An Introduction to 2 Layer PCB

What is a double sided PCB?

The 2 layer PCB ( double-sided PCB )is a printed circuit board with copper coated on both sides, top and bottom. There is an insulating layer in the middle, which is a commonly used printed circuit board. Both sides can be layout and soldered, which greatly reduces the difficulty of layout, so it is widely used.

To use circuits on both sides, there must be a proper circuit connection between the two sides, as shown in the pictures below. The “bridges” between such circuits are called vias. A via is a small hole on the PCB board filled or coated with metal, which can be connected with the circuits on both sides. Because the area of the double-sided board is twice as large as that of the single-sided board, the double-sided board solves the difficulty of the single-sided board due to the interlaced layout (it can be connected to the other side through the holes), and it is more suitable for more complicated circuits than the single-sided board.

We need electronic products with high performance, small size, and multiple functions, which promotes the development of printed circuit board manufacturing to be light, thin, short, and small. With limited space, more functions can be realized, layout density has become greater, and the hole diameter is smaller. The minimum hole diameter of mechanical drilling capacity has dropped from 0.4mm to 0.2mm or even smaller. The hole diameter of the PTH is getting smaller and smaller. The quality of the PTH (Plated Through Hole) on which the layer-to-layer interconnection depends is directly related to the reliability of the printed circuit board.

Double Layer PCB Production Manufacturing process

The production of double-sided board is more complicated than single-sided board. The main reasons are as follows:

(1) The top and bottom layers of the copper coated board/laminate must be layout

(2) The circuits on the top and bottom layers should be connected with PTH.

Particularly critical among these is the PTH, which is also the core process of double-sided board production. The so-called PTH is created by coating/plating a layer of metal on the inner wall of the via to connect the printed circuits of the top and bottom layers. At present, domestic PTH mainly adopts the electroless copper plating process in China. There are two types of electroless copper plating process:

① The thin copper is electrolessly plated first, then the whole board is electroplated to thicken the copper layer, and finally the pattern transfer is performed.

② The thick copper is electrolessly plated first, and then the pattern is transferred directly.

Both of these processes are widely adopted. However, the electroless copper plating method is harmful to the environment, and it will gradually be replaced by more advanced Black hole technology, tin/palladium direct plating technology, and polymer direct plating technology.

2 Layer PCB With HASL-LF /immersion gold surface production process

Cutting —> Drilling —> Sinking/1^ST Copper Plating —> Layout —> Pattern Plating/2^nd Copper Plating —> Etching —> Solder Mask —> Legend Printing —>Immersion Tin (or Immersion Gold) —> CNC Routing —> V Cut (some boards do not need this) —> Flying Probe Test —> Vacuum Packaging

Double-sided PCB with gold plating production process

Cutting —> Drilling —> 1^ST Copper Plating —> Layout —> Pattern Plating/2^nd Copper Plating —> Gold and Nickel Plating —> Etching —> Solder Mask —> Legend Printing —> CNC Routing —> V Cut —> Flying Probe Test —> Vacuum Packaging

Multilayer HASL-LF board/immersion gold board production process

Cutting —> Inner Layer —> Layer Stack —> Drilling —> Sinking/1^ST Copper Plating —> Layout —> 2^nd Copper Plating —> Etching —> Solder Mask —> Legend Printing —> Immersion Tin (or Immersion Gold) —> CNC Routing —> V Cut (some boards do not need this) —> Flying Probe Test —> Vacuum Packaging

Multilayer plate gold plate production process

Cutting —> Inner Layer —> Layer Stack —> Drilling —> Sinking/1^STCopper Plating —> Layout —> 2^ndCopper Plating —> Gold and Nickel Plating —> Etching —> Solder Mask —> Legend Printing —> CNC Routing —> V Cut —> Flying Probe Test —> Vacuum Packaging

1. Pattern plating process

Copper clad board/Laminate —> Cutting —> Punching and drilling benchmark —> CNC drilling —> Inspection —> Deburring —> Electroless thin copper plating —> Electroplating thin copper —> Inspection —> Brushing —> Filming —> Exposure and development (or curing) —> Inspection and repairing —> Pattern plating (Cn + Sn/Pb) —> Film removal —> Etching —> Inspection and repair board —> Nickel-plated and gold-plated plugs —> Hot melt cleaning —> Electrical continuity detection —> Cleaning —> Solder mask —> Curing —> Legend —> Curing —> Shape processing —> Washing and drying —> Inspection —> Packaging —> Finished product.

Thin copper —> Thin copper electroplating —> Electroplating thin copper can be replaced by a single process of electroless thick copper plating; both have their advantages and disadvantages.

The pattern plating —> etching method of double-sided board was typical in the 1960s and 1970s. The process of Solder Mask on Bare Copper (SMOBC) gradually developed in the 1980s, and has become the mainstream process especially in precision double-sided board manufacturing.

