process. There are a lot of applications in the field of communication products, automobiles, and computer peripherals. In the high-speed signal design, Shenyin has also been applied. Since immersion silver has good electrical properties unmatched by other surface treatments, it can also be used in high frequency signals. EMS recommends the use of the immersion silver process because it is easy to assemble and has good checkability. However, due to defects such as tarnishing and solder joint voids, the growth of the silver is slow (but not degraded).


ENIG, Electroless Nickel Immersion Gold






It has been nearly a decade since Shen Xi was introduced into the surface treatment process, and the emergence of this process is the result of the requirements of production automation. Shen Tin does not bring in any new elements in the weld, especially suitable for communication backplanes. Tin will lose solderability beyond the shelf life of the board, and sink tin requires better storage conditions. In addition, the tin-pressing process is restricted in use because it contains carcinogens.


Old wu often finds that the little friends will be confused about the gold and gold plating process. The difference between the underlying gold process and the gold plating process and the applicable scene are compared below.








The crystal structure formed by gold and gold plating is different. The gold plate is easier to weld than the gold plate, and it will not cause poor welding;
The gold plate only has nickel gold on the pad, and the signal transmission in the skin effect is not affected by the signal in the copper layer;
Shen Jin is denser than gold-plated crystal structure and is not easy to oxidize;
The immersion gold plate only has nickel gold on the pad, which will not cause the gold wire to be slightly short;
The immersion gold plate only has nickel gold on the pad, and the solder resist on the line is more firmly bonded to the copper layer;
Shen Jinxian is golden yellow, which is better than gold-plated and yellower;
Immersion gold is softer than gold plating, so it is not as good as gold plating in wear resistance, and gold plating effect is better for gold finger boards.


Attachment: PCB terminology and English comparison


PCB professional English translation term
I. Comprehensive vocabulary
1, printed circuit: printed circuit
2, printed circuit: printed wiring
3, printed board: printed board
4, printed circuit board circuit: printed circuit board (PCB)