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Comprehensive Analysis of Circuit Board Solder Mask

The Structure, Function and Process Details of The Solder Mask

June 27, 2025

What is Solder Mask Made of?

Solder masks are usually made of resin (epoxy resin, polyurethane, or acrylic resin), photopolymers, or liquid UV curing material (LPI). These materials provide heat resistance, insulation, and corrosion protection, firmly adhering to the PCB surface to form a solid protective film.

The Core Function of Solder Mask

In PCB fabrication, the most critical function of solder mask is to prevent solder bridging, ensuring welding quality and circuit safety. During the soldering process, molten solder is highly fluid. Without the restriction of a solder mask, solder may flow between adjacent pads, leading to short circuits or device failure.

The solder mask forms an insulating protective film and guides solder only to the designated pad areas, thus improving welding accuracy.

In addition, the solder mask isolates air and moisture, prevents copper oxidation, and increases long-term durability. Its electrical insulation also minimizes crosstalk and signal interference in high-density designs. For multi-layer PCBs and high-Tg PCBs, solder mask thickness and quality play an important role in ensuring stability and reducing EMI.

Solder mask coating on PCB surface with thickness control

Coating, Exposure and Development on Both Sides of The Circuit Board

Visual Appearance of Solder Mask

The appearance of the solder mask is important but not the only criterion for judging quality. Green is the industry standard, but red, blue, black, white, and yellow are widely used for applications such as medical PCB board fabrications, consumer electronics, and high-end customization.

Color differences may appear due to coating thickness, curing temperature, or copper reflectivity. These minor differences do not affect performance or soldering quality. According to IPC-A-600, acceptable variations include small scratches, bubbles, or rework marks, as long as functional requirements are not compromised.

Relationship Between Component Holes and Solder Mask

In PCB design, the relationship between plated through-holes (PTH) and solder mask strongly affects manufacturability and soldering reliability.

  • For THT components, hole walls must remain free of solder mask to ensure full solder wetting.

  • For microvias or vias under 0.45mm, solder mask thickness and ink fluidity may cause partial residue in holes.

  • When the aperture is less than 0.3mm, solder mask may even fully block the via. This is not a defect but a process-related phenomenon.

Edge Coverage and Solder Mask Retreat Distance

The solder mask should fully cover copper foil but reserve a 0.1–0.15mm retreat distance from pads to avoid covering them. This detail is especially critical for high-frequency PCBs, HDI PCB boards, or BGA packages, where solder mask thickness control directly impacts pad alignment and solderability.

Requirements for Solder Mask Coating Under Different PCB Structures

The solder mask layer is usually applied to both sides of the circuit board in the printed circuit board assembly to form a standard double-sided solder mask structure, which is the most common design form and is suitable for most double-sided circuit boards. In this configuration, both the front and back sides need to be coated, exposed and developed to protect the copper wires, pads and isolate external contaminants.

However, for multilayer PCB and high-density interconnect boards (HDI PCB) with more complex structures, the requirements for solder mask coating are also more refined. This type of PCB usually contains a large number of micro blind holes, laser drilling and dense wiring areas. Local thickening or multiple coating processes may be used in the manufacturing process to enhance the protection of key circuit areas and improve welding reliability and durability.

In addition, in some high-frequency or high-voltage applications, in order to improve the insulation performance of the solder mask layer, customized protection may also be achieved by adjusting the thickness or using special materials (such as ceramic solder mask). Therefore, the number of solder mask layers depends not only on the board structure, but also on the comprehensive influence of electrical performance, packaging density and terminal application requirements.

Available Coating Processes

Solder mask can be applied by using the following methods:

  • Curtain Coating: Applicable to large-volume boards, uniform thickness
  • Inkjet spraying: Suitable for high-precision, low-volume products
  • Screen printing: Suitable for simple graphics, low-cost applications
  • LPI (liquid photoresist): The most commonly used technology, exposure and development after photosensitization

Exposure Process of Solder Mask

The solder mask is developed through a graphic exposure process to ensure that the solder mask pattern is accurately aligned with the pad. The UV exposure equipment uses a mask to align the PCB, and cleans the unexposed area after ultraviolet exposure to form a solder mask opening. Common methods include:

  • Manual Film Exposure: Easy to operate, suitable for small-batch PCB boards production
  • Semi-Automated Film Exposure: Improve accuracy and efficiency through mechanical alignment
  • Fully Automated Film Exposure: With precise alignment and stability, it is suitable for high-precision production of large-scale printed circuit manufacturing
  • Manual Direct Imaging (Laser/LED): Direct exposure without film, manual material processing. Suitable for HDI, finer structures and smaller batches
  • Automated Direct Imaging (Laser/LED): Direct imaging does not require film, and directly uses laser or LED light sources to accurately draw graphics. It is particularly suitable for HDI PCB and fast proofing projects, greatly improving resolution and graphic consistency