2. SMOBC process

The main advantage of SMOBC board is that it solves the short-circuit phenomenon of solder bridging between thin circuits. At the same time, due to the constant ratio of lead and tin, it has better solderability and storage properties than hot melt board.

There are many ways to manufacture SMOBC boards, including the SMOBC process of standard pattern electroplating subtraction and then lead-tin stripping; the subtractive pattern electroplating SMOBC process of using tin plating or immersion tin instead of electroplating lead-tin; the plugging or masking hole SMOBC process; additive method SMOBC technology; etc. The following mainly introduces the SMOBC process and the plugging method SMOBC process flow of the pattern electroplating method and then the lead-tin stripping.

The SMOBC process of pattern plating followed by lead-tin removal is similar to the pattern plating process, and changes only after etching.

Double-sided copper laminate —> According to the pattern electroplating process to the etching process —> Lead and tin removal —> Inspection —> Cleaning —> Solder mask —> Plug nickel plating and gold plating —> Plug sticking tape —> Hot air leveling —> Cleaning —> Legend —> Outline —> Cleaning and drying —> Finished product inspection —> Packaging —> Finished product.

3. Main process flow of the plugging method

Double-sided copper laminate —> Drilling —> Electroless copper plating —> Electroplating copper on the whole board —> Plugging holes —> Film (positive film) —> Etching —> Removing screen printing materials/Removing plugging material —> Cleaning —> Solder mask —> Plug nickel plating and gold plating —> Plug sticking tape —> Hot air leveling —> The following procedures are the same as above to the finished product.

The steps of this process are relatively simple, and the key is to plug the pores and clean the plugged solder mask.

In the hole plugging process, if the hole plugging solder mask is not used to block the holes and the screen printing imaging, but is replaced by a special masking dry film which is then exposed to make a positive image, this is the masking hole process. Compared with the hole plugging method, it fixes the problem of cleaning the solder mask in the hole, but it has higher requirements for masking the dry film.

The basis of the SMOBC process is to first produce the bare copper double layer board PTH, and then apply hot air leveling.

Pore mechanism

Drill holes on the copper-clad board first. It then undergoes electroless copper plating and electroplating copper to form plated through holes. Both play a crucial role in hole metallization.

1. The mechanism of electroless copper

In the manufacturing process of double-sided and multi-layer printed boards, it is necessary to metallize the non-conductive bare holes (NPTH), that is, to implement electroless copper to make them a conductor. The electroless copper precipitation solution is a catalytic “oxidation/reduction” reaction system. Under the catalytic action of metal particles such as Ag, Pb, Au, and Cu, copper is deposited.

2. The mechanism of copper electroplating

The definition of electroplating is to use a power source to push positively charged metal ions in a solution to form a coating on the surface of the cathode conductor. Copper electroplating is an “oxidation/reduction” reaction. The copper metal anode in the solution oxidizes the copper metal on its surface to become copper ions. On the other hand, a reduction reaction occurs on the cathode, and copper ions are deposited as copper metal. Both of them achieve the purpose of perforation through chemical exchange, and the exchange effect directly affects the quality of perforation.

Debris plug holes

In the long-term production control process, we found that when the hole diameter reaches 0.15-0.3mm, the proportion of plug holes increases by 30%.

1. The plugging problem in the process of perforation

When the printed board is processed, most small holes of 0.15-0.3mm still use the mechanical drilling process. In the long-term inspection, we found that impurities remained in the hole when drilling.

The following are the main reasons for drilling plug holes:

When a plug hole appears in a small hole, due to the small hole’s diameter, it is difficult to remove the impurities inside it by high-pressure water washing and chemical cleaning before copper plating, which prevents the exchange of the chemical solution in the hole during the electroless copper precipitation process and makes the electroless copper lose its effect.

When drilling holes, select suitable drill and backing plates according to the thickness of the laminate, keep the substrate clean, and do not reuse the backing plates. Effective dust collection (using an independent dust collection control system) is a factor that must be considered to solve the plugging hole.

Draw circuit diagram

1. There isa variety of dedicated PCB drawing software, such as Protel, etc., which can draw multilayer (including double-sided) circuit board diagrams. The positions of the layers are aligned, and there are vias to connect the layers.The circuit is connected to realize cross circuiting and facilitate typesetting. After the layout is completed, it can be handed over to a professional board factory to become a circuit board.

2. The double-sided circuit board should be drawn into the circuit schematic diagram in turn, which can be divided into two steps. Step 1: Draw the legends of the main components such as IC on the paper according to the position of the circuit board, arrange and draw the circuit of the pins and the peripheral components appropriately, and complete the sketch. Step 2: Analyze the principle and organize the circuit diagram according to the customary drawing method. You can also use the circuit schematic software to arrange the components and connect them, and then use its automatic typesetting function to organize.

