This article is based on the failure mechanism, process technology, and engineering specifications of solder mask via plugging technology. In the form of an FAQ, it helps prevent Design for Manufacturability (DFM) defects at the source of design and manufacturing.

Table of Contents

• Why do most PCB vias require solder mask plugging?
• How is poor solder mask plugging (unfilled/voids) formed?
• Why does incomplete or insufficient solder mask plugging lead to via copper breakage or thinning?
• What are the destructive consequences of thinned or broken via copper? Why is it hard for conventional testing methods to detect?
• How to prevent poor solder mask plugging from an engineering design (DFM) perspective? What is the industry standard for filling rate?
• What via plugging technologies are available at the manufacturing end? How to choose during design?
• How to manage via plugging defects through electrical testing and quality control?

Why do most PCB vias require solder mask plugging?

Among all the vias on the PCB, except for component insertion vias, heat dissipation vias, and test vias, most vias do not need to be exposed. The main reasons and characteristics for applying solder mask via plugging are as follows:

1. Prevent electrical short circuits: The blockage of the solder mask can prevent flux or solder paste from flowing to the component side through the vias during the subsequent component assembly stage, avoiding short circuits caused by this.
2. Save production materials: Applying solder mask via plugging technology can effectively save solder paste.
3. Prevent SMD adhesive loss: Meet the requirements of SMT (Surface Mount Technology) to prevent the SMD adhesive adhering to the surface of IC (Integrated Circuit) components from flowing through the vias.
4. Eliminate cleaning reliability risks: Solder mask via plugging technology can prevent flux, chemicals, or moisture from entering the narrow space between BGA (Ball Grid Array) components and the circuit board, thereby reducing reliability risks caused by cleaning difficulties.

How is poor solder mask plugging (unfilled/voids) formed?

Poor solder mask plugging mainly manifests in the following two situations:

1. Incomplete or insufficient plugging: There is no solder mask ink at the top of the via, and only a small amount of solder mask ink remains at the bottom of the via.
2. Air hole (void) expansion inside the via: There is solder mask on the left side of the via, while on the right side, an air hole extends down the entire hole wall. When it approaches the middle of the via and produces a cross-section, the air hole expands to the left side of the hole wall. At the intersection of the cross-section and the via wall copper foil, the via copper foil is almost broken.

Why does incomplete or insufficient solder mask plugging lead to via copper breakage or thinning?

Once incomplete or insufficient solder mask plugging occurs, micro-etching solutions or acidic solutions may flow into the vias during the subsequent PCB manufacturing process.

Because vias are usually very small (diameter less than 0.35 mm), when poor solder mask plugging occurs, there is almost no solder mask ink left in the hole tent at the via opening, and the solder mask ink remains in the middle or bottom of the via. In this situation, there is no general cleaning solution. The residual micro-etching solution or acidic solution can only hide at the intersection of the solder mask and the via hole wall and cannot be eliminated. Ultimately, this will cause the via copper foil to be corroded, thinned, or broken.

What are the destructive consequences of thinned or broken via copper? Why is it hard for conventional testing methods to detect?

Via copper damage and thinning have extremely high concealment and destructiveness:

1. Conventional testing cannot expose:
   • When the copper inside the via becomes thin, its resistance will reach the milliohm (mΩ) level. Because the resistance change is extremely small, it cannot be tested using the two-wire measurement method, resulting in defective products failing to be exposed.
   • If the via copper foil is very thin and not completely broken, it cannot be found through conventional testing methods (including AOI Automated Optical Inspection, AXI Automated X-ray Inspection, and visual inspection).
2. Fatal consequences during the PCBA stage and long-term use:
   • PCBA stage: During the PCBA (Printed Circuit Board Assembly) stage containing solder, due to high-temperature operation and Z-axis direction expansion, the thinned via copper will be pulled and broken. This will cause the electronic product to be unable to achieve complete functions, or functional instability may occur during long-term use by the customer. Once this problem is discovered, all products belonging to the same production batch must be scrapped, causing huge economic losses.
   • Client stage: For copper breakage or circumferential breakage, although PCB manufacturers can detect some through electrical testing, because the copper etching process of the micro-etching solution is extremely long, the breakage often does not occur until it flows to the customer usage stage. Customers will encounter phenomena such as unstable operation, black screens, or freezing of electronic products, all of which may be caused by via copper damage and disconnection.

How to prevent poor solder mask plugging from an engineering design (DFM) perspective? What is the industry standard for filling rate?

1. Control hole diameter design: After receiving the design files, the engineering department of the PCB manufacturing factory should focus on the hole diameter and its requirements. The hole diameter for plugging should be less than 0.35 mm. An excessively large hole diameter easily leads to incomplete or insufficient plugging.
2. Filling rate (fullness) standard: Although customers usually do not make specific regulations on the integrity of vias, and there is no clear definition of plugging integrity in IPC specifications, according to the requirements set by the most extensive PCB manufacturers, a plugging filling degree of 75% or more is the most suitable.

What via plugging technologies are available at the manufacturing end? How to choose during design?

Currently, the PCB industry has mastered the following four solder mask via plugging technologies:

• Technology #1 (Recommended): Solder mask → Solder mask printing (conducted using aluminum plates together with plugging and exhaust plates).
• Technology #2 (Not recommended): Via plugging and solder mask ink printing occur simultaneously.
• Technology #3 (Recommended): Resin plugging → Solder mask printing.
• Technology #4 (Uncommon): Surface treatment → Via plugging.

Process scheme evaluation:

• Regarding filling integrity: Technology #1 and Technology #3 are recommended because these two methods help achieve a high filling degree. However, their disadvantages are that they require complex aluminum plate and exhaust plate manufacturing processes, require two or more printing machines for simultaneous printing, and the baking of the circuit boards takes more time.
• Regarding Technology #2: Although this technology has high manufacturing efficiency, its fullness is difficult to control. Because an excessively low plugging filling degree will lead to via copper thinning or breakage, this technology is not recommended.
• Regarding Technology #4: This technology is usually not applied in practice, so it will not be discussed in depth.

How to manage via plugging defects through electrical testing and quality control?

1. Limitations and applications of electrical testing: Via open-circuit inspection can indicate whether the via copper is too thin or copper damage caused by insufficient plugging. Electrical testing can rarely detect copper thinning problems, but it can explore circumferential copper breakage problems. If an open circuit is found during electrical testing, it can be used to verify whether the defect is caused by electroless copper (chemical copper), the electroplating process, or poor solder mask via plugging, and corresponding measures can be formulated after finding the cause.
2. Material quality control: The plugging ink (solder mask oil) and plugging resin used in new products must undergo strict technical testing to ensure their quality, and then their performance should be further verified through small-batch testing. If the via plugging quality of the solder mask or resin is too low, air holes will be generated in the vias. Once the micro-etching solution enters these air holes, the via copper will be slowly etched, leading to via copper thinning or breakage. Material quality must not be compromised in pursuit of low cost.