IPC-A-600 standard inspection of PCB solder mask thickness

Classification of Solder Mask Materials

Common solder mask materials are divided into:

● Thermal Curing Type

This type of material needs to be cured at high temperature, has excellent heat resistance and adhesion, and is suitable for demanding high-reliability applications such as multi-layer rigid boards, ceramic PCBs or military and automotive control modules. Its excellent insulation and chemical corrosion resistance make it the first choice for high-frequency and high-power applications.

● UV Curing Type

Rapidly cured with ultraviolet light, suitable for single layer circuit board, large-volume consumer electronic boards such as TVs, power boards, etc. This type of material has high production efficiency and low cost, and is widely used in the field of mid- and low-end electronic products.

● Hot Melt Type

The lowest cost and simple process, but poor heat resistance and mechanical properties. Generally used in simple Aluminum PCBs or low-end lighting products, not suitable for complex or high-temperature working environments.

Acceptance Criteria and Quality Standards for Solder Mask

According to IPC-6012, IPC-A-600, and IPC-SM-840, the criteria include:

  • No cracks or peeling

  • Clear solder mask edges without oil diffusion

  • No pinholes or thickness irregularities

  • Solder mask thickness between 10–30 μm

  • Appearance compliance: matte, semi-matte, or glossy finish

Note on solder mask thickness: Proper thickness ensures balance between solder protection, insulation, and solderability. If too thin, the solder mask may crack or fail to insulate; if too thick, it may cover pads or affect hole integrity.

Matt or Glossy Green Solder Mask Varnish

Depending on the process, the surface of green solder mask can be matte, semi-matte or glossy. Matte is suitable for industrial control PCBs to reduce light reflection, while glossy has better visual effect and is often used in consumer electronic appearance boards. If you or your customers prefer to use conformal coating, or your end customers have requirements for appearance, please contact us.

Through-hole Residue and Solder Mask Contamination

If through-holes are not cleaned thoroughly after soldering, flux or oxide residues may contaminate the solder mask, causing blistering, peeling, or subsequent reliability reduction, so cleaning and baking treatments need to be strengthened.

Solder mask residues in through-holes are mainly caused by 3 factors:

● Through-hole aperture is too small

When the final aperture of the through-hole is less than Ø0.45 mm, especially less than Ø0.3 mm, the solder mask ink is difficult to completely remove during deposition or exposure and development, and it is easy to form residues on the hole wall or inside the hole, or even completely block the hole. This is because the ratio between the surface tension of the ink and the aperture is too large, resulting in incomplete cleaning.

● Too thick solder mask coating

If the thickness of the solder mask is not properly controlled during printing or spraying, especially on high-density boards, the amount of one-time coating is large, which is easy to cause ink to accumulate at the hole mouth. When the subsequent development and rinsing steps fail to completely remove the excess material in the hole, it will cause residues.

● Insufficient development and rinsing process

If the exposure energy of the solder mask is insufficient, the development time is too short, the water pressure is insufficient, or the nozzle angle is not good, the ink in the through hole may not be fully developed and discharged, and the residue will be difficult to remove after solidification. This problem is more likely to occur when the parameters of the automated processing equipment are not matched.

PCB pads with precise solder mask thickness to prevent bridging

A Few Things You Need to Know About Solder Mask

Solder mask is not an insulating material and cannot completely replace the insulating layer design

The thickness of solder mask in high-frequency PCB will affect signal transmission and impedance

Black solder mask is not easy to detect, so it is recommended to use it with caution in industrial products

Some high-temperature applications require white ceramic solder mask materials

As an indispensable part of PCB manufacturing, the design and processing quality of solder mask directly affect the weldability, electrical performance and appearance quality of the finished product. Whether in consumer electronics or aerospace, automobile, or medical equipment, a reasonable solder mask process is the guarantee of high-reliability PCB. Choosing SCSPCBA printed circuit board manufacturers, the right materials, standards, and processes are the important cornerstones to help you move towards high quality.

Why Solder Mask Thickness Matters in Circuit Board Manufacturing Quality

Solder masks’ design, thickness, and process quality directly impact solderability, reliability, and electrical performance. Correct solder mask thickness control ensures long-term durability and high-yield PCB manufacturing, from consumer electronics to automotive, aerospace, and medical applications.

At SCSPCBA, we follow IPC standards to guarantee that solder mask thickness, materials, and coating processes meet the highest requirements for one-stop PCB and PCB assembly manufacturing.

