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Why Are Circuit Board Green?

Nowadays, the circuit board has been widely used in the field of modern information technology and modern industrial production, which can make the circuit miniaturized and intuitive, and plays an important role in the batch production of fixed circuit and the optimization of electrical layout. According to different design principles, circuit boards can be divided into single panel, multi-layer board, flexible board, rigid board and other types. However, have you found that most of the circuit boards on the market are green, other colors are very rare, why is this?


l.What is the use of resistance film?

As surface mount technology (SMT) became the easiest way to mass-produce components, solder resistant layers became critical.

Although hand-welded components do not require a solder layer, a solder layer is essential for mass-produced PCBS. Here we will help you distinguish solder resistance layers and introduce you to the latest industry colors and trends.

The solder barrier is a thin layer of polymer that protects the PCB from oxidation and prevents bridge formation. The welded bridge is an unexpected electrical connection that can cause problems if used alone.


II.Type of PCB solder resistance layer

There are four common types of solder resistant layers. They usually do the same thing, but some materials can cost more. The three main materials used to make a solder barrier are thermally cured to solidify the structure and form a protective barrier.

1. Top and bottom masks

The thermal pad is located between the copper layers of the PCB to manage the heat generated by the copper layers of the board. Pins are needed to connect these planes, and the top and bottom side masks help make it easier to determine where these pins need to be welded. 

2. Epoxy liquid flux resistance

Epoxy is the lowest-cost solder blocking agent and can be printed to printed circuit boards by patterned screen. Despite its name, silk is rarely used in electronics today, but the process remains the same. This printing technique uses woven mesh fibers that produce patterns in open areas to allow ink to pass through.

3. Liquid photoimaging flux resistance

Liquid photoimagable solder resist (LPISM or LPSM or LPI) is made of alkaline electronic materials. LPI is delivered in ink form and can be sprayed or screen-printed onto the circuit board.

4. Dry film can photoimage solder resistance

Dry film photoconductive flux (DFSM) undergoes a photolithography (UV exposure) process similar to LPI. After that, it develops and creates openings in the PCB pattern, then welds the parts onto the copper pad.


III.The importance of solder resistance on PCB

The key function of solder resistance layer is to prevent oxidation and corrosion of copper PCB.

It is also important to use a solder barrier to prevent bridging. A weld bridge is an accidental connection of solder joints on a PCB that can cause damage and short circuiting to the board. The solder resist layer forms a dam between the solder spot of the PCB and other conductive parts to create additional insulation for components on the PCB.

Similar to Bridges, metal whiskers can form on circuit boards, leading to failures and short circuits. Solder resist prevents the growth of metal whiskers, which are tiny wires protruding from the PCB, and can cause system failures. Metal or tin whiskers are most commonly found in lead-free or tin coatings.

In addition to these most critical uses, welded Bridges have many other benefits.

1. Less solder paste is needed during the welding process

2. It may increase the breakdown voltage of PCB dielectric material

3. Prevent contaminants that you may introduce when handling PCBs


IV.PCB solder film color and surface treatment

The circuit board itself is brown, and the green we see is the solder mask layer. Generally speaking, a complete circuit board is composed of pads, holes, solder layer, screen printing layer, copper wire and other parts. The top and bottom layers with color on the surface of the circuit board is made of solder mask coated solder layer. Solder layer color mainly depends on the color of solder mask. In fact, the market solder mask is not limited to a green. We can use red, blue, purple, black, yellow and other colors, but green is the most common.


V.Why most circuit board are green?

As for why we prefer to use green solder mask, it is mainly from the following aspects.

1. It looks better in the yellow light room

In PCB processing, the production of electronic products includes board making and pasting, during which several processes need to pass through the yellow light room, and the green PCB board has the best visual effect in the yellow light room.

2. Convenient for optical positioning correction

In SMT assembly processing, the tin printing, assemble and AOI calibration steps, all need optical positioning calibration, green board instrument identification is better.

3. Good for workers to check the circuit boards

Part of the detection process depends on workers to observe (but now mostly use flying needle test instead of artificial). In the strong light of the non-stop staring board, green is good for the eyes.

4. Environment friendly

Green PCBs are also more environmentally friendly. When the waste board is recycled at high temperature, it will not release toxic gas. Other PCB colors, such as blue and black, are mixed with cobalt and carbon, respectively, which pose a short circuit risk due to their weak electrical conductivity.

5. The price is cheaper than other colors

Because we commonly used color is green, so the factory standby green ink is the most. More demand, more shipments, natural cut low cost, so the cost is relatively low with green.

6. Black, purple and blue PCB colors are too dark, which will increase the difficulty of inspection and maintenance of the motherboard, and the process is not easy to control.

7. Black PCB for example, black PCB in production is most prone to process and raw material ratio problems, a little careless, will cause obvious color difference, resulting in a high defect rate of PCB. Black PCB circuit is also difficult to identify, which makes it difficult for later maintenance and debugging. Many PCB factories do not use black PCB. Even in the field of military industry and industrial control, the products with high quality requirements are mainly based on green PCB. PCB board color, also does not affect its performance.

8. Green ink can do smaller error, smaller area, can do higher accuracy.

In summary, it is not difficult to understand why the PCB substrate color of the circuit board is mainly green.