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PCB Tech

The difference between PCB solder mask and flux layer and design tutorial
2021-10-08
View:23
Author:Downs

What is the difference between PCB soldering layer and solder mask?

1. Solder mask:

Solder mask: refers to the part of the board to be painted with green oil, because it is a negative output, so the actual effect of the part with solder mask is not painted with green oil, but tinned and silver-white. (That is to say, where there is a solder mask, it will not be painted with green oil but tinned)

2. Flux layer:

Paste mask: It is used for machine patching. It corresponds to the pads of all patch components. The size is the same as the toplayer/bottomlayer layer. It is used to open the stencil to leak tin.

Key points: Both layers are used for soldering. It does not mean that one is soldered and the other is green oil; then whether there is a layer refers to the green oil layer, as long as there is this layer on a certain area, it means this Is the area insulated with green oil? For the time being, I haven't encountered such a layer! The PCB board we drew has a solder layer on the pads by default, so the pads on the PCB board are made of silver-white solder. It is not surprising that there is no green oil; but the PCB we drew The wiring part on the board only has the toplayer or bottomlayer layer, and there is no solder layer, but the wiring part on the finished PCB board is coated with a layer of green oil.

pcb board

It can be understood as follows: 1. The solder mask layer means to open a window on the whole piece of solder mask green oil, the purpose is to allow soldering!

2. By default, the area without solder mask must be painted with green oil!

3. The paste mask layer is used for patch packaging! SMT package uses: toplayer layer, topsolder layer, toppaste layer, and toplayer and toppaste are the same size, topsolder is a circle larger than them.

What is solder mask? 

Solder mask, also known as solder mask or solder mask/coating, is a thin layer covering copper traces, eliminating the need for soldering on the printed circuit board (PCB) on the top and bottom sides to help ensure that the PCB is reliable Sex and high performance. Resin is usually selected as the main material of the solder mask because it is excellent in moisture resistance, insulation, solder resistance, high temperature resistance, and aesthetics.  

It is believed that most PCBs are considered green, but are actually the color of solder mask. However, the solder mask can be displayed in different colors, including green, white, blue, black, red, yellow, etc. Apply different colors according to different needs. For example, in the NPI stage (to make them different from mass-produced boards), some RDs tend to pick up the red solder mask for prototypes in the NPI stage. The black solder mask was chosen only to be compatible with the color of the final product shell when these boards need to be partially or fully exposed.  

Even both sides of the same board may contain solder masks of different colors. Take Arduino Uno boarding the enemy as an example:

Function of solder mask 

Due to the market's demand for volume and efficiency, solder masks are becoming more and more popular and important for circuit boards, because the density of sheet rockets and SMT (Surface Mount Technology) have begun to become the leading choices.  

As the name suggests, the solder mask is designed to prevent solder bridges from occurring in the covered area. Reflow soldering plays a key role in SMT assembly because it allows electronic components to be completely and accurately mounted on the circuit board through solder paste. If a solder mask is not used, the copper traces will often connect with the solder paste, which may cause a short circuit. Therefore, the reliability and performance of the assembled PCB will be included.  

In addition to the main responsibility, the solder mask also prevents oxidation, corrosion and fouling of copper traces.

Solder mask manufacturing process 

Some people think that making solder mask is not a cutting-edge technology, and many engineers can do it at home. To realize that this is a complete myth, it is never too late. Soldering mask DIY is only suitable for circuit boards with simple designs. Unless they are formally applied in the final project, it is a bit difficult to ensure the reliability of the product.  

For professional PCB manufacturers, solder mask manufacturing has never been easier. On the one hand, it must comply with strict regulations such as ISO9001, UL or RoHS. On the other hand, solder mask manufacturing consists of several stages, and each stage requires highly mature technology, rich manufacturing experience and the latest equipment.  

The normal procedure of solder mask manufacturing  continues as described in the figure below.

Step 1: Clean the board. The purpose of this step is to clean the surface of the board in order to remove rust or dirt while the surface remains dry.  

Step 2: Solder mask ink coating. The clean board is then loaded into a vertical coater for solder mask ink coating. The thickness of the coating is determined by factors such as the reliability requirements of the circuit board, the field used by the PCB and the thickness of the circuit board. To make matters worse, the surface of the circuit board is not as smooth as expected. The thickness of the solder mask ink varies when it is located on different parts of the board, such as on the traces, on the substrate, or on the copper foil. Due to the consideration of equipment capabilities and manufacturing experience, experienced PCB manufacturers usually specify a specific coating thickness.  

Step 3: Pre-hardening. The pre-hardening is not to completely harden, but to make the coating relatively strong on the board, so that the unwanted coating can be easily removed from the board during the development stage.  

Step 4: Imaging and hardening. At this stage, a transparent film with some circuit images is mounted on the board and then subjected to UV exposure. This process makes the solder mask partially covered by the transparent film harden, while the cross-sectional film covered with the circuit image remains pre-hardened. As a result, when hardening is performed to prevent the exposure of non-designated copper foils from short-circuiting or further affecting the final performance of the circuit board, it is necessary to ensure proper alignment.  

Step 5: Development. Then, put the PCB in the developer to remove the unnecessary solder mask so that the specified copper foil can be properly exposed.

Step 6: Final hardening and cleaning. Implement final hardening so that the available solder mask ink is completely installed on the PCB surface. Then, before further processing (such as surface treatment, assembly, etc.), the board covered with solder mask must be cleaned. 

Solder Mask Design Skills 

In fact, no matter what type of PCB design software you prefer to use, solder masks are optional. By filling in some parameters, the solder mask can be easily designed. Some software can even provide automatic solder masks.  

Before the actual design, it is very necessary to contact the contracted PCB manufacturer in order to correctly understand their ability in the thickness of the solder mask and the minimum spacing between the copper pads. These pads are not for each board. Curing.  

Due to stupid problems of the solder mask (such as insufficient solder mask openings, too many openings, and the number of openings does not match the copper pads in the circuit plane), the circuit board will fail. These problems may be due to negligence or design file modification, but it does take a long time. Some people even cause disasters.