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Learn More About SMT Solutions for Printed Circuit Boards

Efficient smart manufacturing starts with “sticking”

June 14, 2025

In the modern electronic manufacturing industry, surface mounting technology （ SMT ）has become an important circuit board assembly method and is widely used in smartphones, medical equipment, automotive electronics, industrial control, and other fields.

1. SMT Production Line: A Symphony of Intelligence and Efficiency

SMT production line consists of multiple key circuit board manufacturing process sections, each of which is crucial to the quality of the final product:

(1 ) Solder Paste Printer

(2) SPI Inspection

(3) High-speed/Multi-function Placement Machine

(4 ) Reflow Oven

(5 ) AOI Automatic Optical Inspection

(6 ) X-RAY Solder Joint Inspection

2. Solder Paste Storage Cabinet: Quality Control

As a key material in SMT surface mount technology, solder paste is extremely sensitive to temperature and humidity. The PEMS Intelligent Solder Paste Cabinet provides a constant temperature and humidity environment to ensure that the solder paste is stored in an ideal state to avoid oxidation or decreased activity.

SMT patch work area

3. SMT Quality Control

In the SMT production process, the quality management system plays a vital role, which is directly related to the yield and reliability of the circuit board. In order to ensure the welding quality and mounting accuracy, printed circuit board manufacturers generally introduce multi-level detection methods.

( 1 ) SPI Detection

SPI (Solder Paste Detection), located after the printing process, detects key parameters such as solder paste thickness, height, and area, which can effectively prevent defects such as insufficient solder, solder bridges, and cold solder joints, laying a good foundation for subsequent surface mounting links.

( 2 ) AOI Automatic Optical Inspection

AOI automatic optical inspection is performed after patching and reflow soldering. It uses high-definition cameras and image comparison technology to identify problems such as offset, reverse, missing, wrong and poor soldering. It has high accuracy and fast detection speed, which significantly improves the production efficiency and yield rate of board for circuit.

( 3 ) X-ray Detection

X-ray detection can see through the internal structure of solder joints and judge defects such as empty solder joints, solder ball offset, and missing pads. It is an indispensable detection method for high-end printed circuit board manufacturing.

(4 ) MES System Docking

Connect with the MES system to achieve full process data tracking from incoming materials to shipment. Each circuit board has its independent production history, including process parameters, test results, responsible persons and other information, to ensure that problems can be traced, quality can be controlled, and customers can rest assured.

SCSPCBA has the above set of strict quality control systems, which has built the trust foundation of SMT manufacturing and is the high reliability and consistency of printed circuit assembly.

Fully automatic solder paste printer work area

4. SMT Online Quotation System: Get Accurate PCBA Prices

In the rapidly changing electronics manufacturing market, efficiency and transparency have become important criteria for customers to choose SMT suppliers. To this end, SCSPCBA circuit board manufacturers have developed an advanced online quotation system, dedicated to providing customers with fast, intelligent, accurate and transparent one-stop PCBA quotation services, greatly simplifying the procurement process and improving product launch speed.

(1) Upload BOM & Gerber Files for Instant Analysis

Users only need to upload BOM and Gerber files, and the system can instantly analyze key information such as component model, package, usage and PCB board layer structure, and quickly estimate material costs. The system connects to multiple mainstream component supplier platforms, compares spot prices and batch inventory information in real time, and helps customers choose the most cost-effective device solution.

(2) AI Big Data Analysis

Through AI big data analysis, the system can also automatically recommend equivalent alternative components and select available solutions from a database of more than 10,000 products to further optimize the cost structure. In periods of component shortage or price fluctuations, multi-supplier price comparison and alternative suggestions are provided to keep customers in control.

(3) Full-process Quotation

The entire system can generate a full-process quotation including material procurement, PCB manufacturing, SMT patch, and functional testing within 10 seconds, and clearly list the details of each cost. The price is transparent and there is no hidden cost, which greatly enhances customer trust.

(4) Full-process Tracking

Customers can place orders with one click and track the entire process of production orders through the system, including key links such as material completeness, patch progress, test nodes, and delivery time, truly realizing a digital and visual project management experience.

AI Online Quotation System

5. Choosing A Professional SMT Manufacturer Means Choosing Stability and Efficiency

SCSPCBA PCB board manufacturers has created a closed loop of intelligent manufacturing that integrates component procurement, PCB manufacturing, SMT assembly, functional testing, and finished product assembly, thereby achieving:

  • Reducing procurement and management costs
  • Speeding up delivery cycles
  • Improving product consistency and traceability
  • Expanding flexible production capabilities for multiple varieties and small batches

Whether it is a start-up technology company or an international brand, choosing an experienced, advanced equipment, and a sound quality system SMT supplier is a key step in the successful launch of a product.