The circuits on both sides of the board should be accurately aligned. You can use the tips of tweezers, the light transmission of a flashlight, and a multimeter to measure the connection and disconnection and determine the connection and direction of the solder and circuits. If necessary, remove the components to observe the layout.

What is the difference between single-sided PCB and 2 layer PCB ?

Single-sided and double-sided boards differ in the number of copper layers. Double-sided has copper on both sides of the board, which can be connected through vias. However, there is only one layer of copper on one-sided board, which can only be used for layout, and the holes made can only be used for SMT.

Single-sided board is the most basic PCB. The parts are concentrated on one side, and the circuits are concentrated on the other side. Because the circuits are only on one side, this kind of PCB is called single-sided. Single-sided boards have many strict restrictions on the design of the circuit (because there is only one side, the circuits cannot cross and must follow separate paths), so only early circuits use this type of board.

There is layout on both sides of a double-sided circuit board, but to use layout on both sides, there must be a proper circuit connection between the two sides. The “bridge” between such circuits is called a via. A via is a small hole filled or coated with metal on the PCB, which can be connected with the layout on both sides. Because the area of the double-sided board is twice as large as that of the single-sided board, it solves the difficulty of the single-sided board due to the staggered layout (it can be passed to the other side through vias), and is suitable for use in more complicated circuits than single-sided board.

1. Raw Material

Single-sided board: Has copper foil only on one side, such as TV board.

Double-sided board: There are copper traces on both sides, connected by conductive through holes. The price is generally 7 times different (not fully defined due to different materials). There is also a kind of false double-sided board in the industry, which has no through hole connection (much lower cost).

2. Process

Single-sided PCB board: The solder joints are concentrated on one side, and components are usually inserted on the other side. Some products still have SMD components on the copper-clad side. Double-sided board: Both sides can be layout, and both can have plug-in components or SMD components.

False double-sided PCB: Generally, only one side SMT and layout on the other side. The double-sided copper cladding is connected by circuit on both sides of the component foot.

3. PCB circuit board

Single-sided board: The metal circuit that provides the connection of the parts is arranged on an insulating substrate material, which is also a support carrier for installing the parts.

Double-sided PCB : When a single-sided circuit is not enough to provide the connection requirements of electronic parts, the circuit can be arranged on both sides of the substrate, and through hole circuits are deployed to connect the circuits on both sides of the board.

4. Production process

Single-sided board: CAD or CAM CCL cutting, drilling positioning —> Opening punching mold, making screen plate —> Printing conductive pattern, curing —> Etching, removing printing material, cleaning —> Printing solder mask pattern, curing —> Printing marking legends, curing —> Drilling and punching positioning holes, punching and blanking —> Circuit inspection, test —> Solder mask and OSP —> Inspection, packaging, finished product.

2 Layer PCB : AD and CAM CCL cutting/edging —> NC drilling —> PTH —> Pattern plating —> Full plate plating —> Dry film or wet film method masking or plugging holes —> (Negative pattern) (Positive pattern) —> Copper plating/tin lead pattern transfer —> Film removal, etching —> Tin and lead removal, plug plating removal, cleaning —> Printing solder mask/legends —> Hot air leveling or OSP —> Routing/punching shape —> Inspection/testing —> Packaging/finished products.

How do you make 2 layer PCB and precautions

At present, the mainstream circuit board assembly technology in the SMT industry is “full-board reflow soldering (reflow)”. Of course, there are other circuit board soldering methods, and this full-board reflow soldering can be divided into single-sided reflow soldering and double-sided reflow. Single-sided PCB reflow is rarely used now, because double-sided reflow can save space on the circuit board, which means that the PCB can be made smaller. For that reason, most of the boards seen on the market now belong to the double-sided reflow process.

Because the “double-sided PCB reflow process” requires two reflows, there will be some process restrictions. The most common problem is that when the board goes to the second reflow, the parts on one side will be falling due to gravity, especially when the board goes to the reflow zone at high temperatures.

Generally speaking, smaller parts are recommended to be placed on the first side to pass through the reflow oven, because the deformation of the PCB will be smaller on the first pass through the reflow oven, and the precision of solder paste printing will be higher, so it is more suitable to use smaller parts.

Secondly, the smaller parts will not fall off the second time through the reflow oven. Because the parts on the first side will be placed directly on the bottom side of the circuit board, when the board re-enters the reflow zone at high temperature, they are less likely to fall off the board due to excessive weight.

Third, the parts on the first side must go through the reflow oven twice, so their temperature resistance must be able to withstand the heat of the oven twice. The general resistance capacitor is usually required to be able to pass the high temperature at least three times. This meets the requirement that some boards may need to go through the reflow again for repair.

