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To understand the thickness of solder mask, we must first learn what solder mask is. Today's article will lead you to learn about various knowledge points related to solder mask.
1. Definition of solder mask
Solder mask is a coating covering the surface of a printed circuit board, and its main component is solder paste or solder. Solder paste usually contains ingredients such as solder and flux, and its purpose is to be used for soldering electronic components during PCB assembly.
During soldering, in addition to the connection pads (pads) that need to be soldered, solder mask plays a role in preventing solder invasion and bridging (lapping) in other parts, thereby effectively protecting the original characteristics of non-welding parts of the PCB. In fact, in addition to preventing welding of parts that should not be welded, it is more important to protect the safety and reliability of the PCB for a long time during the use of the PCB, such as anti-corrosion (chemical, physical, etc.), anti-moisture, anti-mildew and anti-pollution. At the same time, it also plays a role in participating in controlling the characteristic impedance value. That is to say, the role of solder mask not only plays a role in solder resistance or solder prevention, but also plays a role in "three protections" (anti-corrosion and anti-pollution, moisture and anti-mildew). Solder mask processing currently has screen printing and electrostatic spraying methods.
The impact of PCB solder mask design on PCBA (Printed Circuit Board Assembly) is mainly reflected in the following aspects:
1. Impact on manufacturability
• Poor solder mask coverage: If the solder mask design is unreasonable, such as the solder mask opening is too large or too small, it may cause insufficient exposure of the pad or increase the risk of short circuit, affecting the welding quality.
• Bridging or open circuit problems: Too small solder mask spacing may cause solder bridging, resulting in short circuit; too large may cause unstable welding.
2. Impact on welding quality
• Solder climbing: The thickness, material and window design of the solder mask will affect the distribution of solder. If the solder mask window is too large, it may cause uncontrolled solder flow, resulting in solder short circuit or solder balls.
• Component alignment accuracy: Improper solder mask design may cause uneven tin on the pad, thus affecting the reliability of component mounting and welding.
3. Impact of electrical performance
• Soldermask thickness affects impedance: In high-frequency applications, solder mask thickness affects impedance matching, which in turn affects signal integrity. Usually, high-frequency boards have strict requirements on solder mask thickness to ensure stable electrical performance.
• Insulation protection: Reasonable solder mask design can reduce the risk of electrical short circuits and improve the long-term reliability of circuit boards.
4. Difficulty of rework and repair
• Rework of packaged devices such as BGA: If the solder mask design is unreasonable (such as too large a window), the soldering quality may be reduced due to pad oxidation or solder diffusion during rework, increasing the difficulty of repair.
5. Impact of visual inspection and automatic optical inspection (AOI)
• Impact on detection accuracy: Factors such as solder mask color, contrast, and thickness may affect the recognition accuracy of AOI detection, thereby affecting the defect detection rate.
PCB soldermask thickness standard
The PCB solder mask must have good film forming properties, and its thickness is subject to regulatory requirements. At present, most circuit board manufacturers conduct identification according to the US civil standard IPC-SM-840C. In this standard, the solder mask thickness of Class 1 products is unlimited; the minimum solder mask thickness of Class 2 products is 10um; the minimum thickness of Class 3 products should be 18um. The flame retardancy of solder mask is usually based on the specifications of the US UL agency and must pass the requirements of UL94V-0 (UL94-flammability test of plastics for components in devices and equipment).
Generally, the solder mask thickness in the middle of the line is generally not less than 10um, and the thickness on both sides of the line is generally not less than 5um, depending on the customer's needs.
The importance of solder mask PCB
1. Prevent short circuits: The solder mask plays an important role in preventing short circuits between components on the PCB. When soldering components to the PCB, if there is no protection from the solder mask, solder paste overflow or poor soldering may occur during the soldering process, resulting in short circuits between components. The solder mask can effectively isolate the soldering area to prevent this from happening, thereby improving the reliability of the PCB.
2. Improve welding quality: By controlling the shape and size of the soldering area, the solder mask also helps to improve the accuracy and quality of welding. During the welding process, the solder mask can reduce the welding area, improve the accuracy of welding, avoid uneven or excessive solder flow, and ensure that the welding quality meets the standard requirements.
3. Increase mechanical strength: The presence of the solder mask can increase the mechanical strength of the PCB and make it more durable. As a covering layer, the solder mask can fix components and connect circuits to prevent the external environment from damaging the PCB. At the same time, during the assembly and transportation of the PCB, the solder mask can also effectively protect the surface of the PCB and reduce the impact of mechanical vibration and collision.
4. Improve moisture resistance: The solder mask usually has good sealing performance, which can effectively prevent moisture, dust and other impurities from entering the interior of the PCB, thereby protecting the components on the circuit board from environmental factors. This is crucial for electronic equipment that requires high reliability and long life, especially equipment working in humid environments.
5. Convenient detection and maintenance: The presence of the solder mask makes the surface of the PCB smoother, which is convenient for detection and maintenance operations. When troubleshooting or replacing components, the solder mask can simplify the workflow, effectively protect other parts from damage, and reduce maintenance time and cost.
6. Environmental protection and beauty: The use of solder mask can reduce pollutant emissions during PCB manufacturing and meet environmental protection requirements. In addition, the solder mask can provide color and appearance protection for the PCB, making the PCB look more beautiful and professional.
The appropriate soldermask thickness can not only improve the reliability and stability of the PCB, but also protect the components of the PCB from the external environment, and also facilitate detection and maintenance operations. The application of solder mask can not only improve the performance and reliability of the circuit board, but also help to simplify the manufacturing process, reduce maintenance costs, and improve production efficiency to a certain extent.
Processing of solder mask
Solder mask materials can be used through liquid wet process or dry film lamination. Dry film solder mask materials are usually supplied in a thickness of 0.07-0.1mm, which is suitable for some surface mount products, but it is not recommended for fine pitch applications. Liquid photosensitive solder mask materials are usually specified for surface mount applications due to their economy and precision.
Soldermask thickness is a core parameter that affects PCB electrical performance (such as impedance stability, high-frequency signal integrity), mechanical reliability (peel strength, flexibility) and welding quality (pad wettability). By optimizing the thickness (recommended range: 15–25 μm), the signal attenuation control capability of high-frequency PCBs can be significantly improved (optimization range of 12–18%) and the soldering yield of automotive electronic PCBs can be significantly improved (increased by about 20%).