If you are looking for a trustworthy SMT partner, welcome to experience our 10-second intelligent quotation system, or visit our SMT smart factory to experience the charm of “efficiency + quality” in smart manufacturing.

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One-stop Box Build Assembly Solution: From PCB to Complete Machine Assembly

One-stop Box Build Assembly Solution: From PCB to Complete Machine Assembly

May 20, 2025
 

Box Build Assembly has become an important bridge between electronic design and final products in modern electronic manufacturing. The perfect integration of circuit boards and structural parts determines product performance and affects delivery speed and market competitiveness.

1. What is Box Build Assembly?

Box Build Assembly, also known as system integration or complete machine assembly, refers to integrating multiple electronic, electrical and mechanical components into a complete package. It goes beyond traditional PCB assembly and covers housing installation, cable connection, functional module integration and final testing. It is one of the most complex and closest links to the final product in electronic manufacturing services (EMS).

2. What Are The Components of Box Build Assembly?

Box Build Assembly usually includes the following key components:

  • PCB circuit board: the core of the main control or functional module
  • Cables and harnesses: used for signal and power connections
  • Power module: adapts to various input/output voltage requirements
  • Display/indicator/button: human-computer interaction component
  • Structural parts and housing: protect electronic systems and improve the aesthetics of the whole machine
  • Fasteners and brackets: ensure that each component is firm and aligned
  • Software and firmware: realize functional control and user interaction

The box build assembly includes printed circuit boards, various components, and cables

3. The Core Role of PCB Board In Box Build

In box build assembly process, circuit board manufacturing is the “nerve center” of the whole system. We have mature DIP and SMT mounting technologies in PCB integrated circuit solution, supporting complex requirements from Double Layer PCB, Multilayer PCB, HDI PCB, Aluminum PCB, Ceramic PCB, Thick Copper PCB, High-Tg PCB to mixed assembly.

  • Precise welding to ensure signal integrity and thermal management capabilities
  • Integrated test points and interface design to facilitate subsequent system joint debugging
  • Support the application requirements of flexible PCB and rigid-Flex PCB

4. Our Box Build Assembly Capabilities

Integrated capabilities from circuit design, electronic components procurement, PCB board fabrication, printed circuit assembly, surface mounting technology, plug-in, structural parts customization, to whole machine testing.

  • Full turn-key PCB assembly services: PCB design, PCB fabrication, DFM checking, components procurement, PCB assembly, BGA assembly, firmware programming, functional testing and box-build.
  • Precision structure design support: 3D modeling verification, simulate assembly interference in advance
  • BOM procurement and material management: original factory authorized channels to ensure delivery time and quality
  • Integrated MES traceability system: Each finished product can track component batches and test data

Worker functional testing printed circuit assembly Box Build Assembly process

5. Box Build Assembly Application Industries

Box build assembly manufacturing is widely used in the following industries:

Industries application
Industrial Control Power control box, Instrument housing
Medical Electronics Wearable health equipment, Detector housing system
Communications Router, Antenna box, 5G base station submodule
Smart Home Smart gateway, Central control system, Security terminal
Automotive Electronics In-vehicle controller, Charging pile main control box
Consumer Electronics Audio system, Smart wearable host

Box Build Assembly helps industries such as industry, medical care, and communications

6. Our Box Build Assembly Process

SCSPCBA printed circuit board manufacturer, adopts a standardized and flexible whole-machine assembly process to ensure product stability and consistency:

● Project Evaluation and DFM Support

– Analyze customer drawings, optimize PCB design, and confirm the feasibility of printed circuit assembly

● BOM Procurement and Material Management

– Carry out full-process electronic components procurement and warehouse material preparation according to the customer BOM

● Printed Circuit Board Assembly

– SMT surface mount technology, AOI/X-ray inspection, functional testing

● Cable/Subassembly Manufacturing

– Homemade wiring harnesses, plastic parts, metal brackets and other parts

● Machine Assembly

– Combining manual and automated processes to complete multi-step system integration

● Testing and Verification

– Functional testing, aging testing, waterproofing/drop testing

Packaging and Shipping

– Customized packaging, anti-static/shockproof treatment

Finished product display of scspcba Box Build Assembly

 
 
 
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SUCCESS CIRCUITS GROUP LIMITED

Company established 2013 | Manufacturing sites: Shenzhen, Jiangsu, Jiangxi (China); Vietnam | Service centers: Wuhan (China), USA | All statistics are current as of 2026.

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