Which SMD parts should be placed on the second side through the reflow furnace? This should be the focus.

Large components or heavier components should be placed on the second side to pass through, to avoid the risk of parts falling into the reflow furnace.

LGA and BGA parts should be placed on the second side through as much as possible, so as to avoid unnecessary remelting risks during the second pass, and to reduce the chance of empty/false soldering. If there are smaller BGA parts, it is recommended to put them on the first side through the reflow furnace.

Placing the BGA on the first or second side through the furnace has always been controversial. Although placement on the second side can avoid the risk of remelting the tin and affecting its quality, the PCB will usually deform more seriously when the second side is passed through the reflow furnace. If the PCB is severely deformed, it can be a big problem for the delicate parts to be placed on the second side, because the solder paste printing position and the amount of solder paste will become inaccurate. Therefore, the focus should be to think of a way to avoid PCB distortion, instead of whether to put BGA on the first or second side.

Parts that cannot withstand overly high temperatures should be placed on the second side through the reflow furnace. This is to prevent parts from being damaged by high temperatures.

PIH/PIP parts should also be placed on the second side to pass through the furnace. Unless the length of the solder pin does not exceed the thickness of the board, the pin protruding from the PCB surface will interfere with the steel plate on the second side, so that the solder paste printed steel plate cannot be flatly attached to the PCB.

Some components may use soldering inside, such as a network cable connector with LED lights. It is necessary to pay attention to the temperature resistance of this part to pass the reflow oven twice. If it fails, it must be placed on the second side.

When parts are placed on the second side, it means that the circuit board has already been baptized by the high temperature of the reflow oven. At this time, the circuit board has become somewhat warped and deformed, which means that the tin quantity and printing position of the paste will become more difficult to control, so it is easy to cause problems such as empty soldering or short circuits. Therefore, it is recommended not to place 0201 and fine-pitch parts on the second side through the furnace. For BGA, try to choose a solder ball with a larger diameter.

In addition, in mass production, there are actually many process methods for soldering and assembling electronic parts on the circuit board, but each process is actually determined at the beginning of the circuit board design, because the placement of the parts will directly affect the soldering sequence and quality of the assembly, and the layout will be indirectly affected.

Soldering method of double-sided circuit board

In order to ensure the reliable conduction effect of the double-sided circuit board, the connection hole on the double-sided board (that is, the PTH) should be soldered with a wire, and the protruding part of the connection wire should be cut off to avoid stabbing the hand. This is in preparation for the wiring of the board.

The essentials of double-sided circuit board soldering

1. Devices that require reshaping should be processed according to the requirements of the process drawings:that is, reshaping first and then SMT.
2. The diode should face up after shaping, and there should be no discrepancyin the length of the two pins.
3. When inserting devices with polarity requirements, pay attention to ensure that their polarity is notreversed. Roll integrated block components after SMT. No matterwhether they are vertical or horizontal parts, there must be no obvious tilt.
4. The power of the iron used for soldering must bebetween 25~40W. The temperature of the soldering iron tip should be controlled at about 242℃. If the temperature is too high, the tip is close to useless,and the solder cannot be melted when the temperature is low. The soldering time is controlled at 3~4 seconds.
5. When soldering, generally follow the soldering principle of the device from short to high and from the inside to the outside. Thecorrectsoldering time must be mastered. If the time is too long, the device, as well as the circuits on the copper clad board, will be burnt.
6. Because the solderingis double-sided, a process frame for placing the circuit board should also be made, so as not to squeeze the components for the other side.
7. After the circuit board soldering is completed, a comprehensive check should be carried out to find any missing insertion orsoldering. After quality confirmation, trim the redundant device pins of the circuit board, and then go to the next process.
8. In anyspecific operation, the relevant process standards should be strictly followed to ensure the soldering quality of the product.

Re-soldering techniques for double-sided circuit boards

When re-soldering a double-sided circuit board, it is difficult to repair because it is dirty, messy, and may have faults such as false soldering, disconnection, and poor contact.

1. Observation: Based on drawings or prototypes, get a general understanding of the physical layout.
2. Dismantling: Remove the soldered components, pins andflying leads.
3. Cleaning: Use absolute alcohol to clean the rosin and soldering on the surface of the circuit board. When cleaning, if you use a soldering iron, it will be faster and the effect will be better.
4. Layout: Clarify the layout with reference to your observations. If there is no picture, use the drawing method to assist in annotation.
5. Soldering: Solder according to the clarifiedcircuits. When re-connecting circuit with circuit, try to arrange them on the back.
6. Inspection: According to the results of the drawing or prototype and the analysis of the layout, check whether the soldering is correct and reliable, and whether the process meets the requirements.
7. Power-on test.